Decay Scheme Data of Neptunium Isotopes

A thesis submitted to the University of London for the degree of Doctor of Philosophy

Ian Michael Lowles

July 1991

Imperial College Reactor Centre Imperial College of Science, Technology and Medicine Dedicated to my Parents

Persons attempting to find a motive in this narrative will be prosecuted; persons attempting to find a moral in it will be banished; persons attempting to find a plot in

it will be shot.

Mark Twain The Adventures of Huckleberry Finn. Acknowledgments

I would like to thank Dr. T.D. MacMahon for the help and encouragement he has given during the course of this work, but above all for his friendship.

I am very grateful to Mr. R. Benzing, Dr. A.H. Naboulsi, Mr. I.W. Sinclair and Dr. A.W. Glauert for their many useful suggestions and comments, in areas which included: spectroscopy; statistics; chemistry and computing.

I would also like to thank the Reactor Operations staff, without whose help much of this work would have been impossible.

Thanks also to Dr. A.J. Fudge, Mr. R.A.P. Wiltshire and Mr. I.D. Jackson for their help and advice during my time at Harwell.

Finally, I must thank Alison Smith whose love and support has helped me through the difficult times in this work. She has also had the unenviable task of reading the rough drafts of this thesis, and for that she must be pitied. Abstract

The purpose of this research was to investigate the decay scheme parameters fy\n ryx o of Np and Np, and in particular their half-lives and gamma-ray emission probabilities.

The work was prompted by the International Atomic Energy Agency, Coordinated Research Programme (IAEA-CRP) request for more accurate actinide decay data in order to:

i. assess accurately the effects of these data on thermal and fast reactor fuel cycles

ii. aid in the evaluation of nuclear waste management procedures

iii. provide reliable data for nuclear safeguards

iv. extend the knowledge of actinide decay parameters required in scientific research

Several authors have recently reported precise measurements of alpha- particle, gamma-ray and conversion electron emission probabilities in the decay of 937 Np. An attempt to construct a self-consistent decay scheme from these data has failed. The author believes that the anomalous decay scheme is due, not to what has been measured, but is a consequence of what has previously gone undetected. Prime candidates for these missing data are several very low-energy, high-intensity gamma-ray transitions. An experiment was undertaken to analyse the low-energy gamma-ray spectrum of Np, in an attempt to determine the existence of these transitions, and so resolve the anomalous decay scheme.

The only available precise measurement of the half-life of Np is that of Brauer et al. (1960). As a consequence of this radionuclide's importance in fission reactors, the IAEA-CRP requested a confirmatory measurement of the half-life of 237Np.

In this present determination the specific activity of 37 alpha-particle sources, prepared from known weights of high purity Np solution, have been measured. Accurate concentration analysis of the solution was provided by controlled-potential coulometry. Source activities were measured by a-particle counting in two gas flow proportional counters of known geometry. The measured specific activity and half-life are in excellent agreement with the value reported by Brauer et al. (1960). Moreover, the validity of many of the assumptions used in this present measurement have been verified experimentally.

Attempts to produce a self-consistent decay scheme for Np have relied exclusively on gamma-ray emission probability measurements. However, the precision with which the intensity of the principle gamma-ray transition has been measured, has varied between 1.45 and 3.80%. This was considered unacceptable by the IAEA-CRP, who called for a further measurement of the gamma-ray emission probabilities of 239Np, requesting a precision of 1% for the principle transition.

In this present determination, Np has been prepared by radiochemical noo separation from neutron irradiated U, and by separation from its equilibrium parent, 243Am. Photopeak areas were determined using two different analytical peak fitting routines and source activities were measured using a 4jtp-y coincidence system, which utilised the efficiency extrapolation technique.

The measured gamma-ray emission probabilities are consistent with those of previous workers. Moreover, the intensity of the principle transition has been measured with a precision of greater than 1%, satisfying the original aims of the IAEA-CRP. Contents 4

Contents

Acknowledgements ...... 1

Abstract ...... 2 Contents ...... 4 List of Tables ...... 8 List of Figures ...... 10

1. Introduction ...... 13

1.1 The I A E A - C R P ...... 13 1.2 Nuclear Decay Data of Neptunium Isotopes . 14

2. The Chemistry of Neptunium .... 18 2.1 The Placement of Actinides in the Periodic Table 18 2.2 The Discovery of Neptunium ..... 18 2.3 The Similarities Between the Lanthanide and Actinide Elements 22 2.3.1 Oxidation States ..... 22 2.3.2 The Lanthanide and Actinide Contractions 22 2.4 The Solution Chemistry of Neptunium 25

2.5 Ion-Exchange Chromatography .... 29 2.6 Separation of Neptunium by Ion—Exchange Chromatography 33

3. Counting and Spectroscopy Techniques . 38

3.1 Introduction ...... 38

3.2 Proportional Counters ...... 38 3.2.1 Operating Voltage ..... 39 3.2.2 Alpha-Particle Counting .... 41

3.2.3 Beta-Particle Counting .... 43 Contents 5

3.3 Solid State Detectors ...... 45

3.3.1 Gamma-ray Interactions ..... 45 3.3.2 Photoelectric Absorption ..... 45 3.3.3 Compton Scattering ...... 46 3.3.4 Pair Production ...... 47 3.3.5 Nal(Tl) Scintillation Detectors ..... 48 3.3.6 Semiconductor Detectors . . . . . 51 3.3.7 Peak Area Determination ..... 55 3.3.8 Sampo Routine ...... 57 3.3.9 Omnigam Routine ...... 57 3.3.10 Gamanal Routine ...... 59 3.3.11 Peak Anomalies ...... 60 3.3.12 Detector Efficiency ...... 61 3.4 Coincidence Counting ...... 67 3.4.1 Conversion Electrons ...... 68 3.4.2 Gamma Sensitivity of the p-detector .... 69 3.4.3 Beta Sensitivity of the y-detector . . . . 70 3.4.4 Complex Decay Schemes ..... 70 3.4.5 47tP~y Coincidence System . . . . . 71

3.4.6 Dead Time and Resolving Time Corrections . . . 74

3.4.7 Corrections to the Coincidence Formulae . . . 77

3.4.8 Data Analysis ...... 78

4. Low-energy Gamma-ray Analysis of Np . . . 80 4.1 Introduction ...... 80

4.2 Review of Alpha-Particle Decay Data .... 80

4.2.1 Review of Gamma-Ray Decay Data .... 86

4.2.2 Review of Alpha-Gamma Coincidence Data . . . 88

4.2.3 Review of Gamma-Gamma Coincidence Data . . 91 Contents 6

4.2.4 Review of Conversion Electron Data 95

4.3 Evaluation .... 96 4.4 Experimental .... 99 4.5 Results ..... 106 4.6 Discussion .... 109 4.7 Conclusion .... 113

5. The Half-life of 237Np 114 5.1. Introduction ..... 114 5.2 Review of Previous Measurements 114

5.3 Controlled-Potential Coulometry 116 5.4 Experimental ..... 121 5.4.1 Preparation of Np Standards 121 5.4.2 The Analysis of Neptunium by Controlled-Potential Coulometry 122 5.4.3 Source Preparation 124 5.5 Results ...... 126 5.5.1 Concentration Analysis 126 5.5.2 Source Count Rate 127

5.5.3 Specific Activity 127

5.6 Discussion ..... 136 5.6.1 Alpha-Particle Self-Absorption Correction 136 5.6.2 Source Uniformity Correction . 136 5.6.3 Beta-Particle Sensitivity 140 5.6.4 Alpha-Particle Backscattering . 140 5.7 Conclusion ..... 144

6. Gamma-ray Emission Probabilities of 239Np 145

6.1 Introduction ..... 145 Contents 7

6.2 Review of Previous Measurements . . . . . 145 6.2.1 Half-life Measurements ..... 145 6.2.2 (3-Particle Emission Probabilities .... 145 6.2.3 Internal Conversion Coefficients . . . . 147 6.2.4 y-ray Emission Probability Measurements . . . 147 6.3 Experimental ...... 151 6.3.1 Thermal Neutron Irradiation of 238U02 Spheres . . 151 6.3.2 Thermal Neutron Irradiation of 238U02 Powder . . 155 6.3.3 Separation 239Np from242Am . . . . 159 6.4 Results ...... 161 6.4.1 Absolute Disintegration Rates . . . . . 161 6.4.2 Absolute y-ray Emission Probabilities . . . 163 6.5 Conclusion ...... 168

7. Summary and Conclusions ...... 169

References ...... 173 Contents 8

List of Tables

1.1 Nuclear Decay Data and Their Applications .... 15

2.1 The Periodic Table ...... 20 2.2 Electronic Configuration of the Elements . . . . 21 2.3 The Oxidation States of the Lanthanides and Actinides . . . 23 2.4 Neptunium Ions in Aqueous Solution ..... 28

3.1 Isotopes Used for the Calibration of a Semiconductor Detector . 63

4.1 Measured a-particle Emission Probabilities . . . . 81 4.2 Evaluated a-particle Emission Probabilities .... 83 4.3 a-particle Emission Probabilities determined by CBNM and CIEMAT, Together with Their Recommended Pa Values . . 85 4.4 Energy Levels of 233Pa ...... 85 4.5 Absolute y-ray Emission Probabilities ..... 87 4.6 a-y Coincidence Data (Gonzalez et al., 1979) .... 89 4.7(a) y-y Coincidence Data (Skalsey and Connor, 1976) . . . 92

4.7(b) y-y Coincidence Data (Gonzalez et al., 1979) . . . 92

4.7(c) y-y Coincidence Data (Woods et al., 1988) .... 92 4.8 Conversion Electron Measurements of Woods et al. (1988) . . 95 4.9 Total Transition Probabilities of 237Np ..... 96

4.10 Purity of the Neptunium Sample ..... 99 4.11 Measured Relative y-ray Emission Probabilities of'I'M Np Normalised to P-^(86.5) = 12.3% . . 108

4.12 ICC's for the 5.18 keV T r a n s itio n...... I ll

4.13 ICC's for the 8.69 keV Transition . . . . . I ll

4.14 ICC's for the 36.20 keV T ran sition...... I ll Contents

5.1 Purity of the Neptunium Sample 5.2 Results of the CPC Analysis .... 5.3 237Np Concentration of the Secondary Standards 5.4 Alpha-Source Count Rate .... 5.5 Specific Activities ..... 5.6 The Specific Activity of Each for the Secondary Standards

6.1 A Review of the Half-life Measurements of Np 6.2 239Np End-Points ..... 6.3 A Review of the Absolute y-ray Emission Probabilities of 239Np ..... 6.4 23^U02 Sphere Experiments .... 6.5 The Absolute Disintegration Rates of Each Source 6.6 Measured Absolute y-ray Emission Probabilities of Np Contents 1 o

List of Figures

2.1 Lanthanide and Actinide Contraction of the +3 Cations . . 24

2.2 Neptunium (4n +1) Series ...... 26 2.3 Schematic Representation of a Cation Exchange Resin . . . 29 2.4 Schematic Representation of an Anion Exchange Resin . . 30 2.5(a) Equilibrium Distribution Coefficients for Zr(IV), Nb(V) and Mo(VI) in Hydrochloric-Hydrofluoric Acid Solutions . . 34 2.5(b) Equilibrium Distribution Coefficients for Np(IV), Pu(IV) and U(VI) in Hydrochloric-Hydrofluoric Acid Solutions . . 34 2.6(a) Equilibrium Distribution Coefficients for Zr(IV), Nb(V) and Mo(VI) in Hydrochloric Acid Solutions . . . 36 2.6(b) Equilibrium Distribution Coefficients for Np(IV), Pu(IV) and U(VI) in Hydrochloric Acid Solutions .... 36

3.1 Different Regions of Operation for Gas-Gilled Detectors . . 40 3.2 Theoretical Counting Curve ...... 40

3.3(a) Illustration of a 2k Gas Flow Proportional Counter . . . 42 3.3(b) Schematic Representation of Associated Electronics . . . 42 3.4(a) Illustration of a 4rc Gas Flow Proportional Counter . . . 44

3.4(b) VYNS Thin-Film S o u r c e ...... 44 3.5 Photoelectric Absorption ...... 45

3.6 Compton Scattering ...... 46 3.7 Compton Continuum ...... 47

3.8 Energy Band Structure of an Activated Crystalline Scintillator . . 49 3.9 Illustration of a Scintillation Detector ..... 50

3.10 Band Structure for Electron Energies in Insulators and Semiconductors ...... 52 3.11(a) HPGe Detector Assembly ...... 56 3.11(b) Schematic Representation of Detector Electronics . . . 56

3.12 Background Calculation Method used by Omnigam . . . 58 Contents 1 1

3.13 Cascade y-rays Leading to Coincidence Summing . . . 60 3.14 Absolute Photopeak Efficiency Curve for an Intrinsic HPGe Detector ...... 65 3.15 Absolute Photopeak Efficiency Curve for a p-type HPGe Detector ...... 65 3.16 47t|3-y Coincidence Counting System ..... 72 3.17(a) Variation in the Coincidence Count Rate with Resolving Time . 76 3.17(b) Variation in the Accidental Coincidence Count Rate with Resolving Time ...... 76 3.18(a) Graph of Np vs. Nc/Ny to Determine the Absolute Disintegration Rate of a 60Co Source .... 79 3.18(b) Graph of Np(Ny/Nc) vs. Ny/Nc to Determine the Absolute Disintegration Rate of a ^Co Source . . . 79

4.1 a-y Coincidence Data (Gonzalez et al., 1979) .... 90 4.2 y-y Coincidence Data (Skalsey and Connor 1976, Gonzalez et al. 1979, Woods et al 1988) .... 93 4.3 Proposed Linking Transitions ...... 94 4.4 Decay Scheme Evaluation ...... 91 4.5(a) y-ray Spectrum of 237Np (1-29 keV) .... 101 4.5(b) y-ray Spectrum of 237Np (1-57 keV) .... 102

4.5(c) y-ray Spectrum of 237Np (1-113 keV) .... 103

4.5(d) y-ray Spectrum of 237Np (1-226 keV) .... 104

5.1 A Typical Current-time Curve ...... 117 5.2 Electrolysis Cell ...... 122 5.3(a) and (b) Variation in the Measured Count Rate as a *Y¥J Function of the Mass of Np Deposited on Each Tray . . . 129 5.3(c) and (d) Variation in the Measured Count Rate as a Function of the Mass of Np Deposited on Each Tray . . . 130

5.3(e) and (f) Variation in the Measured Count Rate as a Function of the Mass of Np Deposited on Each Tray 131 Contents 12

5.3(g) and (h) Variation in the Measured Count Rate as a Function of the Mass of 237Np Deposited on Each Tray . . . 132 5.4 Variation in the Specific Activities of the Secondary Standards . 135 5.5 Comparison of Present Measurement with Previous Workers . . 135 5.6 Variation on the Count Rate with Source Thickness . . . 137 5.7 Disposable Electrodeposition Cell . . . . . 139 5.8 Variation in Count Rate with Source Thickness for Electrodeposited Sources ...... 141

6.1 Decay Scheme of 239Np ...... 148 6.2 The Sequential Decay of 243Am ...... 150 6.3 The Sequential Decay of 239U ...... 150 6.4 Irradiation Positions of the ICRC Consort II Experimental Reactor ...... 152

6.5 y-ray Spectrum of Np produced via Neutron Irradiation of 238U02 ...... 154

O ' l Q 6.6 Deconvolution of Np y-ray Spectra by Chemical Purification ...... 157 6.7 y-ray Spectra of Np prepared from ' Different Synthetic Routes ...... 160

6.8 Graph of Np vs. N c/N y for a 239Np Thin Film Source ...... 162

6.9(a) Intercomparison of the Measured Py values for the 106.12 keV y-ray ...... 167

6.9(b)Intercomparison of the Measured Py values for the 277.60 keV y-ray ...... 167 Introduction. 13

Chapter One.

Introduction.

1.1 The IAEA-CRP.

In 1975 the IAEA, in collaboration with the Nuclear Energy Agency of the OECD, convened an Advisory Group Meeting on Transactinium Isotope Nuclear Data (TND) at Karlsruhe, Germany. The meeting was held to review the requirements and status of transactinium nuclear data, relevant to fission reactor research and technology. One of the areas specifically addressed was the current status of the decay data of these nuclides. The meeting concluded that the accuracy of many of these data was not adequate to satisfy a number of needs in such areas as: i. safeguards ii. fuel assay iii. mass determination iv. standards

In response to this situation, the TND recommended that the IAEA implement an international programme to improve the status of these data. The result was the Coordinated Research Programme (CRP) which was established in 1977 to review, measure and evaluate the required transactinium isotope decay data.

Originally five laboratories, all with previous experience in decay data measurements, formally agreed to participate in the work of the CRP, and they were: i. AEA Technology, Harwell, United Kingdom. ii. Central Bureau for Nuclear Measurements (CBNM), Geel, Belgium. iii. Idaho National Engineering Laboratory (INEL), Idaho, USA. iv. Japan Atomic Energy Research Institute (JAERI), Tokai-Mura, Japan. v. Laboratoire de Metrologie des Rayonnements Ionisants (LMRI), Gif-sur-

Yvette, France. Introduction. 1 3

Chapter One.

Introduction.

1.1 The 1AEA-CRP.

However, many other laboratories throughout the world have been engaged in measurements relevant to the four main objectives of the CRP, which were to: assess the status o f the existing data identify the data discrepancies and unfulfilled requirements encourage measurements to meet the requirements evaluate data and ultimately to arrive at a final set of decay data for the transactinium nuclides which would satisfy the required accuracies.

To achieve the above objectives, the CRP continually reviewed existing data, assigned priorities and initiated new measurements. Considerable emphasis was put on collaboration among the participating laboratories and on the intercomparison of results. The participants concentrated on the measurement and evaluation of half- lives and a-particle and y-ray emission probabilities, these being the decay data emphasised at the Karlsruhe TND meeting in 1975 (IAEA, 1986). However, Table 1.1 lists the nuclear decay data of interest to the scientific community at large, together with a review of its useful applications.

1.2 Nuclear Decay Data of Neptunium Isotopes.

Table 1.1 shows that for nuclides which contribute to the radioactivity levels and consequently heat generation, within a nuclear reactor, a comprehensive and accurate knowledge of their decay data is essential. This becomes apparent if one considers the following scenario. Imagine if the primary coolant system of a nuclear reactor were to fail. The subsequent build-up of heat, due to the decay of fission products and actinides, would have to be dissipated by a secondary coolant system. The design of this system would be dictated by the quantity, and expected activity, of the individual nuclides. For this reason, the IAEA-CRP requested more measurements of the decay data of neptunium isotopes, specifically 227Np and O 'l Q Np. Both of these nuclides are produced as waste products in a fission reactor, via the following processes:

the interaction of fast neutrons on U: 238U(n,2n)237U ^ 237Np Introduction.

Nuclear Decay Decay Nuclide Nuclide Assay Environmental/Medical Nuclear Physics Data Heat Identification of Fissionable Materials Tracer Studies Research Half-life * * * * * Q value * * a-particle energy * * * * * a-particle intensity * * * * * p-particle energy * * p-particle intensity * * y-ray energy * * * * * y-ray intensity * * * * * y-ray transition type * * Internal conversion coefficient * * Conversion electron energy * * Conversion electron intensity * * x-ray energy * * * * * x-ray intensity * * * * *

Table 1.1 Nuclear decay data and their applications.

U\ Introduction. 16

n'l c successive thermal neutron capture on U: 235U(n,y)236U(n,Y)237U -4" 237Np

the action of thermal neutrons on U: 238U(n,Y)239U 4 239Np

the radioactive decay of Am: 243Am a 239Np

The activity and heat generation within nuclear waste is dominated, for the first few hundred years, by fission product decay. The major contributors are 90Sr 1 o n and Cs, which effectively become zero after 600 years. Thereafter, and over a non longer period, the radioactivity is largely due to the decay of Np, together with n o n plutonium and americium isotopes. Significant quantities of Np, typically 0.5 kg per ton of uranium fuel, are produced in a pressurised water reactor following three years irradiation (Bindon, 1989). After 100,000 years the radioactivity of a given mass of waste will decrease to a level which is only a few times higher than an equivalent mass of high grade uranium ore. This suggests that radioactive waste management policies should aim at isolating the wastes from the environment for at least 105 years.

Successful modelling of the behaviour of long lived nuclides in waste repositories can only be achieved with accurate decay data. To this end, the IAEA- n o n CRP have invited laboratories to evaluate the current decay data of Np, to produce a self-consistent decay scheme. The organisation has also requested a confirmatory measurement of the half-life of this isotope, since there exists only one precise measurement, which dates from 1960 (Brauer ct al., 1960). 0-30 The determination of accurate decay data for Np (T1/2 = 2.35 d) (Artna- Cohen, 1971) is important to many fields of applied nuclear technology, some of which include: i. the determination Pu accumulation in fast breeder reactors (Mozhaev et al., 1979) nAo OPS) ii. its use as an Am- Np calibration standard for y-ray detectors (Ahmad and Wahlgren, 1972) iii. its use as a chemical yield tracer for other neptunium isotopes (Garraway and Wilson, 1983) Introduction. 1 7

The diversity of this nuclide's applications has initiated numerous measurements of its absolute y-ray emission probabilities (Py). However, of concern to the IA EA -C R P was the fact that the uncertainty in the principle transition varied between 1.45 and 3.80%. This was considered unacceptable by the CRP who called for a further measurement of the y-ray intensities of Np, requesting a precision of 1% for the principle transition.

Imperial College Reactor Centre (ICRC), in collaboration with AEA Technology Harwell, have carried out further measurements of the decay data of 237Np and 239Np, which were in accordance with the IAEA-CRP's requests. This thesis is a result of that programme of research. The Chemistry o f Neptunium.

Chapter Two.

The Chemistry of Neptunium.

2.1 The Placement of Actinides in the Periodic Table.

Niels Bohr's work on the quantised nuclear atom, initiated much discussion into the electronic structure of the heaviest elements. Many scientists believed that the next hypothetical noble gas would have the atomic number 118 and lie 32 elements beyond radon (Z = 86), thus implying another transition group similar to the lanthanides. Indeed there was universal agreement that a transition group existed in the neighbourhood of uranium, but opinions differed as to where the transition began and which electron shells were involved. A number of workers suggested correctly, that the transition involved the filling of the 5 forbitals, analogous to the filling of the 4 f orbitals in lanthanides. However, many of these investigators believed that the filling of the 5 forbitals began with an element beyond uranium, and indeed one that was yet to be discovered (Seaborg, 1949 a).

In 1923 Bohr (1923) suggested that the addition of electrons into the 5 /shell began with element 94. However, other workers predicted that thorium, protactinium, uranium and elements 93, 95 and 99 were just as likely contenders for the initial filling of the 5 fshell. This uncertainty remained for several years and it was not until the discovery of transuranium elements that the electronic structure of the heaviest elements of the periodic table became apparent.

2.2 The Discovery of Neptunium.

The early work by Fermi and others on the irradiation of uranium with slow neutrons had been initially interpreted in terms of the formation of transuranium elements (Quill, 1938). However, Hahn and Strassmann (1939) showed that the resulting radioactive species were isotopes of known elements, produced by the splitting (fission) of the uranium atom into atoms of lower atomic weight. Thus the existence of transuranium elements was viewed with some scepticism.

McMillan (1939) found that irradiating thin uranium foils with slow neutrons, led to the ejection of fission products by their recoil energies. However, two The Chemistry of Neptunium. radioactive species remained in the foil, one with a half-life of 23 minutes and the other with a 2.3 day half-life. The short lived species was identified as an isotope of uranium, while the chemical properties of the 2.3 day activity differed from that of the lanthanides, uranium and all other known elements. It was later identified as an isotope of element 93 (McMillan and Abelson, 1940), the first transuranium nuclide and named neptunium by analogy to the sequence of the planets. The process by which this first transuranium isotope was synthesized is given below:

238U(n,v)239U ■£> 239Np

Although it is now known that the beta-decay of Np produces Pu, the second transuranium element, this fact could only be inferred by McMillan and Abelson at the time (Katz et al., 1986).

It soon became apparent that the solution chemistry of neptunium was similar to that of uranium. Moreover, many of the solid compounds of neptunium, such as the oxides, fluorides and acetates, were isomorphous with the corresponding uranium compounds. A fact that was revealed by the many x-ray diffraction studies carried out by Zachariasen (1949 a,b and c). More important, however, was the fact that the chemistry of neptunium in no way resembled that of rhenium, the element directly above it in the periodic table. This was the first conclusive evidence for the existence of another transition series that was analogous to the lanthanides and which was possibly based around uranium.

The production of an isotope of element 94, plutonium (Seaborg et al., 1946), together with the discovery of a second isotope of neptunium, Np (Wahl and Seaborg, 1948), convinced both Seaborg and Wahl that either thorium or actinium was the zero element in this new series. Seaborg concluded that the evidence suggested that this new transition series begins with actinium, in much the same way as the lanthanide series begins with lanthanum. On this basis it was termed the actinide series and the first 5 f electron would possibly appear in thorium.

Over the decades there have been several modifications to the theories developed by these early workers and our present day understanding of the periodic table and the electronic configuration of the elements are shown in Tables 2.1 and 2.2 (Huheey, 1983). Subshells being completed Principal quantum

ns (/j-i)j n p . eptunium N f o istry Chem The number w ~Y~ Y n n 2 Nonmetals 1 He r Vlll A Light metals Transition metals ______A______.______A_ 2 1 A II A VI A VII A He

3 4 5 6 7 8 9 10 2 Li Be B C N O F Ne

11 12 Vlll B 13 14 15 16 17 18 3 Na Mg III B IV B V B VI B VII B I B 11 B Al Si P S Cl Ar

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 4 K Ca Sc Ti V Cr Mn Fe C'o Ni ('u Zn Ga Oe As Se Hr Kr

37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 5 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 6 Cs Ba La Hf Ta W Re Os Ir Pt An Hg TI Pb Bi Po At Rn

87 88 89 104 105 106 ~v 7 Fr Ra Ac Rf Ha Posttransition metals r

Lanthanide 58 59 60 61 62 63 64 65 66 67 68 69 70 71 series Ce Pr Nd Pm Sm Hu Gd Tb Dy Ho Fr fm Yb Lu

Actinide 90 91 92 93 94 95 96 97 98 99 100 101 102 103 series H i Pa U Np Pu Am Cm Bk Cf Hs Fm Md No Lr

Table 2.1 The periodic table (TIuheey, 1983).

to o The Chemistry of Neptunium. 2 1

Electron E lectron z E lem en t ' configuration Z E lem en t configuration

1 H Is 53 I [K r]4d105s25p5 2 H e I s 2 54 X e [K r]4< i105s25p 6 3 Li [H e ] 2s 55 Cs [X e ]6 s 4 B e [H e ] 2s2 56 Ba [X e ]6 s 2 5 B [ H e ] 2 s 22p 57 L a [Xe]5d6s2 6 C [ H e ]2 r 2 p = 58 Ce [Xe]4/'5d6s2 7 N [He] 2s:2p3 59 Pr [ X e ] 4 f 36 s2 8 O [ H e ] 2 s 22p4 60 N d [ X e ] 4 f 46 s2 9 F [ H e ] 2 s ; 2p 5 61 P m [ X e ^ / ^ s 2 10 N e [He] 2s;2p6 62 Sm [Xe}4f%s2 11 N a [N e ] 3s 63 Eu [Xe]4/ '6s2 12 M g [ N e ] 3 s 2 64 G d [ X e ]4 f '5d6s2 13 A1 [ N e ] 3 s 23p 65 T b [ X e ] 4 / 96 s2 14 Si [ N e ] 3 s 23p2 66 D y [ X e ] 4 / 106s2 15 P [ N e ] 3 s : 3p3 67 H o [X e]4f1 ^ s2 16 S [N e] 3s2 3p4 68 Er [X e]4/1-6s2 17 Cl [N e]3s23p:) 69 T m [X e]^ 1 J6s2 18 A r [ N e ] 3 s ; 3p6 70 Y b [ X e ] 4 / 146s2 19 K [A r]4 s 71 Lu [ X e ] 4 / 145d6s2 '7'*} 20 C a [A r ]4 s 2 H f [ X e ] 4 / 145 26 F e LArJj« 78 Pt [ X e ] 4 f 145 d 96s 27 C o [A r]3 d 4 s2 79 Au [ X e l 4 f 145 d 106s 28 N i [A r ]3 ^ b4s2 80 H g [ X e ] 4 / 145 ^ 106s2 29 C u [ A r ]3 d 104s 81 Tl [ X e ] 4 / 145 d 106s26p 30 Z n [Ar]3^104s2 82 P b [XeJ 4 / 1 ~5d1 °6 s26p2 31 G a [Ar]3d104s24p 83 Bi [ X e J 4 /145 d 106s26p 3 32 G e [Ar]3<7104s24p2 84 P o [X e j4 /'145 d 106s; 6p4 A s [A r ]3 ii104 s24p 3 85 At [X e ]4 /'145 d 106s26p :> 34 Se [A r]3

Table 2.2 Electronic configuration o f the elements (Huheey, 1983). The Chemistry of Neptunium. 22

2.3 The Similarities Between the Lanthanide and Actinide Elements.

The lanthanide series, La to Lu, is characterised by the gradual filling of the 4 forbitals, while the actinide series, Ac to Lr is a consequence of the filling of the 5 forbitals. Thus, the chemistry of the lanthanides is often repeated by that of the actinides. These similarities provided a useful means of predicting the properties of the newly discovered transuranium elements. However, it is important to stress that the chemistry of the actinides is not merely a replay of the lanthanides. There are several significant differences between the two series, due principally to the differences between the the 4 fand 5 forbitals.

2.3.1 Oxidation States.

The characteristic oxidation state of the lanthanides is +3, and although all the actinides, with the exception of Pa and possibly Th, exhibit this oxidation state, it is not necessarily the most preferred. The actinides utilise their f electrons more readily, and exhibit positive oxidation states equal to the sum of the 7s, 6d and 5 f electrons, e.g. Ac(III); Th(IV); Pa(V); U(VI) and Np(VII). As in the first transition series, Sc to Zn, the maximum oxidation state attainable is +7, after which the oxidation state steadily decreases (see Table 2.3). The reduced tendency to use the 5 felectrons, as one progresses along the series, can be demonstrated, for example: U(III) is oxidised with water; Np(III) is oxidised in air; whilst Pu(III) requires a strong oxidising agent such as chlorine (Huheey, 1983). The +4 state is the highest oxidation state for Cm and Bk, whilst beyond that only +2 and +3 oxidation states are observed.

2.3.2 The Lanthanide and Actinide Contractions.

Due to the poor shielding afforded by the 4 fand 5 felectrons, there is a steady increase in the effective nuclear charge, and consequendy a corresponding reduction in size, as one progresses along the respective series. This trend is best observed if one considers the radii of the +3 cations (see Fig.2.1). There are two significant differences between the series. Firstly, although the actinide contraction initially parallels that of the lanthanides, the radii of elements beyond Cm3+ are smaller than expected, probably due to the poorer shielding of the 5 f electrons. Secondly, the lanthanide curve consists of two shallow arcs, with a discontinuity at the spherically symmetrical Gd (4f ) ion. A similar discontinuity is not observed h Ce sr of puim. eptunium N f o istry Chem The

Lanthanides Actinic es Symbol 2 3 4 Symbol 2 3 4 5 6 7 La + Ac + Ce (+) + Th (?) (?) + Pr + (+) Pa + + Nd (+) + (+) U + + + + Pm + Np + + + + + Sm (+) + Pu + + + + (+) Eu + + Am (+) + + + + Gd + Cm + + Tb + (+) Bk + + Dy + (+) Cf (?) + Ho + Es (?) + Er + Fm (?) + Tm (+) + Md + + Yb + + No + + Lu + Lr

Abbreviations: +, in solution; (+), in solid only; (?) in doubt ( Moeller, 1970)

Table 2.3 The oxidation states o f the lanthanides and actinides.

t o CO The Chemistry o f Neptunium.

Fig.2.1 Lanthanide and actinide contraction of the +3 cations (Huheey, 1983). The Chemistry o f Neptunium. 25 at Cm3+. A consequence of this contraction, is that the increase in size corresponding ton = 5 —>n = 6 is lost, with the result that Ho3+ is the same size as the much lighter Y3+, which explains their similar chemical properties. The contraction does not extend to scandium, although its properties may be inferred from those of the lanthanide series.

The main difference between the 4 fand 5 f orbitals depends upon their relative energies and spatial distributions. The 4 forbitals of the lanthanides are sufficiendy low in energy that the electrons are seldom ionised or shared, hence the rarity of the lanthanide (IV) species. Moreover, the 4 forbitals are so deeply buried within the atom that they are hardly affected by the surrounding environment (Jorgensen et al, 1963). In contrast the 5 felectrons of the actinides are readily available for bonding, allowing oxidation states up to +7. Indeed this ease in sharing inner electrons resembles the d electrons of the transition metals and accounts for some similarities between the two series.

The decrease in ionic radii observed along the series affords a means by which the lanthanides and actinides can be chemically separated from each other. A decrease in ionic radius is commonly referred to as a decrease in basicity. This by analogy, is an increase in acidity i.e. an increase in the metal ions ability to accept an electron pair from a donor. Thus, the more acidic the cationic species, be it lanthanide or actinide, then the more readily it will form a complex with a chelating agent.

2.4 The Solution Chemistry of Neptunium.

The following section is concerned only with the solution chemistry of neptunium, leading to separation techniques that were employed during the course of this work. The electrodeposition and electrochemical analysis of neptunium, will be discussed in a later chapter.

The long half-life of 237Np coupled with its relative abundance, has made this an excellent isotope with which to study the chemistry of neptunium. It is the longest lived member of the artificial (4n + 1) radioactive series (see Fig. 2.2) which has been termed the "neptunium family" by analogy with the thorium (4n); uranium (4n + 2) and actino-uranium (4n + 3) families. However, there are a number of fundamental differences between the (4n +1) series and the other families (Seaborg, 1949 b): h Ce sr Npuim. Neptunium f o istry Chem The

to Fig.2.2 Neptunium (4n + 1) series. O n The Chemistry o f Neptunium. 21 i. none of the members in the decay chain are found in nature ii. the series does not include radon iii. the series includes astatine and francium which are not in the main line decay of the other families and hence are missing from, or are very rare in nature iv. the series terminates with a stable isotope of T1 and not Pb

Although the neptunium family is described as the "artificial" (4n +1) series, there is evidence to suggest that this element is found in nature. Several workers (Seaborg and Perlman 1948, Gamer et al. 1948, Levine and Seaborg 1951), non suggested that small amounts of Np were expected to occur in uranium ore, produced by neutron capture in U. They believed that the neutrons were produced from the spontaneous fission of U, from the fission of U by thermal neutrons, or (a,n) reactions on elements of low atomic number. The isolation by Peppard et al. (1951), of small quantities of Pu, from uranium mined m the Belgian Congo, was conclusive evidence that Np did exist in nature. Peppard et 9^7 al. (1952) also reported that they had isolated Np from similar uranium ore.

In aqueous solution neptunium exhibits all oxidation states from IE to VII inclusive (see Table 2.3). The stability of the ions are affected by pH and complexing ligands. In the absence of a complexing agent the trivalent and tetravalent ions exist in acid solution as hydrated Np3+ and Np4+. Both these ions form insoluble hydroxides in the presence of a base. In addition, the hydroxide precipitated from Np(EI) solutions, are rapidly oxidised to the IV state by oxygen.

Pentavalent and hexavalent neptunium form stable complexes in both acidic and basic media. Solutions of neptunium V and VI can be prepared in tetramethylammonium hydroxide to relatively high concentrations, demonstrating the extra stability of these species in basic media. LaChapelle et al.(1949) reported that the solid Np(V) hydroxide existed in at least three forms. However, a recent hydrolysis study of Np(V) showed no evidence for anionic species other than Np0 2(0H)2~ in basic solution (Lierse et al., 1985).

In contrast to the other oxidation states, Np(VII) is stable only in basic solution and its formula is believed to be NpOs^- . In acidic solutions Np(VII) is thought to exist as NpC>3+ (Katz et al., 1986). Table 2.4 lists the chemical formulae of the different oxidation states of neptunuim in both acid and basic media. Oxidation Ion Colour Preparation h Chmityof puim. eptunium N f o istry hem C The state Np4+ + H2 m blue Np3+ Np4+ + e~ (Hg cathode) or violet Np4+ + Zn Np3+ + 02 reduction of Np(V) or Np(VI) with: IV yellow- Np4+ C2O42" + Mn2+ (6M H2SO4) green T (5M HC1) S02 (IM H2S04) Np4+ + HNO3 (heat) V Np02+ green (acid) Np(VI) + NH2OH Np(VI) + stoichiometric I- V Np02(0H)2~ yellow (base) Np02+ + OH- in (CH3)4NOH soln. oxidation of Np(III), Np(IV) or Np(V) with: Ag2+ VI Np022+ pink to Ce(IV) red (acid) B1O3" ocr IICIO4 (evaporate) VI Np042“ pink (base) Np022+ + OH“ in (CH3)4NOH soln. dissolution of U 5NPO5 in dilute alkalis VII Np053- green (base) anodic oxidation of Np(VI) in (GHf^NOH soln. Np(VI) + ozone VII NpQ3+ brown-red acidify NpC>5^- with IICIO4 (acid) dissolution of Li5NpQ6 in dilute acids Table 2.4 Neptunium ions in aqueous solution (Katz et al., 1986).

t o oo The Chemistry o f Neptunium. 29

2.5 Ion-Exchanse Chromatography.

As the term suggests ion-exchange is the transfer of ions of like sign between a solution and an insoluble material. Both natural and artificial materials have the ability to act as ion-exchangers. The prerequisites are that the material must have ions of its own with which to exchange and that the solid has an open permeable structure. This allows ions and solvent molecules to move freely, so affording rapid exchange. However, it is the synthetic organic ion-exchangers which are used extensively in analytical work. These are characterised by being almost insoluble in both water and organic solvents. Furthermore, they contain active or counter ions, that will exchange reversibly with ions in the surrounding solution, without any appreciable physical change occuring in the material. The basic structure of an ion-exchange resin is polymeric. The polymer carries an electric charge that is neutralised by the charges on the counter ions. Indeed, it is the charge on the counter ions that distinguishes between an anion or cation exchange resin. For example, an anion exchange resin contains a polymeric cation with an active anion, and conversely a cation exchange resin contains a polymeric anion with an active cation. A typical example of a cation exchange resin is that which is produced by the copolymerisation of styrene with a small proportion of divinylbenzene, followed by sulphonation (Pepper and Hale, 1955):

- CH-CHr CH- CHr- ch- AAA

VSqHV“ T T + VSqH' CHr-CH-CH2 I I - ch2- ch c h -2-CH ch2 o- \ \ active cation A s

v . , sqH* Fig.2.3 Schematic representation o f a cation exchange resin.

In the above example the sulphonic functional group could be replaced by carboxylic or phenolic groups, and the active ion (Hf), by Na+. The Chemistry of Neptunium. 30

The physical properties of the resins are dictated by the degree of cross- linking. Those resins which are highly cross-linked are often more brittle and more impervious than the lightly cross-linked materials. The preference of the resin for one ion over another is influenced by the degree of cross-linking.

The active ion (H+), in the above example is able to move freely within the solvent-filled pores of the resin. Thus, when a solution containing cations (A+) is in contact with the resin material, the active ions diffuse out of the resin structure, whilst the cations (A+) diffuse in. This diffusion process continues until equilibrium is attained.

The same is true for anion exchange resins, although obviously, the nature of the functional group and active ion are fundamentally different. This type of resin utilises amino (RNH2), substituted amino (R2NH, R3N) or quaternary ammonium groups (R4N+X~). Both the amino and substituted amino groups have only weak basic properties, whilst the quaternary ammonium ion is strongly basic.

The widely used anion-exchange resin, illustrated in Fig.2.4, is produced by the copolymerisation of styrene and divinylbenzene, followed by chloromethylation i.e. introduction of the -CH2CI group in the free para position. The halogen is then replaced by trimethylamine (CH3)3N, which produces the quaternary ammonium ion. As in the previous example the active ion (CF), can be replaced, on this occasion either by hydroxyl or sulphate ions.

H 2NMelCl 3 H2NMejCl Fig.2.4 Schematic representation o f an anion exchange resin. The Chemistry o f Neptunium. 3 1

The basic requirements of an ion-exchange resin can be summarised as follows: i. it must be chemically stable ii. it must be sufficiently cross-linked so as to prevent significant solubility in the solvent iii. it must be hydrophilic so as to permit the diffusion of ions through the structure iv. it must contain a sufficient number of accessible ionic exchange groups

The transfer of ions between the solution and an anion exchange resin is given below (Kitchener, 1955):

(Res.A+)B + C (solution) <=> (Res.A+)C + B (solution)

If the experimental conditions are such that the position of equilibrium is shifted to the right, then the ion C~ is completely fixed to the exchange resin. Moreover, it follows that passing a solution containing B“ ions through the resin will restore it to its original form. Thus, the reaction is said to be reversible. The factors which determine the position of equilibrium are listed below (Vogel, 1961):

(i). Nature o f exchanging ions.

At low aqueous concentrations and at room temperature, the extent of exchange increases with increasing valency, the order being:

Na+< Ca2+< Al3+< Th4+

Whilst, for ions with the same valency, the rate of exchange increases with an increase in charge density, usually seen as a decrease in the size of the hydrated cation:

Li+< H+< Na+< NH4+< K+< Rb+< Cs+ The Chemistry of Neptunium. 3 2

In the case of strongly basic anion exchange resins, univalent anions behave in the same way as univalent cations, in that charge density determines the position of equilibrium:

F"< OH“< HC03“< CT< HS04~< CN~< Br“< N03~< r

The solution concentration plays an important role in determining the position of equilibrium. For example, if a cation is being exchanged for an ion of different valency, the relative affinity of the higher valent ion increases in direct proportion to the dilution. Thus, to exchange a higher valent ion on the exchanger, for one of lower valency in solution, the transfer will be favoured at high solution concentrations. Conversely, if the lower valent ion is in the exchanger and the higher valent ion is in solution, then exchange will be favoured at low solution concentrations.

The pH of a solution, together with the type of functional group present in the resin, also influences the dynamics of the exchange reaction. Strongly acidic cation exchange resins, such as that shown in Fig.2.3, are pH independent (Kitchener, 1955). However, in the weak acid cation exchange resins, such as those containing the carboxylic and phenolic groups, ionisation occurs only in alkaline solution. Thus there is little exchange of ions in solutions below pH 7.

Similarly strongly basic anion exchange resins, such as that shown in Fig.2.4, are independent of the pH of the solution. However, weakly basic amino or substituted amino exchange resins contain little of the hydroxide form in basic solution, so that the position of equilibrium in the following reaction:

(Res.NMe2) + H20 <=> (Res.NHMe2)+OIT

is mainly to the left. In acid solution, however, they behave like strongly basic ion- exchange resins, producing the highly ionised salt form:

(Res.NMe2) + HC1 <=> (Res.NHMe2)+C r The Chemistry o f Neptunium. 3 3

The resin can now be used in acid solution for the exchange of anions, for example:

(Res.NHMe2)+C r + N03~(soln) <=> (Res.NHMe2)+N03_ + Cr(soln)

(ii). Nature o f the ion-exchange resin.

The absorption of ions from the solution will depend upon the degree of cross-linking. Indeed, as the cross-linking increases, the resin becomes more selective towards ions of different sizes. Thus, the ion with the smaller hydrated volume will usually be adsorbed preferentially.

2.6 Separation of Neptunium by Ion—Exchange Chromatography.

To achieve a separation of neptunium from a mixture of actinides and fission products using ion-exchange chromatography, it is essential to know the equilibrium distribution coefficient (Kd) for each of the species:

activity per gram of resin Kd = activity per ml of solution ( 2. 1)

Wish (1959) determined the Kd values for isotopes of neptunium, uranium, plutonium, zirconium, niobium and molybdenum. The experiment consisted of contacting known volumes of the nuclide mixture, which was in HCl/HF acid media, with 1 gram of strongly basic anion exchange resin Dowex 2. This specific "cocktail" was chosen since it is typical of the range of nuclides present in spent fuel. The HCl/HF acid mixture was used since this produced elutions that were much sharper and had little or no tailing, so affording quantitative recoveries.

Fig.2.5(a) shows that Zr(IV), Nb(V) and Mo(VI) adsorb strongly in 0.1M HC1/0.3M HF. Whereas, Fig 2.5(b) shows that Np(IV), Pu(IV) and U(VI) do not adsorb as strongly in low HC1 concentrations. However, as the hydrochloric acid concentration increases, the adsorption of all the species decreases rapidly to a minimum. The two figures show that the equilibrium distribution coefficients of U(VI) and Mo(VI) have the same basic profile, with strong adsorptions observed at The Chemistry o f Neptunium. 34

o o U c .9 3 X ‘h s00 3s •G X) cr3 W

Fig.2.5(a) Equilibrium distribution coefficients for Zr(TV), Nb(V) and Mo(VI) in hydrochloric-hydrofluoric acid solutions.

c *o.9 o0) CJ 3 .9 X3 •c oo Q a

xM HC1 containing 0.3M HF Fig.2.5(b) Equilibrium distribution coefficients for Np(IV), Pu(IV) and U(VI) in hydrochloric—hydrofluoric acid solutions. The Chemistry of Neptunium. 3 5

high HC1 concentrations. Whilst their minimum adsorptions are at 1 and 2M HC1 respectively. Such similarities often make sequential separation of these nuclides difficult. Zr(IV) has a low value above 2M HC1, but is strongly adsorbed in lower hydrochloride concentrations. Np(IV) and Nb(V) are strongly adsorbed in concentrated HC1 and appear to have minimum values at 5M and 6M HC1 respectively, which in practice means that any attempt to recover neptunium from the exchange resin would also result in the elution of niobium and vice versa. The situation is further complicated by the fact that Pu(IV) has virtually no adsorption between 0.5 and 7M HC1 and so cross-contamination by plutonium is inevitable.

The Kd values for the above nuclides in hydrochloric acid have also been determined, using the same anion exchange resin (Wish and Rowell 1956, Bunney et ah 1959). Fig.2.6(a) shows that the values of Mo(VI) increase sharply in low HC1 concentrations, and that the overall adsorption curve of this nuclide is, once again, similar to that of U(VI). However, there is a significant increase in the adsorption of Zr(IV) at higher chloride concentrations than was previously observed in the hydrochloric-hydrofluoric acid media. Fig.2.6(b) shows that the adsorption curves of Np(IV) and Pu(IV) are very similar, with both having minimum values at 1M HC1. However, Nelson et al. (1964) reported that plutonium could be selectively eluted from the column by the addition of a reducing agent, such as NH4I. This would convert Pu(IV) to Pu(III), while Np(IV), U(VI), Mo(VI), Nb(V) and Zr(IV) would remain adsorbed. A successful sequential elution of the remaining nuclides would depend on the separation of niobium from neptunium and of molybdenum from uranium.

Figs.2.5(a) and (b) shows that the elution of neptunium and niobium is rapid below 6M HC1. Wish reported that increasing the acid mixture concentration to 6.5M HC1/0.004 M HF, would allow Np(IV) to elute from the resin, whilst Nb(V) would be retained. The addition of 6M HC1/0.06M HF would result in the elution of Nb(V) from the resin. However, elution with lower fluoride concentrations is desirable, since this would increase the adsorption of the remaining U(VI) and Mo(VI) species. Subsequent elution of U(VI) can be achieved with 0.1M HC1/0.06M HF, however, the addition of the acid mixture at this stage would produce an intermediate 4M HC1 concentration. This would result in the simultaneous elution of Mo(VI). This can be prevented by allowing the resin to air dry, followed by the addition of alcohol. Sequential separation is then achieved by washing the column with 0.1M HCl/0.06 M HF, which elutes U(VI). Mo(VI) can then be removed from the column with 12M HNO3. Equilibrium Distribution Coefficient Equilibrium Distribution Coefficient hydrochloric acid solutions. acid hydrochloric i..() qiiru dsrbto cefcet fr pI) P(V ad (I in U(VI) and Pu(IV) Np(IV), for coefficients distribution Equilibrium Fig.2.6(b) Che sr Ne uni . m iu n tu ep N f o istry em h C e h T i..() qiiru dsrbto cefcet fr rI) N() n M(I in Mo(VI) and Nb(V) Zr(IV), for solutions. coefficients acid distribution hydrochloric Equilibrium Fig.2.6(a) 1 C H M x

36 The Chemistry of Neptunium. 37

The above separation techniques were used successfully to separate neptunium from both actinides and trace fission products. Where necessary the procedures were modified, often to reduce separation time, or if a contaminant was not believed to be present. A summary of each of the chemical separations employed in this work can be found in Chapters 4 and 6. Counting and Spectroscopy Techniques. 38

Chapter Three.

Counting and Spectroscopy Techniques.

3.1 Introduction.

This chapter will review the counting and spectroscopy techniques employed during the course of this research. It will discuss the basic principles which have given rise to proportional counters, scintillation detectors, coincidence systems and finally with the advent of semiconductors, high purity germanium detectors. Modus operandi will be included for each device to avoid repetition in subsequent chapters.

3.2 Proportional Counters.

Proportional counters, which were first introduced in the late 1940s, take their place alongside ionisation chambers and Geiger-Mueller tubes, in that they can be categorised as gas-filled detectors. Their operation relies on the formation of ion pairs produced when incident radiation ionises neutral gas molecules along its path. The charge represented by the original ion pair is amplified by secondary ionisation processes and will eventually result in the phenomenon known as Townsend avalanche. The formation of the avalanche is dependent on the electric field within the gas. At low electric fields, the electrons and ions created by the incident radiation simply drift to their respective electrodes and in this mode the detector is operating as an ionisation chamber. However, during the migration there will be many collisions between the charged particles and neutral gas molecules. The low mobility of positive, or negative ions, ensure that there is very little energy transfer between collisions. Conversely, free electrons are easily accelerated by the applied field and may have sufficient energy to cause ionisation of gas molecules, creating an additional ion pair. Since the average energy of the electrons between collisions increases with increasing electric field, it follows that there is a threshold value for the applied field above which secondary ionisation will occur.

The liberated electron will itself be accelerated by the electric field, and during its drift will collide with neutral gas molecules producing yet another ion pair (NCRP, 1985). Thus, the gas multiplication process takes the form of a cascade in which each free electron, created during a collision, can potentially create more free Counting and Spectroscopy Techniques. electrons by the same process. This has enabled proportional counters to be used in the detection and spectroscopy of low energy radiation, were the number of initially generated ion pairs is small.

Under proper working conditions the gas multiplication will be linear and the collected charge will be proportional to the original number of ion pairs created by the incident radiation. However, increasing the applied electric field introduces nonlinear effects, due to the increasing number of positive ions produced during secondary ionisation processes. These ions are slow moving and so each pulse of incident radiation produces a cloud of positive ions which is only gradually dispersed as it drifts towards the cathode. If the concentration of these ions is sufficiently high, they will represent a space charge that will significantly alter the shape of the electric field within the detector. Increasing the field still further will increase the number of positive ions, so as to reduce the electric field below the point at which additional gas multiplication can take place. The process is then self-limiting and will terminate when the same total number of positive ions have been formed, regardless of the number of initial ion pairs created by the incident radiation. Consequently, each output pulse from the detector is of the same amplitude and no longer reflects any properties of the incident radiation. In such a case the proportional counter is operating as a Geiger-Mueller tube, which can function only as a simple counter of radiation events and cannot be used for pulse height spectroscopy. The different modes of operation for gas-filled detectors are illustrated in Fig.3.1 (Knoll, 1989).

3.2.1 Operating Voltage.

At high values of gas multiplication a single ion pair, created from either an a - or (3-particle, can initiate an avalanche with sufficient secondary ionisation to be detected. However, it is very seldom that a proportional counter with this high degree of sensitivity is used, since the measurement is prone to nonlinearities arising from space charge effects. Furthermore, many background and spurious pulses may also be counted and incorrectly attributed as true events. Hence, lower values for gas multiplication are used which require that the pulse originate from a finite number of ion pairs in order to have an amplitude large enough to exceed the discriminator level of the counting system. Since gas multiplication varies with the applied voltage, a "counting curve" is recorded which will indicate the appropriate operating voltage. This is when the applied field is such that nearly all primary events are counted, resulting in a count rate versus voltage curve which is almost Counting and Spectroscopy Techniques. 40

Fig.3.1 Different regions o f operation for gas-filled detectors.

Fig.3.2 Theoretical counting curve. Counting and Spectroscopy Techniques. horizontal. The plateau is then a measure of the proportional counter's ability to detect all primary ionising events (NCRP, 1985).

Fig.3.2 shows the theoretical counting curve one would expect from a - and (3-sources. The first plateau is the point at which only a-particles are counted and the second where both a - and [3-particles are counted. Since the range of (3— particles greatly exceeds the chamber dimensions, the number of ion pairs formed in the gas is proportional to only a small fraction of the total energy of the particles. The resulting pulses are therefore smaller than those induced by a-particles of equivalent energy. Moreover, they will cover a broader range of amplitude due to the spread in (3-particle energies and variations in possible paths through the gas. This broad distribution of (3-pulses results in a shorter (3-plateau (Knoll, 1989).

3.2.2 Alpha-Particle Counting.

Fig.3.3(a) illustrates a 2n gas flow proportional counter similar to the Simpson (1077B) counters used in Chapters 4 and 5. Detectors of this design are known as windowless flow counters, since the active source is introduced directly into the counting volume of the detector by means of a rotating table. The obvious advantage of this design is that there is no entrance window which could attenuate low energy a-particles. Although the counting geometry is almost 2it, small corrections for a-particle self-absorption and scattering must be determined prior to measuring a-source activities. Thus the effective solid angle, subtended by the active volume of the counter, is evaluated using a calibration source of known activity.

The counter utilised a gold coated tungsten wire of length 125 mm and diameter 0.025 mm, maintained at a working voltage of +950 V relative to the brass body of the counter i.e. the cathode (Wiltshire, 1989). The fill gas was a mixture of 90% argon and 10% methane at a pressure close to that of atmospheric. Methane was used since it acts as a quenching gas which suppresses photon-induced events. These arise when collisions between free electrons and gas molecules result, not in the creation of an ion pair, but simply in an excited gas molecule. These molecules do not contribute to the avalanche but decay to their ground state by the emission of a visible or ultra-violet photon. The deexcitation photons could then create additional ionisation, either in the fill gas or by photoelectic interaction at the wall of the counter and so result in a loss of proportionality. Counting and Spectroscopy Techniques. Spectroscopy and Counting G a s in le t G a s o u t le t

/////////A. I/7 7 /7 ////////////////A V/////////A

Anode wire !

Sample holder

F ig . 3 . 3( h) Illustration o f a 2ngas How proportional counter.

Fig.3.3(b) Schematic representation o f associated electronics. Counting and Spectroscopy Techniques. 4 3

Fig.3.3(b) is a schematic representation of the electronics associated with the 2tc gas flow proportional counter.

3.2.3 Beta-Particle Counting.

Fig.3.4(a) illustrates a 4k gas flow proportional counter similar to that used in Chapter 6. As in the previous example this detector is a windowless flow counter, with the source positioned at the centre of the counting volume by means of a sliding sample holder. If the source is prepared on a backing that is thin compared to the range of the radiation of interest, then particles or photons will emerge from both surfaces of the source, resulting in a detector with almost 4k geometry. Such a device was used, in conjunction with two Nal(Tl) scintillation detectors, to form a 47c(3—y coincidence system which measured the absolute disintegration rate of beta sources mounted on a VYNS thin film backing (see Fig.3.4(b)).

The detector anodes were tungsten wires of 0.3 mm diameter maintained at a working voltage of +1500 V relative to the aluminium body of the counter. The fill gas was again 90% argon and 10% methane. Counting a nd Spectroscopy Techniques. Spectroscopy nd a Counting

Gas inlet Gas outlet

O-ring seal

Fig.3.4(a) Illustration o f a 4n gas flow proportional counter.

Gold-coated VYNS film

Fig.3.4(b) VYNS thin-film source.

4^ 4^ Counting and Spectroscopy Techniques. 45

3.3 Solid State Detectors.

3.3.1 Gamma-ray Interactions.

Of the many ways in which y-rays can interact with matter, there are only three mechanisms which have any real significance to y-ray spectroscopy, and they are: i. photoelectric absorption ii. Compton scattering Hi. pair production

The importance of each of these mechanisms to y-ray spectroscopy dictates that each should be discussed individually.

3.3.2 Photoelectric Absorption.

This process predominates for low-energy y-rays up to several hundred keV and is the result of an interaction between an incident photon and the orbital electron of an absorber atom. The photon completely disappears and is replaced by a photoelectron which is ejected, usually from the K shell of the atom. A free- electron cannot absorb a photon and become a photoelectron since a third body, the nucleus, is necessary for conserving momentum (Siegbahn, 1966).

Fig.3.5 Photoelectric absorption.

The energy of the resulting photoelectron is given by the equation:

Ee- = hv - Eb (3.1)

Eb = binding energy of the photoelectron in its original shell Counting and Spectroscopy Techniques. 46

The vacancy that has been created by the photoelectron is quickly filled by electrons from higher orbitals. This results in the emission of characteristic x-rays or Auger electrons. These secondary radiations will travel a short distance before being reabsorbed through photoelectric interactions, with less tightly bound orbital electrons. Thus, photoelectric absorption results in the liberation of a photoelectron, which appropriates most of the incident y-ray energy, together with one or more low energy electrons. If nothing escapes from the detector, then the sum of the kinetic energies of the electrons will equal the original energy of the incident photon.

3.3.3 Compton Scattering.

This process is observed for all energies. It results from the interaction of an incident y-ray photon with an electron, producing a recoil electron and a scattered photon. The division of energy between the two is dependent on the scattering angle (0). The process is illustrated below:

hv Compton electron

Incident y-ray photon Compton scattered photon Fig.3.6 Compton scattering.

There will be an infinite number of possible scattering angles which will lie between two extremes, namely a grazing angle or a head-on collision. In the case of the former, 0 approaches zero and the resulting energy of the scattered photon is virtually identical to that of the incident y-ray. In such cases the Compton electron takes away very little energy. Conversely, in a head-on collision, 0 approaches n and the incident y-ray is backscattered towards its direction of origin, whereas the Compton electron recoils along the direction of incidence. Such a scenario represents the maximum energy that can possibly be transferred to an electron during a single Compton interaction. The infinite number of scattering angles that lie between these two extremes form a continuum of energies, so that the electron energy distribution is similar to that illustrated below: Counting and Spectroscopy Techniques.

Fig.3.7 Compton continuum and photopeak.

The above discussion was based on the assumption that Compton scattering involves unbound electrons. This allowed us to conveniently ignore the effects caused by the binding energy of the electron to the nucleus prior to scattering. In reality these effects result in a rounding-off of the rise in the continuum close to the Compton edge and is particularly noticeable when using high resolution detectors.

3.3.4 Pair Production.

In this process the energy of a photon is converted, in the nuclear Coulomb 9 field, to an electron-positron pair. An energy of 2moc is required to create the pair and so the process is energetically feasible with incident y-rays of 1.02 MeV or more. Any excess energy will be shared between the particles as kinetic energy, such that:

Ee- + Ee+ = hv - 2moc2 (3.2)

Both particles will travel a few millimetres before losing their energy to the absorbing material. The positron will finally react with an electron and annihilate. If this occurs after the positron has lost virtually all its kinetic energy, then two y-ray photons of energies moc are generated. To conserve momentum these photons are emitted in nearly opposite directions (Debertin and Helmer, 1988). Subsequent Counting and Spectroscopy Techniques. 48 escape of these y-rays from the detector will result in photopeaks located at 511 keV (single escape) and 1022 keV (double escape) below the full-energy peak.

Outlined above are the three fundamental processes by which y-rays interact with matter, and consequently the basic principles which have given rise to pulse height y-ray spectroscopy. Although all detector materials behave identically to the above processes, their response functions are dependent on the nature of the absorbing materials, and more precisely the manner in which charge carriers are created.

3.3.5 NaI(Tl) Scintillation Detectors.

A good scintillation material must have the following properties: i. it should efficiently convert the kinetic energy associated with the radiation into light ii. the conversion should be linear i.e. the light yield should be proportional to the deposited energy iii. the medium should be transparent to the wavelength of its own emission iv. the decay time of the induced luminescence should be short so as to allow resolution of rapid pulses v. the material should be of good optical quality and suitable to be fashioned into many various forms vi. the refractive index of the material should be similar to that of glass so as to permit efficient coupling of the scintillation light to a photomultiplier tube

Unfortunately, no single material fulfils all these prerequisites and so the choice of a scintillator, whether inorganic or organic, crystal or liquid, is always a compromise. Of all the materials available as scintillators, Nal(Tl), which was first used in 1948 (Hofstadter, 1948), remains pre-eminent in the field of y-ray spectroscopy.

The phenomenon of induced luminescence in Nal(Tl), and indeed all inorganic scintillators, is governed by band theory. This predicts that electrons may populate discrete bands of energy in materials. The lower band, called the valence band, represents electrons which are bound to the lattice sites. The upper band, Counting and Spectroscopy Techniques. 4 9 called the conduction band represents those electrons which have sufficient energy to migrate throughout the crystal. The two bands are separated by the forbidden band in which, providing the crystal is absolutely pure, electrons are never found. Absorption of energy will result in the elevation of an electron from the valence band to the conduction band, leaving a hole in the former. The creation of an electron- hole pair may be considered analogous to the formation of ion pairs described in section 3.2. Subsequent deexcitation of the electron back to the valence band, will produce a photon. Unfortunately, the gap width between the two bands are such that the transition would be of too high an energy to lie in the visible region of the spectrum. However, the addition of trace amounts of inorganic impurity (such as thallium), called activators, would modify the energy band structure of the pure crystal, resulting in energy states that lie within the forbidden band. This then provides a route by which electrons can deexcite back to the valence band. Since the energy of these photons are now less than that of the full forbidden gap, the transition will give rise to a visible photon, and hence a scintillation. The mechanism is illustrated below:

Conduction band

T Activator I Electron \ Y excited states Forbidden transfer | band Scintillation photon I v Activator ground state Valence band

Fig.3.8 Energy band structure o f an activated crystalline scintillator.

The next stage is to convert the weak light pulses, which may consist of no more than a few hundred photons, into a corresponding electrical signal and this is achieved by a photomultiplier tube. Fig.3.9 illustrates a typical PM tube, similar to that used in Chapter 6, consisting of a photosensitive layer, called the photocathode, coupled to an electron multiplier. The photocathode converts incident light photons into low energy electrons by a process known as the photoelectric effect (Eisberg and Resnick, 1974). The creation of a photoelectron may be considered in three sequential stages. Firstly, absorption of the incident photon and transfer of energy to an electron, secondly the migration of that electron through the material and finally the ejection of the electron from the surface of the photocathode. The process is far Counting and Spectroscopy Techniques. 50

Nal(Tl) Scintillator Light photons

'Photocathode Photoelectrons

Focusing. .Focusing electrodes electrodes

Fig. 3.9 Illustration o f a scintillation detector. Counting and Spectroscopy Techniques. from efficient, since energy is lost in electron-electron collisions during migration through the material and as the electron tries to escape the potential barrier that exists at the interface between the material and vacuum. Consequently, only a very thin layer close to the surface of the photocathode will give rise to photoelectrons.

Secondary electron emissions are generated by the multiplier portion of the PM tube. Photoelectrons are accelerated towards an electrode called a dynode. The energy deposited by the incident electron will result in the re-emission of more than one electron from the same surface. However, as in the photocathode, only a small fraction of the excited electrons actually contribute to the secondary electron yield. To achieve high electron gains a multiple stage PM tube is employed. Thus electrons leaving the first dynode with an kinetic energy of a few eV, are accelerated across a potential difference to a second dynode resulting in the liberation of more electrons. If the multiplier section consists of N stages, then the overall electron gain is given by the simple equation (Knoll, 1989):

overall gain = aSN (3.3) a = fraction of photoelectrons collected by the multiplier structure 5 = the number of electrons produced per photoelectron

The popularity of NaI(Tl) detectors originates from their high intrinsic efficiency due to the relatively large atomic number of iodine, ensuring a high interaction probability. However, the response function for this detector suffers from poor energy resolution. This is due to significant statistical fluctuations in the relatively small number of photoelectrons produced at the photocathode and confounded by the many inefficient intermediate steps. The only way to reduce the statistical limit in the resolution is by increasing the number of information carriers per pulse and this has been achieved by the use of semi-conductors.

3.3.6 Semiconductor Detectors.

Band theory once more provides the fundamental principles which give rise to semiconductor properties. Consider Fig.3.10 which illustrates the band structure for both insulator and semiconductor materials. Counting and Spectroscopy Techniques. 52

Conduction band bo 5 eV §

Insulator Semiconductor

Fig.3.10 Band structure for electron energies in insulators and semiconductors.

Since electrons must be excited across the band gap into the conduction band, it follows that in the absence of incident radiation or thermal excitation, both insulators and semiconductors have a configuration in which the valence band is completely full and the conduction band empty. Under such circumstances neither material would show any electrical conductivity. However, at non-zero temperatures, thermal energy is sufficient to elevate an electron into the conduction band and this results in the formation of an electron-hole pair. It follows that the smaller the energy gap between the two bands, the greater with be the probability of this phenomenon occuring. Semiconductors are characterised by their small energy gaps, often of the order of 1 eV and fall into three distinctive categories. i) . Intrinsic semiconductors: are pure materials in which the number of electrons in the conduction band is equal to the number of holes in the valence band. The presence of trace impurities will change the electrical properties of the material into its n-type or p-type equivalents. However, improvements in manufacturing processes have made the use of intrinsic semiconductors more widespread. ii) . n-Tyye semiconductors: result when the impurity has more valent electrons than the semiconductor material. Consider the presence of trace amounts of arsenic, which has five electrons per atom, in crystalline germanium, having four electrons per atom. The impurity will occupy a substitutional site within the lattice using four of its valent electrons to bond with Ge atoms, leaving an unpaired electron. The extra electron is lightly bound to the impurity site and can easily be ionised. This will result in an electron moving into discrete energy levels located just below the conduction band (Eisberg and Resnick, 1974). Thus only a little energy is sufficient to elevate it to the conduction band, without the formation of a corresponding hole in the valence band. The presence of pentavalent arsenic in the lattice is regarded as a Counting and Spectroscopy Techniques. 5 3 donor impurity, since it will readily contribute electrons to the conduction band. If the concentration of the impurity is large compared with the expected number of electrons in the conductance band of an intrinsic material, then the electrical conductivity of the semiconductor is determined by the flow of electrons, whilst holes play a minor role. In such a case the electrons are known as majority carriers and holes the minority carriers. in). p-Type semiconductors: are produced when the impurity has fewer valent electrons than the semiconductor material. Consider trivalent gallium occupying a substitutional site within a germanium lattice. Ge will be left with an unsaturated bond and this vacancy represents a hole. The hole can drift through the crystal as successive electrons move to fill the vacancy and in turn create another. The impurity introduces discrete energy levels just above the top of the valance band. Electrons are easily excited into these vacant levels, leaving holes in the valence band without a corresponding electron moving into the conduction band. Thus on average, a hole is created for every acceptor impurity which is added to the material. If this concentration is large compared with the intrinsic concentration of holes, then the electrical conductivity is dominated by the flow of holes. In this case holes are known as the majority carriers and electrons the minority carriers.

Semiconductors which have equal concentrations of donor and acceptor impurities are said to be compensated. Such a material has properties which approach those of an intrinsic semiconductor, since electrons arising from donor impurities are quickly removed at the sites of acceptor impurities.

The impurities described above are often referred to as shallow impurities, since their energy levels lie near the edges of the forbidden band. Other impurities such as gold, zinc and cadmium are classed as deep impurities, since their energy levels are located near the middle of the forbidden band. Such deep impurities can act as traps for charge carriers, so immobilising either a hole or an electron for a relatively long time. Although the trapping centre will eventually release the carrier, the delay is often sufficiently long to prevent the carrier contributing to the measured pulse. Some impurities may act as recombination centres, which are capable of capturing both majority and minority carriers, causing them to annihilate. Again the result is a loss of charge carriers and eventually a reduction in the lifetime of the crystal (Knoll, 1989). So

So far we have considered electron-hole pairs which have been created by thermal excitation. When radiation passes through a semiconductor the effect is the production of many electron-hole pairs along the incident track. Since the energy Counting and Spectroscopy Techniques. 54 required to produce an electron-hole pair in a semiconductor detector is only 3 eV, compared to 30 eV in a gas filled counter and 100 eV in a Nal(Tl) scintillator (Debertin and Helmer, 1988), it follows that a semiconductor will produce many times more charge carriers per incident radiation. The increased number of electron- hole pairs will have a significant effect on the response function of the detector and will result in improved resolution due to reduced statistical fluctuation in the number of charge carriers. An estimate of this fluctuation can be made by assuming that the formation of charge carriers follows a Poisson distribution. If N is the number of electron-hole pairs, then the statistical fluctuation in this number is given simply by Vn. If this were the only source of fluctuation then the response function would have a purely Gaussian shape, since N is typically a large number. The resolution of the peak is defined as the full width of the photopeak at half its maximum height (FWHM), divided by the peak position (Faiies and Boswell, 1981). The width parameter (a), defines the FWHM by the relationship:

FWHM = 2.35a (3.4)

Since the response of the detector is virtually linear, the average pulse amplitude Hq, or peak position is given by:

Hq = KN (3.5)

K = proportionality constant and the standard deviation is expressed as:

0 = k V tT (3.6) hence the limiting resolution R, due to statistical fluctuations in the number of charge carriers is:

FWHM 2.35KVN 2.35 , R~ Ho - KN “VF ( }

Equation 3.7 shows that for a peak resolution of better than 1%, more than 55,000 charge carriers must be collected. As already explained, semiconductor detectors are capable of producing this quantity of charge carriers due to the low ionisation energy required to produce electron-hole pairs. Counting and Spectroscopy Techniques.

In practice the measured resolution of certain types of detectors are lower than that predicted by the above equation. This departure from Poisson statistics is due to the fact that the formations of individual charge carriers are not independent of each other. The Fano Factor has been introduced in an attempt to quantify the deviation of the observed statistical fluctuation from that predicted by the Poisson model.

observed variance in n (3.8) Poisson predicted variance N

The statistical limit on the resolution is now given by the formula:

2.35KV NF (3.9) R " KN

Fig.3.11(a) and (b) shows a HPGe detector system (EG&G Ortec, 1986) similar to that used in Chapter 6. The germanium crystal has a closed-end coaxial geometry which affords an increased active volume and is situated inside the vertical cryostat.

3.3.7 Peak Area Determination.

There is no universally accepted y-ray peak fitting routine, which can claim to be the definitive analytical tool. It is true that many of the commercially available peak fitting programs are similar in design. However, it is the slight variation in the analytical functions used to define the peak shapes, that separates one program from another. For well isolated y-ray transitions, the peak area determined by one routine will be virtually identical to that measured by another. However, significant differences may arise for low intensity transitions or multiplets.

During the course of this research three peak fitting routines were used, namely Sampo (Routti and Prussin, 1969), Omnigam (EG&G Ortec, 1989) and Gamanal (Gunnick and Niday 1972). Counting and Spectroscopy Techniques. 56

y-ray source

Detector system

.Input p - type HPGe detector Preamplifier Main amplifier Pulse height \------> analyser Ortec 137CP2 O rtec 6 7 3 Ortec ADCAM Ortec GEM -30195 * 9 1 8

I ■Preamp power High voltage filter O rtec 1 3 8 lithium contact contact

High voltage bias lectrons supply

O rtec 4 9 5

0-5 kV output ion implantation contact Cross-sectional view of ap - t y p e HPGe coaxial detector.

Fig.3.11(b) Schematic representation o f detector electronics. Counting and Spectroscopy Techniques. 57

3.3.8 Sampo Routine. .

As already stated, the primary factor describing the width of the photopeak is the statistical fluctuation in the number of charge carriers. If this were the only source of fluctuation then the response function would be Gaussian. However, this is seldom the only process observed. For example, incomplete charge collection impairs the resolution and gives rise to low-energy tailing of the photopeaks. Preamplifier noise and instabilities in the main amplifier perturb the line width, especially over long counting periods, again resulting in a reduction in the resolution. Furthermore, random summing of pulses, at high counting rates, broadens the peaks and gives rise to high-energy tailing (Routti and Prussin, 1969).

The analytical peak fitting routine Sampo has, on the basis of the above considerations, represented a photopeak in terms of a Gaussian distribution located at the centroid of the peak, together with simple exponential tails which join either side of the Gaussian. In this model the shape of the peak is defined by three parameters, namely: the width o f the Gaussian and the distances from the centroid to the low and high energy junction points, and these shape parameters vary smoothly with energy. Therefore the parameters of any line in the spectrum may be found by interpolation between the parameters of neighbouring lines. For this purpose, intense and well defined y-ray peaks are used as internal calibration sources and their shape parameters are determined by fitting the peak data to the Gaussian-exponential function. The background continuum is approximated to a straight line, j

Once the shape-calibration fit is performed, the best values for the parameters are stored for each calibration peak. The line shape corresponding to any part of the spectrum is then determined by interpolating the shape parameters linearly with respect to the channel location. The continuum under the peaks can be attributed to Compton events from higher energy y-rays and general background radiation. Consequently the continuum, over a short region, can be described as a continuous, smoothly varying function of energy and is expressed in terms of a polynomial.

3.3.9 Omnigam Routine.

One of the main features of this routine is that the method used to determine the background continuum can be altered depending on the data. If the operator selects the automatic mode, a series of tests is performed until a value for the background channel is obtained which best fits the data. Firstly, a 5-point average Counting and Spectroscopy Techniques. 58

of the channel contents is calculated. The centre channel is located three times the calculated FWHM below the centroid. The average is determined from two channels above and below the central channel and the calculated value is assigned to this channel. If this value is within one sigma (counting statistics) of the actual channel data then it is accepted as the low-energy background value for the peak. The procedure is illustrated below in Fig.3.12.

Fig.3.12 Background calculation method used by Omnigam.

If the average value is not within one sigma of the actual data, then a 3-point average is used. Once again this value is compared with the actual data at the assigned channel. If the 3-point average also fails the test, then the data value at the assigned channel is used as the low-energy background value.

The same process is repeated for the high-energy side of the peak. The background under the photopeak is then simply the straight line between the two points.

Alternatively the operator can select a 5-point average, which is calculated in exactly the same way as before, except the average value is not compared with the actual data of the assigned channel. This method is used for isolated peaks with high scatter in the channel-by-channel data. Similarly a 3-point average may be chosen and this is used in preference to the previous method when the spectrum is complex. The operator can also select a minimum data point within the search width to represent the average background count rate and this is used for complex spectra with good counting statistics. Counting and Spectroscopy Techniques. 59

The background calculation is then used to determine the net peak area of a singlet since:

Net area = Gross area - background area (3.10) and the gross area of the peak is determined using the summation method which sums the contents of each channel between the designated background channels, such that:

h Gross Area = £ C i (3.11) i=l

Q = data value of channel i 1 = low energy channel h = high energy channel

The background calculation for multiplets is analogous to that of singlets, with the exception that the search width is 1.5 times the FWHM below the lowest energy peak of the multiplet and 1.5 times the FWHM above the highest energy peak. A step function is introduced if the background continuum under the multiplet decreases faster than the continuum on the high energy side of the multiplet. The size of the step inserted at each peak centroid is proportional to the peak height.

Peaks to be included in the deconvolution of the multiplet are positioned at the library energy of the component. A pure Gaussian is used to represent each peak, whose line width is determined by interpolating the shape parameters corresponding to the calibration lines. This method requires all suspected y-ray transitions to be included in the nuclide library for a satisfactory deconvolution to take place.

3.3.10 Gamanal Routine.

The main features of this routine are similar to those already described for both Sampo and Omnigam. The peak shape is characterised by a Gaussian distribution, located at the centroid, together with a low-energy tailing function. Counting and Spectroscopy Techniques. 60

Peak shape parameters are determined from calibration sources. Analysis of singlets and deconvolution of multiplets are achieved by interpolating the shape parameters linearly with respect to the channel location.

The background continuum for both singlets and multiplets is represented by a smoothed step function, positioned at the centre of each peak. The size of each step is proportional to the relative height of the peaks.

3.3.11 Peak Anomalies.

Not all photopeaks, that occur in the spectrum, represent the passage of a single y-ray photon through the detector crystal. Additional peaks may arise from the coincident detection of two or more y-rays, which may be in cascade, as illustrated below, or independent of each other.

a - v.or p- particle X

y-ray 1

y-ray 2

Ground State

Fig.3.13 Cascade y-rays leading to coincidence summing.

If one assumes that the lifetime of the intermediate energy level, shown in Fig.3.13, is short, then the two y-rays will be emitted in virtual coincidence. If the response time of the detector electronics are long compared to the time it takes for both photons to deposit their energy in the crystal, then a sum peak will result. The energy of the peak will be equal to the sum of the individual y-ray energies. The number of events in the sum peak is proportional to the product of the branching ratios of the two y-rays, their angular correlation and the solid angle subtended by the detector. Thus, a simple way of minimising the probability of coincidence Counting and Spectroscopy Techniques. 6 1

summing is to increase the source-to-detector distance, thus reducing the subtended solid angle.

Random summing can occur between two, or more, independent y-rays which are emitted in the same time period. For multiple radiations, sum peaks may potentially occur at all possible combinations of any two energies. The probability of this process will increase with enhanced counting rates and so can be reduced by again utilising large source-to-detector distances.

3.3.12 Detector Efficiency.

Charged radiation such as a - and p-particles will cause ionisation immediately upon entering a detector's active volume. Thus it is possible to create a detector with almost 100% counting efficiency. However, the same is not true for uncharged radiation such as y-rays or neutrons, which must first undergo a significant number of interactions before being detected and so have a reduced efficiency. It is necessary to determine the detector efficiency in terms of the number of pulses counted, relative to the number of photons incident on the detector.

Detector efficiency can be divided into two categories, namely absolute and intrinsic, which are defined below:

______number of pulses recorded______(3.12) £ a b s — number of y-ray photons emitted by the source

In this case the efficiency is dependent upon the crystal material and its dimensions, photon energy and counting geometry i.e. the solid angle (£2) subtended by the detector at the source position.

______number of pulses recorded______(3.13) £int —number of y-ray photons incident on the detector

The intrinsic efficiency does not include the solid angle, but is dependent on detector properties alone. The two efficiencies are related by the following formula (Knoll, 1989): Counting and Spectroscopy Techniques. 62

£int — £abs- j (3.14)

The efficiencies can be further categorised depending on the nature of the events recorded. For example, total efficiency records all interactions under the spectrum, regardless of whether they are x-rays, scattering events or simply spurious noise. Photopeak efficiency records only those events which contribute to the full-energy peak.

The most commonly used efficiency is the absolute photopeak efficiency, which is expressed as a function of energy and geometry. The photopeak areas are measured for a series of standard calibration sources, such as those listed in Table 3.1. The analytical peak fitting routine used to measure the peak areas of the calibration sources must be identical to that used for subsequent spectral analysis. The detector efficiency corresponding to a specific y~ray energy is given by the equation:

^ N0.PY.exp(-Xt)tc (3'15)

£y = absolute photopeak efficiency

A y = net peak area (determined by the peak fitting routine) N0 = activity of the source at the time of standardisation PY = absolute y-ray emission probability X = decay constant t = time elapsed since standardisation tc = measuring time (tc is small compared to the half-life)

If t is known precisely, then the uncertainty in the measured efficiency can be calculated by the propagation of errors equation (Bevington, 1969).

(d £ ,,\ (

The measured efficiency values can then be fitted to analytical functions which will interpolate the data over the spectrum energy range. The peak fitting routines used in this research perform the interpolation automatically, using the following mathematical functions: Counting and Spectroscopy Techniques. 63

Isotope Energy (keV) Half-life 241Am 59.537 432.2 ± 0.5 y 133Ba 80.997 10.6 ± 0.2 y 109 Cd 88.034 463.1 ± 0.8 d 57Co 122.061 271.73 ± 0.14 d 57Co 135.474 271.73 ± 0.14 d 139Ce 165.857 137.65 ± 0.03 d 133Ba 276.398 10.6 ± 0.2 y 133Ba 302.853 10.6 ± 0.2 y 51Cr 320.084 27.703 ± 0.004 d 133Ba 356.017 10.6 ± 0.2 y 133Ba 383.851 10.6 ± 0.2 y 113Sn 391.700 115.10 ± 0.17 d 85Sr 514.009 64.85 ± 0.02 d 137Cs 661.660 30.18 ± 0.05 y 54Mn 834.843 312.2 ± 0.1 d 65Zn 1115.546 244.0 ± 0.2 d “ Co 1173.238 5.271 ± 0.001 y 22Na 1274.542 2.602 ± 0.001 y “ Co 1332.502 5.271 ± 0.001 y Table 3.1 Isotopes used for the calibration of a semiconductor detector (Lorenz, 1983). Counting and Spectroscopy Techniques. 64

Sampo 8y = Pi ( E7P2 + P3exp(P4ET) ) (3-17) Omni gam £y= exp(Pi + P2lnEy + P3(lnEY)2 ) (3.18) Gam anal £y = exp (Pi + P2Ey + P3Ey2 + P4Ey3 ) (3.19)

and the error in functions (3.17) and (3.19) are given by the equation:

n n (3.20)

i=l

E = y-ray energy (MeV) a = error in the parameter P(ij) = correlation between Gi.Oj n = number of parameters

The absolute photopeak efficiency curve of an intrinsic HPGe detector (Canberra 770210) used at the Harwell laboratory, is given in Fig.3.14. The electronics were set to analyse the low-energy spectrum (5 to 220 keV) of Np. Efficiency measurements utilised both standard y-ray calibration sources, together with the low-energy x-ray transitions of 54Mn, 57Co, 93mNb, 133Ba, 137Cs, 139Ce, 15-2Eu and 241Am. Energies and emission probabilities for these x-rays have been evaluated by Bambynek (1984). The efficiency measurements were fitted to the 3rd order polynomial described in equation (3.19) and the uncertainty in the calculated value was determined using equation (3.20). Since the detector efficiency changed rapidly between 15 and 50 keV, three polynomials were used to describe the energy regions 5-14, 15-49 and 50-400 keV. 70 Q The y-ray spectroscopy measurements of Np employed the peak fitting routines Sampo and Omnigam. Efficiency measurements corresponding to each program were fitted to the function proposed by Gray and Ahmad (1985), which is represented by a smooth continuous function:

Pi + P2(lnE7) + PsOnEy)2 + P4(lnEy)3 + P5(lnE7)5 + P6(lnE-^)7 e7= (3.21) % *0 < -O .0 •a IS • 3 on 0) O

Both the fitting parameters and their uncertainties were calculated using the CERN library subroutine, MINUIT (James and Roos, 1975), which utilised the least-squares minimisation model. Fig.3.15 shows the photopeak efficiency curve of the p-type HPGe detector used at ICRC. Counting and Spectroscopy Techniques. 67

3.4 Coincidence Counting.

This technique is used to measure the activity of a source which is decaying with the emission of two or more distinguishable radiations, which occur in prompt succession. Two detectors are required and ideally each is sensitive to only one type of radiation (NCRP, 1985). For example the 47t|3-y coincidence counting system used in Chapter Six, contains a 471 gas flow proportional counter, to detect 13- particles, coupled with two Nal(Tl) scintillation detectors, sensitive to gamma radiation. A third counting channel records those events from the two detectors which are in coincidence.

For a simple decay scheme one can assume that: i) . The emission of a (3-particle is accompanied by a y-ray transition which is in prompt time coincidence. ii) . The |3-detector responds only to (3-particles and similarly the y-detector is sensitive only to y-rays. iii). The coincidence unit will register an event only when simultaneous pulses have been recorded in both detectors (Baerg, 1966).

Thus for a point source of activity n0 and detector efficiencies ep and ey, the counting rates in the beta, gamma and coincidence channels are:

np=n0ep ny= n0ey nc = nQepey (3.22)

However, for an extended source of total activity N0, for which the detector efficiencies may vary from one point to another, the analogous expressions for the observed counting rates are:

Np = N0ep NY= N 0eT Nc = N0epey (3.23)

Where ep and Qy are the averages of the elemental volume efficiencies integrated over the extent of the source and are identical to the overall detector efficiencies £p and 8y. However, only when at least one detector has the same efficiency value for the entire source is the mean product of the elemental efficiencies epey equal to 8p£y Counting and Spectroscopy Techniques. 68

(Putman, 1950). In practice this condition is approximated by the y-detector. The resulting individual channel count rates can now be expressed as:

Np = N0£p Ny = N0Sy Nc = No£p£y (3.24)

The above equations are basic to the coincidence method and from them the absolute disintegration rate, N0, can be obtained directly from the observed channel count rates, since:

(3.25)

However, in reality the above equation cannot be applied directly, since the original assumptions which have lead to equation (3.25) can never be attained experimentally. Equations (3.24) must be extended to account for the fact that the p-detector is not exclusively sensitive to one type of radiation and contributions to the P-channel count rate may arise from conversion electrons and y-rays. In cases where the decay scheme is complex, further modifications to equations (3.24) are necessary.

3.4.1 Conversion Electrons.

Electrons arising from the internal conversion of y-rays, contribute a rate, nce, to the observed p-channel count rate. Assuming that the internal conversion process occurs in a time much shorter than the resolving time of the coincidence unit, the rate nce can be expressed as:

nce — N0(l — £p)£ce j + a (3.26)

£Ce = efficiency of the P-detector to conversion electrons, a = internal conversion coefficient

In gas filled detectors £Ce is nearly equal to the solid angle subtended by the counter at the source, since the intrinsic efficiency of the counter approaches unity for high energy electrons (Siegbahn, 1966). However, for less energetic electrons, Counting and Spectroscopy Techniques. 69

events may still be recorded which arise from the detection of x-rays or Auger electrons.

Conversely the internal conversion process will lead to a reduction in the number of events recorded in both the gamma and coincidence channels. Thus, the expressions for the count rates in each channel, analogous to equations (3.24), are now:

Np = Nojep + (1 - £p) £ce a

Np£y Nv = (1 + a) (3.27)

Nc = NQ8E&JLNp£p£y (1 + a)

3.4.2 Gamma Sensitivity o f the /3-detector.

As in the above case, a y-ray that triggers the (5-detector will contribute a rate, npy, to the (3-channel count rate, such that:

N0(l — £p)£py npy = (1 + a) (3.28)

£py = the intrinsic efficiency of the (3-detector to y^-rays

Furthermore, there is an additional contribution, nc, to the coincidence counting rate, which arises from Compton scattering or in the case of more complex decay schemes, from the presence of y-y coincidences. If Eq is the probability of such coincidences, then:

nc — N0(l £p)£c (3.29)

With appropriate energy discrimination levels for the y-channel, the value for £c can be made negligible. Counting and Spectroscopy Techniques. 7 0

3.4.3 Beta Sensitivity o f the y-detector.

The presence of absorber materials between the p— and y-detectors are usually sufficient to prevent p-particles entering the scintillation counters. The introduction of errors in the coincidence method arising from the detection of Bremsstrahlung photons are believed to be negligible and vanish at unit p-detector efficiency (Campion, 1959).

3.4.4 Complex Decay Schemes.

For a decay scheme involving n p-branches, each of fractional intensity ar and with at least one of the branches having one or more associated y-ray transitions, the individual channel counting rates can be expressed as:

Ny — N o l Ur^yr (3.30)

The summations extend over n branches and subscript r refers to the r111 p-branch, so that: £pr = p-detector efficiency for P-particles from the i^1 branch. Eyr = y-detector efficiency for y-rays associated with the r^ branch. £cr = probability of recording a y-y coincidence or Compton scattering event in the coincidence channel. (£(fy)r = P-detector efficiency for y-rays associated with the r1*1 branch. (£ce)r = P-detector efficiency to conversion electrons corresponding to the internal conversion of y-rays associated with the r^ branch.

In general neither the disintegration rate nor the detector efficiency can be determined directly from the observed counting rates unless the terms (1 - £pr) are Counting and Spectroscopy Techniques. 7 1 all zero and one of the detectors is equally efficient for all branches (Putman, 1950). Unfortunately these conditions are not realised in any practical system. However, an Nc evaluation of N0 is possible, if a functional relation between Np and the ratio — 11 y Nc (the efficiency parameter) can be established, such that Np —> N0 as — —» 1 l^y (Baerg, 1966).

(3.31) n P = n «>f (n J)

F —> 1 as “ F*— >1 N y and nPnt _ NnG(*V| (3.32) Nc oC\N cJ

G —> 1 as ->1 Nc

The functions F and G are of an unspecified form and in practice are defined simply as polynomials, which are usually linear or of a low order (NCRP 1985, MacMahon and Baerg 1976). They may be determined experimentally by varying the p-detector efficiency via a low energy discriminator. Then assuming the validity Nc of the function extends to t t= 1, the disintegration rate is obtained in terms of P

3.4.5 47t/3-^Y Coincidence System.

Fig.3.16 illustrates the 47tp-y coincidence system used during the course of this research. Beta-particles were detected by means of a 471 gas flow proportional counter, identical to that described in section 3.2.3. The lower level threshold of the / counter and effectively the p-efficiency, was varied by an automatic digital spectrum scanner (OR TEC 487). ‘

The source and source mount were an integral part of the detector, since good electrical contact must exist between them and the rest of the circuit. Poor conductivity would result in space charges that reduce both the field strength and the Linear Preamp. S.C.A. Amp. A S 16 O R T 488 i and ques. s e u iq n h c e T y p o c s o r t c e p S d n a g tin n u o C N E 4 6 0 3 Digital C lock N E 4 6 5 2

High Voltage N E 4 6 4 6

H.V. Dist. N E 4 6 2 9

Linear Print Preamp. S.C.A. I.B.M. Amp. Control A S 16 O R T 488 Computer N E 4 6 0 3 N E 4 6 1 7

to Fig.3.16 4np-y Coincidence Counting System. Counting and Spectroscopy Techniques. 73

sensitivity of the counter. This would lead to possible discrimination against higher energy p-particles. Consequently, the active material was deposited on a gold- coated VYNS film which was supported on a metal ring.

Two Nal(Tl) scintillation counters, similar to that described in section 3.3.5, were used to detect y-rays. They were positioned either side of the proportional counter to improve the overall efficiency of the system. Although their resolution is / poor relative to semiconductors, they have the advantage that they operate at ambient temperatures and so do not require the elaborate cooling systems associated with HPGe detectors.

All three detectors were assembled on an aluminium frame which was placed inside a lead castle of 50 mm thickness. The inner wall of the castle was covered with a thin sheet of cadmium to absorb lead x-rays, which in turn was covered by an additional layer of copper to absorb Cd x-rays.

Single channel analysers (SCA's) were used to select a window in which the input amplitude had to fall in order to produce an output pulse. Thus the device served to select only a limited range of amplitudes from all those generated by the detectors. Normal SCA's are activated by the leading edge of a bipolar pulse. Thus, pulses which originate at the same time but have different amplitudes will trigger the SCA at slightly different times, giving rise to amplitude walk and large timing uncertainties. Timing SCA's utilise the zero crossover method which is independent of the pulse amplitude (Knoll 1989, Singru 1972). Such devices are widely used in coincidence applications since they produce logic pulses which are closely correlated with the actual event time.

In the case of the y-ray counters, the timing SCA (ORTEC 488) window | was set to correspond only to those events which deposited their energy in the full \ energy peak, often the principle y-ray transition in the decay scheme. Whilst the j SCA window for the p-detector was set automatically by the spectrum scanner.

The logic output pulses from the timing SCA's of both y-detectors were summed in a mixer unit (NE 4618), which produced a single output. This was then mixed with the output signal from the SCA of the beta detector in the coincidence/anticoincidence unit (NE 4651). Whenever two pulses overlapped, the resultant pulse was twice the amplitude of a single one and thus, by exceeding the threshold level of a trigger circuit, was recorded as a coincidence event. The three output signals, beta, gamma and coincidence, were then fed to their respective scaler units which recorded the number of events associated with each channel. Counting and Spectroscopy Techniques. 7 4

The timer scaler (NE 4612) recorded the counting time and at the end of each measurement sent a signal to the spectrum scanner which changed the ^-detector threshold level and initiated a further measurement. The digital clock (NE 4652) recorded the counting time in minutes together with the normal clock time, which was used to correct the measured activity for source decay. Finally, the outputs from all the scalers were transferred to an IBM PC computer for data processing.

3.4.6 Dead Time and Resolving Time Corrections.

The dead time of any detector system, is the minimum time that must separate two events in order that they be recorded individually. Since radioactive decay is a random process, there is a probability that some events will be lost because they occur in close succession. Thus the extent of dead time losses is dependent on the counting rates and the response function of the associated electronics. There are two models of dead time behaviour (Knoll, 1989). i) . Extendable (paralysable): In this model a fixed dead time x is assumed to follow each true event that occurs when the detector is live. Events which occur when the detector electronics are processing a pulse, are assumed to extend the dead time for another period x. ii) . Non-extendable (nonyaralysable): Again a fixed dead time x is assumed to follow each true event, however, in this model, any events which occur during the dead time period have no effect on the behaviour of the detector.

In reality a detector system will have a dead time response which is intermediate between the two models.

A commonly used method for determining the dead time of a detector is known as the paired source technique, which assumes a non-extended dead time model (Baerg 1965, 1966). Two sources A and B, of approximately equal activity, are counted separately and then combined to form C. The dead time is then given in the form of a quadratic equation involving all three counting rates NA, Nb and Nc, together with their background rate b, such that:

(3.33) and A = (NA + NB-N C- b) Counting and Spectroscopy Techniques.

Baerg's adaptation of this technique replaces one of the sources with a periodic pulse generator. This avoids the problem of maintaining the geometry when counting the sources singly or combined. The dead time of the detector is then given by the equation:

(3.34)

Nr = random source rate Np = periodic pulse frequency Nrp = random and pulser combined counts

Assuming that the pulser frequency is invariable, the uncertainty in the dead time can be calculated using the propagation of errors formula, such that:

(3.35)

The coincidence unit will produce a logic output signal if pulses arrive, from the (3- and y-channels, within the resolving time (xr) of the device. The resolving time must be long enough to avoid the loss of true coincidences, arising from fluctuations in time delays, but not so large as to allow accidental coincidences from unrelated beta and gamma events. Fig.3.17(a) shows the variation in the coincidence count rate (Nc) with resolving time. A rapid increase in the count rate is observed between zero and 1 ps. However, from 1 to 5 ps Nc is virtually constant, forming a plateau. Increasing the resolving time still further will, once again, result in a marked increase in the count rate. Compare this with Fig.3.17(b) which is a plot of accidental coincidence counts (Nacc), i.e. those originating from unrelated events, against resolving time. Here the relationship is linear and Nacc increases sharply with increasing values of xr. For example, the observed count rate at 2 ps is twice that at 1 ps. Thus the resolving time of the coincidence unit is set at a value on the plateau region of Fig.3.17(a) which is not associated with a high accidental count rate. The resolving time can be measured by determining the accidental coincidence count rate from two independent input rates Np and Ny, such that: Counting and Spectroscopy Techniques. 7 6

Fig.3.17(a) Variation in the coincidence count rate with resolving time.

Fig.3.17(b) Variation in the accidental coincidence count rate with resolving time. Counting and Spectroscopy Techniques. 11

(3.36) Xr 2Np.Ny

To ensure that the events from both beta and gamma channels are totally uncorrelated, one of the inputs can be replaced by a periodic pulse generator. The uncertainty in the resolving time can be calculated, once again, by the propagation of errors formula, assuming the pulser frequency to be constant.

3.4.7 Corrections to the Coincidence Formulae.

It is now a simple matter to correct both the (3- and y-channel count rates for dead time losses using the equations:

Np' and Ny' denote dead time corrected channel count rates Np" and Ny" denote observed counting rates

The background count rate is similarly corrected and then subtracted to give the true values of Np and Ny. If the isotope is short lived, a further correction is necessary to account for decay during acquisition.

Corrections to the coincidence counting rate, or the ratio Nc/Ny are not as easily obtained as those for the (3— and y-channels. Grigorescu (1973) has reviewed a number of expressions proposed by several authors, most of which assumed that the dead times in both the (3- and y-channels were non-extending.

Bryant (1963) has derived an exact expression which involves only the observed counting rates:

(Nc" - 2xrNp"Ny,l)[ l-(Np" + Ny")x/2 ] (3.38) Ny"( 1 - Np"x)[ 1 - (Np- + Ny" - 2Nc")x/2 - xr(Np"4- Ny") ]

The ratio is obtained by substituting the observed background count rates bp", by" and bc" into the above equation. Counting and Spectroscopy Techniques. 78

the detector efficiency is corrected for background counts by the equation:

Nc N c'/N t’ - [ (bc7bY')(bY7N7")(l-N Y"xY)/ ( l- b y"TY) ] nY ~ 1 - [ (by/NfXl- N y"ty)/(1 - ] xp = imposed p-channel dead time Ty = imposed y-channel dead time Tr = resolving time x = (ffi±l*L

3.4.8 Data Analysis.

A fortran program, 4n;py2, was written to perform the above corrections to the observed channel count rates and to determine the ratios and for Ny Nc varying p-detector efficiencies. The program included a curve fitting routine which utilised the least-squares model to extrapolate the ratios to unit efficiency and so determine the absolute disintegration rate, N0, of the source.

Nc Ny The extrapolation of the ratios rr- and rr^ are shown in Figs.3.18(a) and (b) INy JNc for a ^Co source. Both graphs illustrate, quite clearly, that a straight line fit is sufficient to describe the polynomials F and G. 1 9

------' ------r 1 ------1 1 ------1 1 ------1 ' i- i ------1 ------1 1------1 to determine the absolute disintegration Ny/Nc ------1 1 N y / N c Goodness of =fit Goodness 1.000 ------Least—squares fit to data vs. ------1 1------1 ------1 1 N p ( N j / N c ) ------1 t ------1 ------; ; = No ± 15855 48 Bq . . ' y = - 1.5864e+4 8.5785x of =fit Goodness 1.000 j ;—m m 111 i i i i i ’ ’ y = - 1.5864e+4 8.5785x 1.0 1.2 1.4 1.6 1.8 2.0 '2.2 2.4 - a i S Q o Fig.3.18(b) Graph of rate o fa ^°Co source. ] ] 1.80e+4 1.60e+4 l:70e+4 1.30e+4 “I 1.40e+4 " 60Co source. Fig.3.18(a) Graph ofNp vs. Nc/Ny to determine the absolute disintegration rate o fa Counting and Spectroscopy Techniques. (sdo) ^jjq (sdo) (0jfsl/^iS[)^jiSI Low-Energy Gamma-Ray Analysis o f 237N p .

Chapter Four.

Low-energy Gamma-ray Analysis of 2 3 7Np.

4.1 Introduction.

There have been many measurements of the decay data of 'TYl Np. This is due primarily to its abundance as a waste product in spent fuel, together with the fact that its high radiotoxicity and long half-life make it a potential hazard if released, from a repository, into the environment. This isotope can also be used to determine the fast-neutron flux density, since the Np fission reaction has both a large cross-section and low threshold. Thus, models which predict the behaviour of Np, both in the reactor and the surrounding environment, require accurate decay data. These measurements have included the determination of: specific activity; alpha-particle and gamma-ray emission probabilities and internal conversion coefficients. Alpha-gamma (a-y), gamma-gamma (y-y) and gamma-x-ray (y-x— ray) coincidence studies have also added to the plethora of data available. However, as will be explained in the next section, evaluation of these data to produce an intensity balanced decay scheme has proved difficult.

4.2 Review of Alpha-Particle Decay Data.

The principle cause of the anomalous decay scheme is the discrepancies in the a-particle emission probabilities, (Pa), measured by several workers. The complexity of Np decay, coupled with its low specific activity, has made accurate a-particle spectroscopy difficult (Bortels et al., 1990). The problem has been further confounded by the type of device used for the measurements. For example, magnetic spectrometers, which have excellent energy resolution, suffer from low transmission and so require extensive measuring periods, eventually leading to instrumental drift. Semiconductor detectors, on the other hand, have poorer resolution due to the complexity of the a-line, which is dependent on the nature of the detector, geometry and source thickness (Garcia-Torano and Aetna, 1981).

Table 4.1 lists the measured a-particle energies and intensities from six independent studies. The early work of Magnusson et al. (1955) utilised a gridded ionisation chamber with an overall resolution of 30 keV, and reported a total of nine Low-Energy Gamma-Ray Analysis o f ^ N p . 8 1

Energy Magnusson Kondratev Baranov et al. Browne & Asaro Vara & Gaeta Gonzalez levels et al.(1955) et al. (1957) (1962, 1976) (1968) (1969) et al. (1979) (keV) E a Pa E a Pa Ea P a E a Pa E« Pa E a Pa 4386(?) 0.02 365 4520 0.02 4514.6 0.01 4514 0.04(2) 4514 0.12 4563(8)<0.09 4573.8 0.05 4574(5) 0.1 301 4589 0.5 4581.1 0.02 4577 0.40(4) 4582(5) 0.2 4582 0.5 4595.0 0.09 4595(5)<0.08 279 4598.7 0.06 4598 0.34(4) 4600(5) 0.2 4600 0.57 4621(5) 0.2 4625(5) <0.1 4632(3) 0.3 238 4644 6.0 4639 4639.5 4.66 4640 6.18(12)4641(3) 5.1 4641 6.7 8.3(10) 4659.2 0.58 4656(3) 0.7 212 4664 4664.1 1.62 4665 3.32(10)4665(3) 2.0 4665 3.4 4674 3.3 4681(3) <0.2 4694.5 0.18 4695 0.48(20)4690(5) <0.1 178 4699.3 0.07 4698(3) 0.5 4698 1.8 4708.4 0.30 4708(3) 0.4 165 4713 1.7 4713 2.3(10)4712.4 0.13 4710 1.13(14) 4712 1.5 4719(3) 0.1 4729(5)<0.05 4741.4 0.02 4744(3) 0.1 4755(5)<0.05 108 4767 29 4766 4766.116.8(4) (4766) 8(3) 4767(5) 13 4767 13.1 29.5(40) 103 4771 4771.119.4(4) (4771) 25(6) 4771(5) 19.5 4771 15.5 4780(5) 0.9 86 4787 53 4787 54(4)4788.151.3(8) 4788 47(9) 4788(3) 46 4788 45.5 71 4803.4 1.58 4799 - 3 4805(3) 2.6 4803 3.4 57 4816 3.5 4816 3.4(10)4817.4 1.50 4816 2.4(4) 4818(3) 2.3 4817 2.9 4827(5) 0.5 4835(3) 0.4 4857(3) 0.4 4862.9 0.24 4864 >0.3(1)4862(3) 0.6 4862 1.9 6.7 4870.9 0.93 4870(3) 1.5 0 4872 3.1 4872 2.5(5)4873.4 0.45 4872 2.6(2) 4875(3) 0.8 4875 0.97 Table 4.1 Measured a-particle emission probabilities. n r237^T Low-Energy Gamma-Ray Analysis o f N p . 82

alpha lines. However, the principle peaks, i.e. those populating the 86, 103 and 108 keV energy levels of 233Pa, have been mistakenly identified as doublets. Kondratev et al. (1957) analysed the alpha spectrum using a magnetic spectrometer and observed only six alpha lines, since transitions feeding the 103, 108 and 212, 238 keV energy levels of 233Pa were reported as doublets. Baranov et al. (1962, 1976), using a magnetic spectrometer with a resolution of 4.4 keV, were able to reveal some of the fine structure in the a-decay of 237Np. They identified over 20 monoenergetic a-lines, some of which were separated by no more than a few keV. Deconvolution of the spectrum was achieved by comparing the peak shape characteristics of the multiplets with those of the principle 4788 keV a-transition. Although this work removed many of the anomalies associated with the a-spectra of 237Np, it was unable to quote the multiplet peak energies better than ± 2 keV. Moreover, the intensities of the components could not be estimated accurately, due primarily to the arbitrariness entailed in the graphic resolution of multiplets into their individual components.

The remaining authors in Table 4.1 used surface-barrier detectors to study the a-decay of 237Np. The first of these (Browne and Asaro, 1968), employed a detector with an energy resolution of 17 keV. The values for the principle a - transitions were adopted by Ellis (1971) in his evaluation. Subsequent measurements by Vara and Gaeta (1969) utilised a surface-barrier Au(Si) detector with a resolution of 18 keV, and extended the number of identified a-transitions by a further 13 lines. Finally, Gonzalez et al. (1979) used two detectors of varying active volume, to determine both the principle a-transitions, and those populating the high energy levels of Pa. Unfortunately, they were unable to quote the uncertainties in the emission probabilities, but concluded that a 10% error was associated with peak areas corresponding to transitions which had an intensity greater than 1%.

Table 4.2 shows that evaluations based on the above data have resulted in uncertainties in Pa of between 20 and 40%, for the three major transitions of Np. This was considered unacceptable by the LAEA-CRP, who requested an accuracy of 1% for the emission probability of the principle a-transition (IAEA, 1986).

Two laboratories, closely linked with the CRP, namely CBNM of Belgium, and CLEMAT of Spain, collaborated to perform, what was hoped to be, the definitive measurement of the emission probabilities of Np (Bortels et al., 1990). Using sources prepared at the Harwell Laboratory, the CBNM group collected several hundred a-spectra, using a passivated implanted planar silicon detector Ray Anal i of Np. p N f o sis ly a n A y a -R a m m a G y g r e n E - w o L Energy Nuclear Decay. Sheets IAEA--CRP Table of Radioactive Isotopes Levels (Ellis, 1986) (IAEA, 1986) (Browne and Firestone, 1986) (keV) Ea (keV) Pa (%) E„ (keV) Pa (%) Eot (keV) Pa (%) 4386(?) 0.02 4386.0(25) 0.02 365 4514.5(20) 0.04(2) 4513.5(5) -0.04 4573.8(20) 0.05 4574.7(5) 0.05 301 4581.0(20) 0.40(4) 4581.1(20) 0.40(4) 4577.9(5) 0.40(4) 4594.9(20) 0.09 4595(2) 0.08 279 4598.6(20) 0.34(4) 4598.7(20) 0.34(4) 4598.4(5) 0.34(4) 238 4639.4(20) 6.18(12) 4639.5(20) 6.18(12) 4639.5(5) 6.18(12) 4659.1(20) <0.57 4659.2(20) 0.6 212 4664.0(20) 3.32(10) 4664.1(20) 3.32(10) 4664.6(5) 3.32(10) 4694.4(20) 0.48(20) 178 4699(?) 0.07 4697.1(7) 0.48(20) 4708.3(20) 4707.1(5) 1.0 1.13(14) 165 4712.3(20) 4712.9(5) 0.13 4741.3(20) 0.02 4741.4(20) 0.02 108 4766.0(15) 8(3) 4766.1(15) 8(3) 4766.1(5) 8(3) 103 4771.0(15) 25(6) 4771.1(15) 25(6) 4771.5(5) 25(6) 86 4788.0(15) 47(9) 4788.1(15) 47(9) 4788.4(5) 47(9) 71 4803.3(20) 1.56 4803.4(20) 1.56 4804.0(5) 1.6 57 4817.3(20) 2.5(4) 4817.4(20) 2.5(4) 4817.3(5) 2.5(4) 4862.8(20) 0.24 4862.9(20) 0.24 6.7 4866.9(5) -0.3 4871(7) 0.93 0 4873.0(20) 0.44 4873.1(20) 2.6(2) 4873.4(5) 2.6(2) Table 4.2 Evaluated cc-particle emission prbabilities. oo u> 23 7 Low-Energy Gamma-Ray Analysis of Np. 84

(PIPS), with an energy resolution of 9.1 keV. Operational conditions, such as bias supply and vacuum, were maintained throughout the measurements by means of a purpose built chamber, which isolated the detector when changing a source. A thermostatic bath was used to stabilise the temperature of the detector and preamplifier, which prevented peak drift arising from temperature fluctuation. A small magnetic device was placed between the source and the detector, eliminating coincidence summing between a-particles and conversion electrons.

Each spectrum was checked to ensure that the principle peak was located at the same channel position, and then summed to produce four spectra. The resulting spectra were then analysed independently by CBNM and CIEMAT using their own peak fitting routines. Both programs utilised the experimental observations that: i. the region of maximum intensity is Gaussian ii. the line is asymmetrical due to characteristics such as the type of detector, source thickness and geometry iii. a low energy tail extends to the origin and may be expressed as a percentage of the peak area

Table 4.3 shows the results of the analysis. It should be noted that the same random and systematic uncertainties were applied to both measurements and the recommended values are an evaluation of the two data sets.

The measured values are in close agreement and satisfy the CRP's original request. Low intensity peaks were observed in the residual spectrum, however, they were not included in the fittings since the analysis provided insufficient evidence of their existence. Incorporating low intensity peaks, whose existence was questionable, would result in changes to a number of peak intensities and this has been accounted for by an additional correction to the overall uncertainties.

The recommended values listed above are considered to be the best data currently available for the a-particle emission probabilities of 237Np. The data also provides a means with which to calculate the Pa energy levels. Following a-decay, energy is shared between the recoiling daughter nucleus and the a-particle. If Ea is the energy of the a-particle (mi = 4), and M2 is the mass of the daughter nucleus, then the recoil energy (Er), is calculated by applying the law of conservation of momentum, such that:

(4.1) 237 Low-Energy Gamma-Ray Analysis of N p . 85

Energy Mean Relative Intensity Recommended (keV) CBNM CIEMAT Values 4513.7 0.041(4) 0.042(4) 0.041(4) 4577.5 0.396(20) 0.422(20) 0.41(2) 4598.5 0.374(20) 0.409(20) 0.39(2) 4639.5 6.43(4) 6.47(4) 6.45(4) 4664.6 3.41(4) 3.41(4) 3.43(4) 4698.0 0.52(4) 0.57(4) 0.54(4) 4710.6 1.16(5) 1.23(5) 1.20(5) 4766.4 9.58(30) 9.81(30) 9.70(30) 4771.4 22.94(40) 22.38(40) 22.65(40) 4788.1 47.66(20) 47.77(20) 47.75(20) 4803.6 2.09(5) 2.03(5) 2.06(5) 4817.0 2.47(2) 2.46(2) 2.47(2) 4866.4 0.51(3) 0.51(3) 0.49(3) 4873.0 2.42(3) 2.43(3) 2.43(3) Table 4.3 a-particle emission probabilities determined by CBNM and CIEMAT, togther with their recommended Pa values.

a-particle Ea(keV) Et (keV) Energy level (keV) 0C13 4513.7 4591.2 365.5 ai2 4577.5 4656.1 300.6 an 4598.5 4677.4 279.2 «10 4639.5 4719.1 237.5 ag 4664.6 4744.7 212.0 a s 4698.0 4778.7 178.0 a 7 4710.6 4791.5 165.2 06 4766.4 4848.2 108.4 as 4771.4 4853.3 103.3 0 4 4788.1 4870.3 86.4 <*3 4803.6 4886.1 70.6 CC2 4817.0 4899.7 57.0 a i 4866.4 4949.9 6.7 ao 4873.0 4956.7 0.0 Table 4.4 Energy levels o f2'*'* Pa. 23 7 Low-Energy Gamma-Ray Analysis of Np. 86

Thus the total energy associated with each decay and consequently the energy which must be removed by a de-exciting y-ray photon is given by the equation:

(4.2)

Since the a-particle transition to the ground state is 4873.0 keV (i.e. E j = 4956.7 keV) then all other levels populated by a-decay can be inferred, as shown in Table 4.4.

4.2.1 Review o f Gamma-Ray Decay Data.

Table 4.5, originally compiled by Reich (IAEA, 1986), lists the y-ray emission probabilities reported by several authors. All but two of the measurements are relative. Those of Browne and Asaro (1968) have utilised a-y coincidence studies to normalise their intensities to P^(86.5) = 12.6%. A second measurement by Vara and Gaeta (1969) used a continuous purification process, in which the 237Np source was held on an ion-exchange column, while the equilibrium daughter, 233Pa, was continuously eluted. Although this removed the interfering y-rays, it unfortunately did not prevent the 8%.o and 86.5 keV y-ray transitions being summed as.a doublet. A continuous purification process was also attempted by Skalsey and Connor (1976), however, they abandoned this method and returned to more conventional sources, when they found that it was not possible to achieve the same purification as that of the previous authors. They concluded that following the n'2'y emission of a 5 MeV a-particle, the recoil energy of the Pa nucleus was sufficient to impact it into the resin granules and so make its elution impossible. However, the y-ray spectrum of an evaporated source was measured using planar Ge(Li) detectors, fitted with germanium filters to prevent accidental summing of x- rays with intense y-lines. Relative intensity values were reported by the authors which have subsequently been normalised by Reich (IAEA, 1986), such that P-/86.5) = 12.3%.

Gonzalez et al. (1979) employed both a-y coincidence and direct y-ray spectroscopy measurements to elucidate the fine structure of 237Np y-decay. Although this work positively identified fifty y-ray transitions, only the direct spectroscopy measurements utilised a Gaussian peak-fitting routine, and the values o Enegy Ga - y nlss Np. f o Analysis ay a-R m am G y —E erg n Low

Energy Browne and Vara and Gaeta Skalsey and Gonzalez et al. Banham and Vaninbroukx IAEA (keV) Asaro (1968) (1969) Connor (1976) (1979) Fudge (1984) et al. (1984) (1986) 29.37 14.0(20) 13 16.2(9) 10.3(10) 15.4(2) 15.03(40) 15.3(3) 46.53 0.140(20) 0.1 0.12(2) 0.10(1) 0.104(6) 0.11(1) 0.106(6) 57.15 0.420(38) 0.06 0.433(25) 0.38(4) 0.373(11) 0.39(1) 0.382(11) 86.50 12.6 13 12.3 12.6 12.20(12) 12.44(33) 12.3(2) 88.04 0.160(20) 0.14(4) 0.12(1) 0.138(3) 0.14(1) 0.138(3) 117.68 0.170(20) 0.180(12) 0.151(15) 0.175(2) 0.168(5) 0.173(3) 131.04 0.089(9) 0.1 0.10(1) 0.081(8) 0.086(1) 0.086(2) 134.23 0.071(8) 0.1 0.081(16) 0.063(6) 0.071(1) 0.071(2) 143.21 0.420(40) 0.4 0.462(28) 0.41(4) 0.430(4) 0.434(10) 0.432(8) 151.37 0.249(30) 0.1 0.249(16) 0.227(23) 0.236(2) 0.232(6) 0.234(4) 155.22 0.097(9) 0.097(7) 0.087(9) 0.0917(10) 0.092(2) 169.17 0.076(8) 0.082(9) 0.074(7) 0.0711(7) 0.071(1) 195.09 0.210(20) 0.1 0.1669(21) 0.16(2) 0.184(2) 0.188(5) 0.185(2) 212.42 0.159(20) 0.1 0.166(11) 0.160(16) 0.150(2) 0.155(5) 0.151(2) 238.00 0.068(6) 0.05 0.075(9) 0.063(7) 0.0586(12) 0.059(1) Table 4.5 Absolute y-ray emission probabilities.

oo 2 3 7 Low-Energy Gamma-Ray Analysis of Np. 88 quoted in Table 4.5correspond to those measurements. Once again the relative peak intensities were normalised such that P-y(86.5) = 12.6%.

Until 1980 only relative emission probabilities had been reported, achieving an accuracy no better than 10%. This was unacceptable to the IAEA-CRP, who requested an accuracy of 1% for the principle y-ray. This prompted two laboratories, AEA Technology, Harwell and CBNM, to carry out independent measurements of the absolute y-ray emission probabilities of Np. Banham and y i 7 Fudge (1981) of Harwell, prepared a range of solutions with J Np concentrations of between 14 and 140 mg g-1, determined accurately by Controlled Potential Coulometry (see Chapter Five for an explanation of this technique). A number of spectra were accumulated using an intrinsic HPGe detector, with an energy resolution of 800 eV and spectral analysis was performed with the peak fitting routine Gamanal.

Vaninbroukx et al. (1984) determined the absolute disintegration rate, of vacuum sublimated sources, by means of an a-counter with a well defined solid angle. Gamma-ray spectroscopy measurements utilised a HPGe detector.

Both measurements were in good agreement and were combined to represent the IAEA-CRP recommended values, which have produced an overall uncertainty of 1.96% for the intensity of the principle y-ray transition.

4.2.2 Review o f Alpha-Gamma Coincidence Data.

Table 4.6 lists the a-y coincidence data measured by Gonzalez et al. (1979). Fig.4.1 shows the energy level scheme corresponding to the data. Multipolarities were supplied by Ellis (1986) which were compiled from evaluations by Browne and Asaro (1968) and Hoekstra (1969). Of interest is the a-transition populating the 237.5 keV energy level, which is in coincidence with the 143 keV y-ray, and suggests the existence of a 94.7 keV energy level. Furthermore, y-rays of energy 195 and 201 keV, which depopulate the 201.7 keV energy level, are seen in coincidence with an a-transition feeding the 237.5 keV level. This would suggest the existence of additional y-rays, as yet unreported, linking the 237.5 and 201.7 keV levels, either through a direct transition of 36 keV or via cascade y-rays which utilise the 212.0 keV intermediate energy level.

There is some confusion concerning the population of the 165.2 keV energy level. Neither Browne and Asaro (1968) nor Gonzalez et al. (1979) reported an a - Ener mmaRa ayi of Np. p N f o nalysis A ay a-R m am G y rg e n -E w o L

a-E n erg y Energy y-ray energy (keV) Ge(Li) coaxiadetector Au(Si) (keV) level (keV) 29 47 57 75 86 106 115 118 134 142 143 151 153 155 163 169 4788.1 86 * * * 4771.4 103 * 4710 163 * 4704 169 * * * 4671 202 7 4664.6 212 * * * 4639.5 238 * * * 4616 257 4598.5 280 4577.5 300 * 4567 306 4513.7 366

a-E n erg y a-en ergy y-ray energy (keV) Ge(Li) coaxiadetector Au(Si) (keV) Au(Si) 171 176 191 193 195 197 202 203 209 212 230 238 249 257 262 280 4788.1 86 4771.4 103 4710 163 4704 169 4671 202 4664.6 212 * 4639.5 238 * * * 4616 257 * 4598.5 280 * * * * 4577.5 300 * * * 4567 306 * 4513.7 366 * * * *

Table 4.6 a -y coincidence data (Gonzalez et al., 1979) oo VO 237 Low-Energy Gamma-Ray Analysis of Np. 90

%cl= 100

5 4513.7 0.041(4) t 5 s1 r a■n r ' r r ' VCS 51 c -1 oj c'J c Q h, - c a yl^ ■ —1 M M P—1i—

*“i «3 V2 c 3 ^ r 4577.5 0.41(2) c? Vi3 - c* V O r 3 2 7 /2 + i i i k 1 ______V ^ r* c3 * 3 C 5 4598.5 0.39(2) 1 ci c 3 v£ c ■i v*6 C c rv r - r c32-i i-H ^ t 2 7 9 .2A (7 /2 + L 1\f fAkf d\ftF 1 '1£ r— c* 3 4639.5 6.45(4) C> $”1 ^1 c oc3 --I c< t: c "J Tj c 03 KH / 5 / 2 - 1II A 1 X1 1 ro V I______2 3 1.5 Jr" A S r A s 19 1 P 1 +00+.0 uf) 6l ^5 c- V iri■i r r - 1 C\ vn 5 /2 + k k / «c . ^ 2 1 2 .0 "'1F 11 1 vri vn c 3 Cv 3 /2 + r ■4 ■“"* i 201.7 1 ! 4698.0 0.54(4) o Jr- o / 9

( 9 /2 -) 3 |cs >i >< I *n 108.4 s vc 1 ri 7 /2 + > > > >1 1 '11 ’11 f n>i -r 4788.1 47.75(20) 3/*., 1 VC3 O 013 >1 >< O 86.4I t 4803.6 2.06(51 5 /2 + > j ' 1a|1 ^| / f >f 5 / 2 - . ^ m 4817.0 2.47(21

7 / 2 - > > n >I v is i. o J r 1 O ■a ns CJ 5 . 4866.4 0.49(3)

s i / 4873.0 2.43(3) >< 1/ 2 - > f 6.7 A V f J / >f >r >< v M li ^

233Pa

Fig.4.1 CL—y coincidence data (Gonzalez et al., 1979) 237 Low-Energy Gamma-Ray Analysis of Np. transition to this level, but instead identified two separate transitions, in the same region, feeding the 163.3 and 169.2 keV levels. Unfortunately, they were unable to completely resolve both peaks and so attributed a multiplet intensity of over 1%. Alpha-gamma coincidence measurements support the existence of these levels since the a-transition feeding the 163.3 level is in coincidence with the 106.1 keV y-ray, whilst that populating the 169.2 level is seen in coincidence with the 169.2, 162.5 and 74.7 keV y-rays. There are no reported a-y coincidences corresponding to the 165.2 keV energy level.

4.2.3 Review o f Gamma-Gamma Coincidence Data.

Tables 4.7(a), (b) and (c) list the y-y coincidence measurements of Skalsey and Connor (1976), Gonzalez et al. (1979) and Woods et al. (1988) respectively. The data has been used to construct a level scheme which is illustrated in Fig.4.2. Common to all three measurements are the observed coincidences between the 29.37 and 86.5 keV gates, with the 117 and 143 keV y-rays, known to depopulate the 212 and 238 keV levels respectively. This provides further evidence of the 94.7 keV energy level of Pa and suggests a linking transition of 8.3 keV between this and the 86.4 keV level. The 117 and 143 keV y-rays are also in coincidence with the 57.15 keV gate, providing further evidence of the 8.3 keV linking y-ray. Coincidences between the 57.15, 29.37 and 86.5 keV gates with the 134 keV y-ray transition, which populates the 103.3 keV level, suggests an addition linking photon. Such a transition may occur directly by means of 16.9 keV y-ray photon, or via a cascade which incorporates the 94.7 keV energy level. Gonzalez et al. (1979) report the coincidence between the 86 keV gate and the 191.5 keV y-ray, which feeds the 108.4 keV energy level of 233Pa, and is indicative of a 5.1 keV transition, linking the 103.3 and 108.4 keV energy levels. The proposed linking transitions are illustrated in Fig.4.3.

Gonzalez et al. (1979) reported y-y coincidences between the 36.2 keV y- ray, depopulating the 237.5 keV level, and the 131.0, 195.1 and 201.7 keV y-rays. This provided further evidence for the proposed transition which is believed to link the 237.5 and 201.7 keV levels.

Woods et al. (1988) report coincidences between the 74.7 keV y-ray, of the 169.2 keV energy level, and the 29.37 and 57.15 keV gates. Gonzalez et al.(1979) have observed a coincidence between the 106.12 keV y-ray of the 163.3 keV level, and the 57.15 keV gate. Both measurements verify the existence of the 163 and Table 4.7(a) y-ycoincidence data (Skalsey and Connor, 1976). Enegy Gu - y nlss . p N ' ^ f o Analysis ay a-R m um G y erg n -E w o L y-ray energy y-ray energy G e(Li) detector x-ray delector 2 9 - 5 7 6 2 8 6 9 1 1 1 7 1 3 4 1 4 3 1 5 1 2 9 . 3 7 * * * 8 6 . 5 0 * * * * * * * * 1 1 7 . 0 0 * 1 5 1 . 3 8 * * * 2 1 4 . 0 9 * * * 2 3 8 . 0 4 *

Table 4 .7 (b ) y -y coincidence data (Gonzalez ct a1., 1979). y-ray energy y-ray energy (coaxial detector) Planar detector 2 9 . 4 3 6 . 2 5 7 . 1 8 6 . 5 8 8 . 0 9 4 . 6 1 0 6 . 2 1 1 5 . 4 1 1 7 . 7 1 3 1 . 1 1 3 4 . 3 1 4 3 . 3 1 5 1 . 4 1 5 5 . 2 1 9 1 . 3 1 9 3 . 3 1 9 5 . 0 1 9 6 . 9 2 0 1 . 6 2 1 4 . 0 2 9 . 4 * ? * * * * * * * 3 6 . 2 ? * * * 4 6 . 5 * 5 7 . 1 * * * * * 8 6 . 5 ? * * * * * * * * 8 8 . 0 * * * 9 4 . 6 * * * 1 0 6 . 2 * 1 1 5 . 4 ? ? ? 1 1 7 . 7 * * * *

Table 4.7(c) y-y coincidence data (Woods et a / . , 1988) y-ray energy y-ray energy (y-x detector) y -x detector 2 9 . 4 5 7 . 2 7 4 . 7 8 6 . 5 9 4 . 7 1 0 8 . 6 1 1 5 . 5 1 1 7 . 7 1 3 4 . 2 1 4 3 . 2 1 5 1 . 4 1 5 5 . 2 1 6 9 . 2 1 9 3 . 3 2 0 9 . 2 2 1 4 . 1 2 3 8 . 0 2 9 . 3 * * * * ♦ * ★ 5 7 . 2 * * * * * * * * 7 4 . 7 * * 8 6 . 5 * * * * * * * 9 4 . 7 ♦ * 1 0 8 . 6 * * 1 1 5 . 5 * 1 1 7 . 7 * * * * 1 3 4 . 2 * * * 1 4 3 . 2 * * * * 1 5 1 . 4 * * * 1 5 5 . 2 * * vO 1 9 3 . 3 to 2 1 4 . 1 * 237 Low-Energy Gamma—Ray Analysis of Np. 93

237. N p

% a = 100

4513.7 0.041(4)

9 / 2 + - 3 6 5 . 5

92; ooTT’t 4577.5 0.41(2) «+'4 o\vd o\** n^ 7/2+. « ^ ^ 'O 3 0 0 . 6y '

4598.5 0.39(2) VOCN rn ( 7 / 2 + ) _ ON 2 7 9 . 2

o t-* t-H rn 4639.5 6.45(4) • cn cs cs cm “r o v£ o 5 m ^ mvd 5 / 2 + i-* 1—4 — q o 4664.6 3.43(4) I £ o ^ ^ ^ *o 5 / 2 + -I——--r ^ m • • in' ✓ o o\ cn h 3 / 2 + _ c n •—i *“* >-H 2 0 1 . 7 4698.0 0.54(4)

(9 /2-2 t"; ‘■vt ' y / 4710.6 1.20(5) (1/ 2 +) -4 TT' _ 1 6 9J L r y - . VO,O 1 6 5 . : (11/ 2-1 X jf'

4766.4 9.7(3)

,/4 7 7 1 .4 22.65(40) ( 9 / 2 + 2 • , VO 1 0 8 . 4 5? « 7 / 2 + _ O'O' S' ^ 4788.1 47.75(20) 3 / 2 + — #. >n . cn ■ oovo ONcn J/2+Jf 4803.6 2,06(5)

5/2-- in 4817.0 2.47(2)

7 / 2 - >rv V s i.o Jt

4866.4 0.49(3)

4873.0 2.43(3) 1/ 2- 3 / 2 - „

233Pa Fig.4.2 y-y coincidence data (Skalsey and Connor 1976, Gonzalez etal. 1979, Woods et al. 1988) 237. Np »n CO oo CS CS VD p. N f o Analysis ay a-R m am G y rg e n -E w o L rH CO Tj" fN ON CO H = 100

NO 4^ Fig.4.3 Proposed linking transitions. 237 Low-Energy Gamma-Ray Analysis of Np. 95

169 keV energy levels. There are no reported coincidences with y-rays depopulating the 165.2 keV energy level.

4.2.4 Review o f Conversion Electron Data.

Both Magnusson et al. (1955) and Baranov et al. (1962) have studied the internal conversion following the decay of 237Np. Unfortunately, the measurements are considered unreliable since both groups were unable to produce a source with the required intensity, while still retaining a low surface density.

A more successful study by Asaro et al. (1960) reported that they were able to identify the 86 keV L transitions as anomalous El. The term relates to penetration effects arising when the Schrodinger wave function of the electron falls back into the nuclear charge density. The L conversion lines of the 29 keV transition were too low to permit accurate determination, however, the M lines were measured following an exposure time of nine months and were also assigned as El. Internal conversion coefficients reported by the authors are reliable only to within a factor of two, due to poor resolution arising from source thickness.

Recent conversion electron measurements have been carried out by Woods et al. (1988) using the (3-particle spectrometer at the National Physical Laboratory (NPL). They dispensed with long counting periods by increasing the efficiency of the detector by using a large area multistrip source, together with a sixteen element proportional counter. They were able to determine the internal conversion coefficients and assign multipolarities to five y-rays as shown in Table 4.8.

Ey (keV) OCK CCL ocm CCN+ OCT Mult. 29.37 0.1418 0.048 0.1898 anom. El 57.15 130.70 45.60 11 187.30 E2 86.50 1.125 0.217 0.08 1.42 anom. El 143.21 3.1 2.25 2.06 0.76 8.17 Ml +E 2 151.37 5.3 2.58 3.55 1.12 12.55 Ml +E 2 Table 4.8 Conversion electron measurements o f Woods et al. (1988). Low-Energy Gamma-Ray Analysis o f^N p . 96

4.3 Evaluation. fYlH Evaluating the decay data of Np, in an attempt to produce a self- consistent level scheme, utilised the a-particle energies and intensities determined by Bortels et al. (1990), together with the y-ray emission probabilities evaluated by Reich (IAEA, 1986). Where appropriate the conversion electron data of Woods et al. was employed. However, since they reported ICC's for only five of the major fifteen transitions, and in the case of the principle y-ray only the M and N+ conversion lines were measured, it followed that additional conversion electron data was necessary before an evaluation could take place. These was supplied by Ellis (1986) and were originally compiled from data provided by Browne and Asaro (1968) and Hoekstra (1969). Both the measured and evaluated internal conversion coefficients are listed in Table 4.9, together with the calculated total transition probabilities.

Energy PT (%) (Xt Woods ay Ellis Total transition (keV) CRP (1986) et al. (1988) (1986) Probability (%) 29.37 15.3 0.1898 3.13 63.19 46.53 0.106 0.928 0.20 57.15 0.382 187.303* 179 71.93 86.50 12.3 1.422* 1.4 29.79 88.04 0.138 0.177 0.16 117.68 0.173 13 2.42 131.04 0.086 0.266 0.11 134.23 0.071 8.5 0.67 143.21 0.432 8.17* 7.1 3.96 151.37 0.234 12.55* 6.1 3.17 155.22 0.092 0.177 0.11 169.17 0.071 0.144 0.08 195.09 0.185 0.103 0.20 212.42 0.151 0.0846 0.16 238.00 0.056 0.0651 0.06 Table 4.9 Total transition probabilities o f 237Np. * denotes the ICC values o f Woods et al. (1988), used for the evaluation.

Fig.4.4 illustrates the decay scheme which has been constructed from all of the above data. The low intensity a-transitions to the 365.5, 300.6 and 279.2 keV energy levels have been omitted, since y-rays depopulating these levels will play only a minor role in the scheme. It should be obvious to the reader that this evaluation suffers from some serious discrepancies: Low-Energy Gamma-Ray Analysis o f^ N p . 97

> - <*- ,.o S'* e* 00 CO ?- H o ?- Cv CN vfl5-' CM v$5 CM M3 o O M3 Cv c— 0 0 M3 II II II CM o o d O d d II c II c o II a C- Cl d o d o c d o C-| o Q c. Q Q O Cl o Q (U Q a Q a a a ?*- a V.O # & vOCM CM r-~ co co CO oc GV CN in o r t7 CO CO C >n co CvCM M3 in VO II II II II II II d d d d d c . d| o o c c o c7 Cu cu a c . d- C u Cl. o c ^_. ^— CM OI >n co o r n, CO c n, in \C r- co CM c^ M3 c- G ot C\or or cv oi C-’ CM CM CM C^- CM oT o VO oT vC C VC o r o r o' M3 0C CO t > M3 r~ r— r - ~ o c S c o c M3 o T C- r~ o r oc or . o r . o r o r o r

w m m s \d c

{£6'IL)LVLS [6VZ9 )LZ'6Z I I I [6L’6Z)05‘92 I I {9F01W88 I (oro}es*9fr {X80'0)Z.r69I

{iroJto’iei Fig.4.4 Decay scheme evaluation. I I (0r0}60'561 & I [iVitoULU I ( i r o ) z r s s i I [ 9 Y Q ) Z Y l\l

{L w )w n \

{96'l}\Z'in

{L V D L l'lS l

{ 9 o * o ) o m

+ 4- .CN CN CN CN+ CM ■+■ cn 71

Since we have already established that both a-particle and y-ray emission probabilities used for this evaluation are by far the most accurate, it follows then that the anomalous decay scheme is due, not to what has been measured, but is a consequence of what has not. Prime candidates for these missing data must surely be the low-energy linking transitions, which by their very nature will be highly converted and explains why so many workers have been unable to detect them, even though their existence has been suspected for some time.

In an attempt to formulate a self-consistent decay scheme, an experiment was 9^7 undertaken to analyse the low-energy y-ray spectrum of Np to determine the existence of the elusive linking transitions. 237 Low-Energy Gamma-Ray Analysis of Np. 99

4.4 Experimental.

237NpCl62- (~0.8 g) was provided by Harwell. The material had originally been separated from spent reactor fuel and so contained traces of 238,239,240pu^ which were removed by ion-exchange chromatography, based on a method reported by Jackson and Short (1962) and which is briefly described below.

The ion-exchange column consisted of a 50 ml glass burette which had been cut in half and its base plugged with quartz wool. The anion exchange resin, B io - Rad AG 1x4, was slurried to a depth of 20 ml and then slowly conditioned to 8M HC1.

The neptunium stock was evaporated to dryness under an infra-red lamp and then redissolved in a minimum volume of 8M HC1. The addition of excess NH4I, reduced Np(V) to Np(IV) and similarly Pu(IY) to Pu(III). The resulting solution was then transferred to the column and washed with 8M HC1/0.05M NH4I (100 ml). Pu(in) has a low equilibrium distribution coefficient, in this acid medium and so elutes from the column, whilst Np(IV) is strongly adsorbed. The resin was then washed with 8M HC1 (20 ml) to remove ammonium salts. Np(IV) was eluted from the column, in a tight band, with 0.3M HC1 (20 ml), which represents the acid concentration corresponding to its minimum equilibrium distribution coefficient (see Fig.2.6(b)). All solutions were passed through the resin under gravity and at the rate of approximately 1 ml min-1.

The purified Np solution was taken to dryness and then redissolved m IMHNO3, from which aliquots were removed for a-particle, mass spectroscopy and atomic absorption analysis. The results of the analysis are listed in Table 4.10.

Isotope/Element Analysis Results 237np a-particle spectroscopy 99.98% 236Np Mass spectroscopy 1:5500 238,239,240pu a-particle spectroscopy 0.02% Al:Mg:Si and Fe Atomic absorption <100 ppm Table 4.10 Purity o f the neptunium sample.

The majority of the purified stock was to be used in a second experiment (see Chapter Five), whilst 20 mg was all that was required for y-ray spectroscopy. However, prior to any measurements, it was first necessary to remove the equilibrium daughter, 233Pa, from the sample. This was achieved once again by 2 3 7 Low-Energy Gamma-Ray Analysis of Np. 100 anion exchange chromatography based on a method described by Skalsey and Connor (1976). The separation procedure utilised the fact that Np(IV) is retained on a strongly basic anion exchange resin in 12M HC1/0.5M HF (see Fig.2.5(b)), whilst Pa(IV/V) would be eluted from the resin with a continuous flow of the same acid mixture. The relatively high concentration of hydrofluoric acid necessitated the use of polythene vessels to prevent the formation of silicates. To this end, a pre fabricated polythene exchange column was used for an early separation trial. These columns were supplied by Bio-R ad and were packed with A G lx8 anion exchange resin (2 ml). However, after the column had been conditioned to 12M HC1/0.5M HF, the resin showed signs of severe degradation due to hydrofluoric acid attack and so were discarded. Since previous trials with Bio-Rad AGlx4 had shown it to be a more robust resin, it was incorporated into a second column which consisted of a customised 4 ml polythene pipette with a heat extruded end. The base of the column was plugged with quartz wool and the resin was slurried to a depth of 2 ml.

The 237Np sample (-20 mg) was evaporated to dryness and then redissolved in a minimum volume of 12M HC1/0.5M HF. The resulting solution was then loaded onto the exchange column which had been conditioned to the same acid mixture. The Np(IV) complex remained on the column, whilst protactinium eluted from the resin. The principle y-ray transition in the decay of Pa was used to determine, qualitatively, the removal of this isotope from the sample by analysing the 'YXfl y-ray spectrum of each fraction. Once the eluent was free of Pa the resin was washed with 0.3M HC1 which eluted Np(IV). All solutions were passed through the column under pressure provided by a steady flow of nitrogen gas.

The neptunium sample was evaporated to dryness and any residues were dissolved in a minimum volume of 0.3M HC1. 237Np sources were prepared by depositing between 0.1 and 1 mg of material onto a standard Whatman 9 mm filter /• - paper, which when dried, was sandwiched between two polythene discs of 2.5 cm diameter and 0.25 mm thickness. Each source was positioned 5 cm above a thin- window intrinsic HPGe detector and spectra were accumulated over a period of several days. Spectral analysis was performed using the peak-fitting routine Gamanal. Figs.4.5(a), (b), (c) and (d) show the y-ray spectrum of a 237Np source which was analysed less than four hours from the beginning of the chemical purification. 9^7 It should be noted that the design of the Np sources were identical to that of the calibration materials used to determine the absolute photopeak efficiency of the detector. The efficiency measurements utilised both standard y-ray calibration

237 Low-Energy Gamma-Ray Analysis of Np. 105 sources, together with .the low-energy x-ray transitions of 54Mn, 57Co, 93mNb, 133Ba, 137Cs, 139Ce, 152Eu and 241Am. The efficiency values were fitted to the 4th order polynomial described by equation (3.19). Both the parameters and their uncertainties were determined by means of a least-squares minimisation program and the error in the fitted efficiency value was given by equation (3.20). Since the detector efficiency changed rapidly between 15 and 50 keV, three polynomials were used to describe the energy regions 5-14, 15-49 and 50-400 keV. The resulting efficiency curve is shown in Fig.3.14. Low-Energy Gamma-Ray Analysis o f 237N p . 106

4.5 Results.

The results were derived from eleven spectra corresponding to three sources with active deposits ranging from 0.1 to 1.0 mg. The measurements have been normalised to the 86.5 keV y-ray transition, with the recommended emission probability of 12.3% (IA EA , 1986), such that:

M i 12.3 (4.4) (A^\ where Iy = yZy)

A y = counts per unit time. £y = absolute photopeak efficiency.

The random uncertainty in the relative emission probabilities were determined using the propagation of errors formula:

(4.5)

v/total peak area = uncertainty in the net peak area live time

The values for Py and GpPy were used to calculate the weighted-mean emission probability Py for each y-ray transition.

- Z PT/°Py2 (4.6) 7 =

The goodness of fit of the data to the mean was established by the chi-squared test:

(4.7) 2 3 7 Low-Energy Gamma-Ray Analysis of Np.

Where the data was inconsistent, the random error was reevaluated in one of two ways. Firstly, the calculated uncertainty was inflated by the Birge ratio which forced the data to have an adjusted ratio of unity, whilst the mean remained unchanged. The internal error was then calculated in the usual

-V / Z (P v—Pv72 way. Secondly, the external error which would account for all \ n(n-l) j sources of random uncertainty, was calculated. The larger of the two errors was adopted as the random uncertainty in the measurement, and added in quadrature with the non-random error in the detector efficiency. Table 4.11 list the emission probabilities of 237Np, normalised to P-y(86.5) = 12.3%.

It should be noted that each spectrum was corrected for the ingrowth of 2^Pa, which was inferred from the intensity of the 75.28 keV line, which has an emission probability of 1.32% (IAEA, 1986). Low-Energy Gamma-Ray Analysis o f^N p . 108

Energy (keV) Py (%) Assignment 5.18 0.219 ± 0.005 Np y-ray + (Pa LQ2.15 - Ge3 1K' x-ray escape) 5.84 P aLfli - Ge Kfii* x-ray escape

6.27 Pa LQ2.15 ~ Ge Ka.2 lx-ray escape 6.94 Pa L3 1 - Ge Ka i.2 x-ray escape 8.03 0.014 ± 0.005 Np y-ray 8 .6 8 0.061 ± 0.004 Np y-ray + (Pa Lyi - Ge Kg]' x-ray escape) 9.78 0.36 ± 0.01 Pa Ly\ - Ge Ka i .2 x-ray escape 11.46 Pa La

13.16 Pa L o2 13.36 Pa L a i 15.01 PaLn

15.44 Pa Lg6

16.08 Pa L3 2 .1 5 16.76 Pa L31

18.46 29.37 y-ray - Ge3 Ki' x-ray escape

19.60 Pa L7I + (29.37 y-ray - Ge Ka,2 x-ray l escape) 20.24 P aL^s 29.37 13.7 ± 0.3 Np y-ray 36.20 0.0048 ± 0.0005 Np y-ray 46.45 0.112 ± 0.003 Np y-ray 57.04 0.363 ± 0.003 Np y-ray 63.83 0.0089 ± 0.0008 Np y-ray 75.25 Pa y-ray 86.48 12.3 Np y-ray + Pa y-ray 8 8 .0 1 0.142 ± 0.001 Np y-ray

91.61 Pa Ktt3

. 92.32 Pa Ka 2 • 94.70 Np y-ray + U Ka 2 95.85 Pa Ka i 106.31 0.047 ± 0.001 Np y-ray 107.74 Pa K3 3 108.60 Np y-ray + Pa K3 1

109.12 Pa K3 5

111.67 Pa K3 2 117.94 0.165 ± 0.002 Np y-ray 131.46 0.078 ± 0.001 Np y-ray 134.70 0.0637 ± 0.0009 Np y-ray 143.79 0.382 ± 0.003 Np y-ray 155.96 0.079 ± 0.001 Np y-ray 170.12 0.060 ±0 .0 0 1 Np y-ray 194.65 0.036 ± 0.001 Np y-ray 214.25 0.132 ± 0.002 Np y-ray n? 7 Table 4.11 Measured relative y-ray emission probabilities of Np, normalised to Py(86.5) = 12.3%. (Kai,2 = 9.87 keV and Kpp = 10.98 keV) Low-Energy Gamma-Ray Analysis o f ^ N p . 109

4.6 Discussion.

The measured relative y-ray emission probabilities of 237Np are in good agreement with previous workers. Moreover, the measurements imply the presence of the proposed linking transitions at 5.18, 8.69 and 36.20 keV. There was no evidence of the 8.3 keV transition, whilst the presence of the 8.03 keV peak is j something of an enigma, appearing only in spectra acquired immediately after the j separation from protactinium. It is important to stress that low-energy y-ray i spectroscopy incurs many analytical problems caused by x-rays, x-ray sum peaks, escape peaks etc., and these additional spectral features must first be eliminated, before the suspected 237Np linking transitions can be positively identified. To this end, many of the principle peaks in the spectra have been assigned to specific x-ray transitions (see Table 4.11).

Protactinium x-rays are due to the electron conversion in the decay of Np. As the time from the initial chemical separation increases, so the presence of uranium x-rays, in this case generated by the electron conversion following Pa decay, becomes noticeable. Where x-rays interfere with Np y-rays, the relative emission probabilities have not been calculated. Neither protactinium, nor uranium have x-rays below their Lg, transitions, which are situated at 11.37 and 11.62 keV respectively. Thus, peaks below 11 keV are either y-rays associated with the decay of 237Np, or x-ray escape peaks. As already discussed in section 3.3.2, a characteristic x-ray is emitted by the germanium atom following photoelectric absorption. In most cases the x-ray energy is reabsorbed near the original interaction site, so that the sum of the kinetic energies of the photoelectrons is equal to the original energy of the incident photon. However, if photoelectric absorption occurs near the surface of the detector, the x-ray photon may escape. The result is a new peak which is located at a distance equal to the energy of the characteristic x- ray below the photopeak. As a result of this phenomena, all but the 8.03 keV transition, have been tentatively assigned escape peaks, corresponding to Ge Kai^ ’ and Ge Kpp x-ray escape from the intense Lp2,i5, Lpi and Lyi protactinium x-ray transitions. Thus, the intensities listed in Table 4.11 represent the sum of the y-ray emission and x-ray escape probabilities.

In the absence of experimental data to indicate the true origin of the low- energy peaks, coupled with the fact that the y-ray linking transitions appeared to be the one plausible solution to the anomalous decay scheme of Np, it was decided to report the findings of the above experiment (Lowles et al., 1990). The analysis leading to the published emission probabilities was not as rigourous as that described 2 3 7 Low-Energy Gamma-Ray Analysis of Np. n o in section 4.5 and subsequent reevaluation of the data has produced slightly different values for the combined uncertainty. Only the 8.03 keV transition has an emission probability significantly different from that originally reported, which was due to the intensities of the 8.03 and 8.69 keV peaks being summed. The energies of the proposed linking transitions have shifted slightly from their published values, as a result of the improved a-particle data reported by Bortels et al. (1990).

As already stated the low-energy y-rays would be highly converted and so Pearcey et al. (1990) carried out an experiment to measure the internal conversion lines associated with the suspected linking transitions. They abandoned the gas flow proportional counter used by Woods et al. (1988) in their earlier work and turned instead to Channel Electron Multipliers (CEMs). Although such devices are sensitive to low-energy conversion electrons, their energy calibration in the region of 1-20 keV has proved difficult, whilst their efficiency varies not only between different types of CEM, but also between examples of the same type. In the absence of a suitable calibration source to characterise each detector, a relative efficiency function was used. Pearcey et al. identified the L conversion lines for both the 29.37 and 36.20 keV y-rays of 2^7Np, together with the Li and M1/M2 conversion lines of the 28 keV y-ray of U. Unfortunately, they saw no evidence for the conversion lines associated with the proposed 5.18, 8.30 and 8.69 keV linking y-rays.

The electron binding energies of Sevier (1979) and the internal conversion coefficients of Hager and Seltzer (1968) have been used to calculate the theoretical electron emission probabilities for the 5.18 and 8.69 keV y-rays. The results are shown in Tables 4.12 and 4.13. The L and M conversion lines of the 5.18 keV transition are all less than 2 keV and are approaching the detection limit of the f ey [ \ 7+ CEMs. The transition is between the — 108.4 and — 103.3 keV levels, and so its multipolarity has been tentatively assigned as (Ml + E2). Although the mixing ratio is unknown, we can conclude that the magnitude of the E2 internal conversion coefficients of 2.8 x 106, and hence electron emission probability of 6.1 x 105%, makes the y-ray emission probability of 0.219(5)% unrealistic. However, if the E2 contribution is negligible, then the total transition probability for the 5.18 keV y-ray is 10.2%. This would balance the 108.4 keV energy level, which is populated by an a-particle of intensity 9.7%. The 8.69 keV E2 transition between the ~ 103.3 and the —■ 94.7 keV level, has a total internal conversion coefficient of 4.5 x 105 and a corresponding electron emission probability of 2.8 x 104%, showing the measured y-ray emission probability of 0.061(4)% to be incorrect. 237 Low-Energy Gamma-Ray Analysis of N p . 1 1 1

Table 4.12 ICC's for the 5.18 keV transition Py = 0.219(5). Electron Binding Conversion Theoretical Pe (Ml) Theoretical Pe (E2) Shell Energy (kev) Line (keV) ICC (Ml) (%) ICC (E2) (%) K 112.60 Li 21.11 l 2 20.31 Li 16.73 Mi 5.37 * M2 5.00 m 3 4.17 1.01 35 8 2704000 592176 M4 3.61 1.57 8 2 50986 11166 m 5 3.44 1.74 4 1 37794 8277 Total electron emission probability Ml (%) 10 Total electron emission probability E2 (%) 611619

Table 4.13 ICC's for the 8.69 keV transition = Py0.061(4). Electron Binding Conversion Theoretical Pe (E2) Shell Energy (kev) Line (keV) ICC (E2) (%) K 112.60 Li 21.11 l 2 20.31 U 16.73 Mi 5.37 3.32 3271 200 m 2 5.00 3.69 199180 12150 m 3 4.17 4.52 243203 14835 M4 3.61 5.08 4801 293 M5 3.44 5.25 3735 228 Total electron emission probability E2 (%) 27706

Table 4.14 ICCs for the 36.20 keV transition - 0.0048(5).Py Electron Binding Conversion Theoretical Pe (Ml) Theoretical Pe (E2) Shell Energy (kev) Line (keV) ICC (Ml) (%) ICC (E2) (%) K 112.60 Li 21.11 15.09 61 2.92e-01 21 1.01e-01 l 2 20.31 15.89 7 3.31e-02 660 3.17 U 16.73 19.47 0.4 1.87e-03 617 2.96 Mi 5.37 30.83 18 8.74e-02 9 4.44e-02 m 2 5.00 31.20 2 1.09e-02 289 1.39 m 3 4.17 32.03 0.13 6.24e-04 292 1.40 M4 3.61 32.59 0.02 7.68e-05 5 2.20e-0 2 m 5 3.44 32.76 0.01 4.03e-05 3 1.48e-02 Total electron emission probability Ml (%) 0.43 Total electron emission probability E2 (%) 9.10 2 3 7 Low-Energy Gamma-Ray Analysis of Np. 112

Table 4.14 shows the theoretical conversion line energies and ICC's for the 36.2 keV y-ray, which is believed to de-excite the ~ 237.5 and populate the ~ 201.7 keV levels. Pearcey et al. (1990) reported the multipolarity of this transition to be (Ml + 10(5)% E2), which would result in a total internal conversion coefficient of 280. Using the measured y-ray emission probability of 0.0048(5)%, the resulting transition probability for the 36.2 keV y-ray is 1.3%. This makes the total transition probability of all the y-rays depopulating the 237 keV level to be 9.16%. However, the a-particle feeding to that level accounts for only 6.45%. If however, the internal conversion coefficient for the 151.37 keV y-ray, reported by Woods et al., is replaced by that evaluated by Ellis (1986), namely ay = 6.1, then the discrepancy is reduced, since the total depopulating intensity is now 7.7%. Although the 36.20 keV transition, has for the first time, been proved to account for the population of the 201.7 keV level, its intensity is far greater than that of the y- j rays known to depopulate this level. ‘ 23 7 Low-Energy Gamma-Ray Analysis of N p . 113

4.7 Conclusion.

It would appear that the decay scheme of 237Np is reluctant to give up its secrets and takes a macabre delight in frustrating all those who try to resolve its mysteries.

The measured a-particle and y-ray emission probabilities are now of an accuracy where further measurements would be superfluous. Gamma-gamma and alpha-gamma coincidence studies continually point to the presence of low-energy linking transitions. However, an attempt to determine the presence of these transitions by y-ray spectroscopy has proved inconclusive, since the measurements of Pearcey et al. (1990) show no evidence of the conversion electrons associated with the proposed transitions. However, until the calibration of CEM detectors can be determined with greater accuracy than is currently available, it is unwise to disregard the transitions reported in this work simply as germanium x-ray escape peaks.

y in Further studies into the low-energy linking transitions of Np may benefit from the use of Si(Li) detectors, whose x-ray escape peaks will hopefully not interfere with suspected y-ray transitions. Alternatively the low-energy y-ray spectrum may be deconvoluted by the use of a Compton suppression system. When used in the anticoincidence mode, this device is capable of distinguishing between those events which deposit their energy in the detector, from those which escape from it. Chapter Five.

The Half-life of 2 3?Np.

5.1 Introduction.

To date there have been only two, widely reported, measurements of the half-life of Np. The first (Magnusson and LaChapelle, 1949), resulted in a value of 2.2 ±0.1 x 106 years. The second (Brauer at al., 1960) in 1960, produced a half-life of 2.14 ± 0.01 x 106 years. Responding to the need for accurate decay data for 237Np, the IAEA-CRP requested a confirmatory measurement of the half- life of this radionuclide. The determination was to be carried out independently by AEA Technology, Harwell and CBNM, Belgium (IAEA, 1986).

5.2 Review of Previous Measurements.

9^7 The low specific activity of Np severely restricts the number of experimental techniques by which an accurate determination of its half-life may be made. Direct measurement of the time dependence of a—particle or y-ray decay y i n rates would be totally impractical. Furthermore, measuring the ingrowth of Np from the parent 227U nuclide, which was attempted by Wahl and Seaborg (1949), has proved imprecise, yielding a half-life of 3 x 10^ years, considered to be accurate only to within a factor of two. However, direct measurement of the specific activity (i.e. the activity per unit mass) does provide a means by which the half-life of 'Yl’l Np can be determined, since:

activity _ A.N Specific Activity(S A) = (5.1) mass ~~ NWm/Av

X = decay constant Wm = atomic mass of the nuclide N = number of active atoms Av = Avogadro's number

ln2.Ay and hence Half-life = (5.2) (SA).Wm The H alf-life o f 237Np. 115

Magnusson and LaChapelle (1949) attempted to measure the specific activity of a stoichiometric compound of neptunium. They believed that neptunium would produce acetates that were isomorphous with those of uranium and plutonium. The resulting insoluble compound could be isolated and then converted to the oxide. Following some very elaborate chemistry, both acetate and oxide were prepared and subsequent x-ray analysis by Zachariasen (1949 c), identified the structures to be NaNp0 2(C>2C2H3)3 and Np02 respectively. This was inferred from the fact that their diffraction patterns were isomorphous with the dioxides and acetates of thorium, uranium and plutonium.

' 7 Following x-ray analysis, the acetate was converted to the nitrate. Equal volumes of the resulting solution were placed on two platinum substrates of known mass. The solutions were evaporated to dryness and then the nitrate converted to NpC>2 by igniting the plates at 800°C, until constant masses were attained. This produced two a-particle sources with surface densities of approximately 0.1 mg cm . The corresponding a-particle activity of each source was measured in a 2k counter. The purity corrected mean specific activity was 25.33 Bq Jig-1, which corresponds to a half-life of 2.20 x 10^ years.

Further x-ray diffraction studies by Brewer et al. (1945), showed the oxides of uranium, neptunium and plutonium to be of variable composition. Moreover, the dioxide used for Zachariasen's diffraction experiments was prepared differently from the material used in the half-life measurement. In the latter case the dioxide was prepared from the nitrate, in which Np(V) is the stable oxidation state. Heating the material would produce a compound with a composition approaching that of neptunium dioxide. The uncertainty in the stoichiometry of the dioxide, together with a-particle self-absorption, combined to produce an overall error of 4.5 per cent in the reported half-life value.

The second, and certainly the most referred to, measurement of the half-life of Np was that of Brauer et al. (1960). In this case no attempt was made to produce a stoichiometric compound of neptunium. Instead the mass of 237Np in a standard solution (98.7%) was measured by controlled-potential coulometry (CPC). This technique provides an accurate and precise means of determining the mass of a metal in solution and is independent of the compounds stoichiometry. The Half-life o f237Np. 116

Sources were prepared by evaporating known masses of 227Np solution (14.720 ± 0.007 g 1_1) onto polished platinum discs. The source activities were determined in a low-geometry counter. The disintegration rates of a further two aliquots were measured using a 4n liquid scintillation counter. The purity corrected mean specific activity, derived from six measurements, was 26.03 ± 0.12 Bq Jig-1, which corresponds to a half-life of 2.14 ± 0.01 x 106 years.

The half-life value quoted above, is still the most precise to date. The measurement was underpined by Stromatt's CPC analysis (1959% which reported a precision of 0.05% for the standard solution. However, the above measurement is not supported by a rigourous error analysis, concerning the uncertainties associated with: the calibration of the coulometer; weighing of the active samples and a-particle attenuation arising from self-absorption or from irregular deposits within the sources.

This present measurement has utilised many of the ideas developed by previous workers. Again there has been no attempt to prepare a stoichiometric compound of neptunium, since Stromatt had demonstrated that CPC analysis affords an easy and precise means of determining the mass of Np in solution. Furthermore, the work of Magnusson and LaChapelle had shown that a-particle attenuation in the sources must be quantified if an accurate measurement of the half- life was to be made.

5.3 Controlled-Potential Coulometry.

Coulometric analysis is an application of Faraday's law, which states that: the amount o f substance liberated at the electrodes o f a cell is directly proportional to the quantity of electricity (in coulombs) which passes through the solution. The substance being analysed may undergo direct reaction (oxidation or reduction) at one of the electrodes, which is known as primary coulometric analysis. Conversely it may react with another substance, generated within the cell and this is known as secondary coulometric analysis.

The mass W of a substance produced or consumed during electrolysis, involving Q coulombs, is given by the equation (Vogel, 1961): The H alf-life o f 237Np. 117

WmQ (5.3) nF

Wm = atomic mass of the substance being electrolysed n = number of electrons involved in the electrode reaction F = Faraday's constant (96,494 coulombs)

There are two distincdy different coulometric techniques available, namely controlled-potential or constant current analysis. In controlled-potential coulometry, a divided cell is used in which the working electrode compartment is separated from the secondary electrode by means of a glass frit or ion-exchange membrane. The working electrode compartment is filled with a known mass of solution containing the electroactive species. The potential of the working electrode is fixed at a value for which the reaction under investigation proceeds at a diffusion controlled rate. The current and its integral are monitored as a function of time, usually until the current drops to 1 per cent of its initial value (Southampton Electrochemistry Group). Fig.5.1 illustrates a typical current—time curve.

Fig. 5.1 A typical current-time curve.

In the second method, the substance to be determined is electrolysed at constant current and completion of the reaction is signified by a visual indicator in the solution or by amperometric, potentiometric or spectrophotometric techniques. The total quantity of electricity passed is given by the expression:

Q = it (5.4) i = current (amperes) t = time (seconds) The H alf-life o f 237Np. 118

However, it is controlled-potential coulometry which is of interest to this work, and so a theoretical appraisal of this technique is required.

Consider the reaction of an oxidised species O being reduced to species R at a cathode of constant potential:

O + ne <=> R

If dn0 are the number of moles of O that are reduced to R in time dt, then the quantity of electricity passed is given by Faraday's law:

,dn0 -nF (5.5) it = dt

Fick's first law of diffusion states that: the rate of diffusion of solute O is proportional to the concentration gradient

too _n(Co*-Co°) (5.6) dx " 5

D = Diffusion Coefficient :fe C0 = concentration of species O in bulk solution Cq^ = concentration of species O at the cathode 8 = diffusion layer thickness

If the cathode is made sufficiently negative, then the concentration of O at the cathode is zero i.e. C0° = 0

J = (5.7)

Since the flux J, represents the number of moles of O which cross unit area /'dn0/dt\ perpendicular to the direction of flow in unit time I ——— 1 then equation 5.7 can be

expressed as: The Half-life o f237Np. 119

^ = -A D ^ - (5.8) dt 8 substituting equation 5.8 into 5.5:

C it = nFAD-f- (5.9) 0

If n0 moles of O are present in volume V of the bulk electrolyte at time t, then the bulk concentration can be expressed as:

r * _ Ho. - y (5.10)

and d.w.r.t. time:

3Cq 1 3n0 (5.11) at “ v at substitute equation 5.8 into 5.11:

ac0 _ADC0 (5.12) at “ V5

rearrange and integrate:

Ct f l 9Co* = J9 (5.13) Co V8 Co

% sfc where Co and Ct are the bulk concentrations at time zero and t

( _AD In lc 0 J _ V 51 The H alf-life o f 237Np. 120

. .A D Ct =Co exp [ - yg t (5.14)

Thus the bulk concentration of species O decays exponentially with time, as illustrated in Fig.5.1. substituting equation 5.14 into 5.9 gives Lingane's expression:

nFAD * ( AD n= —g~ Co (5.15)

. . ( AD and hence h = 10 exPl 1 (5.16)

the total quantity of electricity Q passed is simply:

t Q = Jit dt (5.17) 0 which can be measured by integrating the current-time curve shown in Fig 5.1.

Often an appreciable background current (ib) is observed from the supporting electrolyte. In such cases the current decays exponentially to the background level rather than approaching zero. If the background current is assumed constant during electrolysis, then the mass of metal deposited at the electrode is given by the equation:

Wm(Q —ibt) W = (5.18) nF

Analysis of neptunium by controlled-potential coulometry will be discussed in a later section. The Half-life o f237Np. 121

5.4 Experimental.

5.4.1 Preparation o f 237Np Standards.

Accurate determination of the specific activity of a radionuclide demands that the material is carrier free. NpCl^ (-0.8 g), which was provided by Harwell, had originally been separated from spent reactor fuel and so contained traces of 238,239,240pu j^esewere removed by ion-exchange chromatography based on a method reported by Jackson and Short (1962) and which has been described in the previous chapter. The purity of the resulting stock was measured by: a-particle spectroscopy; mass spectroscopy and atomic absorption analysis. Table 5.1 lists the assay results.

Isotope/Element Analysis Results 237Np a-particle spectroscopy 99.98% 236Np Mass spectroscopy 1:5500 238,239,240pu a-particle spectroscopy 0.02% Al:Mg:Si and Fe Atomic absorption <100 ppm Table 5.1 Purity o f the neptunium sample.

The neptunium solution was evaporated to dryness and then redissolved in 1M HNO3 (~20 ml). An aliquot (~14 ml) was removed and placed in a 50 ml volumetric flask of known mass. 1M HNO3 was added to the flask until the neptunium concentration was approximately 10 mg g-1. This was measured by preparing several TEG (tetraethylene glycol) spread sources from the solution and measuring their a-particle activities using a windowless 2tc gas flow proportional counter (Simpson 1077B) of known geometry. An accurate measurement of the concentration of the primary standard was provided by controlled-potential coulometry (see 5.4.2) and the results of the analysis are listed in Table 5.2.

Four secondary standards were prepared by removing a known mass of solution (1 g) from the primary standard and diluting with a known mass of 1M HNO3 (24 g). Each flask was then stoppered, weighed and sealed to prevent evaporation. The concentrations of the secondary standards are listed in Table 5.3. Both the flasks and solutions were weighed using a Sartorius R160P Research Balance, which was calibrated, prior to every use, against a series of National Physical Laboratory (NPL) platinum standard weights. The uncertainties in the weighings were no more than 0.002%. The Half-life o f237Np. 122

5.4.2 The Analysis o f Neptunium by Controlled-Potential Coulometry.

Fig.5.2 illustrates the electrolysis cell (SRDP-R127) designed at Harwell, and used for controlled-potential coulometry. The cell consisted of three compartments separated by an anion exchange membrane. The working electrode was positioned in the central compartment, whilst the auxiliary and reference electrodes were placed in either arm of the cell. The working electrode was a platinum gauze, whilst the auxiliary electrode was a coiled platinum wire. The supporting electrolyte was 1M H2SO4.

Fig.5.2 Electrolysis cell.

The basic method of analysis has not changed since the early experiments of Stromatt in 1959 and utilised the oxidation of the Np(V)/(VI) couple. Since Np(III) and Np(IV) can not be electrically oxidised to the pentavalent state, an excess of cerium sulphate was added to a known mass of the primary standard. This raised the oxidation state of all the species to the hexavalent state. Electrolytic reduction of Np(VI) and unreacted Ce(IV), produced Np(V) and Ce(III) respectively. Subsequent coulometric oxidation produced Np(VI), without changing the oxidation state of the The Half-life o f237Np. 123 cerium ion. The current-time curve corresponding to the last oxidation step was integrated. Blank determinations were carried out in the same way as for the sample.

The Np(V)/(VI) couple was chosen for the analysis, since the reaction is reversible in nitric, perchloric and sulphuric acid. However, sulphuric acid was the preferred medium since the background current was lower, with the added advantage that the redox potentials for the Pu(III)/(IV) and Np(V)/(VI) couples were sufficiently different. This was due to the fact that Pu(IV) formed more stable sulphate complexes than Np(VI). Cerium sulphate was chosen for the oxidation of the neptunium species since the process was both rapid and quantitative.

The mass of 237Np in the sample was given by:

%T , . sample integral - blank integral Np(mg) = ------B— =------— x 237.05 x Fi x F2

F = Faraday's constant Fi = Nemst factor = 1.001, (corresponding to an electrochemical process which is 99.9% efficient) ______100______F2 = Bias factor = mean percentage recovery of reference materials

The bias factor represents the calibration of the device used to integrate the current-time curve (Crossley and Phillips, 1986). Ideally this would use a well- characterised neptunium standard, however, no such material exists and so an alternative calibration standard was required. Normally this material would be electroactive in the same potential range as neptunium, however, a suitable simulant could not be found. The only alternative was to calibrate the device by using two reference materials that undergo different electrochemical reactions, namely high purity iron and potassium dichromate. Solutions of these reagents were made up by mass and recovery determinations were carried out using weighed aliquots. The net current-time curve corresponding to the oxidation of Fe(II) to Fe(III) was recorded and the quantity of iron recovered was calculated using equation 5.18. The determination of chromium, in K2Cr2C>7, was based on the direct reduction of Cr(VI) to Cr(III).

The calculated bias factor can be found in section 5.5.1. 5.4.3 Source Preparation.

The backing trays of a-particle sources must be made of a material with as low an atomic number as possible, so as to minimise a-particle backscattering (Ballaux, 1985). However, the choice of substrate is often dictated by its resistance to certain acids. Stainless steel is corroded by hydrochloric acid and so is used to prepare a-particle sources from nitric acid only, whilst tantalum is stable in both hydrochloric and nitric acid solutions. In this experiment electro-polished stainless steel trays (27 mm diameter x 0.35 mm thick) were employed. However, each tray had to undergo an elaborate cleaning process before it was ready to be used as a source backing. This required the trays to be boiled in concentrated nitric acid and then washed several times in de-mineralised water, followed by acetone. Firing the trays over a meaker burner produced a thin oxide layer which was found to improve the uniformity of the deposit, as well as enhance the sources appearance. The active area of the substrate was defined by applying Zapon lacquer around the rim of the tray. The prepared source trays were then covered and stored in a dry, dust free, non-corrosive environment.

The secondary standard solutions were moved into the work area and allowed to thermally equilibrate. Approximately 4 ml of solution was drawn up into a plastic pycnometer. This was then transferred to the balance, allowed to thermally equilibrate and then the initial mass recorded. Approximately 120 mg of solution was dispensed onto a stainless steel tray and on returning the pycnometer to the balance the observed mass difference indicated the quantity of neptunium solution transferred. It was important to ensure that no solution remained in the neck of the pycnometer or that any liquid had adhered to the outer surface of the vessel, since this would result in an erroneous weighing. To the tray, which was placed under an infra-red lamp, was added a spreading agent (10% TEG) which reduced the surface tension of the liquid and so ensured that the active deposit (-16 |ig cm ) was uniformly distributed across the surface of the tray. Once the majority of the liquid had evaporated, the tray and its protective syndanyo square, were gently warmed over a meaker burner. If at any time the liquid came into contact with the Zapon ring, or boiled, then the source was rejected. When all the liquid had finally evaporated, the tray temperature was slowly increased until the Zapon ring began to char. At this point the tray was removed from the syndanyo square and fired to red heat. On cooling, the tray was covered and labelled.

Ten a-particle sources, with activities in the range 1.0-1.9 kBq, were prepared from each of the four secondary standards. Three sources were rejected from the set since their active deposits were likely to cause severe a-particle attenuation.

The a-particle activity of each source was measured independently in two windowless 2k gas flow proportional counters (Simpson 1077B). The geometry counter was determined by means of a decay-corrected 241Am calibration source. The uncertainty in the absolute disintegration rate of this source was 0.25%, estimated following an intercomparison of the source activity at AEA Technology Harwell and NPL (Fudge, 1991).

The mass of neptunium deposited on each tray, its a-particle activity and finally the calculated specific activity are given in the next section. 5.5 Results.

5.5.1 Concentration Analysis.

Table 5.2 lists the results of the controlled-potential coulometric analysis of the primary standard.

Analysis of samples 237Np (mg g 1) sample 1 9.996 sample 2 10.013 sample 3 10.030 sample 4 10.016 Table 5.2 Results o f the CPC analysis.

The mean concentration for the standard was 10.014 ±0.014 mg g- (RSD= 0.14%). This was added in quadrature with a bias factor (F2), of 1.00247 ± 0.00110 (RSD = 0.11%). The resulting neptunium concentration was 10.014 ± 0.018 mg g-1.

Mass loss due to evaporation from the time the primary standard was prepared, to the CPC measurement amounted to 0.019%. Hence, the corrected concentration of the standard was 10.012 ± 0.018 mg g-1.

From this measurement the concentration of the secondary standards were:

Secondary Standard 237Np concentration (mg g *) Solution A 0.5191 ± 0.0009 Solution B 0.4164 ± 0.0007 Solution C 0.4391 ± 0.0008 Solution D 0.4703 ± 0.0008 Table 5.3 Np concentration of the secondary standards.

The uncertainty in each concentration was quoted as 0.18%, derived from the quadrature addition of the uncertainty in the CPC measurement, together with the non-random error associated with the analytical balance. The Half-life o f237Np. 127

5.5.2 Source Count Rate.

Table 5.4 lists the mass of 237Np deposited on each tray, together with its background corrected count rate. Figs.5.3(a) to (h) show the variation in the measured count rate as a function of the mass of 237Np deposited on each tray. The relationships are linear, with correlations of between 1.000 and 0.996. This indicates that the prepared a-particle sources were of a high standard, since inaccurate weighing of solutions or loss of liquid during preparation, would result in points deviating from the line. Furthermore, it would appear that no significant corrections are to be made for the self-absorption of a-particles, since there is no evidence of the count rates for the larger deposits falling short of the line. However, this assumption will be tested experimentally in a later section.

5.5.3 Specific Activity.

The first stage of the analysis was to ascertain whether there were any discrepancies in the measurements arising from different solutions or proportional counters. To this end the data were separated into eight sets. The weighted-mean specific activity (ji), of each data set was determined by the equation:

E * iM 2 (5.19)

, .r activity (Bq) xi = measured specific activity = ------,— r - 1 F J mass (jig) Gi = uncertainty in the background corrected count rate

The results are listed in Table 5.5.

The goodness of fit of the data to the mean was established by the chi- squared test:

(5.20)

n = number of measurements in each set

Activities were calculated for each data set by multiplying the recorded count rates by the measured detector efficiencies. The Half-life o f237Np. 128

Source Weight of Weight of Count rate Count rate number solution (g) 237Np (mg) Simp.l (cpm) Simp.2 (cpm) 3A 0.14117 0.07328 58365 ± 73 57861 ± 45 4A 0.10984 0.05702 45358 ± 61 44896 ± 39 5A 0.12463 0.06470 51018 ± 6 8 50146 ± 42 6A 0.09562 0.04964 39673 ± 53 39014 ± 40 7A 0.12729 0.06608 52774 ± 61 52240 ± 31 8A 0.09552 0.04959 38915 ±31 38313 ± 33 9A 0.13234 0.06870 54357 ± 30 53447 ± 44 10A 0.13361 0.06936 54679 ± 54 53877 ± 40 IB 0.11061 0.04605 36856 ± 33 36301 ± 33 2B 0.15182 0.06321 50465 ± 36 49949 ± 51 3B 0.11279 0.04696 37536 ± 34 36931 ± 29 4B 0.09806 0.04083 32496 ± 33 32074 ± 31 5B 0.09750 0.04060 31908 ± 33 31327 ± 33 6B 0.09616 0.04004 31908 ±28 31168 ± 2 6 7B 0.10506 0.04374 34875 ± 29 34426 ± 33 8B 0.12153 0.05060 40172 ± 52 39336 ± 35 9B 0.14294 0.05951 47551 ±45 46936 ± 40 10B 0.10462 0.04356 34647 ± 48 34145 ± 28 1C 0.14160 0.06217 49902 ± 44 49006 ± 43 2C 0.12371 0.05432 43597 ± 39 43022 ± 37 3C 0.11432 0.05019 40392 ± 39 39684 ± 36 4C 0.11181 0.04909 38935 ± 34 38334 ± 34 5C 0.11655 0.05117 40998 ± 33 40248 ± 37 7C 0.10673 0.04686 37616 ± 27 36916 ± 24 8C 0.10483 0.04603 36966 ± 32 36405 ± 33 9C 0.09136 0.04011 32144 ± 25 31639 ±31 IOC 0.11654 0.05117 40940 ± 40 40288 ± 33 ID 0.09409 0.04425 35306 ± 27 34931 ± 30 2D 0.13448 0.06325 50657 ± 43 49946 ± 36 3D 0.11092 0.05217 41755 ± 32 41120 ±38 4D 0.15336 0.07213 57682 ± 30 56579 ± 28 5D 0.12799 0.06020 48148 ± 22 47496 ± 39 6D 0.13403 0.06304 50476 ± 25 49426 ± 41 7D 0.11531 0.05423 43447 ± 24 42729 ± 38 8D 0.14042 0.06605 52823 ± 38 51987 ± 47 9D 0.12138 0.05709 45594 ± 21 44679 ± 37 10D 0.12678 0.05963 47749 ± 46 46785 ± 39

Table 5.4 Alpha-source count rate. Corrected Count Rate (cpm) Corrected Count Rate (cpm) 231Np. f o alf-life H The i..() n () aito i temaue cutrt a a ucin h ms of o mass the f o function a as rate count measured the in Variation (b) and Fig.5.3(a) 7 ? 2 pdpstdo ah tray. each on deposited Np 129 Corrected Count Rate (cpm) Corrected Count Rate (cpm) h l-ie 237Np. f o alf-life H The i..() n () aito i temaue on ae s fnto oftems f o mass the f o function a as rate count measured the in Variation (d) and Fig.5.3(c) nn n pdpstdo ah tray. each on deposited Np 130 Corrected Count Rate (cpm) Corrected Count Rate (cpm) 237Np. f o alf-life H The 2N eoie nec tray. each on deposited 227Np i..() n () aito i te esrdcutrt a a ucin h ms of o mass the f o function a as rate count measured the in Variation (f) and Fig.5.3(e) 131

Corrected Count Rate (cpm) . Corrected Count Rate (cpm) 237Np. f o alf-life H The i..() n () aito i temaue on ae s ucin h ms of o mass the f o function a as rate count measured the in 7 Variation 3 2 (h) and Fig.5.3(g) pdpstdo ec tray. each on deposited Np Mass of Neptunium (jig) 132

The Half-life o f237Np. 133

Source Activity Spec. Act. Activity Spec. Act. number Simp.l (Bq) (Bq jig-1) Simp.2 (Bq) (Bq w f1) 3A 1907.58 26.03 ± 0.03 1906.52 26.02 ± 0.02 4A 1482.47 26.00 ± 0.03 1479.32 25.95 ± 0.02 5A 1667.45 27.77 ± 0.03 1652.30 25.53 ± 0.02 6A 1296.65 26.12 ± 0.03 1285.52 25.90 ± 0.03 7A 1724.83 26.10 ± 0.03 1721.32 26.05 ± 0.02 8A 1271.87 25.65 ± 0.02 1262.40 25.47 ± 0.02 9A 1776.57 25.87 ± 0.02 1761.10 25.63 ± 0.02 10A 1787.08 25.77 ± 0.03 1775.25 25.60 ± 0.02 IB 1205.20 26.17 ± 0.02 1200.33 26.07 ± 0.02 2B 1650.22 26.10 ± 0.02 1651.65 26.13 ± 0.03 3B 1227.45 26.13 ± 0.02 1221.18 26.00 ± 0.02 4B 1062.60 26.03 ± 0.03 1060.58 25.98 ± 0.03 5B 1043.38 25.70 ± 0.03 1035.88 25.52 ± 0.03 6B 1043.38 26.07 ± 0.02 1030.63 25.75 ± 0.02 7B 1140.40 26.07 ± 0.02 1138.37 26.02 ± 0.03 8B 1313.53 25.97 ± 0.03 1300.72 25.70 ± 0.02 9B 1554.92 26.13 ± 0.02 1552.02 26.08 ± 0.02 10B 1132.97 26.02 ± 0.03 1129.05 25.92 ± 0.02 1C 1634.28 26.28 ± 0.02 1626.18 26.15 ± 0.02 2C 1427.82 26.28 ± 0.02 1427.60 26.28 ± 0.02 3C 1322.83 26.35 ± 0.03 1316.85 26.23 ± 0.02 4C 1275.15 25.97 ± 0.02 1272.03 25.92 ± 0.02 5C 1342.70 26.23 ± 0.02 1335.57 26.10 ± 0.02 1C 1231.92 26.28 ± 0.02 1224.98 26.13 ± 0.02 . 8C 1210.63 26.30 ± 0.02 1208.05 26.25 ± 0.02 9C 1052.70 26.25 ± 0.02 1049.87 26.17 ± 0.03 IOC 1340.78 26.20 ± 0.03 1336.88 26.13 ± 0.02 ID 1155.08 26.10 ± 0.02 1154.47 26.08 ± 0.02 2D 1657.32 26.20 ± 0.02 1650.72 26.10 ± 0.02 3D 1366.08 26.18 ± 0.02 1359.03 26.05 ± 0.02 4D 1887.00 26.17 ± 0.02 1869.93 25.92 ± 0.02 5D 1575.25 26.17 ± 0.02 1569.73 26.08 ± 0.02 6D 1651.40 26.20 ± 0.02 1633.52 25.92 ± 0.02 ID 1421.43 26.22 ± 0.02 1412.20 26.03 ± 0.02 8D 1728.18 26.17 ± 0.02 1718.18 26.02 ± 0.02 9D 1491.68 26.13 ± 0.02 1476.63 25.87 ± 0.02 10D 1562.20 26.20 ± 0.03 1546.25 25.93 ± 0.02 Table 5.5 Specific Activities. The Half-life o f237Np. 134

The total chi-squared value for each set was higher than its predicted value at the 95% confidence limit, indicating that the random uncertainty was underestimated. To make the data consistent it was necessary to increase the random error in one of two ways. Firstly, the calculated uncertainty was inflated by the Birge ratio 3 n-1J which forced the data to have an adjusted ratio of unity, whilst the mean remained unchanged. The internal error was then calculated in the usual way. V XiM'2

X ( x j - M - ) ' Secondly, the external error V v n(n- 1) which would account for all sources of random uncertainty, was calculated. The larger of the two errors was then added in quadrature with the non-random uncertainties, namely: 0.18% from the CPC analysis of the primary standard; 0.002% attributed to the analytical balance and 0.25% corresponding to the uncertainty in the activity of the 241 Am calibration source. Finally the calculated specific activity value for each data set was corrected for the presence of 0.02% a-emitting contaminants. The results are listed in Table 5.6.

Secondary Spec.Act. (p.) Inflated Error Combined Error Standard (Bq qg_1) (Bq Hg-1) (Bq Hg-1) A-Simp.l 25.85 0.07 0.10 •B-Simp.l 26.05 0.05 0.08 G-Simp.l 26.23 0.03 0.08 D—Simp.l 26.13 0.02 0.08 A-Simp.2 25.78 0.08 0.12 B—Simp.2 25.92 0.07 0.10 C-Simp.2 26.15 0.03 0.08 D—Simp.2 25.98 0.03 0.08 Table 5.6 The specific activity for each of the secondary standards.

Fig.5.4 shows that there is no significant difference between the calculated specific activities of each secondary standard or proportional counter. Thus all eight data sets were combined and analysed in the same way as described above. This produced an overall specific activity of 26.03 ± 0.08 Bq jig-1, which corresponds to a half-life of 2.144 ± 0.007 x 106 years (assuming an ephemeris year, i.e. 365.244 days/year). Fig.5.5 shows that this present measurement of the half-life of J 'Np agrees exactly with that reported by Brauer et al. The Half-life o f^N p . 135

26.50 “

2 6 .4 0 -

26.30 - 1 &J0 ZL 26.20 - cr . 0 . 2 6 .1 0 - >> - 2 6 .0 0 “ o - < 2 5 .9 0 - ■ M 0-) 25.80 - a, . 00 n Simpson 1 25.70 - A Simpson 2 2 5 .6 0 -

25.50 -* T D Secondary Standard Solutions Fig.5.4 Variation in the specific activities o f the secondary standards.

2.32e+6

Magnusson and LaChapelle 2.28e+6 ” 2.2 ± 0.1 x 10 6 (a)

2.24e+6 -

2.20e+6 ” i► Brauer et al. 2.14 ± 0.01 x 10 6 (a) 2.16e+6 “ } i x Present work 2.12e+6- 2.144 ± 0.007 x 106 (a)

2.08e+6 1940 1950 1960 1970 1980 1990 Fig.5.5 Comparison o f present measurement with previous workers. The Half-life o f237Np. 136

5.6 Discussion.

5.6.1 Alpha-Particle Self-Absorption Correction.

_n The mass of neptunium deposited on each substrate (~16 p.g cm ) was chosen so as to minimise a-particle attenuation. This was based purely on experience and had to be validated experimentally. A previous attempt to quantify the self-absorption of a-particles in various materials, had resulted in the Bragg- Kleeman relationship (Nuclear Engineering Handbook, 1958). This predicted that 17 mg cm-2 of NpC>2 was required to completely stop an a-particle of 4.87 MeV. However, this is considered valid only to within 15% and so a more accurate estimate of self-absorption, based on observation, was necessary. This was achieved by measuring the variation in a-particle count rates with increasing source thickness.

Using the same stock as in the previous experiment, a solution with an approximate 237Np concentration of 0.430 mg g-1 was produced. From this, weighed TEG spread sources, with surface densities of between 3 and 85 |ig cm , were prepared as described previously. The a-particle count rate corresponding to each source was measured in a 2n proportional counter.

Fig.5.6 shows the graph of background corrected count rate as a function of —9 source thickness. Above 30 |ig cm the data points begin to deviate from the straight line. This may not necessarily be due to a-particle self-absorption, but is almost certainly a consequence of some of the practical difficulties encountered when preparing a-particle sources from large solution deposits.

The a-particle sources prepared for the half-life experiment had, on average, /■ ) a source thickness of 17 jig cm . This is within the proportional region of Fig.5.6 and so an additional correction to the reported half-life value is not necessary.

5.6.2 Source Uniformity Correction.

It was also assumed that the active deposit of each a-particle source, prepared in the half-life measurement, was uniform. By far the easiest and most widely used method of achieving a uniform deposit on a metal substrate is by electrodeposition. Although this technique is not new, it is currently experiencing a renaissance being used to manufacture sources from environmental and biological Corrected Count Rate (cpm) i.. Vrain n on ae ih ore thickness source with rate count in Variation Fig.5.6 ure hickness T rce ou S

. Half-life of o e f i l - f l a H e h T The H alf-life o f 237Np. 138 samples (Talvitie, 1972), as well as from active waste leachates containing large quantities of dissolved solids (Hough, 1990).

The electrodeposition of actinides as hydrous oxides from ammonium sulphate, has been studied by Talvitie (1972) and Hallstadius (1984). Unfortunately, the critical pH adjustment required to ensure total recovery of the actinide requires practice. In contrast, the sulphate salt electrolyte (NaHS0 4 -Na2 S0 4 ) acts both as a buffer, maintaining a pH of 2 and reduces the resistivity of the cell (Kressin, 1977). Furthermore, since NaHSC>4 and Na2 SC>4 are known to be stable, both as salts and in solution, it followed that the decomposition products of these reagents would not affect the electrodeposition of the actinides. It was also found that mg amounts of NO3- , C2O42- and PO4- could be tolerated.

Fig.5.7 is an illustration of an electrodeposition cell used in this experiment. The cathode was a stainless steel source tray, identical to those used previously. Weighed amounts of Np solution (430 |ig g ) were added to the cell containing the electrolyte. The anode-cathode distance was not critical, but it was no more than 10 mm, since greater electrode distances would increase the electrical resistivity of the solution. Previous trials had shown that with a constant current of 1 amp and a deposition time of 90 minutes, the expected recovery of neptunium was 94.5%. Increasing the current or deposition time did not increase the recovery. Stirring the solution, to reduce the concentration gradient at the cathode, was not necessary since the solution was vigorously agitated by the evolution of oxygen and hydrogen at the anode and cathode respectively. Shortly before the end of the deposition, 4M KOH (2 ml) was added to the electrolyte. This raised the pH of the solution and prevented the active deposit from redissolving. On completion the anode was removed from the cell before the power supply was turned off. If the electrodes were in solution without an external power source, a galvanic cell would be produced and the result would be a counter e.m.f. flowing in the opposite direction and so causing the metallic deposit to pass back into solution.

Before removing the source tray from the cell, it was first washed gently with 1% NH4OH. This removed any trace acid and so fixed the deposit to the substrate. After washing the tray several times with acetone, it was removed from the cell and dried. Finally it was fired gently to red heat. Great care was taken in this final stage since the electrodeposited layer was fragile and could easily have been removed from the substrate by vigorous washing.

The a-particle activity of each source was measured in a gas flow proportional counter. The observed count rate was corrected by a factor of 1.06 to The H alf-life o f 237Np. 139

Fig.5.7 Disposable electrodeposition cell. The H alf-life o f 237Np. 140 represent 100% recovery of neptunium. The variation in count rate as a function of source thickness for both electrodeposited and TEG spread sources, is illustrated in Fig.5.8. The two types of sources showed no significant difference in their count rates, indicating that the active deposits of TEG spread sources are uniform over the range 10-30 p.g cm-2.

5.6.3 Beta-Particle Sensitivity.

The sensitivity of gas flow proportional counters to different types of radiation is largely dependent on the detector operating voltage. Thus, it was important to establish whether any of the events, recorded by the counters, were due to P-particles arising from the decay of 233Pa. This could be established by measuring the observed count rate from a pure source of Pa. However, the short half-life of this nuclide (27.0 ±0.1 d) (IAEA, 1986) prevented its use as a p— particle source. Consequently a solution of 137Cs, with an activity of 11.51 ± 0.15 kBq g_1 (standardised by NPL), was used to prepare weighed TEG spread sources, with total p-particle activities in the range 1.47-1.50 kBq. Measuring the sources in each detector produced count rates which were no greater than the detector's normal background levels.

1 Since neither (3-particle nor conversion electron transitions of Cs were detected, it follows that the equivalent Pa transitions, which are of lower energy, would not contribute to the detectors observed count rate.

5.6.4 Alpha-particle backscatter.

Alpha-particles incident upon thick metal backings occasionally undergo a large angle single scattering, identical to that which led Rutherford to propose his revolutionary model of the atom. However, the majority of a-particles experience multiple scattering at small angles, close to the surface of the backing. When an a - particle source is placed in a 27t proportional counter, some of the particles initially emitted downwards are backscattered into the sensitive volume of the counter. Conversely, some a-particles which were originally travelling upwards are backscattered into the source (Hutchinson et al., 1976). This results in an increase in the measured counter efficiency. For many years this increase was thought to be negligible, and often quoted as 1 in 8000, based on a scattering experiment performed by Geiger and Marsden (1909). However, Cunningham et al. (1943) Corrected Count rate (cpm) Fig.5.8 Variation in count rate with source thickness for electrodeposited sources. electrodeposited for thickness source with rate count in Variation Fig.5.8

ure Thickness rce ou S 4 . p N ^ f o e f i l - f l a H e h T ^ The H alf-life o f 237Np. 142 measured the activity of natural uranium on platinum backings and found that they were unable to interpret their results unless they assumed a detector efficiency of 52%. A theoretical analysis of the phenomenon by Crawford (1949% showed that for a-particles with a range of 3.68 cm in air, the calculated backscattering, from platinum substrates, was between 3 and 3.5%. Moreover, he postulated that the extent of the backscattering depended on the nature of the material used as a substrate and varied proportionally with Z .

These calculations were supported experimentally by Cunningham et al. 900 (1949), who concluded that the specific activity of Pu, mounted on a platinum substrate, was 4% higher when measured in a 2k proportional counter, than in a low-geometry counter. Qualitative support for Crawford's hypothesis was furnished by the observation that the measured activity of Pu mounted on quartz, was 3% lower in a 2 k proportional counter than identical samples mounted on platinum.

Of interest to the half-life experiment is the difference in a-particle 9 A 1 backscatter between the Am calibration source, used to determine the geometry of the proportional counters, with those sources used to measure the specific activity of 907 Np. However, rather than replicate the already considerable experimental and mathematical data available, it was considered prudent to estimate the difference in backscatter, if there be any, from previous work.

Lucas and Hutchinson (1976) developed a simple Gaussian model which calculated the geometry of the detector as a function of the source backing, a - particle energy and surface density. Using a weightless 210Po source (Ea = 5.31 MeV) mounted on a stainless steel substrate, they reported the detector efficiency to be 50.52%. Ballaux (1985) used an identical model and showed that sources of 238U (Ea = 4.19 MeV) and 242Cm (Ea = 6.10 MeV), mounted on stainless steel, produced detector efficiencies of 50.67 and 50.48% respectively.

Recently Ferrero et al. (1990) used a Monte Carlo simulation to model the scattering of a-particles in an isomorphous media. The simulation was based on the total stopping powers and the Rutherford scattering laws. The model was for OJU (Ea = 4.4 MeV) sources, of varying surface densities, on polished stainless steel trays. The detector efficiency, at low surface densities, was shown to be approximately 51%.

All of the above data are consistent with the findings of the present work. An 241 Am (Ea = 5.28 MeV) calibration source, mounted on a stainless steel backing, produced an average detector efficiency of 50.94%. Both the Am and The Half-life of 237Np. 143

237Np sources could be regarded as weightless and were of similar activities. Thus, there is no reason to assume that a-particle backscattering from the calibration source was significantly different from that of the TEG spread sources used to determine the half-life of 237Np. 5.7 Conclusion.

The above experiment has resulted in a new value for the specific activity of 227Np, namely 26.03 ± 0.08 Bq fig-1, which corresponds to a half-life of 2.144 ± 0.007 x 106 years. As shown in Fig.5.5 this determination is in exact agreement with that reported by Brauer et al.

This measurement has been supported by a detailed study of all corrections and error analysis. Moreover, the validity of many of the assumptions used in this present measurement have been verified experimentally. • • 239 Gamma-Ray Emission Probabilities of Np. 145

Chapter Six.

Gamma-ray Emission Probabilities of 2 3 9Np.

6.1 Introduction.

As already stated in Chapter One, the determination of accurate decay data for transuranium isotopes is not merely an academic exercise, but is of importance to many fields of applied nuclear technology. This is particularly true in the case of Np, whose radioactive decay affords the following applications: i. for decay heat calculations and determination of Pu accumulation in fast breeder reactors (Mozhaev et ah, 1979) ii. as a calibration standard for y-ray detectors (Ahmad and Wahlgren, 1972) iii. as a chemical yield tracer for other neptunium isotopes (Garraway and Wilson, 1983)

The diversity of this nuclide's applications has prompted numerous studies of its radioactive decay. These have included the measurement of P-particle and y-ray emission probabilities, internal conversion coefficients and half-life.

6.2 Review of Previous Measurements.

6.2.1 H alf-life Measurements.

Table 6.1 lists the half-life measurements performed by several workers. A review of the data (Artna-Cohen, 1971) has resulted in a weighted-mean half-life value of 2.355(4) days. This is currently the IAEA-CRP recommended value for the half-life of 239Np.

6.2.2 [3-Particle Emission Probabilities.

Table 6.2 lists the Np p-particle end-points. The values reported by Hollander (1956, 1957) have been inferred from y-ray emission probabilities, whilst Author(s) Production Route Half-life (d) . p vN f o ilities b a b ro P n io s is m E y a -R a m m a G Philipp and Riedhammer, 1946 238U(n,Y)239U(p“) 2.31 (11) Wish, 1956 238U(n,Y)239U(p~) 2.345 (4) Connor and Fairweather, 1959 238U(n,Y)239U(p“) 2.34 (2) Cohen et al., 1959 238U(n,Y)239U(p“) 2.366 (3) Qaim, 1966 238U(3He,pn or d) 2.354 (8) Bigham et al., 1969 243Am(a) 2.346 (4) O ' l Q Table 6.1 A review of the half-life measurements of Np.

705 (7%) 435 (46%) 301 (47%) Graham and Bell, 1951 715 654 440 330 Tomlinson et al., 1951 715 (4.8%) 655 (1.7%) 441 (31%) 380 (10%) 329 (52%) Freedman et al., 1953 723 (7%) 655 439 (21%) 382 (27%) 327 (45%) Baranov and Shlyagin, 1956 -650 (5%) 440 (45%) 330 (45%) 212 (5%) Hollander et al., 1956-57 713 (6.5%) 654 (4%) 437 (48%) 393 (13.5%) 332 (28%) Connor and Fairweather, 1959 Table 6.2 239Np end-points.

4^ O n 239 Gamma-Ray Emission Probabilities of Np. 147 the intensities quoted by the remaining authors are based on the analysis of (3— spectra. There is some agreement in the end-point energies, but the relative intensities vary considerably between different observers. These differences have meant that all subsequent evaluations of (3-particle intensities have been inferred from the y-ray emission probabilities.

6.2.3 Internal Conversion Coefficients.

The conversion electron spectrum of Np has been studied by several authors (Graham and Bell 1951, Baranov and Shlyagin 1956, Hollander et al. 1956, Ewan et al. 1959, Connor and Fairweather 1959, Davies and Hollander all of1965) which produced relative internal conversion coefficients.

Schmorak (1983) utilised the relative ICC values of Ewan et al. (1959) to produced a self-consistent decay scheme for Np. The y-ray emission probabilities were those quoted by Heath (1974) and the (3-particle intensities were inferred from the total transition probabilities (Py + Pe). The resulting decay scheme is illustrated in Fig.6.1.

6.2.4 y-ray Emission Probability Measurements.

Precise values for the y-ray emission probabilities of 239Np are central to our understanding of its decay scheme. Table 6.3 lists the measured y-ray emission probabilities (Py) of 239Np and shows that previous determinations have fallen into two distinctive categories. Several workers have utilised the fact that Np is in secular equilibrium with 243Am and have measured its y-ray emission probabilities whilst in-situ with the parent nuclide. Indeed it is the Am-Np pair which is OOQ to be used as the standard calibration source mentioned in section 6.1. Np with its range of well defined high intensity peaks, is somewhat limited as a calibration standard due to its short half-life. However, when in equilibrium with Am (7370 ± 15 a) (IAEA, 1986) a calibration source is produced which has an energy range of between 43 and 334 keV, together with a substantial shelf-life.

Since 239Np has a short half-life, it follows that measuring the rate of decay of 243Am is the same as determining the rate of decay of239 Np, by virtue of the fact that the two are in equilibrium (see Fig.6.2). Thus the absolute disintegration rate of Gamma-Ray Emission Probabilities o f2j9Np. 148

239. N p 2.355 d 9 3 \ 277.599 (35.9) %(3- = 1 0 0 * X y-ray energy Total transition probability (Py + Pee) per 100 decays of parent CN CS <—( vo c n m oO o—'

(165.8) 0.0046

T 5 s S vocn^n \ 5 5 6 . : (210.2) 1.8 (7/2-)^’r;'^'_0.'°'§"^"2-?}------• .-'rr r- s o o o o o ~ (216.8) 0.0072 \o <—i w-< o o o err v , . rjoocs oqvqw-jo o o 522 (229.8) 0.019 1-4 r-~, t*-. ov _ X '—’.^7-0 m, 5 1 1 . 8 1 ov cs

o ov p i t n 438 53 vo ov oo >n vf in h p. in t-> 00 OV . oo r - cs o cs cs cs cs 2 8 5 . 4 3 X ^ 5 / 2 ( + )

S OO S §

vo? s o ®0 *-( oo 1 6 3 . 7 3 9 / 2 ( + )

cn Ov (646.3) o° CT s S a (664.8) 2 ^ vo _ •”* 7 5 . 6 8 7 / 2 ( + ) - cs s -, C— Ov v-> s - CS 57.25 713 4 ■vo----- moo V t 4 7 . 8 3 ^ .3 /3 2 / 2(+ ( + ) . y •l/2(+)« £ 239 94Pu

Fig.6.1 Decay scheme o f 229Np. Ray E sin rbblte of Np. N f o Probabilities ission Em y a -R a m m a G

N o m in a l E y A h m a d an d Starozhukov Vaninbroukx Ewan et al.c Yurova et al ^ M ozhaev et al.a A h m a d a Chang et al.c (k e V ) __ ( 1 9 5 9 ) Wahlgrena (1972) ( 1 9 7 4 ) et al.a (1977) ( 1 9 7 9 ) (1 9 8 2 ) et al.a (1984) ( 1 9 8 6 ) 4 4 .6 6 0 .1 3 (1 ) 4 9 .4 1 0 .1 1 (1 ) 5 7 .2 8 0 .1 3 5 (7 ) 6 1 .4 6 1 .2 9 (6 ) 1 .2 9 (2 ) 8 6 .7 1 0 .3 4 0 ( 1 5 ) 1 0 6 .1 2 2 2 .7 8 2 7 .8 (9 ) 2 6 .6 (1 0 ) 2 6 .4 (8 ) 2 7 .5 0 (4 0 ) 2 6 .0 8 (3 8 ) 1 8 1 .7 1 0 .0 7 5 (8 ) 0 .0 8 3 (4 ) 0 .0 7 (1 ) 2 0 9 .7 5 4 .0 8 3 .4 2 ( 1 0 ) 3 .3 6 (1 4 ) 3 .3 0 (1 0 ) 3 .4 6 (5 ) 3 .2 8 (5 ) 2 2 6 .4 2 0 .2 4 (3 ) 0 .2 9 0 (1 6 ) 0 .2 8 (2 ) 2 2 8 .1 8 1 2 .6 8 1 1 .4 (3 ) 1 1 .7 8 (4 4 ) 1 1 .2 (3 ) 11 .2 1 (1 8 ) 1 1 .0 5 ( 1 4 ) 2 5 4 .4 1 0 .1 1 (1 ) 0 . 1 1 0 ( 6 ) 0 . 1 2 ( 1 ) 2 7 2 .8 4 0 .0 8 (1 ) 0 .0 7 7 (4 ) 0 . 0 8 ( 1 ) 2 7 7 .6 0 1 4 .0 7 6 1 4 .5 (4 ) 14.1 (4 ) 1 5 .0 (5 ) 1 4 .3 0 (2 4 ) 1 4 .5 (4 ) 1 4 .3 8 (2 1 ) 14.2 1 (1 3 ) 2 8 5 .4 6 0 .6 2 9 0 .7 6 (2 ) 0 .9 3 (6 ) 0 .7 9 0 (2 5 ) 0 .7 7 (2 ) 0 .7 6 5 (9 ) 3 1 5 .8 8 1 .4 9 6 1 .5 2 (5 ) 1.63 (7 ) 1 .6 0 (5 ) 1 .6 0 (3 ) 1 .5 5 (2 ) 3 3 4 .3 1 1 .9 8 9 1 .9 5 (7 ) 2 .1 (1 ) 2 .0 6 (6 ) 2 .0 8 (3 ) 1 .9 9 (2 ) Table 6.3 A review of the absolute y-ray emission probabilities of Np.

a 243Am-239Np equilibrium pair, without chemical separation. H Irradiation of U, without chemical separation. c Irradiation of 238U, followed by radiochemical separation of 239Np from uranium and fission products.

vo • 239 Gamma-Ray Emission Probabilities of Np. 150 the 243Am-239Np pair .is determined by accurate a-particle counting. This then negates the need to measure direedy the (3-particle activity of jyNp.

243 Am 95

2 3 9 7370 ± 15 a Np 93

56.52 ± 0.10 h 239 Pu 94

(2.411 ± 0.003) x 10 a

Fig.6.2 The sequential decay of Am

Alternatively, the nuclide can be produced via thermal neutron irradiation of 238U, as illustrated below:

Once again the y-ray intensities can be determined whilst in-situ with the irradiated uranium sample or, following radiochemical separation from uranium and fission products, measured independently (see Table 6.3). The (3-particle activity of the resulting pure 239Np source can be determined using a 47t(3—y coincidence system, identical to that described in section 3.4.5.

Of concern to the IAEA-CRP was the fact that the uncertainty in the intensity of the principle 106.1 keV transition varied between 1.45 and 3.80%. As a consequence of this radionuclide's importance, the CRP called for a further measurement of the y-ray intensities of Np, requesting a precision of 1% for the principle transition. • r 2 39 Gamma-Ray Emission Probabilities of Np. 151

6.3 Experimental.

This present determination of the y^-ray emission probabilities of 239Np has utilised both methods of synthesis. However, in the interests of clarity, the different production routes will be treated as individual experiments.

6.3.1 Thermal Neutron Irradiation o f 238UO2 Spheres.

on o 9^5 Highly depleted UO2 spheres (-6 mg) with 10 ppm U, were provided by Harwell. The spheres were placed in various irradiation positions, and consequently varying flux densities, of the Imperial College CONSORT II nuclear reactor (see Fig.6.4). The theoretical 239Np activity A(t') was determined using the following equations (Holloway, 1983):

A(f) = N(t’)A2 (6.1)

and

N0.(J).gi N(f) = Y " (l-e~^2T) e_^2' ( 6.2) (Ki - ^1) A-2

T = irradiation time t' = decay time Xi = 239U decay constant

X2 = 239Np decay constant Gi = 238U thermal neutron cross-section N0 = number of 238U nuclei present

Following a sufficient delay, the Np activities were between 4.1 x 10 and 1.1 x 106 Bq. Gamma-ray spectroscopy measurements were performed using a HPGe detector, identical to that described in section 3.3.12. The source-detector distance was 25 cm, which was chosen to minimise both coincidence and random 239 Gamma-Ray Emission Probabilities o f Np. 152

1 80 ° face

Fig.6.4 Irradiation positions o f the ICRC Consort II experimental reactor. • • • 23 9 Gamma-Ray Emission Probabilities of Np. 153 summing corrections. Table 6.4 lists the irradiation facilities and experimental conditions employed during these preliminary experiments.

Irradiation Thermal Flux Fast Flux Irradiation y-ray analysis Position (n cm- 2s-1) (n cm- 2s_1) Time (sec) (sec) CAS (Al) 1.20 x 1012 0.52 x 1012 1200 3600 CAS (Cd) 1.92 x 1010 0.41 x 1012 1200 3600 CT 5 (Posn.4) 1.19 x 1012 0.33 x 1012 16080 10000 CT 6 (Posn.3) 1.15 x 1012 0.33 x 1012 23820 30000 CT 6 (Posn.3) 1.15 x 1012 0.33 x 1012 26520 90000 CT 6 (Posn.3) 1.15 x 1012 0.33 x 1012 26880 100000 CT 8 (Posn.3) 0.23 x 10u 0.22 x 1012 26700 30000 0° FACE 1.46 x 1011 1.18 x 1010 27480 30000 VTC 2.7 x 108 <7 x 106 27600 30000 VTC 2.7 x 108 <7 x 106 54480 30000 O'lQ Table 6.4 UO2 sphere experiments.

non Fig.6.5 shows a typical y-ray spectrum of Np. The principle 106.12 keV y-ray was not easily resolved from the Kai^ plutonium x-rays, which were non produced by electron conversion in the decay of Np. The spectrum was further complicated by the presence of uranium x-rays, resulting from y-ray photon interaction with inactive uranium. The sample also contained materials arising from the fission of trace 235U, and this was observed in all irradiation positions, regardless of whether the flux was thermal or epithermal. The following isotopes 91,92Srj 91m,92Yi 95 ,9 7 ^ 9 5 ,9 7 ^ 9 9 ^ 99mT(.; 131,132,133,135j 1 3 2 ^ 140Ba ^ I40La were present in all spectra. This would infer a great many more contaminants qq ni 109 by virtue of their radioactive decay. Moreover, Mo, I and Te had y-ray transitions at 181.1, 284.3 and 228.2 keV respectively, which interfered with transitions of 23^Np. These preliminary measurements showed that a precise determination of the y-ray emission probabilities of Np, prepared from neutron irradiated U, was feasible only if: i. the 238U starting material had a reduced 235U content ii. inactive 238U was chemically removed from the sample prior to y-ray analysis 0 6 0 8 8 - V 0 N - 0 1 N O 10: 00 56: T A D E T C E L L O C A T A D CT8.SP COUNTS X 1000 130.0 Y G R E N E 220.0 keV 310.0

.0 0 0 4 Lh p. N f o Probabilities ission m E y a -R a m m a G 239 Gamma-Ray Emission Probabilities of Np. 155

6.3.2 Thermal Neutron Irradiationof2^UC >2 Powder.

238UC>2 powder (99.999%) containing approximately 2.4 ppm 23**U was provided by Harwell, in the hope that this material would fulfil the first prerequisite. The oxide (10 mg) was dissolved in 8M HNO3 (~5 ml), together with a few drops of H2O2 (20 vol.). The resulting uranyl nitrate, (U0 2(N0 3)2), was evaporated to dryness, to remove traces of hydrogen peroxide, and then redissolved in nitric acid. The uranium solution was transferred to polythene capsules ready for irradiation in the Imperial College CONSORT II nuclear reactor. The solution was always irradiated in the positions of highest thermal neutron flux, i.e core tube four positions 3 & 4 (1.39 x 1019 n cm—9 s — 1 ). The theoretical activity was calculated once again using the above equations, with irradiation and decay times of 7.5 and 16 99Q hours respectively, producing a jyNp activity of 2.1 x 10 Bq.

Despite the purity of this new starting material, the production of trace fission products was inevitable and as already explained in section 2.6, the most probable contaminants would be zirconium, niobium and molybdenum. These were removed sequentially by ion-exchange chromatography based on a method described by Wish (1959). A theoretical account of that work can be found in section 2.6, whilst the experimental procedure is outlined below.

The irradiated solution was transferred to a beaker and evaporated to dryness over a hot-plate. The residue was then redissolved in 12M HC1/0.06M HF (~2 ml). The resulting solution was again evaporated to dryness and then redissolved in the acid mixture. This process was repeated several times to ensure the formation of anionic chloride complexes. The solution was then loaded onto a strongly basic anion exchange resin Bio-Rad AGlx4 (~2 ml). The resin, though already in the chloride form, was conditioned slowly to the same acid mixture. Washing the resin with the 12M HC1/0.06M HF (5 ml) removed trace Zr(IY). However, Np(IV), U(VI), Mo(VI) and Nb(V) were all strongly adsorbed (see Figs.2.5(a) and (b)). Np(IV) was then eluted by washing the column with 6.5M HC1/0.004M HF (5 ml). The eluent was collected in 1 ml fractions, with the majority of the activity found in the first three samples. This meant that the last two fractions could be discarded since they probably contained trace Nb(V), whose minimum K

The final steps of the above procedure may be omitted if a quick separation was required, or conversely the flow rate through the resin may be increased by pressurising the column with a steady stream of nitrogen gas. However, it is the precision with which the acid solutions are made, that determines whether the separation is successful, and this is particularly true if an efficient decontamination of Nb(V) from Np(IV) is desired. The sharp elutions afforded by low concentrations of hydrofluoric acid, far outweighs the problems caused by the action of this acid on the glassware.

The purified neptunium fractions were grouped together and evaporated to dryness under an infra-red lamp. The residues were then redissolved in a minimum volume of 2M HNO3. Sources were prepared by depositing an aliquot (5 |il) of 239Np solution onto a gold coated VYNS film. A similar volume of spreading agent (0.1% TEEPOL) was added to the active solution, which reduced the surface tension of the liquid and so produced a uniform layer. The deposit was then allowed to air dry. The resulting 239Np source was placed 25 cm from the end-cap of the HPGe detector identical to that used in the previous experiment. The y-ray analysis was started simultaneously with the timing device of a 47t(3-y coincidence counting system. This represented the reference time and so the measured y-ray emission and absolute disintegration rates were corrected back to this time.

Photopeak areas were determined by the peak fitting routines Sampo and Omnigam. Shape parameters were calculated using a limited multi-element standard containing the isotopes 38C1, 56Mn, 60mCo and 198Au. The detector efficiency calibrations were performed using standard sources provided by LMRI and Amersham International. The measured efficiency values were then fitted to the analytical function proposed by Gray and Ahmad (1985) given in equation 3.21. Both the parameters and their uncertainties were calculated using the CERN library subroutine MINUITwhich utilised the least-squares minimisation method.

Fig.6.6 shows the y-ray spectra corresponding to two 239Np sources. The top spectrum was obtained from a source which contained both inactive uranium and fission products. The bottom spectrum was chemically purified prior to y-ray COUNTS X 1000 8 8 - V 0 N - 0 1 N O 10: 00 56: T A D E T C E L L O C A T A D 3. 200 310.0 220.0 130.0 keV Y G R E N E

400.0 - y sin rbblte of JNp. f o Probabilities ission m E ay a-R m m a G '-J U\ 239 Gamma-Ray Emission Probabilities of Np. 158 analysis. This second spectrum shows that the 90 to 120 keV region of the spectrum has been simplified by the removal of uranium x-rays. Furthermore, there appears to be no measurable quantities of fission products, indicating that the original aims of this experiment had been achieved.

Following y-ray analysis, the absolute disintegration rate of the source was determined using a 47tp-y coincidence system, identical to that described in section 3.4.5. Once the source had been placed inside the proportional counter, the chamber was flushed with fill gas (90% argon: 10% methane) to remove air. The gas flow was then lowered and the counter pressure was slightly greater than atmospheric. Once the electronics had stabilised, the voltage of the y- and p—detectors were set at 1000 and 1500 volts respectively. The output signals from both beta and gamma amplifiers were checked for overloading using an oscilloscope. The minimum beta threshold was set above the electronic noise level, whilst the gamma window was fixed, using a timing single channel analyser, to the 277 keV y-ray transition. Both the dead time and resolving time of each detector were determined as described in section 3.4.6. The measuring time was set to 300 seconds, which was of sufficient duration to record the maximum number of events in the beta channel without either the gamma or coincidence channels saturating. After each measurement the recorded beta (Np"), gamma (Ny") and coincidence counts (Nc") were transferred, together with the measuring and real clock times, to an IBM PC computer. The beta channel threshold was then moved automatically to the next level, by a digital spectrum scanner and this initiated a further measurement. The scanner was set to 64 steps over a range of 3 volts which represented one complete cycle. The process was repeated several times, typically over one half-life so as to improve counting statistics. The source was then removed from the chamber and a background measurement taken over one cycle. Finally the recorded data was transferred to the mainframe computer (CYBER CY855) based at the Imperial College Computer Centre, where the Fortran program 4npy2 was used to determine the absolute disintegration rate of the source.

Both the y-ray emission and absolute disintegration rate measurements were repeated with a second source from the original purified sample. However, preparing a sufficiently active source, several days after irradiation, required a large deposit. This raised the probability of self-absorption and so on average only two O'lQ Np sources were prepared from each irradiation.

The absolute y-ray emission probabilities of Np, prepared from several 'I'lQ neutron irradiations of high purity UO2, are listed in section 6.4. • 239 Gamma-Ray Emission Probabilities of Np. 159

6.3.3 Separation o f239Np from 242Am.

As already mentioned, Np is in equilibrium with Am which is a readily available material obtained in the reprocessing of spent reactor fuel. AmCl3 (3.4 x 106 Bq) was supplied by Harwell. The solution was evaporated to dryness under an infra-red lamp and redissolved in 7M HC1. This process was repeated several times to ensure the formation of the anionic Np(IV) complex. The active solution was then loaded onto an anion exchange resin Bio-Rad AGlx4 (5 ml) which had previously been conditioned to the same acid molarity.

Am(III) does not form anionic complexes and is eluted from the resin in 7M HC1 washings. The principle 74 keV y-ray transition of 24^Am made it possible to determine qualitatively the removal of this isotope from the column, by analysing nA'Z the y-ray spectrum of each fraction. Once the eluent was free of Am the resin was washed with 1M HC1, which corresponded to the minimum equilibrium distribution coefficient of Np(IV) (see Fig.2.6(b)). The separation procedure was not quantitative as the yield of Np was between 65 and 70% due to the fact that some Np(V) is eluted simultaneously with Am(III) (Garraway and Wilson, 1983). Addition of reducing agents, such as NH4I, to maintain the Np(IV) complex would result in the formation of ammonium salts, whose presence would impair the detector resolution of low energy y-rays and the efficiency of the 47tP~y coincidence system.

The resulting purified Np fractions were grouped together and evaporated to- dryness under an infra-red lamp. The residue was then redissolved in a minimum volume of 2M HNO3. Sources was prepared in an identical way to that described in section 6.3.2. Both the y-ray emission and absolute disintegration rates for each source were measured as above.

Fig.6.7 compares the y-ray spectra of two sources. The top spectrum 9^0 OA'X corresponds to a source of Np which was "milked" from Am. The bottom spectra was obtained from material which had been separated from inactive uranium and fission products. There are no obvious differences between the spectra.

The absolute y-ray emission probabilities of Np, separated from JAm, are listed in the following section. COUNTS X lOOO 0 9 - N A J - 2 1 N O 15: 16: 00 T A D E T C E L L O C A T A D 3. 220.0 130.0 kev Y G R E N E 310.0

400.0

0 6 1 Ray Emiso Poaiiis 3Np. 239N f o Probabilities ission m E y a -R a m m a G 23 9 Gamma-Ray Emission Probabilities of Np. 161

6.4 Results.

6.4.1 Absolute Disintegration Rates.

As stated above, the program 47tpy2 was used to perform the corrections to the channel count rates and to calculate the ratios Nc/Ny and Np.Nc/Ny (see Chapter Three). The experimental data was fitted to a straight line using the least-squares method and extrapolated to 100% detector efficiency, which corresponded to the absolute disintegration rate, N0 of the sources. Fig.6.8 shows the graph of (Np vs. Nc/Ny) for source F. Table 6.5 lists the calculated disintegration rates for each of the sources.

239Np Maximum P Absolute Disintegration Source Efficiency (%) Rate (Bq) A 95 29882 ± 454 B 96 18362 ± 295 C 95 17842 ± 260 D 93 60307 ± 477 E 94 59476 ± 769 F 93 20908 ± 129 Table 6.5 The absolute disintegration rates o f each source.

Sources A to E were prepared from irradiated 238UC>2 powder, whilst source F resulted from the chemical separation of 239Np from 243Am. The quoted values for N0 are the weighted-mean of several measurements:

_ Xxj/CTi2 (6.3) XVCTi2

xi = fitted value for N0, when Nc/Ny = 1. C[ = uncertainty in the fitted value.

The goodness of fit of the data to the mean was established by the chi- squared test:

n E (xi - M-)2 ( 6 . 4 ) a,2 i= 1 NP (cps) i.68 rp ofNj vs. Nfj f o Graph 6.8 Fig. N^Nyfor Nc/Ny

a^N ttn im source. film tltin ^^Np a G\ p. *N ~J f o Probabilities ission m E ay a-R m am G to 23 9 Gamma-Ray Emission Probabilities of N p . 163

1 Since each data set was found to be consistent, the internal error X i M 2 was used to determine the overall uncertainty in the absolute disintegration rate.

The significance of the maximum p-efficiency, (N c/N y), is simply that as the ratio approaches unity the extrapolation length decreases and consequently the least- squares fitted value for N0 becomes more accurate.

6.4.2 Absolute y-ray Emission Probabilities.

The initial stage of the analysis was to determine whether there was any significant variation in the emission probabilities determined by the two peak fitting routines. To this end the net peak areas determined by Sampo and Omnigam were treated as individual data sets. The peak areas were first corrected for counting losses, occuring during acquisition, by the equation:

* t ______A y X t R (6.5) y 1 - exp(-taR)

Ay = net peak area (counts per second)

X = Np decay constant (per second) tR = real time (second)

If the real time, tR, of the counting system was known precisely, then the uncertainty in the corrected peak area was determined using the propagation of errors formula:

(6 .6 )

V total peak area GAy = uncertainty in the net peak area live time GX - uncertainty in the decay constant 239 Gamma-Ray Emission Probabilities of Np. 164

Since the reference time was set to the beginning of the y-ray acquisition, a further correction to account for decay was not necessary.

The y-ray emission probabilities (Py), from each of the six sources, were calculated using the following equation:

pY= (6.7)

£y = absolute photopeak efficiency

The random uncertainty in each measurement was determined by the quadrature addition of ' and such that:

The values for Py and Gp^, were used to determine the weighted mean emission probability (Py) of each transition. As in the above example, the goodness o f fit of the mean to the data was established using the chi-squared test. Where the data was inconsistent, usually in the case of low intensity transitions, the random error was reevaluated in one of two ways. Firstly the calculated uncertainty was inflated by the Birge ratio ^ which forced the data to have an adjusted ratio of unity, whilst the mean remained unchanged. The internal error was then calculated in the usual way. Secondly the external error, which would account for all sources of random uncertainty, was calculated as follows:

-V / (P-* - P y ) 2 G (6.9) \ n (n -l)

Finally the larger of the two errors was adopted as the random uncertainty in the measurement. The y-ray emission probabilities and the calculated random errors, derived from the two peak fitting routines, are listed in Table 6.6. mmaRa E sin rbblte of Np. f o Probabilities ission Em ay a-R m am G

y-ray Energy Py (Omnigam) Py (Sampo) Py (Combined) Py (IAEA, 1986) Py (Nichols, 1991) (keV) (%) (%) (%) (%) (%) 61.46 1.24 ± 0.02 1.30 ± 0.01 1.27 ± 0.03 1.29 ± 0.02 106.12 25.6 ± 0.1 25.6 ± 0.1 25.6 ± 0.2 27.2 ± 0.4 26.7 ± 0.4 181.71 0.088 ± 0.004 0.088 ± 0.003 0.088 ± 0.002 0.081 ± 0.004 209.75 3.42 ± 0.02 3.54 ± 0.02 3.47 ± 0.03 3.42 ± 0.05 226.42 0.25 ± 0.01 ? 0.25 ± 0.01 0.28 ± 0.02 228.18 11.54 ± 0.05 ? 11.54 ± 0.05 11.27 ± 0.18 11.12 ± 0.15 254.41 0.115 ± 0.006 0.111 ± 0.003 0.113 ± 0.004 0.11 ± 0.01 272.84 0.040 ± 0.003 0.16 ± 0.01 0.077 ± 0.004 277.60 14.51 ± 0.07 14.42 ± 0.07 14.46 ± 0.10 14.38 ± 0.21 14.31 ± 0.20 285.46 0.787 ± 0.004 0.815 ± 0.005 0.80 ± 0.01 0.79 ± 0.02 315.88 1.578 ± 0.008 1.626 ± 0.008 1.60 ± 0.01 1.60 ± 0.03 334.31 2.03 ± 0.02 2.06 ± 0.02 2.05 ± 0.02 2.07 ± 0.03 nQQ Table 6.6 Measured absolute j-ray emission probabilities of Np.

0\ U\ r 239 Gamma-Ray Emission Probabilities of Np. 166

There appear to.be only two inconsistencies between the data sets. Firstly, the peak fitting routine Sampo is unable to separate the 226.41 and 228.18 keV transitions. Instead it analyses the peaks as a singlet. Previous data analysis showed that Omnigam also failed to deconvolute these peaks, unless both were listed in the nuclide library (see section 3.3.9). Secondly, there is a significant difference between the measured values for the 272.84 keV transition. This peak, though clearly visible, is situated on the low energy side of the intense 277.60 keV y^-ray. The emission probability determined by Omnigam is reasonably consistent with that of previous measurements, whilst that of Sampo is over twice the IAEA—CRP recommended intensity (IAEA, 1986). Obviously the photopeak area is overestimated by Sampo, probably due to its close proximity to the 277.60 keV y- ray.

The remaining emission probabilities were consistent and so were combined to form one complete data set. The y-ray intensity was expressed as a weighted mean, derived from twelve measurements. The random uncertainty was calculated as described above and then added in quadrature with the non-random error associated with the detector efficiency. Table 6.6 lists the measured y-ray emission probabilities resulting from the combined data set, together with the LAEA-CRP evaluated y-ray intensities (IAEA 1986, Nichols 1991).

* y i g In general the measured values for the y-ray emission probabilities of Np are in good agreement with those of previous workers (see Table 6.3). The principle 106.12 keV transition is in agreement with the most recent value reported by Chang et al. (1986), whilst the intensity of the 277.60 keV y-ray is consistent with all previous measurements (see Figs.6.9(a) and (b)). w £ X O J £ c3 e o c o c3 Ji >.

y-ray Emission Probabilities (%) . p 239N f o bilities roba P n issio m E ay a-R m am G 24.00 25.00 - 25.00 - 0 .0 6 2 00- 0 .0 7 2 28.00 00- 0 .0 9 2 30.00 .15.40 13.60 13.80 14.00 14.20 14.40 14.60 14.80 15.00 15.20 15.60 15.80 90 95 90 95 90 1995 1990 1985 1980 1975 1970 90 95 90 95 90 1995 1990 1985 1980 1975 1970 Fig.6.9(b) Intercomparison o f the measured Py values for the 277.60 keV y-ray. keV 277.60 the for values Py measured the f o Intercomparison Fig.6.9(b)

Fig.6.9(a) Intercomparison o f the measured Py values for the 106.12 keV y-ray. keV 106.12 the for values Py measured the f o Intercomparison Fig.6.9(a) ha and Ahmad ha and Ahmad Wahlgren Wahlgren

Yurova et al. et Starozhukov Starozhukov et al. et

et al. et

Mozhaev et al. et

Ahmad Ahmad Vaninbroukx Vaninbroukx et al. et et al. et Chang

et al. et } Chang

t al. et

mesaurement measurement Present Present 167 239 Gamma-Ray Emission Probabilities of Np. 168

6.5 Conclusion.

The present determination of the y-ray emission probabilities of Np has utilised different analytical peak fitting programs and production routes, yet the measured values are consistent with those of previous workers. Moreover, the intensity of the principle transition has been measured with a precision of greater than 1%, satisfying the original aims of the IAEA—CRP.

Further measurements of (3-particle emission probabilities and internal /)'1Q conversion coefficients are essential if a self-consistent decay scheme of Np is to be produced. Furthermore, the development of an analytical peak fitting routine, capable of distinguishing between the different peak shapes associated with y- and x-rays, would provide a means by which the principle 106.12 keV y-ray could be completely resolved from interfering plutonium Kai,2 x-rays. Summary and Conclusions. 169

Chapter Seven.

Summary and Conclusions.

The work carried out in this thesis was prompted by the IAEA-CRP request for more accurate decay data for Np and Np. Both these nuclides contribute to the radioactivity levels, and consequendy heat generation, within a fission reactor. Furthermore, Np with its long half-life, high radiotoxicity and relative abundance represents a potential waste hazard. Thus, models which predict the behaviour of these isotopes, both in a reactor and the surrounding environment, require accurate decay data.

The objectives of the CRP are not just to encourage measurements, but also include the evaluation of data. Chapter Four described an attempt to construct a self-consistent decay scheme for Np. The evaluation was based on precise a - particle, y-ray and conversion electron emission probabilities, yet produced a decay scheme which contained many anomalies. The conclusion was that the difficulties arise not from what had been measured, but what had previously gone undetected. This assumption was supported by numerous coincidence measurements which pointed to the possibility of low-energy linking y-rays. Such transitions would be highly converted and explains why so many workers had been unable to detect them. Consequently, an experiment was undertaken to analyse the low-energy y-ray spectrum of Np in an attempt to prove, once and for all, the existence of the low-energy transitions.

High purity 917 Np sources were chemically separated from trace plutonium contaminants and Pa. Gamma-ray spectroscopy measurements were carried out using an intrinsic HPGe detector and spectral analysis was performed using the peak fitting routine Gamanal. The measurements implied the presence, and assigned relative emission probabilities, to y-rays at 5.18, 8.69 and 36.20 keV. Theoretical internal conversion coefficients were calculated for each transition, and showed that the measured relative emission probabilities of both the 5.18 and 8.69 keV transitions were overestimated. One possible explanation for this, was that these y- rays interfered with x-ray escape peaks from intense protactinium Lp2,i5, Lpi and Lyi transitions.

In response to this work the National Physical Laboratory (NPL) used Channel Electron Multiplier detectors, to measure the conversion electrons associated Summary and Conclusions. 170 with the proposed linking transitions. They identified conversion lines for only the 36.20 keV y-ray. It is possible that the L and M conversion lines of the 5.18 and 8.69 keV y-rays are approaching the detection limit of the CEMs, or conversely, that the proposed linking transitions simply do not exist. If the latter is true, then the decay scheme of 237Np remains an enigma.

Many workers in the field of radionuclide metrology believe that the measured a-particle and y-ray emission probabilities of Np are now of an accuracy where further measurements would be superfluous. However, a comprehensive study of the conversion electrons following the decay of 237Np is still required. Woods et al. (1988) of NPL, determined the internal conversion coefficients and assigned multipolarities to five of the major fifteen y-ray transitions. Obviously more measurements are required, particularly of the low-energy conversion lines associated with the principle 29.37 keV y-ray. Advances in CEM detectors could make this possible, but not until their calibration can be determined with a greater accuracy than is currently available.

Further studies into the low-energy linking transitions of Np may benefit from the use of Si(Li) detectors, whose x-ray escape peaks will hopefully not interfere with the suspected transitions. Alternatively the low-energy y-ray spectrum may be deconvoluted by the use of a Compton suppression system. When used in the anticoincidence mode, this device is capable of distinguishing between those y- rays which are absorbed by the detector, from those which escape from it.

For many years the only precise measurement of the half-life of Np dated from 1960 (Brauer et al., 1960). As a consequence of this radionuclide's importance to fission reactors, the IAEA-CRP requested a confirmatory measurement.

In this present determination, which is described in Chapter Five, the specific activities of 37 sources, prepared from known masses of high purity Np solution, were measured. Concentration analysis of the primary solution, was provided by controlled-potential coulometry. Source activities, in the range of 1.0-1.9kBq, were determined by a-particle counting in two gas flow proportional counters of known geometry. Summary and Conclusions. 171

This measurement was supported by a detailed study of all corrections and error analyses. Many of the assumptions used to design the experiment were tested to check their validity.

The measured specific activity was 26.03 ± 0.08 Bq Jig-1, which corresponds to a half-life of 2.144 ± 0.007 x 106 years, in excellent agreement with that reported by Brauer.

The determination of accurate decay data for 239Np is important to many fields of applied nuclear technology, some of which include: 270 i. the determination of decay heat and Pu accumulation in fast breeder reactors 9 A'l OQQ ii. its use as an Am- Np calibration standard for y-ray detectors iii. its use as a chemical yield tracer for other neptunium isotopes

The diversity of this radionuclide's applications has prompted numerous measurements of its y-ray emission probabilities. However, the uncertainty in the intensity of the principle 106.1 keV transition, reported by these authors, has varied between 1.45 and 3.80%. This was considered unacceptable by the IAEA-CRP who called for a further measurement of the y-ray emission probabilities of Np, requesting a precision of 1% for the intensity of the principle transition.

In this measurement, described in Chapter Six, high purity Np sources were prepared via two production routes. Firstly, by chemical separation from neutron irradiated U, which ensured that neither uranium isotopes nor fission products were present in the sources. Secondly, by chemical separation from 2A7 Am. Gamma-ray spectroscopy measurements were performed using two analytical peak fitting routines Sampo and Omnigam. The absolute disintegration rate of each source was determined in a 47t(3-y coincidence system, whose performance had been calibrated against a ^Co reference material. The principles behind coincidence counting techniques and the calculations necessary to determine the absolute disintegration rate are discussed in Chapter Three.

The measured values for the absolute y-ray emission probabilities of Np, which were derived from different analytical peak fitting routines and production routes, were in good agreement with those of previous authors. Moreover, the Summary and Conclusions. I l l intensity of the principle transition has been determined with a precision of greater than 1%, satisfying the original aims of the IAEA-CRP.

However, we are still some way from producing a self-consistent decay scheme for 2^Np. Beta-particle emission probabilities have, in the past, been inferred from y-ray intensities and all previous measurements of internal conversion coefficients have been relative. Obviously these areas require further investigation.

This work has also highlighted the need for an analytical peak fitting routine, capable of distinguishing between the different peak shapes associated with y- and x-rays. This would resolve the principle 106.1 keV y-ray of Np, from interfering plutonium Kai}2 x-rays. References. 173

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