NEUTRON INDUCED PEAKS IN GERMANIUM DETECTORS By ERMIAS GETE B.Sc., University of British Columbia., 1992

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE

in THE FACULTY OF GRADUATE STUDIES Department of Physics

We accept this thesis as conforming to the required standard

THE UNIVERSITY OF BRITISH COLUMBIA October 1994 © Ermias Gete, 1994

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Peaks from fast and thermal neutron interactions in a HPGe detector have been studied using neutrons from Cf,232 the 28Si(r,un) and the 209Bi(ir,n) reactions. In the latter case, it was possible to separate1 the neutron induced events using the TOF method.

The peaks from (n,n’) interactions in the the detector crystal have a peculiar triangular shape of about 40 keV across due to the recoiling nucleus, The two ma jor prominent peaks at 596 and 691 keV corresponding to T4Ge(n,n’) and 72Ge(n,n’) have been studied for the above three neutron sources. The triangles have have been fitted to an exponential function to compare the peak shapes and a difference of less than 20 % has been observed between the three different cases.

In addition, many peaks resulting from (n,-y) and (n,n’) reactions in the mate rials surrounding the detector have been observed and identified. The ‘-y-raysfrom Cf252 fission fragment isotopes and from the 209Bi(7r ,xn) have also been analyzed and identified.

II 2 Acknowledgements List List Abstract 1 Contents 2.3 2.2 2.1 Experimental 1.4 Introduction 1.3 1.2 1.1 of of Figures Tables Electronics h Experimental The The Neutron Exotic 7-ray 1.3.2 Detection 1.3.1 1.2.3 1.2.1 1.1.2 1.2.2 1.1.1 TRIUMF Spectroscopy Atoms Pionic Muonic Major Physical Detection Interaction Detection energy Mechanism Methods features atoms Cyclotron atoms spectrum properties of and Setup of with 7-rays and photons measurement and of of Photons Semiconductor germanium pion from nuclear of using semiconductors with absorption 111 252 Cf germanium muon matter of detectors photons Detectors capture by the detectors as nucleus 7-ray spectrometers . . viii vii 22 25 24 22 21 19 16 16 11 vi ii 1 5 5 4 9 2 3 2.3.1 Stopping telescope 25

2.3.2 Compton suppression 26

2.3.3 Ge electronics .. 27

2.3.4 Strobe 28

2.3.5 Timing logic 29 2.4 Experimental Set Up for the First Cf252 Run 29 2.5 Data Acquisition System. 30

3 Methods of Data Analysis 32 3.1 Introduction 32

3.1.1 Peak fitting 33

3.1.2 Energy calibration 33

3.1.3 Time of flight discrimination .. . 35

3.1.4 Background reduction 37

4 Results and Discussion 39

4.1 Introduction 39 4.2 7-Rays from ir Absorption on Bi209 40 4.2.1 The total 7-ray spectrum 40

4.2.2 The prompt spectrum 44

4.2.3 The neutron spectrum 51

4.2.4 The delayed spectrum 53 4.3 Neutron Induced 7-rays from the Cf252 Run 55 4.3.1 Lines from the Ge(n,n’7) and Ge(n,7) reactions 55 4.3.2 The 7-rays from (n,n’) and (n,7) reactions on surrounding

materials 59 4.4 Comparison of Neutron Triangles from , ir, and Cf252 Runs 62

iv 5 7-rays from Cf252 70 5.1 Introduction 70

5.1.1 Nuclear fission 70

5.2 Transitions of the fission fragment isotopes observed from Cf252 . .. 72

6 Conclusion 87

Bibliography 88

V List of Tables

3.1 Calibration energies used . 34 4.1 A list of 7-rays from Bi209 run ,.... 46 4.2 Excited states of Ge isotopesir observed 53 4.3 Abundance and capture cross-section of Ge isotopes 57 4.4 7-rays from Ge(n,’y)reactions 58 4.5 7-rays from (n,n’) and (n,7) reactions from the Cf252 run 60 4.6 Peak shapes of the 596 and 691 keV lines 65 5.1 A list of 7-rays observed from the Cf252 spectrum 75 5.2 A list of the even-A fission fragment isotopes observed 81 5.3 A list of the odd-A fission fragment isotopes observed 85

vi List of Figures

1.1 Band structure of a) insulators, b) semiconductors . 6 1.2 Energy levels produced in semiconductors by impurity atoms ... 8 1.3 A diagram of a planar Ge(Li) Detector 11 1.4 A diagram of an HPGe detector...... 12 2.1 The TRIUMP cyclotron 23 2.2 Experimental set up for the Bi209 experiment . . . . 25 2.3 A block diagram for the pionirtelescope . . .. 26 2.4 A block diagram for the electronics of the Compton suppressor . . 27 2.5 A block diagram of the Ge electronics .. 27 2.6 A logic diagram for the strobe signal 28 2.7 Experimental set up for the Cf252 experiment 30

3.1 Deviation of the linear calibration energy from the literature energy 36 3.2 Deviation of the energy from eq. 3.4 from the literature energy . 36 3.3 Time spectrum of the ir on Bi 37 3.4 Compton suppressed and unsuppressed spectra from the Cf252 run 38 4.1 7-ray spectrum from the background run 41 4.2 7-ray spectrum from the ir on Bi209 with no timing cut 43 4.3 Prompt 7-rays from ir absorption on Bi209 (low energy) 45 4.4 Prompt 7-rays from r absorption on Bi209 (high energy) 45 4.5 The neutron induced spectrum from the ir on Bi209 54 4.6 The neutron induced spectrum from the ir on Bi209 54 4.7 The delayed 7-ray spectrum from the ir on Bi209 55 4.8 A portion of the 7-ray spectrum from the Cf252 .. 57 4.9 ”Ge(n,n’) line from the irBi run 66 4.10 774Ge(n,n’) line from the [iSi run (Gel) 66 4.11 T4Ge(n,n’) line from the 1rSi run (Ge2) 67 4.12 T2Ge(n,n’) line from the irBi run 67 4.13 72Ge(n,n’) line from the Cf252 run 68 4.14 72Ge(n,n’) line from the 1uSi run (Gel) 68 4.15 72Ge(n,n’) line from the uiSi run (Ge2) 69

vii Acknowledgements

I would like to express my sincere gratitude and appreciation to my supervisor Professor David F. Measday for his direction, support and encouragement through out the progress of this work.

I would also like to thank my fellow graduate students; B, Moftah, M. Saliba and T. Stocki for their valuable assistance in the experiments.

viii correlation of angle using measuring the could sure We neutron from cause detector. 50

Germanium

Introduction Chapter the keV shall neutrino the In muon The between be serious a emitted to high order interactions measured now induced experiment Such several the and capture, detectors following resolution systematic describe neutrino). the to angular is effects minimize therefore 7-ray pseudoscalar MeV. by a

in 1 on measuring are serious the the germanium cannot correlation n the and HPGe errors. In muon Hence, commonly muon the reaction an a function problem experiment detector. systematic easily capture Thus recoiling coupling the the capture crystals between 7-ray 2 SSi(t_,v) 28 A1* Doppler used it be of arose on became nucleus gp distinguished experiment. The constant to lineshape at error 28 5i to the from detect [6] TRIUMF broadening understand 7-ray (E570) - clear on 1229 (which neutron [10]. of 7-rays the is lineshape that [6]. keV the a has from which measured function is of these This it in weak opposite been induced de-excitation the was the 7-ray was is angular background de-excitation performed interaction, necessary energy of a value detecting reactions the to function effects the 7-21 correlation range of 7-ray direction to and gp, angular to effects. of 7-rays in gp study 7-ray mea from it and can the the by is essential to fully understand the origin of any background especially around the line of interest. This work was started in an effort to understand the plateau region which existed in the 1210-1230 keV region (mainly caused by (n,n’) reaction in the detector crystal itself). It was then extended to study of y-rays resulting from neu tron inelastic scattering and thermal neutron capture inside the germanium crystal as well as its surrounding materials by using a Cf252 source and neutrons from pion absorption on Bi.209 The latter experiment was performed since it was possible to separate the neutron induced events by using the time of flight method. In addition to the neutron induced events, the x and 7-rays following r absorption on and the fission fragment 7-rays have also been analyzed and identified,

The remainder of this chapter is devoted to the discussion of the basic idea of 7-ray detection techniques using Ge detectors, and the fundamental properties of muonic and pionic atoms have also been given. A discussion about Cf252 has been postponed until chapter 5 where the 7—rayspectra will be explained.

1.1 Detection Mechanism of Photons

When 7-rays interact with matter, they produce high energy electrons. These elec trons deposit their energy in the medium by ionization or excitation depending on the nature of the medium. The detection and measurement of 7-rays is performed by measuring the ionization and excitation produced by these electrons. The basic interaction mechanisms of photons with matter and their detection mechanisms are discussed below,

2 1.1.1 Interaction of photons with matter

When passing through matter, photons can interact with the atoms in various ways. The main interaction processes relevant to 7-ray spectroscopy are:

1. The photoelectric effect.

2. The Compton effect.

3, Pair production. The relative importance of these effects depends on both the photon energy and the atomic number of the absorbing medium.

The Photoelectric effect

In this phenomenon the photon interacts with a bound electron and all of the photon energy is absorbed. This results in the ejection of an electron with a kinetic energy equal to the difference between the the photon energy and the binding energy of the electron. The cross-section of this interaction is significant at low energies and it also increases rapidly with the Z value of the medium and is approximately given by: Zn = const.- (1.1) where n varies between 4 and 5.

The Compton scattering

In the Compton scattering, the photon transfers a portion of its energy to the electron and the remainder appears as a secondary photon. It is the predominant interaction mechanism for 7-ray energies of about 0.5 to 5 MeV. The probability of Compton scattering per atom of the absorber depends upon the number of electrons available as scattering targets, and is therefore proportional to Z.

3 Pair production

In this phenomenon, the energy of the photon is converted in the nuclear coulomb field to an electron-positron pair. For the pair-production to occur, the incident photon must have an energy at least equal to the sum of the rest masses of the electron and the positron i.e, 1.02 MeV, and this process dominates at high energies

(greater than 5 MeV). The excess photon energy is transformed into the kinetic energy of the electron and the positron. For the pair production cross-section, no simple expression exists but its magnitude varies approximately as the square of the absorber atomic number.

1.12 Detection and measurement of photons

In all the three cases discussed above, free electrons are generated, and as these electrons are slowed down on their path through matter, they create excited molec ular states (for example scintillating crystals), electron-ion pairs (for example gases) or electron-hole pairs (for example semiconductor crystals) . In many photon de tectors, one makes use of these information carriers; i.e, charge pairs or the light emitted in the de-excitation of the molecular states either to detect the passage of a photon or to determine its energy by measuring the quantity of charge produced.

In all the cases of photon interaction discussed, the interaction cross-section is dependent on the Z value of the medium. Hence, materials with high Z are chosen for the detection of photons above 100 keV. NaI(Tl) detectors are the most common detectors and are often chosen due to the high Z-value of iodine.

The other factor to consider for a detector material is the average number of the information carriers generated per photon energy. For example, the aver

4 age energy required to produce an electron ion pair in gas ionization chambers is about 30 eV whereas the average energy needed to produce an electron-hole pair in semiconductor detectors is 3 eV, an order of magnitude lower. Since the energy resolution of the detector is inversely proportional to the number of information carriers generated, semiconductor detectors have superior energy resolution.

For 7-ray detection semiconductor (germanium) detectors are preferred over NaI(Tl) detectors because of their superior resolution. Germanium, however, must be operated at low temperature (typically 77K) and is more expensive, so it is used only when its advantages are needed.

1.2 7-ray Spectroscopy with Semiconductor De tectors 1.2.1 Physical properties of semiconductors

Semiconductors and insulators have the characteristic property that the highest filled energy band of the electrons bound to the atom is separated from the band of the electrons not bound to the atom by an energy known as the band gap, Eg (Fig

1.1). In semiconductors, the size of the band gap is small enough (about 1 eV) such that a few electrons are excited to the conduction band by thermal energy at room temperature. In insulators, the bandgap is large (about 6 eV) so that at normal temperatures the electrons are all in the valence band since thermal energy is not sufficient to excite the electrons to the conduction band.

At any non zero temperature, it is possible for an electron in the valence band to be excited to the conduction band by thermal energy. The excited electron leaves a vacancy (called a hole) in the valence band. The combination of the two

5 (a) (b)

Empty conduction band

Conduction band

Ej6eV E1eV

Valence Band

Filled Valence Band

Figure 1.1: Band structure of a) insulators, b) semiconductors is called an electron-hole pair. When an electric field is applied across the crystal, the electron and the hole drift in an opposite direction. Thus, the motion of both the electrons and the holes contribute to the conductivity of the material.

The charge carrier concentration (the concentration of electrons or holes) at a given absolute temperature T is given by:

n = AT3/2exp( (1.2) Where A is the proportionality constant2 of the material, k is Boltzman’s constant and Eg is the bandgap energy. From )the above equation, it can be seen that the con centration of charge carriers is a strong function of temperature for a given material.

Thus if the material is cooled to low temperature, the charge carrier concentration decreases drastically.

6 Under the action of an external electric field, both the electrons and the holes undergo a net migration parallel to the direction of the applied field. For low and moderate electric field, the drift velocities of the electrons and the holes are propor tional to the applied electric field, the proportionality constants being the mobilities of the electrons and the holes respectively. At higher electric field values, the drift velocity increases more slowly with the field until a saturation value is reached. Many semiconductor detectors are operated with electric field values such that to result in a saturated drift velocity which is of the order of 1O m/s.

Effects of impurities

In a pure semiconductor crystal, the allowed energy levels are present only in the valence or conduction bands. Such a material is called an intrinsic semiconductor.

In practice, however, it is not possible to achieve perfect semiconductor crystals free of impurities, and the existence of these impurities perturbs the energy band structure by adding additional levels in the forbidden energy gap as shown in Fig 1.2. If the energy levels created by the impurities are localized near the conduction or the valence band, they are called shallow impurities; whereas deep impurities produce deep lying levels near the center. Deep lying levels are introduced by impurities belonging to transition metals. Such an impurity could capture an electron from the conduction band and then it may capture a hole while still holding the electron, allowing them to annihilate. This process is called recombination. Hence such impurity sites are effectively re combination centers. The existence of these recombination centers could affect the performance of the radiation detector since a charge loss could occur as a result of recombination which results in the degradation of the resolution.

7 Valence band

Donor levels — —

Deep lying impurities /

Acceptor levels

Conduction band

Figure 1.2: Energy levels produced in semiconductors by impurity atoms

On the other hand, some deep impurities are capable of capturing either only electrons or only holes. Such centers hold the electrons or holes and release them after a certain characteristic time. If this time is on the order of the charge collec tion time, charges will be lost and incomplete charge collection will result.

Besides impurities, structural defects of the crystal, namely point defects and dislocations could also act as trapping and recombination centers. Structural de fects could arise when growing the crystal or due to radiation damage.

Impurities by elements with 3 and 5 valence electrons produce shallow levels near the conduction or the valence band respectively. The effect of impurities with three valence electrons such as boron, aluminum, gallium or indium is the intro duction of free holes within the crystalline structure. These are called acceptor impurities, since the holes can accept electrons. Similarly, impurities with five va

8 lence electrons, such as phosphorus, arsenic and antimony introduce free electrons. These are called donor impurities, since they donate electrons. Materials in which the acceptor impurities predominate are called p-type materials, those with primar ily donor impurities are known as n-type materials.

When donor and acceptor impurities are both introduced in a crystal lattice, the material is called compensated. The excess electron from the donor atoms no longer find an absence of empty states, since states are available at the acceptor levels. These will be filled preferentially by the electrons from the donor levels, as a result, the electrical effects of the impurities present is neutralized. Thus the material retains its intrinsic property.

Thin layers of semiconductors that have an unusually high concentration of impurity are often given a special notation. Thus n and p designate heavily doped n-and p-type layers that, as a result have very high conductivity. These layers are often used in making electrical contact with semiconductor devices.

1.2.2 Detection of 7-rays using germanium detectors

When a photon interacts in the crystal, it produces high energy electron(s) by one of the processes discussed. These high energy electrons deposit their kinetic energy in the crystal by creating electron-hole pairs. If the secondary electrons are suffi ciently energetic, they create additional electron-hole pairs. The over-all effect of this process is the production of many electron-hole pairs along the track of each high energy electron created. These electron-hole pairs are then free to be collected at the electrodes which are in electrical contact with the electrodes that generate a strong electric field along the crystal of the order of 1000 V/cm.

9 In practice, however, the presence of acceptor or donor impurities makes the operation of 7-ray detectors more complex than discussed above. When an electric field is applied across a semiconductor crystal with donor or acceptor impurity, an electric current based on the electrons or holes from the impurity results. The sta tistical fluctuations in this current results in a noise which masks the pulse resulting from photon interaction.

One way of reducing this steady-state current which results from the impurities is to reduce the impurity concentration by drifting lithium ions into the germanium crystal. Lithium, being an interstitial donor compensates the acceptor impurities and the resulting material has an electrical property similar to the intrinsic mate rial. Germanium detectors made by this process are called Ge(Li) detectors. The structure of a planar Ge(Li) detector is given in Fig. 1.3. The excess lithium on the upper surface resulted in a highly doped n+ layer which served as an electrical contact, and a thin uncompensated layer remained on the opposite side. A major drawback of Ge(Li) detectors is that the detector must be kept cold at all times in order to prevent further migration of the lithium ions which will ruin the donor- acceptor compensation obtained.

If the impurity concentration in the germanium crystal could be reduced to about 1010 atoms/cm the intrinsic region can be achieved by creating a diode structure. This3 structure is obtained by doping one surface of an ultrapure p type germanium, with lithium, creating an n+ layer on one side. Hence, the bulk of the crystal consists of a “p” region as well as a thin heavily doped + layer. When a reverse bias is applied to this ii+p region, electrons and holes are pulled out of an intermediate region called the depletion layer, and current cannot flow across the junction except for some leakage current. The thickness of the depletion

10 + Uncompensated n layer p—type/ Ge

7

Figure 1.3: A diagram of a planar Ge(Li) Detector layer is related to to the applied voltage and the impurity concentration in the material. A detector made in this way is called an intrinsic or high purity germanium

(HPGe) detector and is illustrated in Fig. 1.4. One major advantage of high purity germanium detectors is that they can be stored at room temperature and cooled when they are in operation. Because of thermal noise, however, germanium detectors must be operated at 77 K.

1.2.3 Major features of germanium detectors as 7-ray spec trometers

The most important parameters characterizing a 7-ray spectrometer are the resolu tion, linearity, efficiency and timing characteristic and these properties are discussed below:

Energy resolution

The major factor which makes germanium detectors the best 7-ray spectrometers is their excellent energy resolution (typically about 1.8 keV Full Width at Half Max at 1.33 MeV 7-ray energy as compared to about 90 keV for NaI(Tl) at the same

11 n P contact /contact

7

Figure 1.4: A diagram of an HPGe detector energy.)

The total energy resolution of a germanium detector is determined by the following factors:

1. The inherent statistical fluctuation of the charge carriers.

2. The efficiency of the charge collection process.

3. The electronic noise from the spectrometer system.

The intrinsic energy resolution due to statistical fluctuations of the charge collection process (wi) is given by:

w = 2.35v’ (1.3)

Where e is the average energy for electron-hole creation, F is the Fano factor and E is the 7-ray energy.

The contribution to the energy resolution due to the charge collection effi

12 ciency (wv) is caused by trapping and recombination which result in the loss of information carriers.

The noise from the electronic system also contributes to the energy resolution

(we). This contribution mainly depends on the leakage current and the capacitance of the detector.

The total energy resolution is given by the combination of the above three.

(1.4)

Linearity

Another major advantage of germanium detectors is their linearity over a wide en ergy range. Due to the non linearities in the electronics, however, a slight deviation from linearity can be observed during high precision measurements over a wide energy range.

Efficency

Compared to NaI(Tl) detectors, germanium detectors have a small efficiency since they are much smaller in size than NaI(Tl) detectors. The efficiency of germanium detectors is usually measured relative to 7.6 cm x 7.6 cm NaI(Tl) detector at 25 cm from the source. Typical germanium detectors have efficiencies ranging from 10 to

50 % although 100 % detectors have been made recently by Ortec.

Timing characteristics

The timing resolution of germanium detectors is much worse than that of scintil lation detectors. The pulse risetime of germanium detectors is of the order of 100

13 nsec and is restricted by the time required for the charge carrier collection. In ad dition, the detailed pulse shape of the rise from germanium detectors can vary from one event to another depending on the site of the creation of the charge carriers. Hence, output pulses show variation in their leading edge. With special electronic processing the time of arrival of a 7-ray can be determined with a resolution of 8 to 10 ns, but a NaI(Tl) can achieve 1 to 2 ns.

Neutron interaction in Ge detectors

When germanium detectors are used to measure 7-ray spectra from reactions where fast neutrons are produce as outgoing particles, peaks resulting from inelastic neu tron excitation of the nuclei of the various isotopes of germanium appear in the measured 7-ray spectra. These peaks have a peculiar triangular shape of about 40 keV across because the recoiling Ge nucleus deposits its energy inside the detector, which adds to the energy of the 7-ray deposited. The presence of these peaks have first been recognised by Chasman et al. [24]. In addition, a further investigation of these peaks was done by Bunting and Kraushaar [21].

In addition to peaks arising from neutron inelastic excitation thermal neutron capture 7-rays could also be observed if thermal neutrons are produced as back ground. The thermal neutrons capture 7-rays are not broadened since the recoiling nucleus has an energy of the order of a few hundred electron volts.

Since the purpose of this work is to investigate these various peaks, a detailed discussion will be given in Chapter 4.

14 Radiation damage

As a semiconductor detector, germanium detectors are sensitive to radiation dam age. The transfer of energy from the incident radiation to the crystal lattice can cause lattice defects by knocking the atoms from their normal positions. The deep lying energy levels resulting from these lattice defects serve as recombination centers and hole traps. Hence, in a radiation damaged detector, some charge will be lost before collection due to trapping and recombination and this results in a subsequent degradation of energy resolution and low energy tailing.

Fast neutrons as well as heavy charged particles such as ce-particles could produce a significant radiation damage. On the other hand, 7-rays and electrons produce very little radiation damage since they mainly interact with the atomic electrons. A neutron fluence of about i0 2n/cm could bring a significant deteri oration of resolution, and a neutron fluence of about 1010 2n/cm could make the detector totally unusable [26].

It is found that HPGe coaxial detectors fabricated from high-purity n-type germanium crystals where electrons are the carrier type are much more resistant to performance (about 30 times) degradation when compared with HPGe co-axials made from high purity p-type germanium where holes are the carrier type [17]. This is because of the fact that the damaged sites preferentially trap the holes instead of the electrons. The n-type crystals also have the advantage of a thinner dead-layer from the electrical contact and so can be used for lower energy photons (down to a few keV where as the p-type do not work below 40 keV). Unfortunately, n-type crystals are more expensive.

15 1.3 Exotic Atoms

When a negatively charged meson is slowed down in matter and eventually stopped, it can replace an electron in an atom forming a mesonic atom. The process involved from the time the particle enters matter until the mesonic atom is formed can be explained as follows:

1.In the first stage, the free meson loses energy by collisions and it is slowed down to a few keV in about 10_il to i0 s. 2. Now the meson has the same velocity as electrons in the atom. It interacts with electrons and is eventually bound to a particular atom in a highly excited state.

During this time, its energy is reduced from 2 keV to 0 in lO_15to iO’ s. 3. The meson then cascades down in series of “hydrogen like” bound states. Tran sition accompanied by Auger electrons initially and later by x-rays in iO’ s.

The slowing down and capture of slow pions and muons is essentially the same process for both. This is due to the fact that the slowing down and capture process depends almost entirely upon the charge and mass of the meson. (The charges are the same and the masses are only about 30 percent different). However, the absorption processes of the muon and the pion are quite different and are discussed below.

1.3.1 Muonic atoms and nuclear muon capture

Once a muonic atom is formed, the muon cascades down to lower lying orbitals through Auger transition and x-ray emission. After the muon reaches the iS state, the weak interaction comes into play, and it can either decay as a free muon would, or it can interact with one of the protons in the nucleus resulting in the capture of

16 the ,u by the nucleus:

+A —* +A — 1)* r (Z) VIL (Z (1.5)

The elementary process for the above reaction is:

(1.6)

The nucleus is normally left in an excited state, and de-excites by emitting 7- rays, neutrons and sometimes protons or heavier particles. In heavy nuclei, however, because of the coulomb barrier it is almost always the neutrons that are emitted. The neutron energy spectra following r capture on nuclei have been mea sured by several authors [12-13]. In Fig 1.5 is given the neutron energy spectra from muon capture on 0, Si, Ca and Pb [12]. Unfortunately the peak at a few MeV has not been measured accurately.

This spectrum consists of: 1, Low energy neutrons (about 10 MeV or less) from de-excitation of the the nucleus formed after the capture i.e:

(1.7)

2. Intermediate Energy Neutrons (between about 10 to 25 MeV) which result from the elementary process itself. i.e. muon absorption on a single proton;

(1.8)

3. High energy neutrons (greater than 25 MeV) which result from r absorption on correlated nucleon pairs. This is often hypothesized as resulting from a muon transforming into a virtual pion via inverse pion decay, then the ir interacts with a quasi-deuteron to form a pair of neutrons.

17 _____

101

.0 12 0 Si •Co I> opb

-4- -0- . ,3 —4--4- w ±

-5 10

106 I 0 20 40 60 80 —+ E( Fv1V)

Figure 1.5: Neutron energy spectrum from t on O,Si,Ca Pb From Ref. [12]

18 1.3.2 Pionic atoms and pion absorption by the nucleus

Similar to muonic atoms, the pion cascades down from higher excited states to lower ones by emission of Auger electrons and x-rays. However, since the pion is a spinless particle, the fine structure observed in muonic x-ray spectra is not present in the pionic x-ray spectra. In addition, unlike muonic atoms where the weak muon- nuclear interaction is quite negligible, the pion is quickly absorbed by the strong pion-nuclear interaction which produces an important effect on the intensity of the x-rays and the position and width of pionic atom energy levels. The strong inter action causes:

1. The x-rays below the absorbing level to be missing.

2. The energy of the last x-ray to be broadened due to the short lifetime of the level.

3, The energy of the x-ray transitions to be shifted.

Pion absorption by the nucleus occurs when the pion is captured by corre lated np or pp pairs in the nucleus. Absorption of a ic on a single nucleon is highly unlikely because to conserve energy and momentum, the Fermi momentum of the nucleon must be about 500 MeV/c, but such a high momentum cannot be obtained from the Fermi motion of a single nucleon in the nucleus.

The absorption thus occurs on correlated np or pp pairs: 1. r 2p —* np. 2. iC np —* nn.

(but absorption on the np pair is the most probable). Hence, the pion absorption by the nucleus results in one or two fast neutrons depending on the above reactions. In addition, neutrons from the de-excitation of the resulting nucleus are also produced.

19 A neutron energy spectrum from pion absorption on Pb is given in Fig 1.6 [14]. For’ very light nuclei, the direct neutrons from the absorption process are more visible as a shoulder at higher energies. In Fig 1.6 the extrapolation to zero energy (in the

inset) is purely fictional; in reality the curve has to turn over somewhere, and go to zero probability at zero neutron energy. Similar to r capture the portion of the peak in the probability function has not been measured.

IoI —

00 —

I I •. 0 I 2 3 . EIMeVI Ic

0 - LEAD . .

1.0 10 100 200 E 1MeV)

Figure 1.6: Neutron energy spectrum from ir absorption on Pb from Ref [14].

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(1.9)

well be Chapter 2 Experimental Methods

The aim of the experiments performed in this work is to study the background 7-rays resulting from neutron interactions inside the Ge crystal as well as its surroundings using neutrons from a Cf252 source and neutrons resulting from r absorption, and then to compare them with the peaks in the muon capture experiment.

The Cf252 run was performed in the M9B area during the shutdown period in September 1993 and its experimental setup is described in Section 2.4. The pion run was performed in March 1994 during the polarized beam period of the M13 Channel at the TRIUMF cyclotron using a Bi209 target. Another Cf252 was also performed during this run. The M13 Channel and the experimental geometry are discussed in section 2.2. The electronics used in these experiments is given in section 2.3. The experimental set up for the Cf252 run performed in September 1993 is discussed in section 2.4.

2.1 The TRIUMF Cyclotron

The TRIUMF facility (Fig. 2.1) is a variable-energy isochronous cyclotron which accelerates H ions to produce protons of peak energy of 520 MeV. The H ions are stripped of their two electrons by passing the ion beam through a thin

22

PROPOSED

EXISTING

FACILITIES

EXPERIMENTAL AND

LINES BEAM

SOURCE ION

I [1

LY

M15(ii) POLARIZED NH

SERVICE 3 SOURCE

ION

HALL MESON

THERMAL

RATHO SOURCE HION II

EXTENSION

\\ AHE*X

M9(fl/p) M1301/u)

ITY

HALL

cILL

BLIB(P) BL2A(P)

L4(P) CYCLOTRON VAULT

P%UCTION

$-i

b

ERIDGE

—SERVICE

FACILITY

ING HANOI

REMOTE

______

______foil of carbon There are two stripper foils which are 180 degrees apart and each of them can be adjusted to provide its own beam of protons with independent energies up to 520 MeV, and currents up to 140 tA in 5 nsec bunches with a separation time of 43 nsec, corresponding to the 23 MHz cyclotron frequency. The first proton beam is used for proton-induced reactions while the other one is directed to a pion production target (usually carbon or beryllium).

2.2 The Experimental Setup

A schematic of the experimental setup for the pion run is shown in Fig. 2.2 Si,

52, S3 and S4 are thin plastic scintillators used to define the pion stop signal. The dimensions of these scintillators are 15 cm x 20cm x 0.31 cm, 11.3 cm x 0.31 cm, 10 cm x 10 cm x 0.31 cm, 19.4 cm x 13.3 cm x 0.31 cm respectively. In order to slow down the pions coming from the channel , a 2 mm thick Al degrader was inserted between Si and S2, and a 2.5 cm thick Al degrader was inserted between S2 and

S3. The optimum degrader thickness was determined by adjusting the thickness of the Al degrader so that the number of pions stopped in the target was maximized.

A 10 %HPGe with a resolution of 2 keV full width at half maximum (FWHM) at 1.33 MeV was used to observe the 7-rays. The Ge detector was surrounded by a

NaT annulus to detect 7-rays Compton scattered from the Ge detector.

The Ge detector was placed at 50 cm from the target in order to have a time of flight separation between the neutrons and the 7-rays reaching the detector.

Typical flight times over this distance for neutrons of 1 to 100 MeV are about 60 and 7 ns respectively compared with 1.7 ns for photons. The time resolution of the

Ge-detector is about 7 ns FWHM so that the neutrons can effectively be separated

24 S=Scintillctor T=Target D=Oegroder

Nol

7V

Si Dl S2 D2 S3 S4

Figure 2.2: Experimental set up for the irBi209 experiment in the time spectrum.

2.3 Electronics

The essential features of the electronics and data acquisition system are illustrated in Figs. 2.3-2.6. The electronics is composed of the beam telescope, the Compton suppression and the germanium detector. Part of the electronics resided in the experimental area and the other part in the counting room.

2.3.1 Stopping telescope

Signals from the telescope scintillators Si, S2, S3 and S4 were taken from the exper imental area to the counting room through 50 ohm cables. The signals were then fed to a leading edge discriminator.The threshold of the discriminator corresponding to

25 D.B. = Delay Box D=Leading Edge Discriminator

To TDC

(384 us)

Figure 2.3: A block diagram for the pion telescope

Si was set just above the electronic noise so that every incoming charged particle that is incident on Si could be detected. S2 was adjusted to trigger mainly on pions by adjusting the threshold of its discriminator in such a way that it ignored particles with a lower energy loss, like electrons and muons. The time of a pion stop was defined by S3, which also mainly triggered on pions. A veto signal from S4 completed the telescope logic Si.S2.S3. defining a pion stop. This signal was then fed to a CAMAC TDC after being delayed by 384 nsecs. The time interval between the stopping of the pion in the target and the detection of the 7-rays in the detector is then determined by looking at the spectrum from this TDC slot

2.3.2 Compton suppression

For each of the 6 PMTs to the NaT annulus, the signals were fed to an amplifier and the amplifier’s output was fed to CFDs whose threshold was set at about 150 keV, just at about the difference between the Compton edge and the full energy peak.

The output of the CFD was then fed to the CAMAC TDC after 80 nsec delay.

26 ______

To TDC Stop NaI(TI)

(80 ns)

Figure 2.4: A block diagram for the electronics of the Compton suppressor

PA. = Preamplifier SA. = Spectroscopic Amplifier T.F.A.= Timing filter amplifier CFD = Constant fraction discriminator

To AD:

Ge

—E::::EzL (To Strobe)

Figure 2.5: A block diagram of the Ge electronics

2.3.3 Ge electronics

The two signals from the pre-amplifier of the germanium detector were sent to a spectroscopic amplifier and a timing filter amplifier to provide energy and timing signals respectively. The output of the spectroscopic amplifier was then fed to a spectroscopic ADC. The ADC was gated by a 12 is signal which was generated by the strobe. The germanium energy electronics were in the experimental area to avoid a distortion of the pulse amplitude due to electronic noise.

The timing signal from the TFA output was sent to the counting room where it was fed to a Constant Fraction Discriminator (CFD). The output of the CFD was then used to generate the strobe.

27 G.G = Gate generator

To Spect. ADC

S)

[1 msec)

INHIBFF

Computer Busy

Figure 2.6: A logic diagram for the strobe signal

2.3.4 Strobe

The strobe signal was generated by the anti-coincidence of the “Ge.t” signal and the INHIBIT signal. (The INHIBIT signal stops any further event processing until the data acquisition from the previous event is completed). The INHIBIT signal was generated by the computer busy signal, the strobe and a 1 msec. protection gate between the strobe and the generation of the computer busy signal. The strobe was then routed to:

1. Produce a delayed gate which defined the Spectroscopic ADC’s digitizing window.

2. The INHIBIT.

3. A gate generator to generate a protection gate for the INHIBIT. 4. Start TDCs in the CAMAC crates.

5. The Starburst to start the data acquisition system after going through an LRS

688 level adapter.

28 2.3.5 Timing logic

There are two main timing signals of interest incorporated into the electronics cir cuitry: the NaT time and the time between the pion stop signal and the Ge signal.

The Nal time logic

The NaT timing signals described previously in the NaT Compton suppression logic section were input to the CAMAC TDC after 80 ns delay. During the data anal ysis, a software cut of the Compton scattered events was made by selecting events corresponding to both the Ge and the NaT(Tl) firing at the same time.

Time of pion logic

The S1.S2.S3. signal was sent to a TDC after being delayed by 384 nsecs. The time interval between the stopping of the pion in the target and the detection of the 7-ray in the detector is then determined by looking at the spectrum from this TDC slot. 2.4 Experimental Set Up for the First Cf252 Run

The tests with the californium source were run when the cyclotron was not running. The physical setup is shown schematically in Fig. 2.7. A 0.5 mCi (1.01 g) Cf252 source with a yield of about 2.3 x 106 neutrons per sec. was contained in a bunker made of concrete and wax. The bunker had a concrete collimator which was 27 cm long and approximately 1.6 cm in diameter. The source was about 3 cm away from this collimator and the Ge detector was placed at about 50 cm away from the collimator.

Several runs were taken by putting Pb, polyethylene, concrete and wax in front

29 Ge detector 127 C Concrete 2Cf bunker 1.6 cm

3 cm

Figure 2.7: Experimental set up for the Cf252 experiment

of the collimator to change the relative flux of neutrons and 7-rays coming through the collimator. This was done to discriminate events induced by neutrons against

7-rays coming from the source. In one run the Ge detector was housed in the NaT annulus. This was done to see what effect this would have on the background lines.

2.5 Data Acquisition System

The data acquisition system used in this experiment consisted of the VDACS data acquisition program running on a DEC VAX computer, the Starburst (a PDP-11 computer module in the main CAMAC crate), and the CAMAC modules contained in crate 1.

The Starburst reads the data from the CAMAC modules in the crates and sends to VDACS. The VDACS then transfers the data to the VAX where it can be written to VCR tapes or read by the online program running (Display). A user defined TWOTRAN program which is compiled on the VAX and executed in the Starburst instructed the Starburst on reading events from CAMAC modules and processing the data.

30 The overall control of the acquisition was performed by the VDACS program. The user could interactively start, pause and stop acquisition; load and unload VCR tapes.

31 Chapter 3 Methods of Data Analysis

3.1 Introduction

The data analysis of a 7-ray spectrum from a Ge detector is generally carried out by the determination of the channel positions, x of the observed peaks followed by determination of the energy vs channel relation, E(x). In this work, similar steps have been taken to analyze the data obtained from the various runs. These steps could be outlined as follows.

1. Determination of the energy and intensity of the 7-rays obtained from the various spectra.

2. Identification of each 7-ray peak with a specific nuclear de-excitation and deter mination of the interaction responsible for the excitation.

3. Analysis of the peak shapes resulting from neutron interactions inside the Ge detector from the r and Cf252 runs.

The data obtained from the different runs were analyzed using the TRIUMF Vax cluster of computers. The sorting analysis program Display was used to read the data from the VCR tapes and sort the data into the respective energy and time spectra. The energy spectra corresponding to different time windows were then analyzed and fitted using the program Displot. PLOTDATA was also used for

32 fitting and producing the final spectra.

3.1.1 Peak fitting

With the exception of the neutron induced peaks inside the Ge crystal, a majority of the y-ray peaks were quite symmetric and they could be fitted by a gaussian function (Equation 3.1) whose height, width and position are allowed to vary,

(xx )2 F(x) = Ae 22 + Bx + C (3.1)

The peak is represented by the first expression which is a gaussian with centroid at .x0 The Full Width at Half Max of the peak (w) is determined from u using the relation: w = 2vo- = 2.35cr (3.2)

The area under the gaussian peak is determined from Area = 42Aw (3.3) The spectral background is represented by a straight line which is not allowed to vary. This line is generally chosen to have a zero slope although occasionally a non-zero slope is used for peaks below 300 keV. In addition, it was necessary to use double gaussian fits for closely spaced peaks.

3.1.2 Energy calibration

The energy calibration for the spectra from the ir and Cf252 runs performed in March 1994 was done using the strong background peaks from ‘52Eu and 60Co. These came from induced radioactivity in the experimental area and cannot be avoided. Although the appearance of these peaks in the spectra was undesirable, they proved to be very useful for calibration since they are well known lines which

[19], [20] which cover a wide range of energy (Table 3.1). Moreover, it was possible

33 Table 3.1: Calibration energies used [19], [20] Radionuclide Energy (keV) EU152 344.277(2) 778.903(2) 964.043(7) 1112.063(7) 1408.000(7)

60Co 1173.238(4) 1332.502(5)

to calibrate the spectrum under experimental conditions because of these peaks, hence avoiding the possible variations of the spectrometer system which could arise from conditions changing with time. For the spectrum from the j run, the calibra tion was done using muonic x-rays and the well known 41Ar line from air activation. The 511 keV annihilation line was not used since it has natural width of 1.8 keV due to the Doppler effect arising from the electron motion at the annihilation point

[22]

Calibration relation For most practical purposes, it is assumed that the relationship between the - ray energy and the channel position is linear. Due to nonlinearities in the detection system, however, this relationship is, in general, not linear. A special large deviation from linearity was observed at low (below 300 keV) and at high energy (above 2

MeV) . To account for the nonlinearity at the low energy side for the Cf252 run, the calibration was done using the relation given by equation 3.1 as suggested by Dryak

[21],

34 E(x)= a(l/x)+b+cx-{-dx (3.4)

Where E is the energy, x is the peak position a,b,c and d are constants de rived by least square fit of the well known2 calibration peak energies to the respective channels.

A -y-ray spectrum of a radiothorium source was analyzed to test whether the deviation of the detection system from linearity could adequately be compensated by using equation 3.4 as calibration equation instead. Fig 3.1 and 3.2 show the deviations of 7-ray energies obtained from calibrations using a linear calibration equation and equation 3.4 respectively. In figures 3.1 and 3.2, the data points that are marked by an asterisk are points which are used in calibrating the spectrum whereas the square data points are those 7-ray energies which are not used for calibration. The error bars represent only the statistical uncertainties. Comparing fig. 3.1 and 3.2, a deviation of as much as 1.2 keV was observed for the linear calibration, whereas the maximum deviation observed in fig 3.2 is about 0.13 keV. For the ir and for the 1tr runs, a quadratic calibration was sufficient since the energy region of interest was 400 to 2000 keV.

3.1.3 Time of flight discrimination

A time spectrum representing the time between the 7t stop and a Ge signal is shown in Fig. 3.3. The relation between the channel position and time is 0.244 ns/channel. The time resolution of the Ge detector is about 7 ns FWHM. The time spectrum consists of the following events:

1. A “prompt” peak which results from the pionic x-rays and capture 7-rays occur ring almost immediately after the pion stop.

2. A tail-like continuum from neutron induced 7-rays, which take few nanoseconds

35 — ——————————— I I 0.6- I

0.4-

—‘> ci) 0.2-

Li

0 Li 0 0.0-

—0.2 - 2

—0.4 - I I 0 500 1000 1500 2000 2500 3000 Energy (keV)

Figure 3.1: Deviation of the linear calibration energy from the literature energy 0,15

0,09

0.03

L

—0.09

—0.15 1000 1500 2000 Energy (keV)

Figure 3.2: Deviation of the energy from eq. 3.4 from the literature energy

36 ______

3000- I—

Prompt 2400- N Neutrons 1800-

° 1200-

Background Delayed 600-

-

U I I 0 100 200 300 400 500 Time (ns)

Figure 3,3: Time spectrum of the ir on Bi to reach the Ge-detector and its surroundings after being produced from r ab sorption in the target.

3. A wavy continuum from delayed 7-rays, “slow” neutron and background events.

The waviness is caused by the R.F. structure of the proton beam

4. The continuum to the left of the prompt peak which is due to random background events not related to the pion absorption.

3.1.4 Background reduction

As described in chapter 2, one of the factors that determine the accuracy of a fit to a 7-ray peak is the signal to noise ratio. In addition to distorting the peaks the background could obscure peaks which are relatively weak. A reduction of the continuum resulting from Compton interactions in the Ge crystal was done by rejecting the events from the NaT annulus surrounding the Ge detector which

37 C D 0 C.)

2400 2500 2600 2700 2800 2900 3000 Energy (key)

Figure 3.4: Compton suppressed and unsuppressed spectra from the Cf252 run are in coincidence with the signals from the Ge crystal. As shown in fig. 3.4a, it was possible to reduce the Compton background by a factor of 3 practically without affecting the full energy events. It is also evident from fig. 3.4b that some peaks which were not observed in the unsuppressed spectrum appeared clearly in the suppressed spectrum. This technique also suppresses single and double escape peaks which are very prominent for high energy 7-rays (but also act as a useful energy calibration). Another useful aspect of Compton shields is that background lines that are part of a cascade also get suppressed.

38 Chapter 4 Results and Discussion

4.1 Introduction

As discussed in Chapter 1, the main purpose of this work is to determine the neutron induced events in the Ge detector as well as the surroundings. The Cf252 run was performed for the purpose of accomplishing this aim. However, there were many shortcomings associated with the data from this run:

1. The 7-rays from the fission fragment isotopes produced a background.

2. Most of the neutrons from the source were thermalized by the concrete shield.

3. There were not sufficient statistics to observe the relatively weaker transitions in the Ge isotopes, especially the 1204 and the 1216 keV peaks.

The pion run was performed in the hope of obtaining a spectrum which was mainly due to neutron interactions in the Ge detector and its surroundings by us ing the time of flight discrimination technique. Spectra of the prompt, delayed and neutron events from the ir absorption on Bi209 target, as well as the background 7-rays associated with many different origins in the area, have been studied exten sively and are given in section 4.2. The neutron induced peaks from the excited states of the various Ge isotopes and the surrounding materials are also studied in

39 this section. Since one of the major goals of this work is to examine closely the 1200-1230 keV region, some detailed study of this region has also been given.

Most of the 7-rays from (n,-y) reactions in the Ge crystal and the surrounding materials have been observed from the September 1993 Cf252 run. These 7-rays have been identified and explained in section 4.3. Because the 7-ray spectrum from the Cf252 source is quite complex, a full analysis of the 7-ray spectrum and the identification of the peaks is given in Chapter 5.

A comparison between the peak shapes for the neutron induced Ge lines from the Cf,252 ir and the jr runs was made in order to determine whether there is a dependence of peak shape on the neutron energy spectrum. This investigation will be discussed in Section 4.4.

4.2 7-Rays from ir Absorption on Bi209

Consistent with the explanation given in Section 1.3, the pionic cascade via emis sion of x-rays is followed by a prompt absorption by the nucleus, resulting in the emission of neutrons, charged particles, and nuclear 7-rays. The total 7-ray energy spectrum as well as the energy spectra corresponding to different windows in the time spectrum have been analyzed separately and are given in the following sections.

4.2.1 The total 7-ray spectrum.

This spectrum was obtained by imposing no condition on the timing. Many peaks associated with the ir absorption as well as neutron induced 7-rays have been observed in this spectrum. Since the 7-rays resulting from ir absorption in the target and neutron interaction will be discussed shortly, only the background peaks

40 keV

£LUV

60 1650

L0 C 1100 0 Eu mEU Na.rEu / I /K Jj3EJJJ__ z4 I 820 1020 1220 1420 1620 keV

Figure 4.1: 7-ray spectrum from the background run will be discussed here. Many of the lines which have been identified as background have also been observed in the spectrum obtained from background runs which were performed before and after the ir runs. A portion of the spectrum from the background run is given in Fig. 4.1, and the total 7-ray spectrum from r on Bi209 is given in Fig. 4.2. A complete list of the observed 7-rays in the prompt, neutron, delayed and general spectrum is given in Table 4.1. The 7-ray energies determined from a fit are given to two decimal places with the errors indicating the statistical uncertainties only. For the peaks which were too small to be fitted, the energies are given to one decimal place only and the corresponding errors are estimated to be of the order of 0.5 keV. In the last column of this table is indicated in which spectrum the particular 7-ray 1S observed.

41 The strong lines at 778.9, 867.4, 964.1, 1085.8, 1112.1 and 1408.0 indicated in the general spectrum as well as some other peaks given in Table 4.1 have been identified with the radioisotope 52Eu, In addition, the weak peaks at 723.4, 756.8, 996.4 and 1596.58 keV have been‘ assigned radioisotope These isotopes to the 54Eu. produced from the (n,7) concrete blocks ‘ M13 are reaction in the surrounding the area. The aggregate from which these concrete blocks are made contains a small amount of europium which has an isotopic composition of 47.9% ‘51Eu and 52.1% 53Eu. Since the area is near the target where the pions are produced, these blocks ‘ exposed to a large of are flux thermal neutrons. Due to the large neutron capture cross-section of 151 5800 barns) is produced by the (n,7) reaction. Be Eu ( ,‘52Eu cause of the relatively small cross-section of Eu153 (380 barns), a smaller amount of ‘54Eu is produced (7% of 52Eu). ‘52Eu 112t=13.2 years) /3-decays to 52Gd* /9+decays (t and also to 52Sm*; ‘ ‘54Eu (t112=8.55 years) 9-decays to 54Gd*, lThese excited isotopes de-excitel by 7-ray emission. l

The very strong lines at 1173 and 1333 keV belong to 60Co. The production mechanism for these isotopes is probably via spallation reactions on copper in the magnetic coils. In addition, the positron annihilation line at 511 keV; the K,40 the 41Ar and the H‘H(n,’y) lines at 1460, 1293 and 2223 keV respectively have been observed. The2lines at 583.14 and 2614.4 keV belong to T1208 which is a daughter isotope of Th.228 These lines originate from a Th228 source which was contained and shielded in the area.

7-rays from ir interactions in the Al degrader have also been observed in this spectrum. These are the 472, 1274 keV peaks which are produced from 27Al(7r 24Na;,p2n) the peak at 1368 keV is from 27A1(ir 24,3n)Mg; and the peaks at 984 keV and 1633.5 keV are from 2720A1(7r,3p4n) and 2720NeA1(ir,2p5n) respec F 42 500

Figure 4.2: 7-ray spectrum from the ir on Bi209 with no timing cut tively. All these peaks were not observed in the prompt and neutron spectra.

The peaks at 843, 1014 and 2211 keV belong to the first, second and third excited states of 27A1 respectively. These peaks result from the excitation of the Al nuclei by the ir in the aluminum degrader, and they are also prominent in the neutron spectrum. The peaks appearing in the neutron spectrum, however, are due to (n,n’) reactions. It is interesting to compare the widths of the 2211 and 2223 keV peaks (Fig. 4.2). The peak at 2211 keV is Doppler broadened due to its short lifetime (28 fs), but the hydrogen capture line is not broadened.

The strong peak at 1808 keV is from Mg.A1(ir,irp) This peak was also observed in the prompt spectrum. The reason2726 why it was observed in the prompt

43 spectrum is from the pions which escape the degrader and fire the ir stop signal. The peak at 1778 keV, is a result of 28Si(n,n’) interaction in the concrete blocks. 4.2.2 The prompt spectrum Most of the 7-rays in the prompt spectrum have been identified with Pb200207 iso topes. These isotopes are produced from the reaction )Pb209Bi(ir,xn)( for x from 2 to 9. The primary reaction proceeds on correlated np pairs forming 2o7Pb* which de-excites by emission of a 7-ray or a few neutrons. Proton and charged par ticle emission are strongly inhibited because of the large Coulomb barrier associated with heavy nuclei.

The 7r-atomic transitions (5g —* 4f), (6g —* 4f), (7g —* 4f) have been observed in the prompt spectrum. The strong peak at 589 keV which belongs to the (5g—*4f) transition is notable because of its linewidth, due to the pion’s very short lifetime in the 4f level as a consequence of the r absorption by the strong interaction.

The strong line at 718.5 keV and the peak at the 1021.8 keV are identified with B10 which is produced from C(ir,2n)’°B in the styrofoam which was used to support the target. ‘2 The peaks at 896.4, 992.1 and 1608.6 keV results from Bi*Bi(n,n) re action in the target. Although these 7-rays result from neutron29O interaction, they appear in the prompt spectrum because the reaction occurs in the target itself.

44 640-

(I) 480- z 8320-

160

Figure 4.3: Prompt 7-rays from r absorption on Bi209 (low energy)

180

0 120

keV

160

, 120 0 ° 80

1900 2000 2100 2200 2300 2400 keV

Figure 4.4: Prompt 7-rays from r absorption on Bi209 (high energy)

45 Table 4.1: A list of 7-rays from Bi209 run. N= Neutron Spectrumir P= Prompt Spectrum D= Delayed Spectrum G= Spectrum with no timing requirement A= Observed in all spectra

Peak Identification Lit. value Transition Observed 411.4 52Eu ,152Gd* 411.126(3) 755,4 —* 344.3 G 412.7 ‘ G 127j 418.10(5) 417.95(10) 418.0 —* 0 N,D,G 422.3 Pb202 422.13 1382.8 — 960.7 N,D 439.99(1) 23Na 439.991(10) 439.9 —* 0 N,D,G 443.99(2) Eu152 152Sm* 443.965(4) 1529.9 —* 1085.8 G 458.1 N,D 462.4 Pb200 462.34(13) 1488.4 —+ 1026.2 N,D,G 472.3 24Na 472.207(9) 472.2 — 0 G 477.64(2) 7Li 477.605(3) 477.6 — 0 G 506.30(4) N,P,G 511.00(1) annh. 511.0034(14) A 516.3 Pb206 516.18 2200.2 —* 1684.1 G 541.2 G 562.5 76Ge 562.93 562.9 —+ 0 N 564.1 Eu152 152Sm* 563.983(4) 684.8 —+ 121.7 G 569.64(6) Pb207 569.65 569.6 —* 0 N,G 583.1 T1208 ÷2O8pb* 583.191(2) 3197.7 —+ 2614.6 D,G 589.87(2) irBi 589.89(.06) 5g —* 4f A 593.08(6) 127j 593.3(2) 650.9 — 57.6 N 596.0 T4Ge 595.8 595.8 —* 0 N 600.2 D,G 604.77(3) G 609.5 G

46 Table 4.1 (continued)

Peak Identification Lit, value Transition Observed 127j 618.31(6) 618.5(2) 618.5 —* 0 N,D,G 628.69(3) 127J, Pb201 628.6(2),628.2 628.6 — 0 A 628.2 —* 0 636.4 G 657.6 1,2720 658.90(11),656.0 716.5 —* 57.6 N,P,G F 656.0 —* 0 666.39(5) ‘ G 669.9 P 683.77(9) P,D,G 688.8 52Eu Z152Sm* 688.674(6) 810.5 — 121.8 G,D 692.6 72Ge‘ 691.3 691.3 —* 0 G,D 703.4 Pb205 703.4 703.4 —* 0 P 718.26(5) ‘°B 718.29(9) 718.3 —* 0 N,P,G 723.4 Eu154 _154Gd* 723.356(22) 1719.6 —* 996.3 G 127j 744.70(4) 744.70(1) 744.7 —* 0 N,G 756.6 Eu154 _*154Gd* 756.808(22) 1127.8 — 371.0 G 760.01(7) P 778.929(8) 52Eu 152Sm* 778.920(4) 1123.2 —* 344.3 G 785.6(1) ‘ P 786.9 Pb202 786.95 2169.8 —* 1623.1 G 794.8 N 795.74(3) P,G 803.1 Pb206 803.1 803.1 —* 0 P 810.3 G 825.2 Pb203 (6.1 s) 825.21 825.2 —* 0 G

47 Table 4.1 (continued)

Peak Identification Lit. value Transition Observed 834.93(1) D,G 836.3 N 838.62(6) A 843.76(3) 27A1 843.76(3) 843.7 —* 0 A 846.77(3) 56Fe 846.754(20) 846.8 —* 0 A 866.87(13) P 867.43(2) 52Eu 152Sm* 867.390(6) 1233.8 —÷ 366.5 A 873.45(5) 54Eu‘ .+154Gd* 873.230(18) 996.3 —* 123.0 G 881.03(8) ‘Pb206 881.0 1684.1 —* 803.1 P,G 888.8 A 896.39(6) Bi,209203 896.2(1),896.85 896.2 —> 0 A Pb —* 896.85 0 899.1 Pb204 899.15(10) 899.2 —* 0 A 904.88(8) irBi 904.82(6) 6g —*4f A 911.6 Pb204 (65.9 m) 911.7 2185.5 — 1273.9 G 912.65(11) A 914.6 D,N 916.6 D,N 917.2 P 960.75(5) Pb202 960.67 960.7 —+ 0 P 964.09(1) 52Eu 152Sm* 964.055(4) 1085.8 —* 121.8 A 977.70(9) ‘ P,N,G 984.21(12) Pb204 F20 984.0,983.8 2257.8 —÷ 1273.9 P,G , 983.8 —÷ 0

48 Table 4.1 (continued)

Peak Identification Lit. value Transition Observed 987.5 Pb205 987.7 987.7 —* 0 P,G 992.5 Bi209 992.6(1) 2601.2 —* 1608.6 P 996.38(6) 54Eu _154Gd* 996.329(18) 996.3 —* 0 G 1005.03(3) 52EU‘ 152Sm* 1005.06(12) 1371.6 —* 366.45 G 1014.42(2) ‘27A1 1014.45(3) 1014.4 —* 0 A 1021.8 1021.78(14) 1740.2 —* 718.3 P 1026.47(4) Pb200 1026.2 1026.2 —ì 0 A 127J 1044.38(15) 1044.2(2) 1044.0 —÷ 0 N 1040 70Ge 1040.6(5) 1040.6 —* 0 N 1063.55(12) Pb207 1063.64 1063.6 — 0 A 1085.87(1) Eu152 Z152Sm* 1085.80(8) 1085.8 —* 0 G,D,N 1089.79(5) 52Eu _152Gd* 1089.767(14) 1434.2 —* 344.3 G 127j 1094.61(14) ‘ 1094.40(12) 1094.4 —* 0 N 1108 76Ge 1108.41(8) 1108.4 —* 0 N 1112.08(1) 52Eu 152Sm* 1112.087(6) 1233.8 —* 121.8 D,G 1120.1 ‘ G 1147.32(11) D 1173.254(3) 60Co 6ONj* 1173.238(4) 2505.7 —* 1332.5 G,D,N 1208.0 N 1212.97(5) Eu152 Z152Sm* 1212.89(9) 1579.5 —* 366.45 G 127j 1218.0 1218.4(2) 1218.4 —* 0 N 127J 1228.5 1228.9(2) 1228.9 —* 0 N 1238.7 56Fe 1238.255(26) 2085.1 —* 846.8 N,G 1257.3 G

49 Table 4.1 (continued)

Peak Identification Lit. value Transition Observed 1267.3 G 1274.54(1) 22Na 1274.542(7) 1274.5 —* 0 D,N 54Eu 1274.54(3) 1397.5 —* 123,1 1293.46(4) 41Ar‘ 41K* 1293.609(8) 1293.6 —* 0 G 1299.15(4) Eu152 1s2Gd* 1299.152(9) 1643.4 —* 344.3 G 1332.521(4) 60Co _*60Ni* 1332.501(5) 1332.5 —* 0 D,N,G 1368.60(5) 24Mg 1368.675(6) 1368.7 —* 0 N,G 27J 1401.7 1401.6(2) 1401.6 —* 0 N 1408.038(8) Eu152 152Sm* 1408.011(14) 1529.9 — 121.8 G,N,D 127J 1413 1413.4(2) 1413.4 —* 0 N 152Sm* 1457.6 52Eu 1457.619(15) 1579.5 —+ 121.8 G 1460.82(1) K40‘ 40Ar* 1460.859(5) 1460.9 —+ 0 G,D 1528.5 Eu152 152Sm* 1528.106(19) 1650.2 —* 121,8 G 1547.3 Bi?209 1546.7(1) 2442.8 — 896.2 P 1596.61(6) 54Eu _154Gd* 1596.582(20) 1719.6 —* 123.1 G 1608.66(10) 209‘Bi(n,n’) 1608.6(1) 1608.6 —* 0 P,G 1633.5 20Ne 1633.8 1633.8 —* 0 G 1658.54(15) G 1764.45(6) Pb205 1764.30(10) 1764.3 —÷ 0 G 1778.84(5) Si28 1779.030(11) 1779.0 —* 0 N,G 1808.74(12) 26Mg 1808.70(6) 1808.7 — 0 P,N 2211.0 27A1 2211.1(6) 2211.1 —* 0 P,N,G 2223.08(23) H(n,-y) 2223.247(17) 2223.2 — 0 G 2614.42(7) T1208 2O8pb* 2614.533(13) 2614.5 —* 0 G 2753.7 24Mg 2754.030(14) 4122.8 —* 1368.6 G

50 4.2.3 The neutron spectrum

The neutron spectrum mainly consists of 7-rays produced by (n,n’) reactions on 70727476Ge, 127J, 27Al and 56Fe, The various excited states of these isotopes observed in the neutron spectrum are discussed below. ’ 70Ge(n,n’) Referring to Fig. 4.6, the triangular peak at 1040 keV belongs to the first excited state of 70Ge, In the complicated structure existing in the 1200-1240 keV region, the edge below the 1218 keV 127J peak is identified with the second excited state of 70Ge (1216 keV). It is this peak which initiated this enquiry.

72Ge(n,n’) The first excited state of 72Ge (693.6 keV) has not been observed in the neutron spectrum. However, this peak is very prominent in the delayed spectrum (Fig 4.7). and is marked by its triangular shape. This peak is a result of the production of the 0+ first excited state of 0e72 by (n,n’) reaction, and it can only de-excite by emission of conversion electrons. The peak appears at the full transition energy because the energies of the conversion electrons and subsequent x-rays are summed in the detector and both are detected with 100 percent efficiency. This peak is very dominant in the “delayed” spectrum and it is not seen in the neutron spectrum due to the long lifetime (0.42 us) of this state.

The triangular peak which lies underneath the cluster of 7-ray peaks in the 830-850 keV region belongs to the second excited state of T2Ge (833.95 keV).

74Ge(n,n’) The prominent triangular peak observed at 596 keV (Fig. 4.5) belongs to the first

51 excited state of 74Ge . The second excited state (1204 keV) could also be seen at the edge of the 1200-1240 keV region. The peak at 608 keV which belongs to the

(1204 —* 596 keV) transition could not be observed distinctly in the neutron spec trum since it is hidden beneath the tail of the 596 keV peak. However, this peak is observed from 73Ge(n, reaction in the Cf252 run (Fig. 4.8). It is also evident that the triangular)7 596 keV peak is wider than the peak at 691 keV because it is a combination of the 596 and 608 keV triangles (Sect. 4.4).

76Ge(n,n’) The peak at 563 keV, which belongs to the first excited state of 76Ge has been ob served clearly. The bump at about 1108 keV (Fig. 4.6) could be due to the second excited state of 76Ge (1108.4 keV).

All the excited states of the Ge isotopes observed from (n,n’) reaction have been summarized in table 4.2.

27Al(n,n’) The strong peaks at 843 and 1014 keV as well as the peak at 2211 keV, which have also been observed in the prompt spectrum (section 4.2.2), appear in the neutron spectrum as well. These peaks are due to neutron interaction in the aluminum cans surrounding the Nal annulus as well as in the aluminum degrader.

56Fe(n,n’) The strong peak at 846.8 keV belongs to the first excited state of 56Fe. The rela tively weak peak at 1238 keV also corresponds to the transition of 56Fe from the second to the first excited state.

52 Table 4.2: Excited states of Ge isotopes observed Isotope Energy (keV) Lit. value Reference

70Ge 1040 1040.6(5) [37] 1216 1216

72Ge 691 691.3 [30] 835 834.4

74Ge 596 595.8 [36] 608 608.4 1204 1204.3

76Ge 563 562.93 [18] 1108 1108.4

271(n,n’) There ‘were many strong peaks observed which are identified with the excited states of 127J which came from the NaT surrounding the detector. There are strong peaks occurring at 593, 618, 628, 658, 744 keV. In addition, weaker peaks at 1094,

1218,1228 1401 and 1413 keV were observed.

4.2.4 The delayed spectrum

As shown in Fig 4.7, most of the peaks due to (n,n’) reactions which were observed in the neutron spectrum appear in this spectrum too. This is because of the interaction of slow and delayed neutrons. The dominant peak at 692 keV from 72Ge(n,n’) has been discussed in Section 4.2.3. Many of the strong background lines which were observed in the general spectrum have also been observed in the delayed spectrum, but not all of them, because the general spectrum has better statistics, so some lines are hard to identify in the delayed spectrum.

53 0,

D 0 (-)

0, C D 0 C)

Figure 4.5: The neutron induced spectrum from the ir on Bi209 80

60

C,) c D 00

20

0 1000 1050 1100 1150 1200 1250 1O0 Energy (keV)

Figure 4.6: The neutron induced spectrum from the ?t on Bi209

54 800 511 / Ce(n,n) i, 600

500 600 700 800 900 1000 keV

8:o I 600 I I

:::

1000 1100 1200 1300 1400 1500 keV

Figure 4.7: The delayed 7-ray spectrum from the ir on Bi209 4.3 Neutron Induced 7-rays from the Cf252 Run The data from the Cf252 has essentially two major parts. The first one is the - ray emission from de-excitation of the fission fragment isotopes, and the second is the 7-ray spectrum from (n,n’) and (n,7) reactions. The 7-rays from the fission fragments will be discussed in in detail in Chapter 5.

In this section, only the 7-rays from (n,n’) and (n,’y) reactions observed in the Cf252 7-ray spectrum will be discussed. 4.3.1 Lines from the Ge(n,n’7) and Ge(n,7) reactions

One special feature of the data from the Cf252 source is the observation of excited Ge isotopes resulting from (n,n’) and (n,-y) reactions. It was possible to make the distinction between the two reactions which result in the excited states of the same

55 isotope (for example 73Ge(n,7) and 74Ge(n,n’)) because the 7-rays produced fol lowing inelastic neutron excitation are noticeably broadened on the high energy side, producing a peculiar triangular shaped peak, whereas the thermal neutron capture 7-rays appear to be oniy slightly broadened since the recoiling nucleus has an energy of less than a keV. Most of the excited states of 707274Ge isotopes from (n,n’) reactions that were observed from the r run were also’ observed in the Cf252 run. Because of the relatively high capture cross-section of T3Ge (Table 4.3), many lines from 73Ge(n,7) reactions were also observed even though it is one of the less common isotopes. The peaks from the (n,n’) reaction were enhanced in the runs where the Pb shield was used, while peaks originating from the (n,7) reaction were much stronger in the runs where polyethylene and wax were used as shields of the collimator hole (Chapter 2). A portion of the 7-ray spectra from the Cf252 source when 5cm lead and 4 cm polyethylene were used to shield the collimator is given in Figs. 4.8a and 4.8b. As could be seen from fig 4.8a the triangular peaks at 596 and 691 keV are evident whereas in Fig. 4.8b they are much weaker.

Ge(n,n’)

The peaks at 596, 691 and 1040 keV which belong to the first excited state of 727074Ge were observed. In addition, the peak at 835 keV (second excited state of 72Ge)’ was observed.

Ge(n,-y) The thermal capture 7-rays from 7073T4Ge are listed in Table 4.4. The strongest peaks observed were seen from capture’ on 73Ge. This is because of the relatively high capture cross-section of 73Ge. The narrow peaks at 493, 595 608, 868 and 1204 keV are due to the 773Ge(n, reaction. The peaks at 174.9, 198.5, 326.5, 499.8 and ) 56 Energy (key)

Figure 4.8: A portion of the 7-ray spectrum from the Cf252

Table 43: Abundance and capture cross-section of Ge isotopes

Isotope Abundance ( %) cr (barns) Ac’-C (relative intensity) T0Ge 20.5 3.25 66.63 72Ge 27.4 1.0 27.4 T3Ge 7.8 15 117 74Ge 36.5 0.52 19.0 76Ge 7.8 0.16 1.23

57 Table 4.4: 7-rays from Ge(n,7)reactions Reaction Energy (keV) Lit. value Reference 70Ge(n,7) 174.9 174.88(5) [31] 198.5 198.35(7) 326.5 326.0(2) 499.8 499.85(6) 708.5 708.16(8) 831.1 831.3(1) 1094.8 1095.8(3) 1298.8 1298.8(3) 73Ge(n,7) 492.7 492.936(6) [36] 595,7 595.847(6) 608.2 608.353(5) 868.2 867.898(6) 961.7 961.055(10) 1101.2 1101.267(12) 1204.0 1204.208(12) 740e(n,-y) 139.7 139.2(1.0) [32] 574.9 574.7(10)

708.5 due to 70Ge(n,’y) reactions and the peak at 139.7 keV due to 74Ge(n,’y) have been reported in the work by Bunting and Kraushaar [23].

58 4.3.2 The 7-rays from (n,n’) and (n,7) reactions on sur rounding materials

The major components in the experimental area for the first Cf252 run were the con crete (the shield and the collimator) and the detector system. Concrete is mainly made of water, 2Si0 and 3CaCO The detector system has several components made of Al and Fe; while the Ge. crystal has an indium layer beneath it to which the high voltage is applied. Many 7-rays which were attributed to (n,n’) and (n,-y) reactions on H,Al, Si, Fe, In and I were observed. The strong peak at 2223.3 keV is due to the ‘H(n,’y) reaction. It has been seen in many other experiments where there are ther mal neutrons . The lines at 471.6 keV and 1368.4 keV arise from 2724Na.A1(n,a7) The strong peaks at 843 keV and 1014.3 keV due to (n,n’) reactions on 27A1 have also been observed in the Cf252 spectrum. The peak at 1778.8 keV arises from the reactions 2728A1A1(n, .+28Si and 28Si(n,n’). The lines at 1273.3 keV and 2093.5 keV are from)7 the 28Si(n,7) reaction. The peaks at 1942.6, 2001.2 and 2010 keV result from (n,7) reactions on 40Ca. There is little doubt about the identification of these lines as the intensity ratios agree with reference [39]. The peaks at 847 and 1238 are from (n,n’) reactions on 56Fe, In addition the 1613 and 1726 lines arising from 56Fe(n, ) reaction have been observed.

The most intense lines from 151n(n, ) observed in this experiment are listed in table 4.5. In the work of Schaller‘ et al., [41] some lines from 1151n(n, 7)reaction have been reported as background lines resulting from thermal neutron capture in In.

59 Table 4.5: 7-rays from (n,n’) and (n,7) reactions from the Cf252 run Isotope Energy (keV) Lit. value Reaction Reference ‘H 2223.3 2223.247(17) H‘H(n,7) 2 [28] Al27 472.2 472.207(9) A1(n,a7) 843.9 843.76(3) 2724NaAl(n,n’) 1014.5 1014.45(3) 27A1(n,n’) 1368.3 1368.633(6) A1(n,a) —+ 24Mg 1778.5 1779.030(11) Al272824Al(n,’y)Na —* Si28 Sj28 1273.3 1273.398(11) 28Si(n, [28] 1778.5 1779.030(11) 287Si(n,n’) 2092.7 2093.027(12) 28)Si(n,-y)

40Ca 1942.3 1942.61(11) 40Ca(n,-y) [28] 2001.1 2001.49(31) 40Ca(n,-y) 2009.5 2009.8(2) 40Ca(n,7) Fe56 847.2 846.764(6) 56Fe(n,n’) [18] 1612.5 1612.70(10) 56Fe(n,-y) 1724.6 1725.05(10) 56Fe(n,7) “51n 272.8 272.9 1n(n,7) [39] 417.1 417.2 1n(n,7)5 819.2 819.3 1n(n,7)5“ 1097.2 1097.0 5“1n(n,7) 1293.3 1293.4 115“1n(n,7)5 1507.3 1507.5 1fl(fl,7)“ 2111.9 2112.1 “5151n(n,7) J127 133.4 133.6 ‘271(n,-y) [39] 202.5 202.94(10) 271(n,n’7) [35] 417.7 417.95(10) 271(n,n’7)‘ [35] 442.9 442.9 271(n,7) [39] 618.0 618.5(2) ‘ [35] 271(n,n’7) 628.5 628.6(2) ‘ [35] 271(n,n’y)‘ 745 744.70(10) ‘271(n,n’7)

60 The various peaks resulting from 1271(n, y) and 1271(n, n’) were observed only in the run where the NaT suppressor was in place, a further confirmation that these lines come from this reaction. The peaks from 271(n, n’) have also appeared more clearly in the spectrum from the ir run and it ‘was possible to identify the weaker and higher lying transitions from this spectrum (Sec. 4.2.3).

61 4.4 Comparison of Neutron Triangles from ,u, ir, and Cf252 Runs

In this section, the 596 keV and the 691 keV peak shapes from the first excited states of 74Ge*Ge(n,n) and 72Ge*Ge(n,n) have been studied. A comparison of these peak shapes from the 28Si(r,nv), Cf252 and 209Bi(ir,xn) spectra has been made. This comparison is based on the different neutron energy spectra arising from these reactions. The average neutron energy emitted from Cf252 is about 2 MeV. The muon capture reaction produces neutrons ranging in energy from about

1 MeV to 10 MeV. The neutrons from the pion absorption could have an energy as high as 60 MeV or more, but also peak at a few MeV.

The purpose of doing this comparison is to determine whether there is any dependence of the peak shape on the neutron energy. This was needed for the

1216 keV region for which knowledge of the shape of the neutron background is of particular importance. Unfortunately, however, this comparison could not be done from the existing data for the 1216 keV region, due to lack of sufficient statistics.

This comparison was made by fitting the peaks to the following function.

[(x [(x Y(x) = ERFC x1)]EXP x1)J+E [_(x — X)2]+F O2 i=1 a1 (4.1) In the above equation, the first expression corresponds to a complementary error function which basically determines the edge of the Ge(n,n’) peak. The second expression determines the tail of the peak which was assumed to be exponential. The expression inside the summation corresponds to a Gaussian function which corresponds to any symmetric peak which might be sitting in the region of inter

62 est. There were up to three peaks sitting on the tail which are related to different sources. The last expression corresponds to a background which was assumed to be flat for that region.

A least squares fit was done by allowing ,a1 a, ,u2 ,x1 x0 to vary in the above expression. The parameter which was compared is a2 since it determines the characteristic of the decaying exponential tail.

The various fits are given in Figs. 4.9-4.15. The values of 2cr for the 596 and 691 peaks are given in Table 4.6. The errors given in Table 4.6 are the statistical errors only obtained from the fit. Although statistical errors obtained from the fit are less than 7 % , systematic errors arising from the lack of knowledge of the pre cise shape of the background, as well as due to other possible peaks existing in the region of the fit, contribute to the uncertainty in the results obtained. This can be seen in Fig. 4.9 where a fit was done using different regions of the same peak and a difference of 20 % in the value of a2 was observed. In addition, it was observed that including weak peaks which exist in the region of the fit could give results which are different by more than 10 % (Fig 4.13).

A discussion of the results from the two peaks is given below.

691 keV

The values of a2 found for the Cf252 and the 7rBi209 are very similar (15.2 and 15.4 keV respectively) whereas the values for Si28 are different by about 15 % For the Si28 study, there was a 7-ray peak at 691 keV which was superimposed on the 691 keV triangle. In addition there were two more peaks at 718 and 731 keV.

q63 On the other hand, the Bi209 and Cf252 spectra are cleaner (Figs 4.12-4.15). The differences observed could mainly be due to the systematic errors discussed. Hence, we conclude that no measurable difference has been observed.

In the work of Skoro et al., [42] they fitted the 691.3 keV peak from the following neutron sources to a similar function to ours.

1. Environmental neutrons using a 10 cm lead shield.

2. Environmental neutrons using 20 cm lead shield. 3. Neutrons from a Cf252 source.

The values of cr2 they found are 20.8, 12.0 and 20.4 keV respectively. They explained the significantly different result in the second case to be caused by the different neutron energy spectra resulting from the different thickness of the shielding used. However, these differences could probably be caused by the poor statistics and poor fit to the background as well as due to the contribution of small 7-ray peaks which were not included in the fit.

596 keV As given in Table 4.6, the values of 2cr are higher than for the 691 keV. This is because this peak is composed of the 596 keV and the 608 keV peaks which are both from 72Ge(n,n’) reactions. Due to background lines, it was not possible to fit the 596 keV triangle from the Cf.252

64 Table 46: Peak shapes of the 596 and 691 keV lines. Note: Errors include statistical errors from the fit only. Neutron Source (596 keV) (691 keV) u2(keV) u2(keV) Cf252 * 15.2(1.0) 209Bi(7r Pb209,xn) 19.2(1.1) 15.4(L0) 28Si(.c 28A1,xnv) (Gel) 23.0(1.0) 18.8(0.5) 28A1Si(,xnv) (Ge2) 22.9(1.0) 17.5(0.4) * Not fitted due to background and poor statistics.

65 300 1(nn) 2 = 21.0 keV 27 250

200

150

100

50 590 600 610 620 630 64•0 Energy (keV)

300 = 17.4 keV 250 o

200

150

100

50

590 600 610 620 630 640 650 660 Energy (key) Figure 4.9: 74Ge(n,n’) line from the 7rBi run 10000 I I I I I

1271 (n , n’) 1(n,n) 9000 - 127

1

C/) 8000 - C :3 0 0 7000 -

6000 -

5000-— I I 590 600 610 620 630 640 650 660 keV Figure 4.10: 74Ge(n,n’) line from the 1cSi run (Gel) 66 14000 - I I (n,n’) 13000 - 127

12000 -

11000- - C 10000 -

9000 -

8000 -

7000-—- I I 590 600 610 620 630 640 650 660 keV Figure 4.11: 74Ge(n,n’) line from the 1cSi run (Ge2) 160

140

120

100 Cl) C 80 00 60 40 20

0 0

Figure 4.12: 72Ge(n,n’) line from the irBi run 67 I I I I I 500 I I I I (C) 450 450

400 400

350 350

300 300

250 250 -

200 zUut 6 0 695 700 705 710 715 720 7: 5 680 690 700 710 720 730 740 R Energy (key) Energy (key)

500 500 (d 450 14.33 keV 450 400 400 keV. 350 350• Içl 300 300 Ji 250 250 52 200 — ZJ1 I I I I 690 695 700 705 710 715 720 725 660 690 700 710 720 730 740 750 Energy (key) Energy (key) Figure 4.13: 72Ge(n,n’) line from the Cf252 run 14000 .

12000 -

_10000 -

08000 B10

J

6000 -

4000-— I I I 680 690 700 710 720 730 740 750 keV Figure 4.14: 72Ge(n,n’) line from the 1iSi run (Gel) 68 16000 I I I

14000 -

120OO -

010000 B10

8000 -.

6000-— 680 690 700 710 720 730 740 750 keV Figure 4.15: 72Ge(n,n’) line from the 1rSi run (Ge2)

69 Chapter 5 7-rays from Cf252

5.1 Introduction

Californium-252 is a fissioning radionuclide which has many applications in nuclear, radiological and medical sciences and technology. It is made by leaving U238 in a reactor for a long time where the U238 nucleus undergoes 14 consecutive (n,7) reac tions and some /3-decay.

5.1.1 Nuclear fission

Nuclear fission results mainly from the competition between nuclear and Coulomb forces in heavy nuclei. Fission can occur spontaneously as a natural decay process or it can be induced by adding energy to the nucleus either through the absorption of a relatively low-energy particle such as a neutron or a photon or from a high energy charged particle such as a proton or a pion. This produces excited states or compound nuclear states that are high enough in energy to overcome or more easily penetrate the barrier. Nuclear fission is a very complex process. Although certain aspects can be explained by various theoretical models, there is, at present, no comprehensive theory that explains all aspects.

70 The sequence of the process of fission and the time scales involved could be outlined as follows:

Instantaneous neutron emission occurs during the fission process (scission neu trons) and very shortly after it (less than 10—13 secs). Neutron emission is followed by emission of 7-rays, conversion electrons and x-rays in from 10_14 secs to iO secs.

Some of the fragment isotopes could de-excite by ,6-decay. This process is much slower compared to neutron and 7-ray emission with the half lives being between about 10_i secs to 102 sec. Delayed neutron emission proceeds with half- lives ranging from about 10_i secs to 102 secs. Delayed neutrons are those emitted promptly from the nuclides after a delayed /3 decay. The resulting nucleus is in an excited state above the neutron binding energy and the observed neutron activity follows the half-life of its /3-decay precursor.

Mass distribution

The fission of a nucleus into two fragments does not occur uniquely, and the fragment mass distribution covers a wide range. The mass distribution of fission of heavy nuclei have been studied extensively using radiochemical and mass spectrometric techniques. Spontaneous fission and fission that is induced by low-energy particles are less likely to split into equal masses, while fissions induced by high energy particles are more likely to split into equal-mass fragments.

Energy distribution

The total energy release in fission is very large, about 200 MeV, and is simply equivalent to the difference in mass between the fissioning nucleus and the sum of the fragment masses. The energy is manifested in the kinetic and excitation energies

71 of the fragments. The excitation energy is dissipated through neutron and ‘-y-ray emission and relates directly to the shape of the fragments at scission.

Neutron and 7-ray emission

Fission is accompanied by instantaneous neutron emission, and these neutrons are called prompt neutrons. The average number of prompt neutrons emitted per fis sion is called v, and is characteristic of the particular fission process. For Cf,252 v=3.9. In addition to prompt neutrons, there are delayed neutrons emitted after

,8-decay of the fission products. The number of delayed neutrons emitted is about

1 per 100 fission.

A relatively large average number of 7-rays are emitted per fission, and these - rays are not isotropically distributed about the emitting fragment. This anisotropic distribution of the 7-rays signifies the presence of fragment angular momenta which correlates with Coulomb induced rotation of the distorted fragments. Measurements of the transition energies and lifetimes verify the existence of high fragment spins and identify the spins with a particular fragment isotope. Hence, a study of 7-rays from fission gives information about states which normally are not populated in radioactive decay or nuclear reactions. A limitation is that the 7-ray spectrum is very complex because it consists of several 7-rays emitted from a great number of fission fragment isotopes.

5.2 Transitions of the fission fragment isotopes observed from Cf252

The total 7-ray spectrum from a Cf252 source is very complex because of several fission fragments having too close 7-ray energies to be resolved by the detector.

72 This complexity has at times made it difficult to make an accurate determination of the energy of the 7-rays and hence to make a proper identification.

A list of the 7-rays observed in this work is given in table 5.1. The 7-rays constitute:

1. Background.

2. Neutron induced.

3. Direct fission fragments and beta delayed products.

Since the background and the neutron induced peaks have been discussed in the previous chapter, only the 7-rays from the de-excitation of the fission fragments will be considered here. The fission fragment 7-rays have two origins.

1. Prompt 7-rays resulting from the de-excitation of the fission fragments.

2. 7-rays from ,6-decay of fission fragment isotopes. In the present experiment these cannot be distinguished, but there have been experiments in which a coincidence with a fission fragment is demanded. It is thus possible to make the identification if needed. In some cases, a particular 7-ray may come from both effects.

Most of the fission 7-rays have been identified with the 2 —÷ 0 transition 4+ of the even A isotopes. In some cases it was possible to observe up to 6+ * transitions. A list of these isotopes and the respective 7-rays is given in Table 5.2.

A help in the identification of these isotopes comes from the work of Chieftz et. al

[48], [47] and also other works [49], [46], [44].

The /3-delayed 7-rays have been identified with the help of the information

73 obtained from the fission fragment isotopic yield [43], and from [50]. A majority of the -de1ayed 7-rays are identified with odd A isotopes and are given in Table 5.3.

74 Table 5.1: A list of 7-rays observed from the Cf252 spectrum. BG = Background, N Neutron induced, F = Fission fragment 7-ray, Peak Identification Lit, value Comments 75.69(2) Nd,152 56Sm 76.0,75.9 F 77.9 ‘ 110.1 121.98(1) Eu152 121.7824(4) BG 123.3 Eu154 123.070(4) BG 126.1 133.8 Pr44 133.544(5) F 138.2 8‘°‘Tc 138.4 F 140.3 99Tc, 4‘°Zr 140.50,140.3 F 145.5 Pr141 145.4441(14) F 150.4 2‘°Zr, p85 151.9, 151.18 F 158.98(3) Ce148 158.7 F 162.6 Nd154 162.4 F 171.9 106Mo, 171.7 F 174.9 74Ge(n,7) 174.89(5) N 181.1 46Ba 181.0 F 185.3 65Cu(n,7)‘ ? 185.91 N 190.2 Ba141 190.2 F 192.4 ‘°Mo 192.3 F 197.47(4) 4 Xe136 197,33 F 199.22(5) Ba144 199.3 F 211.71(2) ‘°°Zr, 34Te 212.7,210.8 F 218.0 Pr46 ‘ 218.3 F 228.6 132j‘ 228.6(6) F 231.8 42La 231.52 F 240.83(4) ‘10Ru 240.8 F 242.29(4) 8‘°‘Ru 242.3 F 244.81(3) Eu152 244.6989(10) BG 249.78(2) 35Cs 249.79 F 255.3 ‘42La 255.12 F

75 Table 5.1 (continued)

Peak Identification Lit. Value Comments 258.56(2) 38Cs146Ce,’ 258.7,258.3 F 263.0 268.4 269.87(3) 6‘°Ru 270.3 F 275.6 41La 277.0 F 283.0 ‘ 293.2 Pr143 293.26 F 295.98(1) Ce148Mo, 296.0,295.7 F 302.7 7‘°Pd 302.8 F 304.1 2 304.2 F 14La 306.74(2) Pd?‘°‘Tc,’° 306.86, 306.1 F 312.0 133j 312.1 F 314.3 5Nd147 314.7 F 316.59(3) 5‘°Rh , 6‘‘Pr 316.4, 316.8 F 319.1 Pd105 319.2 F 321.7 324.6 326.59(3) Zr102 326.6 F 330.77(4) 44Ba 331.0 F 332.80(3) ‘Pd, Ba146 332.9, 332.7 F 336.0 “4 340.0 Pd,116 51Pm 340.4, 340.07 F 343,4 1‘‘La ‘ 343.7 F 344.20(1) 52Eu 344.2811(19) BG 348.56(3) 12Pd‘ 348.8 F 350.5 6‘°Mo‘ 350.8 F 351.7 Zr,100 Sr95 352.6, 352.2 F 357.78(4) 4‘°Ru 357.8 F 359.37(5) Ba,142 271(n,n’)? 359.7,360.3 F,N 364.1 Xe131 364.480(20) F 368.3 ‘ 369.5 Mo104 368.8 F 373.9 Pd110 1271(n,n’)? 373.8(5), 374.9 F,N

76 Table 5.1 (continued)

Peak Identification Lit. value Comments 376.3 40Xe 376.8 F 381.2 ‘36Xe 381.5 F 387.99(2) ‘ 397.17(2) 44Ce 397.3(2) F 400.19(4) ‘Mo 400.0 F 2133j‘° 407.4 407.9 F 409.6 Ce,146 152Eu?? 409.7, 411.126(3) F,BG 417.71(3) 271(n,n’7) 417.95(10) N 422.6 ‘°”°Ru 423.0, 423.1 F 426.4 8‘ 431.2 Ba144 431.7 F 433.9 8Cs, Te134 434,5, 434.8 F 439.78(3) 23Na(n,n’) 439.9 N 443.8 52Eu 443.965(4) BG 446.7 Mo??‘ 447?? F 453.6 2‘°Te,131 46Nd 452.4, 453.8 F 457,3 40Xe ‘ 457.8 F 462.6 ‘Ba138 462.785 F 465.7 469.0 5‘°Ru 469.2 F 472.2 27A1(n, 472.207(9) N 474.9 72Baa) 475.0 F 477.5 ‘ 477.605(3) N 481.6 483.3 38Xe 483.7 F 486.97(4) ‘Cd, 40Ce 487.8, 487.018(9) F 496.93(2) 3‘°Rh“8 ‘ 497.080(13) F 499.8 74Ge(n,-y) 499.85(6) N 510.96(3) annihilation 511.0 BG 519.49(5) 527.6 Ba140 528.26 F 530.0 Xe133 529.872 F 535.1 36Te 535.8 F 537.3?? La142‘ 537.27 F 540.97(4) Ce144 541.1 F 546.4 “°Pd, Ba138 546.3, 546.9 F 571.1 ‘42Cs 571.66(2) F

77 Table 5.1 (continued)

peak identification Lit. value Comments 574.9 74Ge(n, 575.0(8) N 582.7 7 584.9 )44Ce 585.0 F 588.76(4) ‘Xe138 588.9 F 596.3 74Ge(n,n’-y) 596.3 N 602.06(4) Ba140 602.2 F 608.5 Ge(n,n’-y) 608.4 N 74 , 618.2 1271(n,n”-y) Cd112 618.5(2), 617.4 N 625.5 628.5 271(n,n’-y) 628.6(2) N 631.1 42Ba 632 F 641.19(4) Ce142‘ 641.17 F 647.9 ‘ 658.2 1(n,n’), 9TMo 658.90(11), 657.92 N,F 661.532(8) 37Cs27 661.660(3) F 667.6 ‘Xe132 667.73(5) F 676.6 676.4 F 692.9 72Ge(n,n’7) 691.3 N 696.9 Te132 696.8 F 706.0 708.5 T4Ge(n,7) 708.2(1) N 724.04(3) ‘°Rh, 54Eu 724.199(5), 723.356(22) F,BG 729.6 5 ‘ 738.9 743.2 271(n,n’)?, 97Nb 744.70(10), 743.36 N,F 756.5 95Nb 756.729(12) F 765.6 95Mo 765.807(6) F 772.6 132‘ 772.68(5) F Xe 778.83(2) 52Eu 778.920(4) BG 798.6 ‘ 815.2 96Sr, Ce140 815.5, 815.766(4) F 831.4 Ge(n,n’),’ 831.2(1), 830.8 N 835.03(6) 72Ge(n,n’7)38T4Xe 834.4 N 844.43(20) Al(n,n’ Ce144 843.76(3), 844.9 N 27 134J 846.95(4) ),56Fe(n,n’7), 846.764(6), 847.02 N,F 867.47(5) 774Ge(n,n’7)??, ‘52Eu 867.8, 867.390(6) N,BG

78 Table 5.1 (continued)

peak identification Lit. value Comments 873.4 54Eu 873.230(18) BG 875.7 ‘38Xe 875.3 F 884.16(6) ‘134j 884.09 F 894.9 42La 894.9, 894.85 F 912.6 ‘ 919.0 Zr94 918.8 F 925.0 40Ce 925.2 F 948.6 ‘42La 948.8 F 964.06(2) ‘52Eu 964.055 BG 974.32(6) ‘Te132 973.9 F 996.5 Eu154 996.329(18) BG 1004.90(10) 54Eu 1004.775(21) BG 1009.84(10) ‘38Ba 1009.7 F 1014.45(6) 27A1(n,n”y)‘ 1014.5(3) N 1040.4 70Ge(n,n’7) 1039.6 N 1085.85(3) 52Eu 1085.842(4) BG 1089.8 ‘52Eu 1089.767(14) BG 1094.8 74Ge(n,7)‘ 1095.7(2) N 1112.03(2) 52Eu 1112.087(6) BG 1139.3 74‘Ge(n,-y) 1139.3(2) N 1173.262(8) 60Co 1173.238 BG 1180.4 1213.06(10) Eu152 1212.970(13) BG 1220.0 1222.8 1261.2 Xe135 1260.4 F 1274.56(2) 22EuNa,152 1274.542(7), 1274.54(3) BG 1279.25(8) 34Te 1279.1 F 1294.1 ‘ 1298.8 Ge(n,7)?? 1298.3(2) N 1299.3 74 52Eu 1299.152(9) BG 1313.04(5) Xe136‘ 1313.0 F 1332.530(8) 60Co 1332.501 BG 1408.09(2) Eu152 1408.022(4) BG

79 Table 5.1 (continued)

peak identification Lit, value Comments 1428.2 1435.03(18) Cs138 1435.7 F 1435.89(10) Ba138 1435.9 F 1457.93(27) Eu152 1457.619(15) BG 1460.91(6) K40 1460.895(5) BG 1596.31(4) 40Ce, 154Eu?? 1596.182(20), 1596.582(20)?? F,BG 1612.4 56Fe(n,’y)‘ 1612.70(10) N 1632.9 1778.9 28Si(n,n’ 1779.030(11) N 2016.1 7 2217.5 2223.21(5) ) 2223.247(17) N H(n, 2239.6 172H 2397.75(16) Ce142 2397.7 F 2613.76(27) ) 2614.533(13) BG Th232 2639.0 2777.8 2789.65(25)

80 Table 5.2: A list of the even-A fission fragment isotopes observed Isotope Energy (keV) Lit. value Transition

Zr94 919.01 918.8 2 —* 0

‘°°Zr 211.9 212.7 2 —* 0 352.4 352.6 4 ,‘ 2

Mo102 295.9 296.0 2 —* 0 400.19 400.0 4+ —+ 2 2‘°Zr 152.8 151.9 2 —* 326.59 326.6 4+ —÷ 2 4‘°Mo 191.9 192.3 2 —* 4‘°Ru 357,78 357.8 2 — 0 4‘°Zr 140.25 140.3 2 —* 0 ‘°Mo 171.88 171.7 2 —+ 0 6 4+ —* 351.66 350.08 2

6‘°Ru 269.87 270.3 2 — 0 Tc108 138.22 138.4 2 —* 0

RU108 242.29 242.3 2 —* 0

“°Ru 240.83 240.8 2 —*

81 Table 5.2 (continued)

Isotope Energy (keV) Lit. value Transition

“°Pd 373.89 373.8 2 —* 0 546.35 546.3 4+ ‘ 2

Pd112 348.56 348.8 2 —* 0 ‘12Cd 618.21 617.4 2 — 0 “4Pd 332.80 332.9 2 —* Cd118 486.97 487.8 2 —* 0

Te132 974.32 973.9 2 —* 0 696.97 696.8 4+ —* 2

132J 228.65 228.2 ‘32Te —* J132 132 ‘32Xe 667.58 667.68 j —p ‘34Te 1279.25 1279.1 2 —* 0 34Xe 846.95 847.02 134 — j134 ‘ 884.16 884.09 j —+ Te136 535.07 535.8 2 —+

36Xe 1313.04 1313.0 2 —* 0 ‘ 381.22 381.5 4 —* 2 197.47 197.33 6 —* 4 Ba138 1435.89 1435.9 2 - 0 462.63 462.785 4 —* 2 1009.84 1009.8 ‘38Cs —* ‘38Ba

82 Table 5.2 (continued)

Isotope Energy (keV) Lit, value Transition

38Xe 588.76 588.9 2 — ‘ 483.33 483.4 4+ .‘ 2 831.38 830.8 J138 — Xe138 138 875.67 875.3 J —*

38CS 258.56 258.31 38Xe —* ‘ 433.88 434,49 ‘38Xe —* 40Xe 376.30 376.8 2 —* 0 ‘ 457.33 457.9 4 —÷ 2

Ba140 602.06 602.3 2 —* 527.60 528.26 4 —+ 2 ‘40La 537.3 537.27 Ba140 —* °La Ce140 1596.31 1596.49 2 —* 486.97 487.03 4+ —* 2 ‘42CS 571.06 571.66 ‘42Xe —* Ba142 359,4 359.3 2 —+ 475.2 475.0 4 —* 2 631.14 632 6 —+ 4

La142 231.85 231.52 Br42 — 42La 255.28 255.12 ‘Br142 —* ‘La142 894.96 894.9 Br42 —* 948.55 948.8 ‘Br42 —÷ La142 Ce142 641.19 641.17 42La —* 2397.75 2397.7 ‘La142 —* ‘42Ce

83 Table 5.2 (continued)

Isotope Energy (keV) Lit. value Transition

44Ba 198.5 199.4 2 —* 0 ‘ 330.9 331.6 4 .‘ 2 431.17 431.7 6 ,‘ 4 44Ce 397.17 397,3 2 — ‘ 540.97 5411 4 —* 2 844.43 844.9

584.90 585.0 6 .‘ 4+

46Ba 181.12 181.0 2 —p ‘ 332.80 333.0 4 —+ 2 46Ce 258.6 258.7 2 —* ‘ 409.59 4097 4 —* 2 Pr46 218.0 218.3 46Ce —* ‘ 316.59 316.8 ‘Ce146 —* ‘Pr46 ‘46Nd 453.59 453.9 6Pr —* Nd146 48Ce 158.98 158.7 2 — ‘ 295.98 295.7 4 —÷ 2 ‘52Nd 75.69 75.9 2 —* 0 ‘54Sm 72.7 72.8 2 —+ 0 Nd154 72.7 72.8 2 —* 0 162.2 162.4 4 — 2 ‘56Sm 75.69 76.0 2 —+ 0

84 Table 5.3: A list of the odd-A fission fragment isotopes observed Isotope Energy (keV) Lit. value Transition 85Rb 150.37 151.18 85mJ(r — 5Rb 95Nb 724.0 724.18 Zr95 — 95Nb 756.48 756.71 Zr95 —* 95Nb

95Mo 765.63 765.79 95Nb —* 95Mo

97Nb 743.17 743.36 Zr97 —* 97Nb

97Mo 658.20 657.92 97Nb —* 97Mo

99Tc 140.25 140.51 99Mo —* Tc 1‘°Ru 306.74 306.80 Tc101 —+ Rh103 496.93 497.08 Ru103 —*

Rh105 316.59 316.4 Ru105 —÷ Rh105 469,03 469.2 5‘°Ru —* 5‘°Rh 724.04 724.2 5‘°Ru — Rh105 Pd105 306.74 306.1 Rh105 —* 319.13 319.2 T‘°Pd 302.69 302.8 107Rh—* Pd107 131J ‘31Xe 364.05 364.47 —* ‘31Xe 133j 133j 312.0 312.1 ‘33Te —*

85 Table 5.3 (continued)

Isotope Energy (keV) Lit, value Transition Xe133 530.04 529.872 j133 — ‘33Xe Xe135 1260.1 1260.4 j135 135)( ‘35Cs 249.78 249.79 Xe135 — La141 190.21 190.2 Ba141 —* 41La 275.57 277.0 ‘ 304.14 304.2 343.38 343.7 Pr141 145.52 145.44 ‘41Ce —+ ‘Pr43 293.19 293.26 Ce143 —* ‘Pr Nd147 314.3 314.67 ‘Pr47 —+ Nd147 Sm151 340.03 340.08 Pm151 —* ‘51Sm

86 Chapter 6 Conclusion

In this work, the nature of several peaks from (n,n’) and (n,7) reactions in a Ge crystal has been studied when the germanium detector was submitted to a mixed flux of neutrons and 7-rays. An identification of these peaks is very useful when germanium detectors are used to detect 7-rays in an environment where fast and thermal neutrons are present as a background. In addition, several peaks from the (n,7) reaction in the NaT crystal surrounding the detector and the other materials in the experimental area have been analyzed and identified.

By fitting the (n,n’) peaks at 596 and 691 keV from three different neutron sources, it was found out that no significant energy dependence was observed for the peak shape. This is surprising and was not expected. Unfortunately the neutron spectrum from the three tests is not known precisely.

The 7-rays from ir absorption on Bi,209 the fission fragment 7-rays and the background 7-rays m the area will be useful as a reference for future work on similar accelerators.

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90