Radioactive Decay Data: Powerful Aids in Medical Diagnosis and Therapy, Analytical Science and Other Applications

Radiochim. Acta 100, 615–634 (2012) / DOI 10.1524/ract.2012.1959 © by Oldenbourg Wissenschaftsverlag, München

Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications

By A. L. Nichols1,2,∗

1 Department of Physics, University of Surrey, Guildford, GU2 7XH, UK 2 Manipal University, Madhav Nagar, Manipal 576104, Karnataka, India

(Received January 31, 2012; accepted in revised form April 10, 2012) (Published online July 30, 2012)

Radioactive decay / Decay data measurements / sition types; electron-capture and β+-particle energies, Decay data evaluations / Decay data files / transition/emission probabilities and transition type (also Reactor operations / Fuel cycle applications / EC/β+ ratios when appropriate); γ-ray energies, emis- Non-energy applications / Nuclear medicine sion probabilities and internal conversion coefficients (also internal-pair formation coefficients for β+β− when appropriate); Auger- and conversion-electron energies and emission Summary. Decay data are commonly used to characterise probabilities; X-ray energies and emission probabilities; and quantify radioactive material, and provide an important spontaneous fission properties (branching fraction and recoil means of understanding the properties and structure of the energies); delayed-neutron energies and emission probabili- nucleus. Experimental measurement techniques are reviewed, ties; delayed-proton energies and emission probabilities; and with the emphasis placed on recent developments that repre- comprehensive quantification of the uncertainties associated sent a potential quantum leap in advancing our knowledge, particularly by means of γ-ray spectroscopy. A select number with all of the above atomic and nuclear parameters. of internationally-accepted decay-data evaluations and com- Additional ancillary data requirements can be met from pilations are also discussed in terms of their contents. Both the above, including various total mean energies which need energy and non-energy related applications require the input to be quantified and adopted for particular applications: of well-defined decay data, and such activities have been mean heavy-particle energy (includes mean α, neutron, pro- reviewed. Various important decay-data issues are assessed, ton, fission fragment, and associated recoil energies); mean and note taken of any significant requirements for better light-particle energy (includes mean β−, β+, Auger-electron quantified data. and conversion-electron energies); and mean electromagnetic energy (includes mean γ, X-ray, β+β− annihilation 1. Introduction radiation and internal bremsstrahlung). While more exotic modes of decay have been detected (e.g. double-beta (ββ) In-depth assessments, evaluations and measurements of ra- and cluster/heavy-ion decay), these low-probability phe- dioactive decay data have been requested and undertaken nomena are not considered further in this review. over many years. Recommended decay data are normally The need for well-defined radioactive decay data was derived from all relevant publications that include quantifi- recognised over 80 years ago with the publication of a paper cation of decay-scheme data primarily by means of direct by the International Radium-Standards Commission which measurement but also by calculation. The measurement and included such world-renowned scientists as Marie Curie, derivation of such recommended data sets are welcomed by Otto Hahn, Hans Geiger and Lord Rutherford [4]. Recom- nuclear physicists and engineers (a) to define the status and mended radioactive constants were proposed with no un- our current knowledge of particular decay parameters, and certainties, based predominantly on known measurements determine whether there is a need for further investigation by members of the Commission and their co-workers. This and study, and (b) hopefully to provide highly reliable input work led on to more extensive nuclear reaction data listings data for modelling codes in order to quantify the operational by Fea [5] and Livingston and Bethe [6], and the first recog- characteristics and behaviour of irradiated fuel and other nizable Table of Isotopes format by Livingood and Seaborg materials with reasonable confidence. in 1940 [7] that appeared every four or five years in Re- Atomic and nuclear decay-data parameters encompass views of Modern Physics until 1958. Subsequent editions of the following [1–3]: half-life; total decay energies (Q- the Table of Isotopes have been published at regular inter- values); branching fractions (if more than one known de- vals up to an including the 8th edition in 1996 [8], which also cay mode); α-particle energies and emission probabili- contains a CD-ROM of the full contents. Recommended nu- ties; β−-particle energies, emission probabilities and tran- clear structure and decay data for this particular edition of the Table of Isotopes have primarily been extracted from the *E-mail: [email protected]. Evaluated Nuclear Structure Data File (ENSDF, see below). 616 A. L. Nichols

Katharine Way began collecting and compiling nuclear data in the early/mid 1940s, and a compilation of her work first appeared in 1950 [9] – no specific values were recommended, nor uncertainties given. Nevertheless, this work evolved into Nuclear Data Sheets (as published by Aca- demic Press, and subsequently by Elsevier Inc.) and the Evaluated Nuclear Structure Data File (ENSDF) [10]. Eval- uations of nuclear structure and decay-data measurements were carried out at regular intervals of time, and formatting codes were developed to display the recommended nuclear data in a clear, concise and well-defined manner. These studies continue as a multinational work programme, with bien- nial meetings held to discuss both managerial and technical issues under the auspices of the Nuclear Data Section of the International Atomic Energy Agency [11].

2. Experimental techniques Radioactive nuclides of interest are normally prepared by means of either reactor irradiation or charged-particle acceleration and controlled bombardment of carefully prepared targetry. Isotopic enrichment of the target material and purification of the resulting product represent important requirements when striving to measure accurate decay data. Various radiochemical procedures have been successfully adopted to achieve elemental separation of the irradiated target, including anion-exchange chromatogra- Fig. 1. Alpha-particle spectra of thin mass-separated sources of (a) phy, application of many forms of liquid-liquid extraction, 237Np, and (b) 243Am measured by means of a 450-mm2 passivated im- and dry distillation [12–14]. For example, the adoption planted planar silicon (PIPS) detector – main α peaks are labelled in of various radiochemical techniques to achieve high lev- keV energy units [18]. els of radionuclidic purity was very important in forming the basis for accurate measurements of the positron emission probabilities of 64Cu, 76Br and 124I for medical Significant developments have recently occurred with re- applications [14]. spect to improvements in energy resolution by means of Long-established experimental techniques can be used cryogenic microcalorimetry: to quantify in detail specific features of a decay scheme, 1. Detector system consisting of a superconducting transi- ranging from α, γ and electron spectroscopy operated in sin- tion-edge sensor (TES) with Mo:Cu bilayer and an ab- gles and various coincidence modes, time-dependent stud- sorber of superconducting tin has been shown to give ies of these emissions to determine important parameters an energy resolution of (1.06 ± 0.04) keV FWHM for such as half-lives, and angular correlation measurements for 5.3MeVα particles [19, 20]. greater structural detail. The more substantive techniques are 2. Sensor of gold doped with a small concentration of er- briefly discussed below, along with some thoughts on future bium (Au:Er) for which the magnetization changes as developments. a function of modification in temperature by α-particle absorption – energy resolution of (2.83 ± 0.05) keV 2.1 α-spectroscopy FWHM was determined for 5.5MeVα particles [21]. Obviously, measurements of α spectra play an important Such ultra-high resolutions are a significant improvement role in quantifying and defining the decay schemes of α- beyond the theoretical limit of conventional silicon detec- particle emitting nuclides, and impact most significantly on tors. Alpha-particle measurements with this type of detector studies of the many heavy elements and actinides. One loss system would greatly reduce uncertainties in decay schemes over recent years has been the decline in maintenance of and specific aspects of their decay data, with an inevitably dedicated magnetic spectrometers that offer extremely good beneficial knock-on effect involving the accuracy and effi- energy resolution. Precise, well-defined studies of α spec- cacy of their application. tra were feasible with homogeneous-field magnetic spectro- graphs [15, 16]. Silicon-based ionization detectors such as 2.2 X- and γ -ray spectroscopy the silicon barrier detector (SBD) and passivated implanted planar silicon (PIPS) detector are now much more com- The extremely successful development and adoption of sil- monly used to measure the energies and emission probabil- icon and germanium crystals as detectors in X- and γ-ray ities of α particles [17]. Good resolution α spectra obtained spectroscopy has contributed immensely to our understand- by means of a 450-mm2 PIPS detector are shown in Fig. 1 ing of the atomic nucleus across the known Chart of the for mass-separated sources of 237Np and 243Am [18]. Nuclides. Since the late 1960s, Si- and Ge-based detectors Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications 617 have offered the best compromise between energy resolution and efficiency to ensure sound, accurate and reliable X- and γ-ray spectral studies. More specifically, major advances into the 1980s were associated with increased volume and improved purity of Ge crystals, while the more recent introduction of segmented Ge systems has further improved their detection capabilities and performance characteristics. A significant amount of nuclear structure data measured and evaluated for inclusion in recommended decay databases originates from γ-ray spectroscopy undertaken with single-crystal Ge(Li) and HPGe detectors which operate satisfactorily below 110 K (and therefore are normally maintained by means of liquid nitrogen at 77 K). Directly measured data include X- and γ-ray energies and emission probabilities, with the added potential to derive the spins, parities and lifetimes of excited states, determine γ transition types and mixing ratios, and calculate directly other features of the decay scheme under investigation. Low-energy photon spectrometers (LEPS) based on a planar small-area HPGe crystal have also been specifically developed to measure γ-ray spectra over the low to intermediate energy range from 3 to approximately 300 keV – these detector systems operate at high resolution, and an example γ spectrum of a chemically-purified 233Pa source is shown in Fig. 2 [18]. The division of a Ge crystal into sections offers additional information identified with the γ interactions, referred to as γ-ray tracking expressed in terms of energy, time and location – the net result has been the twin achievements of unprecedented efficiency and energy resolution. The development and operational characteristics of single-crystal Ge detectors and more advanced Ge detector arrays can be Fig. 3. Half of the modular layout of HPGe detectors for (a) Gamma- found in the substantial review articles of Lee et al. [22], and sphere, and (b) Euroball. Eberth and Simpson [23]. Only a brief semi-historic summary is given below. Rapid progress in the discovery and understanding of tirely related to the advent of Ge(Li) detectors in γ-ray spec- nuclear structure and decay data in the 1970s was almost en- troscopy. Such beginnings led on to technological advances in the growth of large high-purity Ge crystals to increase the peak-to-Compton ratio, reduce background effects, and improve the coincidence rate and spectral statistics. Escape- suppression shields were introduced in order to reject partly absorbed events from the γ spectra, and the number of escape-suppressed Ge detectors was increased into arrays during the course of the 1980s to compensate for the commensurate loss in coincidence efficiency [23]. Such HPGe arrays with double gating and ancillary detectors opened up the possibility of measuring γ-γ-γ coincidences to quantify weak and complex γ cascades. Second-generation systems consisting of 4π solid-angle arrays of germanium detec-

tors and BGO (bismuth germanate (Bi4Ge3O12)) shields have proved to be very costly, and therefore resulted in the evolution of only two recognised primary projects (Fig. 3): Gam- masphere – multi-laboratory programme based in the USA; and the Euroball collaboration in Europe. Gammasphere and Euroball are seen as achieving the highest feasible goals for 4π solid-angle arrays with escape-suppressed Ge detectors. Their impressive achievements in the continued evolution and resolution of nuclear structure and decay scheme data can be found on the Web [24, 25]. Fig. 2. Gamma-ray spectrum of a chemically-purified 233Pa source measured by means of a 2-cm2 ×1-cm planar low-energy photon spec- Scientific interest has tended to focus in recent years on trometer (LEPS), at a source-to-detector distance of 5 cm – main γ extraordinary N/Z ratios close to the proton and neutron peaks are labelled in keV energy units [18]. drip lines as a consequence of the growing need to support 618 A. L. Nichols the study of nuclear reactions generated by emerging ra- crystals compares with the full AGATA 4π geometry of 60 dioactive ion beam facilities (RIB). Theoretical analyses and triple clusters (180 Ge detectors)). Complementary analy- experimental studies on Ge-based detector design has led to sis systems are available to operate in conjunction with the the concept of γ-ray tracking in terms of the position, energy AGATA sub-array, and so extend the measurement capabili- and time of all γ-ray interaction points within Ge detector ties to a broader range of nuclear physics topics [28]. systems: Assembled stacks of planar double-sided Ge strip detectors (DSSD) possess sufficient pixelation to achieve γ-ray – GRETA (Gamma Ray Energy Tracking Array, USA [22]; positional resolution [22]. Possible drawbacks are envis- geodesic configuration of 120 to 130 segmented HPGe aged, such as the existence of dead layers at the edge of detectors); each planar crystal and the need for mechanical structure to – AGATA (Advanced GAmma Tracking Array, Europe ensure the provision of sufficient cooling to the HPGe detec- [23, 26]; geodesic configuration of 180 hexagonal and 12 tors. Nevertheless, such systems are being studied for their pentagonal segmented HPGe detectors). γ-ray tracking potential and possible application in high- Identification of the interaction points and quantification resolution photon imaging. of the energy deposited at these locations can be achieved The ability to discriminate between γ rays of slightly dif- with highly-segmented Ge detectors and pulse-shape ana- ferent energies by means of a scintillator is of importance lyses. However, the sequence of a γ-ray scattering process in medical imaging, γ-ray spectroscopy and X-ray astron- is too fast for measurement compared with the time reso- omy. Both NaI(Tl) and CsI(Tl) have been the scintillators lution of a Ge detector, and therefore tracking algorithms of choice for over 50 years because of their reasonable en- based on the underlying interaction processes have to be ergy resolution, while other scintillators such as Bi4Ge3O12 used. While GRETA and AGATA will permit access to the in positron emission tomography (PET) and PbWO4 in high- farthest reaches of the Chart of the Nuclides, other areas of energy nuclear physics have found appropriate application. nuclear application will also benefit from the operational ad- However, lanthanum halides doped with Ce3+ have also vances in Ge detector technology. Thus, the development been shown to exhibit significant promise based on good of position-sensitive γ-ray detectors for nuclear structure energy resolution in combination with fast luminescence de- studies will have important repercussions in astrophysics, cay [29–31]. The superior energy resolution of LaBr3(Ce) medical imaging and nuclear safeguards. scintillator has successfully been used with Gammasphere to GRETINA represents a significant stepping-stone to- quantify with good precision the lifetimes of nuclear levels wards GRETA, and consists of coaxial Ge crystals of tapered between 50 ps and 1 ns, as populated in the IT and β− decay hexagonal shape that make up seven modules, each contain- of 177mLu [32]. Potentially new and improved scintillators ing four 36-fold segmented crystals (Fig. 4). This set of 28 are constantly being assessed for adoption in high-resolution Ge crystals covers a quarter of the 4π solid angle envis- γ-ray spectroscopy, and studies of their performance pro- aged for GRETA, and will be extensively tested in a series vide essential guidance as to their suitability and applica- of nuclear structure, reaction and symmetry studies [27]. tion [33]. Similarly, an AGATA sub-array consisting of five triple clus- Another noteworthy technique for accurate X- and ters of highly-segmented Ge detectors has been assembled low-energy γ-ray detection is the X-ray microcalorime- at Laboratori Nazionali di Legnaro, Padova (total of 15 Ge ter [34, 35]. Effectively, the detector system consists of 36 microcalorimeters in 6 × 6 array, although only 32 are elec- tronically active: 28 pixels use 8-µm thick HgTe absorbers, and the remaining four pixels use 30-µm thick Bi absorbers. Maximum sensitivity is achieved when this device operates at 60 mK to give an energy resolution of 6 eV (FWHM) from 0 to 10 keV, while an operating temperature of 90 mK gives FWHM of ∼ 26 eV up to about 60 keV. This device senses and quantifies the heat deposited by incident photons on the HgTe absorbers, and has been used with impressive accuracy to determine an energy split of only (7.6 ± 0.5) eV between the ground and first excited states of 229Th, based on precisely measured differences in the more substantial γ rays populating both members of the doublet [36]. Total Absorption Gamma-ray Spectroscopy (TAGS) has been successfully used to determine the β− strength functions of a significant number of complex decay schemes [37–41] on the basis of calculations involving the data derived from 4πγ-ray spectroscopy. Normally, the main spectrometer consists of a substantial well-type NaI(Tl) detector in which the γ-ray emitting source is contained, along with an auxiliary NaI(Tl) detector that fits into and closes the well π Fig. 4. Quarter-based layout of the GRETINA detectors (foreground) to achieve an approximately 97% 4 solid-angle geometry. compared with the full 4π solid-angle system proposed for GRETA Various other ancillary γ-ray detectors may also be used (background). to study coincidence gating. More recent assemblies have Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications 619 involved the assessment, design and construction of other γ-counting techniques. The half-life of the radionuclide of suitable detector materials and arrays. Good overall detec- interest is determined by substituting the results of the two tion efficiency and energy resolution are required, along forms of measurement into the basic equation of radioactive with a sound spectral understanding in order to disentangle decay. the decay components from each other and any background Half-lives of approximately 20 to 50 years and above activity. TAGS has proved to be of importance in the devel- can prove awkward to measure by standard spectroscopic opment of a more correct and comprehensive understanding techniques, and have been shown to benefit from quan- of the nucleus, reaching beyond some of the specific limi- tification via ultrasensitive mass-spectrometric measure- tations of γ singles and coincidence measurements [39, 40]. ments of isotopic concentrations. Examples of the suc- Furthermore, TAGS studies have been used to determine the cessful adoption of this approach include 32Si (half-life average β− and γ energies of particular fission produces, of 133 ± 9a [45]), 44Ti (half-life of 54.2 ± 2.1a [46]), aiding considerably in calculations of the decay heat emitted 60Fe (half-life of (1.5 ± 0.3) ×106 a[47]), 126Sn (half-life by irradiated reactor fuel [38, 41] – see also Sect. 4.2. of (2.07 ± 0.21) ×10+5 a[48, 49]and(2.35± 0.07) ×10+5 a [50]). Similar studies have also been carried out by a combination of inductively-coupled plasma mass spectrome- 2.3 Electron spectroscopy try (ICP-MS) and specific activity measurements to de- Appropriate studies of β, Auger-electron and internal termine the half-lives of 79Se (half-life of (3.77 ± 0.19) × conversion-electron emissions have yielded important data 10+5 a[51]) and 126Sn (half-life of (2.33 ± 0.10) × 10+5 a in the resolution of difficulties associated with the popu- [52]and(1.98 ± 0.06) × 10+5 a[53]). Further mass-spectro- lation and depopulation of daughter nuclear levels and the metric studies should be encouraged to address existing derivation of a normalisation factor to convert relative γ-ray anomalies and uncertainties that remain in half-life values emission probabilities to absolute values. Various types of for long-lived radionuclides of recognised importance (i.e. detector system have been used, e.g. permanent-magnet 180◦ 32Si, 60Fe, 59Ni, 79Se and others). spectrograph, permanent-magnet pre-accelerating√ spectrograph, six-gap spectrometer and iron-free π 2 double- focusing spectrometer [42–44]. While sound quantification 3. Recommended decay data of electron emissions has proved important input to the evo- Compilations and evaluations of nuclear data have been pro- lution of fully consistent decay schemes and well-defined duced since the early 1930s to inform and assist nuclear decay data, this type of complex spectrometric study has physicists, α and γ spectroscopists, and basic nuclear sci- been seriously neglected in recent years. A major problem ence researchers. More specifically, extended libraries of is the bulky nature of the magnet systems, coupled with the reaction cross sections, fission yields, nuclear structure and delicate nature of the multi-element detector assemblies – decay data have evolved in well-defined formats for appli- their maintenance and operation have historically proved to cation by nuclear engineers and plant operators in power be costly. generation, fuel reprocessing and waste management. As β− intensity distributions are normally determined from outlined below, such libraries have historically been updated γ any calculated imbalances in the population and depop- over agreed time intervals either by means of international ulation of nuclear levels of the decay schemes assembled consensus, or more localised efforts based on immediate na- γ and quantified by means of -ray spectroscopy. However, tional needs. Regular improvements to and subsequent con- such experimental studies can be fraught with difficulties trolled studies of the contents of a decay-data library also γ resulting from the failure to detect all of the weak transi- provide a rapid means of assessing their applicability and tions. Faced with this basic problem of omission in the study suitability – such activities assist greatly in defining further of complex decay schemes, Total Absorption Gamma-ray requirements to improve the existing decay data. Spectroscopy (TAGS) has been used to determine unam- biguous β− feeding data, as outlined above in Sect. 2.2. 3.1 Decay-data files Consideration has been given to the most comprehensive 2.4 Specific activity and mass spectrometry databases that are judged to be most relevant to defining the Accelerator mass spectrometry (AMS) and thermal ioniza- status of and existing requirements for improved decay data: tion mass spectrometry (TIMS) have been successfully used 1) ENSDF (Evaluated Nuclear Structure Data File) – to determine the half-lives of long-lived radionuclides by consists of nuclear structure and decay data for all known counting the number of ions of interest after acceleration, radionuclides (www.nndc.bnl.gov/ensdf/). Complete mass and relating these data to the specific activity of the ra- chains are evaluated at regular intervals (nominally every dionuclide in the ion source. Normal procedures involve seven to ten years) by members of the International Net- irradiation processes to generate the radionuclide of inter- work of Nuclear Structure and Decay Data Evaluators, est, followed by purification through the removal of un- and published in Nuclear Data Sheets [10]. The strength desirable elements/species by chemical separation. Small of ENSDF is to be found within the completeness of this fractions of the resulting purified solutions are used to pre- comprehensive database (e.g. see Fig. 5 for the IT decay pare targets for mass spectrometric studies in order to de- data of 99mTc in ENSDF). NuDat represents an electronic termine radionuclide/element ratios, and measurements are chart of the nuclides based primarily on the contents of also made on carefully weighed bulk samples to deter- ENSDF and constitutes a user-friendly means of access- mine their specific γ-ray activities by means of standard ing decay data (www.nndc.bnl.gov/nudat2/), while MIRD 620 A. L. Nichols

Fig. 5. Nuclear data sheet for the IT decay of 99mTc (ENSDF); comparison with equivalent data in Ref. [54] reveals some subjective differences – e.g. 140.511-keV γ emission, with recommended Pγ of 89(4)% in ENSDF and 88.5(2)% in DDEP. Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications 621

Table 1. Sub-libraries of decay-data ﬁles in ENDF-6 format for nuclear applications.

Sub-library Year No. of Comments nuclides a

JENDL-FP b [68] 2001 1229 From ENSDF + gross β theory

JEFF3.1.1 [69] 2007 3852 From NUBASE [72], DDEP, UKPADD6.7 [73] c, UKHEDD2.5 [74] d and ENSDF + TAGS data e

ENDF/B-VII.1 [71] 2011 3817 From ENSDF + extension of atomic radiation + delayed-neutron emissions + TAGS data e a: Speciﬁed number of nuclides also includes ﬁles of stable nuclides. b: New version in preparation: ENSDF + TAGS data + delayed neutrons (Fukahori, T., JAEA, private communication, May 2011). c: More recent version available as UKPADD6.11 [75]. d: More recent version available as UKHEDD2.6 [76]. e: Average β and γ energy data obtained from various dedicated gross βγ and TAGS studies (Total Absorption Gamma-ray Spectroscopy).

(Medical Internal Radiation Dose) generates the result of 211At/211Po, 223Ra (within 227Ac decay chain), 241Am and processing ENSDF decay data through the RADLST code 252Cf [66]. to give the energies and intensities of all emitted radia- Other nuclear data libraries have been assembled and tion, along with their dose rates (www-nds.iaea.org/mird/). subsequently updated in various ways, based on the guid- NSR (Nuclear Science References) is an ancillary bibli- ance and requirements of the nuclear power industry and ographic database through which mass-chain evaluators related fuel-cycle activities. These decay-data sub-libraries and other users can accesses all publications of interest have been prepared via a combination of evaluation and (www-nds.iaea.org/nsr/index.jsp). direct adoption of recommendations from respected data 2) DDEP (Decay Data Evaluation Project) – contains sources (e.g. data extraction from ENSDF, DDEP and other comprehensive atomic and nuclear decay data evaluations evaluated atomic and nuclear data files). These recog- of selected radionuclides, with emphasis placed on the com- nised nuclear applications databases are maintained in pleteness of each decay scheme and the derivation of X-ray the internationally-accepted ENDF-6 format [67], and are and electron decay data [54–60]. Multinational evaluations briefly described in Table 1.JENDL-FPfileshavebeen are submitted via a coordinator to the custodian of both the assembled by staff working at the Japan Atomic Energy Re- database and associated comments files at the Laboratoire search Institute [68], and now part of the Japan Atomic En- National Henri Becquerel, CEA Saclay, France. Compre- ergy Agency; JEFF3.1.1 constitutes a combination of multi- hensive decay data for 194 radionuclides are contained national assessments and evaluations carried out under the within the DDEP files (www.nucleide.org/DDEP_WG/ auspices of the Nuclear Energy Agency of the OECD [69]; DDEPdata.htm), as observed on 4 June 2012. ENDF/B-VII.1 is a predominantly USA initiative with 3) Relevant IAEA databases – a number of nuclear a number of significant international inputs (latter are mainly data initiatives organised as Coordinated Research Projects files dedicated to nuclear reaction excitation functions) and (CRPs) under the auspices of the International Atomic En- retention of various data from the earlier ENDF/B-VII.0 ergy Agency (IAEA) have resulted in the generation of high- library [70, 71]. The decay data include half-lives, decay quality decay data. modes and Q-values, branching ratios, average decay ener- a) Update of X ray and gamma ray decay data stan- gies, α, β,EC/β+, Auger-electron, conversion-electron, γ, dards for detector calibration and other applications, 1998– X-ray, etc. energies and emission probabilities, and various 2005 [61–63], www-nds.iaea.org/xgamma_standards/, other nuclear parameters (e.g. internal conversion coeffi- www-nds.iaea.org/publications/tecdocs/sti-pub-1287_Vol1. cients), along with comprehensive uncertainty data. pdf, www-nds.iaea.org/publications/tecdocs/sti-pub-1287_ Communication links with many electronic decay-data Vol2.pdf. Nuclides within this particular database of high files are rapid and comparatively easy to implement in relevance to nuclear medicine include 60Co, 64Cu, 66Ga, the age of the PC, Internet, CD and DVD. As mentioned 67Ga, 99mTc, 111In, 123I, 125I, 131I, 137Cs, 153Sm, 166Ho, 169Yb, above, NuDat provides a user-friendly means of extract- 192Ir and 201Tl [62, 63]. ing decay data from ENSDF (www.nndc.bnl.gov/nudat2/); b) Updated actinide decay data library, 2005–2011 LiveChart also accesses ENSDF, and offers the user a wide [64–66] – data also forwarded for inclusion in the DDEP range of procedures to interrogate this database and dis- database, and listed on the web site www.nucleide.org/ play the nuclear parameters of interest in various ways DDEP_WG/DDEPdata.htm. Nuclides within this particular (www-nds.iaea.org/relnsd/vchart/index.html). JANIS con- decay database address requirements in (i) nuclear power stitutes an equivalent software platform for the rapid inspec- generation with the inclusion of all important Th, Pa, U, Np, tion and display of numerical data within the JEFF nuclear Pu, Am and Cm radionuclides and their natural decay prod- data library (www.oecd-nea.org/janis/). ucts, and (ii) nuclear medicine applications identified with Dedicated catalogues of γ-ray spectra have been as- 212Bi (228Th decay chain), 213Bi (229Th/225Ac decay chain), sembled to assist greatly in the characterisation and quan- 622 A. L. Nichols tification of the radioactive content of materials [77, 78]. 4. Applications in reactor technology Systematic studies of the γ-ray emissions of individual ra- Along with various neutron-induced cross sections as dionuclides have been carried out by means of NaI(Tl) a function of neutron energy, the decay data of actinide scintillation, Si(Li), Ge(Li) and HPGe detectors, and these fuels, their (n,γ) reaction products and heavy-element de- “fingerprint” spectra are of immense value in spectroscopic cay chains, fission products and activation products are analyses. With the advent of the Internet, these γ-ray spec- important inputs to the calculation of the radionuclide in- trum catalogues and enhancements have conveniently been ventories generated by irradiated fuel bundles within the made available in electronic form through the Web site reactor core of a power reactor. Such inventory data are http://www.inl.gov/gammaray/catalogs/catalogs.shtml,for essential in the plant design (particularly the provision of NaI – http://www.inl.gov/gammaray/catalogs/pdf/naicat.pdf, adequate shielding), operational safety measures that in- and for Ge and Si(Li) detectors – http://www.inl.gov/ clude annual shutdown procedures, decommissioning, fuel gammaray/catalogs/pdf/gecat.pdf. handling, storage and reprocessing, and waste disposal. Fur- thermore, studies of efficient transmutation by means of the 3.2 Status of decay data recycling of high burn-up fuel need to be assessed from the points of view of power production and reducing the Important work continues or is being planned to explore quantities of long-term actinide and fission-product waste further structural facets of the nucleus, particularly through inventories. the use of radioactive ion beams to study specific areas of Stable and radioactive nuclides introduced into inventory the chart of the nuclides in a highly focussed manner. Fur- calculations are of the order of 120 actinides and heavy- thermore, significant efforts are constantly being made to element nuclides within their decay chains, 1050 fission respond fully to the demands of decay-data users in support products, and 700 possible activation products. While the of their energy and non-energy applications. Accurate meas- decay data of the important actinides (identified as specific urements and sound evaluations of decay data have been Th to Cf radionuclides), their heavy-element decay-chain proposed and are underway to provide information to assist products and the commonly-occurring activation products in basic nuclear physics research and ensure the efficacy and are judged to be sufficiently well quantified for purpose, validity of nuclear data as applied to fission and fusion reac- a number of notable fission products merit improvement and tor physics, nuclear medicine and analytical science. further experimental study (see below). Some of the sub-libraries of recommended decay data are primarily dedicated to reactor operations and fission- based fuel cycles (JENDL-FP, JEFF3.1.1 and ENDF/B- 4.1 Normal operational characteristics of reactor VII.1), although they can be used further afield for fusion systems research studies and various non-energy applications, if de- Many technical features of reactor operations such as crit- sired. Updating of these ENDF-6 decay-data files is predom- icality and power distributions are basically dependent on inantly reliant on the regular evaluation efforts within the neutron physics (neutron-induced cross sections, angular ENSDF initiative, although some sub-libraries do contain distributions, secondary energy distributions, and ν¯ (average a limited number of recommended decay-data files that have prompt neutron emissions)). The importance of well-defined been individually evaluated and assembled in a more ded- decay data arises when consideration is given to the follow- icated manner (e.g. within JEFF3.1.1). DDEP evaluations ing: (a) shielding requirements, (b) proposed utilisation of are based on the implementation of a series of agreed eval- recycled and high burn-up fuel, and (c) irradiated fuel stor- uation procedures, with the aim of assembling high-quality age and reprocessing, along with the subsequent needs of files of recommended decay data of immediate value as stan- waste management. dards in radionuclide metrology; these particular data sets Power, rod worth and structural damage are regularly as- have also been assessed and are now supported by the Bu- sessed during plant operation, and in this regard the decay of reau International des Poids et Mesures (BIPM) as a means short-lived fission and activation products need to be known. of encouraging worldwide adoption. Furthermore, changes occur in the fuel, structure and ab- Specific inadequacies and uncertainties remain to be sorber materials under neutron irradiation, and the half-lives addressed with respect to the decay-scheme data for a num- and emissions of all major fission products (of the order of ber of radionuclides, whether considering either a particu- 90 radionuclides) and the actinide decay chains are required lar radionuclidic application (e.g. nuclear medicine), or as inputs in such analyses. Modelling codes have been de- encompassing a relatively large number of radionuclides veloped to determine neutron and gamma transport within (e.g. shielding and decay heat calculations). These envis- and in close association with the reactor core. The gamma aged needs are discussed in Sects. 4 and 5, below, covering decay data are normally grouped into bins for ease of cal- such disparate issues as power reactor control and safety, culation, covering the energy range from zero to 10 MeV. shielding/storage of irradiated fuel, radiation dosimetry, nu- Half-lives and branching fractions are also required as in- clear medicine, radioactive dating, and astrophysics. Further put and, together with inventory calculations, provide the work is required to build on previous initiatives such as means of performing time-dependent studies of the neces- IAEA CRPs through consideration of possible decay-data sary shielding requirements. Equivalent shielding studies are needs over the next 5 to 15 years. Improvements to some also undertaken for irradiated fuel storage, transport, repro- of the existing data are required, along with new additions cessing and disposal. All of the required decay data originate to ensure that some pre-emptive measures are in place to from ENDF-6 files [68, 69, 71], and are judged to be accu- address our future needs. rately known. Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications 623

4.2 Decay heat of γ-ray singles data to calculate β− transitions by means of gamma population-depopulation balances of the proposed A sound knowledge of the time-dependent energy release nuclear levels [82]. TAGS (Total Absorption Gamma-ray from radioactive nuclides is extremely important in formu- Spectroscopy) measurements can overcome these difficul- lating safe operational procedures for nuclear power reac- ties, and provide the necessary mean beta and gamma decay tors, identified primarily with the need to maintain cooling to energies for sound decay heat calculations [37, 38, 41, 83]. previously irradiated fuel. Accurate estimates of the resulting decay heat are needed in safety assessments of all types of reactor and fuel-handling plant, the storage of spent fuel, the transport of fuel-storage flasks, and the intermediate- term management of any resulting radioactive waste. Actinide and fission-product inventories are calculated for the specified conditions during reactor operation and the subsequent cooling period. These data are used in conjunction with the radionuclidic half-lives, and all heavy- particle, light-particle and electromagnetic radiation characteristics to determine the total energy release rates for heavy-particles, light-particles and electromagnetic radiations [79–81]: M ( ) = λT ( ) i HHP t i Ni t EHP i=1 M ( ) = λT ( ) i HLP t i Ni t ELP i=1 M ( ) = λT ( ) i HEM t i Ni t EEM i=1 where HHP(t), HLP(t) and HEM(t) are the total decay heat of the heavy-particle, light-particle and electromagnetic radia- λT tions, respectively, at time t after reactor shutdown; i is the total decay constant of radionuclide i; Ni (t) is the number of i i i atoms of radionuclide i at time t;andEHP, ELP and EEM are the energy release rates for heavy-particle, light-particle and electromagnetic radiations, respectively, per disintegration of radionuclide i. The heavy-particle component includes α, neutrons, protons, recoil nuclei and fission fragments; light particles are defined as β−, β+, Auger electrons and internal-conversion electrons; and electromagnetic radiation is identified with γ, X-rays, annihilation radiation and internal bremsstrahlung. Neutron-induced cross sections, fission yields and radioactive decay data constitute the input to the summation calculations used to determine the release of decay heat as a function of time after the termination of neutron-induced fission [79–81]. These calculations require the inclusion of mean α, β and γ energies derived normally from the discrete α, β and γ energies and emission probabilities for a significant number of fission products, and the results are compared with experimental decay-heat benchmarks. However, the determination of β− emission probabilities has long been problematic in decay-scheme studies. Although γ-ray emission probabilities and internal Fig. 6. Decay-heat calculations for irradiated 239Pu as a function of − conversion coefficients can be used to derive β feeding to cooling time in terms of (a) total energy release rate per fission second, daughter nuclear levels, all forms of singles-based Ge de- (b) light-particle/electron energy release rate per fission second, con- β− β+ tector possess low intrinsic efficiency for the detection of stituting the , , Auger and conversion electron components, and (c) electromagnetic energy release rate per fission second, constituting high-energy γ rays above ∼ 1.5 MeV that undermines such γ − the , X-rays, annihilation radiation and internal bremsstrahlung com- an exercise. Furthermore, the determination of direct β de- ponents. Tobias (1989) denotes benchmark data for 239Pu as determined cay to the ground state of the daughter nucleus can pose even by a least squares fit to all available measured data [84]; JEFF311 more serious problems for various other reasons. Hardy et defines the calculated decay heat when the JEFF3.1.1 decay-data sub- ∼ γ library was adopted [69] in which TAGS data from Greenwood et al. al. have demonstrated that 20% of the true -ray intensity had been included [38]; JEFF311+ TAGS are equivalent decay-heat above 1.7 MeV for a fictional radionuclide (Pandemonium) calculations in which TAGS data from Algora et al. had also been may remain undetected, impacting significantly on the use added [41]. 624 A. L. Nichols

Table 2. Requested TAGS measurements for decay-heat studies of Th-U and U-Pu fuels.

Radionuclides Common to Th-U and U-Pu fuel Additional for U-Pu fuel Additional for Th-U fuel

86Br, 87Br(β−n), 88Br(β−n), 89Kr, 90mRb a, 89Sr, 97Sr, 105Mo, 85Se, 86Se, 84Br, 89Br, 87Kr, 91Kr, 88Rb, 90Kr, 92Rb, 96Y, 99Zr, 100Zr, 98Nb, 105Tc, 106Tc, 107Tc, 94Rb, 92Sr, 96mY, 97Y, 98Zr(?), 99mNb, 99Nb, 100Nb, 101Nb, 102Nb, 103Mo, 142Cs(β−n), 145Ba a, 143 La a, 100m Nb, 102mNb, 101Mo, 128mSb, 129mSb, 102Tc, 103Tc, 104Tc, 132Sb, 135Te, 136I, 145La a 130m Sb, 133Sb, 138Xe, 139Ba, 141La, 146mLa 136mI, 137 I(β−n), 137Xe, 139Xe, 140Xe a: TAGS measurement recommended in repeat of equivalent studies by Greenwood et al.[38].

As determined from the TAGS measurements of Green- bound levels of nuclei. Extensive studies of delayed-neutron − ¯ wood et al. [38], total mean β and γ energies (Eβ− and and related gamma-ray emissions have been undertaken by ¯ Eγ ) for twenty-nine fission products of importance in decay- means of singles and coincidence counting techniques in heat calculations have been incorporated into the JEFF3.1.1 order to provide level-density information and well-defined − decay-data sub-library [69] to replace inadequate values that evidence of complex β n decay for many important fission- arose from the evaluation of gamma-ray spectroscopic stud- product nuclides [89–93]. However, various short-lived fis- − ies. However, even with these more reliable data in place, sion products that undergo β n decay still remain to be significant disagreements are still observed when the refor- characterised in a similar manner, along with other delayed- mulated decay-heat calculations are compared with avail- neutron emitting nuclides of both lower and higher mass to able benchmark data [84]. As shown in Fig. 6, calculations assist in astrophysical and basic nuclear structure investiga- of the total and component energy release rate per fission tions [94]. − second for thermal-neutron irradiated 239Pu, in which the ad- Antineutrinos are emitted as part of the β -decay process justed JEFF3.1.1 decay data were used, differ significantly of fission products generated in the reactor core: from the decay-heat benchmark at cooling times between A → A + − + ν,¯ 5 and 8000 seconds (i.e. up to 2.2 h). Under these circum- Z X Z+1Y e stances, WPEC Sub-group 25 (SG-25) of the NEA-OECD addressed the requirements for additional experimentally- and undergo virtually no attenuation in their flux over long derived fission-product decay data of relevance to 235Uand distances. Therefore, their unchanged spectral signature 239Pu reactor systems, and emphasised the need for further could be used to monitor the efficacy of reactor opera- TAGS measurements [85]. A similar assessment exercise tions based on the nature of the irradiation, particularly with has been undertaken for Th-U fuel in which 233U(n,f) oc- respect to the detection of clandestine procedures and unan- curs – while U-Pu and Th-U fuels exhibit much in common, nounced fuel movements. Various collaborations have aided there are also a number of notable differences [86]. The two in the development of antineutrino detection techniques to sets of proposals for new TAGS studies are brought together probe reactor operations non-invasively [95, 96]. Reliable in Table 2. Some of the listed radionuclides represent repeat fission-product databases are also required that contain ra- measurements of Greenwood et al. as a sensible means of dionuclides from which the uncertainties identified with cross checking different sets of TAGS data [38]. A conse- Pandemonium have been removed by measurement (see β− quence of these particular assessments is that recent dedi- Sect. 4.2), allowing -decay data to be used with confi- cated TAGS studies have focussed on the β− decay of 101Nb, dence in the derivation of predicted antineutrino spectra for 105Mo and 102,104,105,106,107 Tc [41]. As depicted in Fig. 6,these comparison with and interpretation of the measured antineu- additional measurements have clearly addressed much of the trinos [88]. long-standing discrepancy in the γ component of the decay 239 heat for Pu between cooling times of 5 and 8000 s [87]. 5. Non-energy based applications However, the same cannot be said for the equivalent 235U decay-heat calculations – very little change arises from the The nature and properties of radioactivity can be success- introduction of the new decay-energy data, and significant fully used in a significant number of non-energy based ap- disagreements with the decay-heat benchmark remain for plications. Some applications are highly significant because cooling times up to 4000 secs. This significantly differing there are no satisfactory alternatives, while others represent impact of thermal-neutron fission on the release of decay important supportive procedures in various forms of inte- heat from 235Uand239Pu can be attributed to the marked grated multi-method approach (e.g. combinations of medical differences between the light-peak fission yields of the two treatment: surgery-chemotherapy-radiotherapy). systems. While further TAGS measurements are underway β− 87,88 94 137 to study the decay of Br, Rb and I[88], much 5.1 Radiation dosimetry remains to be done as specified above. Quantification of the absorption of atomic and nuclear radiation is important in defining the dose received and dam- 4.3 Other reactor-related issues age inflicted on structural materials and human tissues, and Delayed-neutron decay is an important process in the control hence the consequences of exposure to all forms of radioac- of nuclear fission, as well as providing insights into uncer- tive emission. Radiation dose refers to the amount of energy tain features of nuclear structure and the population of un- deposited in matter and the resulting biological effects that Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications 625

Table 3. Radionuclides used commonly in nuclear medicine for gamma imaging of body functions: relevant decay properties [10, 54–59].

Radionuclide Half-life Noteworthy gamma emissions: Eγ (keV), Pγ (%)

67Ga 3.2613(5)d 93.307(12), 38.1(7)%; 184.577(17), 20.96(44)%; 300.232(21), 16.60(37)%; 393.528(20), 4.59(10)% 81Rb/81m Kr generator 81mKr: 13.10s 190.46(16), 67.66(32)% 81Rb: 4.572(4)h 82Sr/82Rb generator 82Rb: 1.2575(2)min 511 annihilation, 190.9(4)%; 776.52(1), 15.08(16)% 82Sr: 25.34(2)d 99Mo/99m Tc generator 99mTc: 6.0067(5)h 140.511(1), 88.5(2)% 99Mo: 2.7479(6)d 111In 2.8049(4)d 171.28(3), 90.61(20)%; 245.35(4), 94.12(6)% 123I 13.2234(37)h 158.97(5), 83.25(21)% 131I 8.0233(19)d 80.1850(19), 2.607(27)%; 284.305(5), 6.06(6)%; 364.489(5), 81.2(8)%; 636.989(4), 7.26(8)% 133Xe 5.2474(5)d 80.9979(11), 37.0(3)% 201Tl 3.0421(17)d 135.312(34), 2.604(22)%; 167.45(3), 10.0(1)% are dependent on such parameters as radionuclidic activity, Specific radionuclides have been identified as potentially energy of the radiation, time of exposure, and distance from suitable for various life-saving applications. While their pro- the source. Data requirements for dosimetry studies are as duction routes and decay properties need to be defined with follows: confidence, some deficiencies remain, especially with regard 1. Isotopic abundances for all target elements, to the optimum generation of specific radionuclides, either 2. Cross-section data for perceived nuclear reactions, the minimization or elimination of impurities, and the accu- 3. Radiation damage cross sections, and rate quantification of various decay parameters [102–104]. 4. Decay data of all reaction product nuclides. The latter requirements include a sound knowledge of the half-life and α, β−, Auger-electron, conversion-electron, β+, Overall nuclear data requirements need to cover the oper- γ ation of accelerators, reactor cores and their fuel cycles, and X-ray energies and emission probabilities as appropri- and all relevant features of nuclear medicine involving diag- ate for patient dose-rate calculations. nosis, therapy, magnetic resonance imaging and associated Various radionuclides of the desired purity and decay research studies. characteristics have been adopted for the diagnostic imag- Safe operations of reactor power plant, related fuel-cycle ing of physiological and biochemical processes within the processes and non-energy-based nuclear procedures are de- human body. Nuclear techniques such as Single-Photon termined from the perspective of the envisaged radiation Emission Computer Tomography (SPECT) and Positron dose rates and the need to undertake reputable calculations Emission Tomography (PET) complement other imaging in the design and introduction of mitigating features such methods (e.g. magnetic resonance and ultrasound) to fur- nish an extremely powerful means of detecting functional as appropriate shielding. Dedicated libraries of nuclear data γ have been assembled to assist in such analyses, and would abnormalities and disease. A number of -emitting radionu- normally include sub-sets of reaction-product decay data. clides possess decay characteristics that are highly suitable The International Reactor Dosimetry File IRDF-2002 was for diagnostic studies by means of gamma cameras (Table 3) released in 2005/2006 [97], and has become internationally- – their chemical form is particularly important in ensur- recognised as a validated source of data assembled in the ing efficient and precise delivery to the body function(s) of desired format for safety studies of fission-reactor systems. interest. This file contains decay data for 85 radionuclides (58 ground Charged-particle cyclotrons and linear accelerators are states, 25 first isomeric states, and two second isomeric increasingly being used to generate radionuclides for both states), including half-lives, decay modes and intensities diagnostic and therapeutic purposes [105, 106], along with as taken from ENSDF (see Sect. 3.1,andRef.[10]) and nuclear reactors employed to produce various activation and reformatted for nuclear applications. Proposals have been fission products for similar applications. Sealed sources of 60 137 γ formulated to extend IRDF-2002 further as IRDFF-1.0 (In- Co and Cs provide external beams that penetrate ternational Reactor Dosimetry and Fusion File) in order the body and constitute the means of controlled therapeu- to encompass fusion systems [98, 99] – good progress has tic treatment to internal tumours. Studies of heavy-ion beam been made in this respect with the introduction of newly- therapy have provided striking evidence for the accurate evaluated excitation functions and related decay data [100]. delivery of a well-defined, localised dose to a particular site, and carbon-ion beams exhibit particular promise in this respect. Brachytherapy involves the use of sealed sources 5.2 Nuclear medicine placed in or close to the tumour, while attempting to en- Both diagnostic and therapeutic nuclear procedures extend sure minimal damage to healthy tissue – 103Pd, 125I(asX- across a wide range of medical activities to address and ben- ray emitters), 137Cs and 192Ir (as γ emitters) with effective efit human health in a safe and efficacious manner [101]. ranges of a few centimetres are commonly used, while β− 626 A. L. Nichols emitters include 32P, 90Yand188Re with effective ranges of the recommended measurements and evaluations of excita- a few millimetres. Radio-immunotherapy involves the co- tion functions in order to optimize the production of these valent bonding and labelling of monoclonal antibodies and radionuclides [111]. peptides with radionuclides such as 90Y, 131I, 153Sm and 213Bi for injection into the bloodstream, followed by transport and 5.2.1 Diagnostic γ -ray emitters attachment to tumours. PET has become an extremely note- 99m γ worthy and successful technique for the diagnosis of cancer Reactor-produced Tc is the most commonly used -ray – the most popular radionuclide for such studies has been 18F emitting radionuclide for diagnostic purposes, and both the (half-life of 1.83 h), which requires the radionuclide produc- cross-section and decay data are well known. Decay data for 67Ga, 97Ru, 111In, 123I, 147Gd, 201Tl and 203Pb are also rea- tion and patient treatment facilities to be close to each other 67 111 geographically. sonably well defined. Both Ga and In possess relatively The proposed use of radionuclides in nuclear medicine long half-lives and are rarely used anymore for diagnostic purposes, although they have been adopted as therapeutic necessitates a sound knowledge of their production cross 99m sections, half-lives and decay schemes, and much effort has radionuclides (see Sect. 5.2.4); Tc: Auger-electron and other low-energy electron decay data are required to assess been expended to derive such data through measurement and 123 evaluation. IAEA initiatives have included a Coordinated possible application in microdosimetry; I: Auger emis- Research Project (CRP) on “Charged particle cross-section sions may become an issue, if therapeutic applications arise database for medical radioisotope production: diagnostic ra- in the future. dioisotopes and monitor reactions” to consider the nuclear data requirements for diagnostic radionuclides [107], fol- 5.2.2 Positron emitters lowed by an equivalent study for therapeutic radionuclides Excitation functions for the four most commonly used short- defined as “Nuclear data for the production of therapeu- lived positron emitters in PET studies have been recently tic radionuclides” [108]. While both projects were primar- evaluated and well quantified (11C, 13N, 15Oand18F[107]). ily dedicated to measurements and evaluations of relevant Noteworthy decay data needs are as follows: 57Ni, 66Ga, cross sections for product nuclides and impurities, limited 72As, 73Se, 75Br, 76Br, 77Kr, 81Rb, 82mRb, 83Sr, 86Y, 89Zr, amounts of recommended decay data were tabulated from 94mTc, 120Iand121I: decay-data measurements and eval- ENSDF [10]. uations are required; 44Ti/44Sc, 52Fe/52mMn, 62Zn/62Cu, An IAEA consultants’ meeting was held from 3 to 5 72Se/72As and 140Nd/140Pr generators: decay-data measure- September 2008 in Vienna, Austria, dedicated to “High- ments and evaluations are required. precision beta-intensity measurements and evaluations for specific PET radioisotopes”. Participants assessed the decay 5.2.3 Therapeutic β−, X-ray and γ -ray emitters data for approximately 50 positron-emitting radionuclides, and recommended a series of measurements and evalua- Comprehensive production cross sections and relevant decay − tions to improve the known decay characteristics of ex- data for β -emitting radionuclides used to perform internal isting and potential PET nuclides [109]. Furthermore, an radiotherapy have undergone study in a recent IAEA CRP IAEA consultants’ meeting was held at the same venue from (see IAEA Technical Reports Series No. 473 for the produc- 21 to 24 June 2011 to discuss “Improvements in charged- tion of 32P, 89Sr, 90Y, 131I, 186Re, 188Re and others [108]). particle monitor reactions and nuclear data for medical iso- The decay data of sources used commonly in external radi- tope production”, and formulate a work programme for ation therapy (e.g. 60Co and 137Cs) and brachytherapy (e.g. − a Coordinated Research Project aimed at improving the ex- 192Ir) are also well defined. Focus is best placed on β citation functions of charged-particle monitor reactions and emitters with known discrepant data, and radionuclides be- the nuclear decay data for a number of medical radionu- lieved to possess future therapeutic potential (specifically − clides [110]. low-energy β and X-ray emitters): 67Cu, 103Pd, 161Tb, 169Er Continued developments in medical imaging and ther- and 175Yb: decay-data measurements and evaluations are re- apy will involve the utilization of improved diagnostic and quired; 191Pt/191mIr generator: require decay-data measure- therapeutic techniques, along with the production of po- ments and evaluation of 191Pt parent. tentially more effective and suitable radionuclides. Such an envisaged set of circumstances merits the consideration of 5.2.4 Therapeutic Auger-electron emitters future needs to expand and improve the content of nuclear 125I is the most commonly used Auger-electron emitter for databases for medical applications. Thus, a further tech- internal radiotherapy – both reactor-production and decay nical meeting was organised by the IAEA from 22 to 26 data are well characterized. The following radionuclides are August 2011 in which invited specialists were asked to as- also identified as potentially suitable for application with re- sess “Intermediate-term nuclear data needs for medical ap- spect to microdosimetry at the molecular and cellular levels, plications: cross sections and decay data” [111]. A sum- and therefore would require much improved Auger-electron mary of their recommendations for decay data is given decay data: 67Ga, 71Ge, 77Br, 99mTc, 103Pd, 111In, 123I, 140Nd, below, based on the envisaged requirements for (a) diag- 178Ta, 193mPt, 195mPt and 197Hg. nostic γ-ray emitters, (b) positron emitters, (c) therapeutic β−, X-ray and γ-ray emitters, (d) therapeutic Auger- 5.2.5 Therapeutic α emitters electron emitters, and (e) therapeutic α emitters, and covering an estimated timescale of approximately fifteen years. 230U/226Th: decay-data measurements and evaluations are The reader is also referred to the original IAEA report for required. Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications 627

5.3 Neutron activation analysis the number of suitable nuclear reactors is declining – with a lack of irradiation facilities, the technique has declined When nuclear research reactors became more accessible in in popularity, become more expensive, and has been re- an increasing number of countries during the 1960s, neutron placed to a significant degree by ICP-MS/AES (inductively- activation analysis (NAA) evolved as a common method of coupled plasma mass spectrometry and atomic emission determining low-level concentrations of the elements. Thus, spectroscopy) and PIXE (proton-induced X-ray emission a flux of 2 × 1013 n cm−2 s−1 provided the means of achiev- spectroscopy). ing detection limits of 10 ng or less for about 50 elements, given that any radiation from many interfering radionuclides can be removed by radiochemical separation. 5.4 Radioactive dating The introduction of solid-state Ge crystals as gamma- Natural radioactive decay can be used to determine the age ray detectors refined simultaneous detection considerably, of samples that contain a rather select number of specific and instrumental neutron activation analysis (INAA) and radionuclides and/or their related in-growth radioactive or prompt gamma-ray neutron activation analysis (PGAA) stable daughter nuclides. Atomic bomb tests in the atmo- became extremely powerful, non-destructive techniques. sphere over the course of the 1950s and early 1960s, along These neutron-activation procedures are capable of simul- with other nuclear activities and accidents, have also gener- taneous, in-situ multi-element assay in a rapid, quantitative ated discrete radioactive markers which provide the means manner across almost all of the Periodic Table from hydro- of dating certain environmental materials and samples ac- gen to uranium. Elemental coverage of PGAA complements curately. Such changes in nuclidic content are described by that of conventional, delayed INAA, and the list of mea- the standard radioactive decay laws, and provide very accu- surable elements includes the low Z and high abundance rate dating mechanisms based entirely on the half-lives of elements in organic and geological materials, along with the radionuclides involved [119–122]. high cross-section elements such as B, Cd, Sm and Gd. Analyses of hydrogen and boron are especially important 5.4.1 Time markers because of the paucity of other reliable analytical techniques for trace levels of these two elements. Together, PGAA and Atmospheric testing of large nuclear devices at specific INAA can measure all elements except oxygen in most com- times has deposited discrete amounts of radioactivity in the mon materials. environment. Under certain circumstances, some of these ra- Nearly every neutron capture involves the (n,γ) reac- dionuclides will remain firmly at their original location, and tion, and therefore the yield of prompt γ rays per neutron can be subsequently used as time markers. Thus, 3H, 14C, is greater than that of delayed γ rays. Unfortunately, PGAA 137Cs and 239Pu are known to mark the years 1962–63 ac- usually has a poorer sensitivity compared with that of INAA curately, and are firmly absorbed onto soils, sediments and because the neutron flux is some five orders of magnitude other materials, with the possibility of being buried by sed- lower in an external reactor beam than for irradiation ad- imentation and soil-forming processes, as well as by snow jacent to the core. Over the previous 20 years, the adop- precipitation. tion of PGAA has increased because of the growing avail- Cosmogenic radionuclides are produced in the earth’s ability of high-flux thermal and cold beams from neutron atmosphere by the interactions of high-energy cosmic radi- guides [112]. Such guided beams can be made free of fast ation with various available atoms such as nitrogen and oxy- neutrons and superfluous γ rays to give improved signal- gen. An important nuclide in this respect is 14C within the to-background ratios and spectral quality. Comprehensive natural CO2 bio-cycle, for which the sensitivity of detection review articles and books dedicated to PGAA have also been has increased dramatically with the advent of accelerator regularly published [113–115]. mass spectrometry (AMS) and the potential to analyse very A major advance in PGAA was the assembly of a com- small specimens. The most popular radioactive time mark- prehensive compilation of more than 10,000 neutron-capture ers based on direct measurements are listed in Table 4, along γ rays of the elements [116], quantified in terms of their with their recommended half-lives. energies, abundances and cross sections. Furthermore, a substantial computer readable sub-set of these data was made 5.4.2 Radioactive parent and stable decay product available on diskette (International Nuclear Geophysics Database-90 (INGD-90)), along with an IAEA Technical Specific radionuclides possess half-lives sufficiently long for Report [117]. This work continued through an IAEA initia- a significant fraction of their atoms to be present today, many tive from 1999 to 2005, designed to improve the quality and years after their formation and incorporation into the solar quantity of the nuclear data in order to apply PGAA reliably system. Identified as geochronometers, their existence pro- and with much greater confidence in such fields as materials vides an accurate means of quantifying the absolute ages science, chemistry, geology, mining, archaeology, environ- of events that go back to the formation of the solar system, mental monitoring, food analysis and medicine [118]. with even the potential to date events prior to this partic- Despite what has been written above, there are recog- ularly important process. Several such long-lived radionu- nised to be two drawbacks to the use of all forms of neutron clides are listed in Table 5, and their suitability is determined activation analysis: (a) even though the technique is es- by a number of critical factors: sentially non-destructive, the neutron-irradiated sample may – Maintenance of a closed system during the course of ra- remain radioactive for years after analysis, requiring the dioactive decay to ensure no escape of parent and daugh- need to implement strict handling and disposal protocols; (b) ter products from the geological feature/sample. 628 A. L. Nichols

Table 4. Radioactive time markers and their recommended half-lives [10, 54–59].

Parent radionuclide Half-life Comments

3H 12.312(25)y Time marker in glaciology and hydrology 10Be 1.387(12)× 106 a Lunar, meteor and marine geosciences, and glaciology 14C 5.70(3)× 103 a Archaeology, geology, glaciology, hydrology, etc. 26Al 7.17(24)× 105 a Lunar, meteor and marine geosciences 32Si 153(19)y Half-life covers time span inaccessible by 14C and 210Pb studies 36Cl 3.01(3)× 105 a Cosmology, geoscience and hydrology 129I 1.61(7)× 107 a Hydrology, and other possible applications 137Cs 30.05(8)a Recent sedimentation rates in aquatic systems, and accumulation rates of glaciers 226Ra-222Rn-210Pb 210Pb 22.23(12)a Dating over the previous 100 to 150 years (e.g. lake (226Ra 1.600 (7) × 103 a) sediments and glacier ice) 239Pu 2.4100 (11) × 104 a Recent sedimentation rates in aquatic systems, and accumulation rates of glaciers

Table 5. Long-lived radionuclidic chronometers applicable to geological and cosmological dating [10, 54–59].

Parent- Parent half-life Measured Comments daughter isotope ratio

40K(EC)-40Ar 1.2504(30)×109 a 40Ar/38Ar EC branch of 10.75(15)%, and β− branch of 40K(β−)-40Ca 40Ca/42Ca 89.25(15)%; tendency towards preference for 40Ar/39Ar ratio method (see Sect. 5.4.2) 87Rb(β−)-87Sr 4.81(9)×1010 a 87Sr/86Sr 138La(EC)-138Ba 1.02(1)×1011 a 138Ba/137Ba EC branch of 65.6(5)%, and β− branch of 138La(β−)-138Ce 138Ce/142Ce 34.4(5)%; not commonly used 147Sm(α)-143Nd 1.060(11)×1011 a 143 Nd/144Nd 176Lu(β−)-176Hf 3.76(7)×1010 a 176Hf/177Hf 187Re(β−)-187Os 4.33(7)×1010 a 187Os/188Os 190 186 11 186 188 186 15 Pt(α)- Os 6.5(3)×10 a Os/ Os T1/2 ( Os(α)) of 2.0(11) ×10 a; not commonly used

Th and U Noteworthy half-lives Measured Comments decay chains isotope ratio

232Th-208Pb 232Th 1.402(6)×1010 a 208Pb/232Th Dating based on secular equilibrium 228Ra 5.75(4)a 228Th 1.9126(9)a 235U-207Pb 235U 7.04(1)×108 a 207Pb/235 U Dating based on secular equilibrium; 231Pa 3.267(26)×104 a 235U/231Pa disequilibrium studies 227Ac 21.772(3)a 238U-206Pb 238U 4.468(5)×109 a 206Pb/238 U Dating based on secular equilibrium; 234U 2.455(6)×105 a 234U/230Th various disequilibrium studies 230Th 7.54(3)×104 a 226Ra 1.600(7)×103 a 226Ra/230Th 210Pb 22.23(12)a

– No daughter material incorporated into the geological have benefitted from determining particular isotopic ratios feature/sample during or after formation. of the daughter element (Table 5), and using these data – Able to quantify the parent and daughter products to the to calculate the date of sample formation based on the desired accuracy. assumption that the system has been closed to both par- – Well-defined parent half-lives. ent and daughter mobility from the time of interest to the present. These requirements present significant technical challenges Major natural phenomena prevent the retention of inert to geologists, analysts and measurers of decay data (lat- argon (and 40Ar) within the crystalline structure of many ter in their determination of the half-lives of such long- forms of geological feature, and also result in argon absorp- lived radionuclides). However, developments in analyti- tion from the atmosphere. Faced with such serious uncer- cal techniques such as high-precision mass spectrometry tainties in the long-term stability of argon content for sample and inductively-coupled plasma optical emission spec- analysis and the need to undertake two separate analytical troscopy have assisted greatly in improving the accuracy of procedures in order to measure parent potassium and daugh- geochronology. Furthermore, given the difficulty in measur- ter argon, research scientists have increasingly preferred to ing the absolute abundance of a specific nuclide, researchers date their samples of interest by the more sophisticated Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications 629 quantification of 40Ar/39Ar ratios that arise within samples 5.5 Other industrial and research applications irradiated in a well-defined fast-neutron flux. Within the carefully controlled conditions of a suitable nuclear reactor, Extremely unfortunate incidents and terrorist activities over 39 39 39 Ar can only be produced via the K(nf, p) Ar reaction, recent years have underlined the essential need to detect, which is dependent upon the amount of 39K within the ir- identify, localise and minimise/eliminate nuclear and radio- radiated sample, length of irradiation, neutron-flux density logical threats at both the national and international levels. and neutron-capture cross section of 39K, whereas the 40Ar Thus, there exists the need to develop increasingly user- content arises from the natural EC decay of 40K. Mass- friendly nuclear detection and identification systems in order spectrometric measurements can be made more precisely on to improve support towards ensuring the safety and security isotopes of the same element (argon), although corrections of the populace. Recommended nuclear data are a very im- may still be required to allow for the presence of atmo- portant aid in addressing criticality safety and sound stock- spheric argon and interference from other reactor-produced pile stewardship, and preventing the illicit trafficking of nu- isotopes. Under these better considered circumstances, stud- clear materials. Such onerous requirements include the need ies of 40Ar/39Ar ratios are more commonly undertaken in to adopt agreed sub-sets of recommended decay data from preference to 40Ar/38Ar ratio measurements. Other debat- a respected database such as ENSDF [10]. Thus, a particu- able assumptions and various analytical difficulties also exist larly specific set of well-defined data have been adopted with within the other systems listed in Table 5 [120–122], al- a reasonable degree of confidence in their correctness and though they will not be considered further in this particular permanency [124], hopefully to ensure a consistently correct review. approach through these parameters towards the maintenance When deemed to be in secular equilibrium, the three of the desired level of national and international security. naturally-occurring decay chains of 232Th, 235Uand238U Decay data adopted in such circumstances include the ra- to particular stable Pb nuclides constitute independent dionuclide half-life, major radiations, and energies of the geochronometers, based on the parent radionuclides pos- main γ-ray emissions to be effectively used as fingerprints sessing much longer half-lives than their respective daugh- for any radioactivity detected. ters (Table 5). Measurements of sample 208Pb/232Th, 207Pb/ Radioactive tracers are often used to measure the rates 235Uand206Pb/238U ratios in conjunction with their of reaction of chemical processes for (a) basic research pur- 208Pb/204Pb, 207Pb/204Pb and 206Pb/204Pb ratios, respectively, poses, (b) optimisation of industrial production routes, and provide a means of standardising and dating closed geo- (c) determination of the movement of molecules through logical systems accurately for ages greater than 30 million biological systems and tissue to obtain important physio- years [120, 122]. Non-radiogenic 204Pb is used as the refer- logical data and understanding for application in human ence isotope within such samples, possessing a very weak health, livestock control and plant husbandry. A number of radioactive half-life for α decay of ≥ 1.4 × 1017 a, which radionuclides have been commonly applied to studies of effectively permits the nuclide to be defined as stable. complex biochemical, biological and metabolic behaviour Externally-driven geological events can significantly dis- to resolve existing uncertainties, particularly the more suit- turb secular equilibrium. If measurements can be made of (a) able radionuclides of H, C, P, S and I. Research studies into subsequent decay of a radionuclide that has been separated reaction kinetics and mechanisms have also benefited im- from precursors, or (b) ingrowth of a radioactive daugh- mensely from these labelling techniques. Obvious care has ter, the resulting data can be used to establish the date of to be taken to adopt radiotracers that permit the phenom- the original geological disturbance (e.g. time at which par- ena of interest to be successfully monitored (e.g. by means ent separated from daughter nuclei during a sedimentation of γ-ray spectroscopy), with decay-data parameters defined process, and initiated radioactive disequilibrium). Important sufficiently well that they do not hinder accurate studies of information on the geological and geochemical history of the chemical reactions and transport mechanisms of inter- geographical features and deposits can be determined in this est [125–127]. manner [123]. Accurate information describing the spatial distribution The decay data of importance in radioactive dating are of a wide range of minerals in the Earth is an essential re- the half-lives and relevant branching fractions (e.g. for the quirement for the development of economic routes in the EC and β− decay modes of 40Kand138La). Geologists, geo- exploration, extraction and processing of oil, coal, metallif- chemists and other specialists in the field of geochronometry erous and non-metalliferous materials, including such prop- believe that these parameters need to be known with greater erties as total clay content, grain density and porosity. Rapid accuracy in many cases. Uncertainties in the parent half-life on-line elemental analysis of natural products such as oil and range from 1.0% for 138La and 147Sm, 1.6% for 187Re, 1.9% coal (H, C, N, O, Al, Si, S, Ca, Fe), raw minerals for con- for 87Rb and 176Lu,toasmuchas4.6% for 190Pt – half-lives struction (O, F, Na, Mg, Al, Si, P, S, K, Ca, Ti, Mn, Fe), for these long-lived radionuclides need to be determined and mixed ores for manufacture (Al, Mn, Fe, Ni, Cu, Ag, Au with improved accuracy in order to derive more precise ages. and others) can be achieved by means of nuclear borehole The equivalent data for both 40Kand87Rb, and the Th-U- logging [117, 127]. The popular neutron activation sources Pb decay chains are known to higher precision, and therefore are 252Cf, 241Am-Be and 14-MeV neutron tubes based on can be adopted in age calculations with greater confidence – d(t, n)4He. Decay-data needs that arise from such remote ir- however, there are other more substantial issues to consider radiations can be readily adopted from ENSDF [10]: natural in such studies that relate to analytical difficulties and the abundances, half-lives, decay modes and branching frac- extremely important prerequisite for sample stability over tions, and the energies, intensities and uncertainties of the many years. γ rays emitted following neutron capture were collected to- 630 A. L. Nichols gether in the early 1990s to create the International Nuclear 1. combination of PET and radiotherapy derived from ra- Geophysics Database (INGD-90) [117]. dioimmuno reactions, and Nuclear astrophysics constitutes productive collaboration 2. Auger-electron and α-particle therapy at the cellular and between astrophysicists and nuclear physicists to study the molecular levels. γ nuclear reactions of relevance to stellar modelling, -ray, Long-term decay-data needs in nuclear medicine will de- optical and X-ray astronomy, and the formation and evolu- pend upon the relative success of such proposed develop- tionary existence of the cosmos. A primary aim is to improve ments and the commensurate identification of the most suit- our understanding of the origins of the universe through able radionuclides in these studies and medical treatments. the concept of nucleosynthesis, which can be defined in Overall, demands are expected to gravitate towards positron terms of the formation mechanisms of the elements and the emitters and appropriate therapeutic radionuclides, as out- commensurate generation of energy in the stars. Various lined in Sect. 5.2. An increased adoption of metallic-based proposed cosmological phenomena need to be satisfacto- positron emitters can be envisaged to occur as a consequence rily addressed through nuclear physics (helium fusion; s, of improved coordination chemistry (especially for Ti, Ga r and rp processes; X-ray processes; etc.) in order to im- and Cu radionuclides) arising from advances in the prep- prove our understanding of the driver mechanisms for ac- aration of organometallic compounds. The consequences of tive galaxies, core collapse, supernova neutrinos, neutron adopting highly-focused therapeutic treatment through mi- stars and black holes. Spectral studies of complex nuclear crodosimetry techniques will also require improved char- structure play an important role in addressing the funda- acterisation and quantification of the atomic and nuclear mental nature of the universe, and will continue to do so decay data of the most suitable low-energy Auger-electron for many years to come. Recent and future advances in our emitters. ability (a) to generate unstable radionuclides close to the Another noteworthy decay-data issue is the requirement proton and neutron drip lines, and (b) to detect and quan- for more accurately known half-lives of specific long-lived tify their decay characteristics will aid considerably in the radionuclides that are used as geochronometers in the dating advance of basic research within both nuclear physics and of localised geological events (see Sect. 5.4) and nucleosyn- astrophysics. thesis within the solar system. The half-lives and associated uncertainties of 87Rb, 176Lu, 187Re and 190Pt may be significant in this respect, although other features of such work are 6. Concluding remarks and future perspectives of considerable importance, particularly the requirement for sample stability over many hundred-million years and po- An extremely important requirement in the adoption and ap- tentially serious inadequacies in the adoption of a particular plication of radioactive decay data is that such numerical analytical technique to quantify multi-element and multi- data and their uncertainties should be sound and sufficiently isotope content. accurate. These needs have been satisfactorily addressed Requests for new nuclear data measurements are driven with respect to the main fission products and actinides pro- primarily by the results of methodical evaluations to iden- duced in power reactors, although highly-specific decay- tify inconsistencies and unforeseen gaps in the desired data. data measurements and evaluations remain outstanding to Thus, a healthy synergy exists in which αβγ spectroscopy address particular needs in decay-heat calculations and the laboratories undertake decay-data studies driven by data in- potential future implementation of various high burn-up fuel adequacies highlighted by decay-scheme evaluations of dir- strategies. Other significant issues that have arisen in recent ect relevance to basic nuclear physics research and appli- years include the provision of improved delayed-neutron cation in the industrial, medical and environmental sectors. β− data, and the beneficial requirement for well-defined Under these circumstances, the communication and trans- spectra to assist in non-invasive studies of the antineutrino fer of such data and information has improved considerably, emission from operational reactors. and large quantities of nuclear data can be rapidly accessed A wide range of activation products are identified with within seconds, thanks to the work undertaken over many various proposed designs of fusion reactor. Their decay years by staff at the various national and international nu- properties also need to be sufficiently well known to ad- clear data centres. dress standard operational requirements, and study radiation dosimetry, longer-term radiotoxicity, and the bulk handling of activated structural materials as radioactive waste. While 7. Data uncertainties much of the decay data would appear to be readily available from various well-established databases, the quantity of Data and their uncertainties are presented throughout the ( ) such data would suggest a need for careful assessment to text and tables in the form 1234 x ,wherex is the uncer- ensure all envisaged requirements are met. tainty in the last digit or digits quoted in the measured or Significant diagnostic and therapeutic developments con- evaluated number. This uncertainty is generally expressed at σ ( ) ± tinue to occur in both organ imaging and radiotherapy. Thus, the 1 confidence level. Examples: 1739 8 means 1739 . ( ) ± . ( ) × +6 efforts are underway to couple positron-emission tomogra- 8; 0 0171 22 means 0.0171 0.0022; 1 13 17 10 ( . ± . ) × +6 phy (PET) with X-ray computed tomography (CT) and mag- means 1 13 0 17 10 . netic resonance imaging (MRI), with the aim of achieving Acknowledgment. Advice and information has been gratefully received comprehensive and detailed organ assessment. New possi- from Roberto Capote Noy (IAEA, Vienna, Austria), Christopher J. bilities for improved internal radiotherapy are also being Dean (Serco, Winfrith Newburgh, Dorset, UK), Filip G. Kondev (Ar- explored on the basis of the following procedures: gonne National Laboratory, USA), Robert W. Mills (National Nuclear Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications 631

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