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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 2 Department of Physics, University of Surrey, Guildford, GU2 7XH, UK 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 / Decay data evaluations / Decay data files / Reactor operations / Fuel cycle applications / Non-energy applications / Nuclear medicine Summary. Decay data are commonly used to characterise and quantify radioactive material, and provide an important means of understanding the properties and structure of the nucleus. Experimental measurement techniques are reviewed, with the emphasis placed on recent developments that represent a potential quantum leap in advancing our knowledge, particularly by means of γ -ray spectroscopy. A select number of internationally-accepted decay-data evaluations and compilations are also discussed in terms of their contents. Both energy and non-energy related applications require the input of well-defined decay data, and such activities have been reviewed. Various important decay-data issues are assessed, and note taken of any significant requirements for better quantified data. 1. Introduction In-depth assessments, evaluations and measurements of radioactive decay data have been requested and undertaken over many years. Recommended decay data are normally derived from all relevant publications that include quantification of decay-scheme data primarily by means of direct measurement but also by calculation. The measurement and derivation of such recommended data sets are welcomed by nuclear physicists and engineers (a) to define the status and our current knowledge of particular decay parameters, and determine whether there is a need for further investigation and study, and (b) hopefully to provide highly reliable input data for modelling codes in order to quantify the operational characteristics and behaviour of irradiated fuel and other materials with reasonable confidence. Atomic and nuclear decay-data parameters encompass the following [1–3]: half-life; total decay energies (Qvalues); branching fractions (if more than one known decay mode); α-particle energies and emission probabilities; β − -particle energies, emission probabilities and tran*E-mail: [email protected]. sition types; electron-capture and β + -particle energies, transition/emission probabilities and transition type (also EC/β + ratios when appropriate); γ -ray energies, emission probabilities and internal conversion coefficients (also internal-pair formation coefficients for β + β − when appropriate); Auger- and conversion-electron energies and emission probabilities; X-ray energies and emission probabilities; spontaneous fission properties (branching fraction and recoil energies); delayed-neutron energies and emission probabilities; delayed-proton energies and emission probabilities; and comprehensive quantification of the uncertainties associated with all of the above atomic and nuclear parameters. Additional ancillary data requirements can be met from the above, including various total mean energies which need to be quantified and adopted for particular applications: mean heavy-particle energy (includes mean α, neutron, proton, fission fragment, and associated recoil energies); mean light-particle energy (includes mean β − , β + , Auger-electron and conversion-electron energies); and mean electromagnetic energy (includes mean γ , X-ray, β + β − annihilation radiation and internal bremsstrahlung). While more exotic modes of decay have been detected (e.g. double-beta (ββ) and cluster/heavy-ion decay), these low-probability phenomena are not considered further in this review. The need for well-defined radioactive decay data was recognised over 80 years ago with the publication of a paper by the International Radium-Standards Commission which included such world-renowned scientists as Marie Curie, Otto Hahn, Hans Geiger and Lord Rutherford [4]. Recommended radioactive constants were proposed with no uncertainties, based predominantly on known measurements by members of the Commission and their co-workers. This work led on to more extensive nuclear reaction data listings by Fea [5] and Livingston and Bethe [6], and the first recognizable Table of Isotopes format by Livingood and Seaborg in 1940 [7] that appeared every four or five years in Reviews of Modern Physics until 1958. Subsequent editions of the Table of Isotopes have been published at regular intervals up to an including the 8th edition in 1996 [8], which also contains a CD-ROM of the full contents. Recommended nuclear structure and decay data for this particular edition of the Table of Isotopes have primarily been extracted from the Evaluated Nuclear Structure Data File (ENSDF, see below). Unauthenticated Download Date | 6/12/17 1:06 PM 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 Academic Press, and subsequently by Elsevier Inc.) and the Evaluated Nuclear Structure Data File (ENSDF) [10]. Evaluations 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 biennial 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 chromatography, application of many forms of liquid-liquid extraction, and dry distillation [12–14]. For example, the adoption of various radiochemical techniques to achieve high levels of radionuclidic purity was very important in forming the basis for accurate measurements of the positron emission probabilities of 64 Cu, 76 Br and 124 I for medical applications [14]. Long-established experimental techniques can be used to quantify in detail specific features of a decay scheme, ranging from α, γ and electron spectroscopy operated in singles and various coincidence modes, time-dependent studies of these emissions to determine important parameters such as half-lives, and angular correlation measurements for greater structural detail. The more substantive techniques are briefly discussed below, along with some thoughts on future developments. 2.1 α-spectroscopy Obviously, measurements of α spectra play an important role in quantifying and defining the decay schemes of αparticle emitting nuclides, and impact most significantly on studies of the many heavy elements and actinides. One loss over recent years has been the decline in maintenance of dedicated magnetic spectrometers that offer extremely good energy resolution. Precise, well-defined studies of α spectra were feasible with homogeneous-field magnetic spectrographs [15, 16]. Silicon-based ionization detectors such as the silicon barrier detector (SBD) and passivated implanted planar silicon (PIPS) detector are now much more commonly used to measure the energies and emission probabilities of α particles [17]. Good resolution α spectra obtained by means of a 450-mm2 PIPS detector are shown in Fig. 1 for mass-separated sources of 237 Np and 243 Am [18]. Fig. 1. Alpha-particle spectra of thin mass-separated sources of (a) 237 Np, and (b) 243 Am measured by means of a 450-mm2 passivated implanted planar silicon (PIPS) detector – main α peaks are labelled in keV energy units [18]. Significant developments have recently occurred with respect to improvements in energy resolution by means of cryogenic microcalorimetry: 1. Detector system consisting of a superconducting transition-edge sensor (TES) with Mo:Cu bilayer and an absorber of superconducting tin has been shown to give an energy resolution of (1.06 ± 0.04) keV FWHM for 5.3 MeV α particles [19, 20]. 2. Sensor of gold doped with a small concentration of erbium (Au:Er) for which the magnetization changes as a function of modification in temperature by α-particle absorption – energy resolution of (2.83 ± 0.05) keV FWHM was determined for 5.5 MeV α particles [21]. Such ultra-high resolutions are a significant improvement beyond the theoretical limit of conventional silicon detectors. Alpha-particle measurements with this type of detector system would greatly reduce uncertainties in decay schemes and specific aspects of their decay data, with an inevitably beneficial knock-on effect involving the accuracy and efficacy of their application. 2.2 X- and γ -ray spectroscopy The extremely successful development and adoption of silicon and germanium crystals as detectors in X- and γ -ray spectroscopy has contributed immensely to our understanding of the atomic nucleus across the known Chart of the Nuclides. Since the late 1960s, Si- and Ge-based detectors Unauthenticated Download Date | 6/12/17 1:06 PM Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications 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 233 Pa 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 found in the substantial review articles of Lee et al. [22], and Eberth and Simpson [23]. Only a brief semi-historic summary is given below. Rapid progress in the discovery and understanding of nuclear structure and decay data in the 1970s was almost en- Fig. 2. Gamma-ray spectrum of a chemically-purified 233 Pa source measured by means of a 2-cm2 × 1-cm planar low-energy photon spectrometer (LEPS), at a source-to-detector distance of 5 cm – main γ peaks are labelled in keV energy units [18]. 617 Fig. 3. Half of the modular layout of HPGe detectors for (a) Gammasphere, and (b) Euroball. tirely related to the advent of Ge(Li) detectors in γ -ray spectroscopy. 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. Escapesuppression 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 detectors and BGO (bismuth germanate (Bi4 Ge3 O12 )) shields have proved to be very costly, and therefore resulted in the evolution of only two recognised primary projects (Fig. 3): Gammasphere – 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]. Scientific interest has tended to focus in recent years on extraordinary N/Z ratios close to the proton and neutron drip lines as a consequence of the growing need to support Unauthenticated Download Date | 6/12/17 1:06 PM 618 the study of nuclear reactions generated by emerging radioactive ion beam facilities (RIB). Theoretical analyses and experimental studies on Ge-based detector design has led to the concept of γ -ray tracking in terms of the position, energy and time of all γ -ray interaction points within Ge detector systems: – GRETA (Gamma Ray Energy Tracking Array, USA [22]; geodesic configuration of 120 to 130 segmented HPGe detectors); – AGATA (Advanced GAmma Tracking Array, Europe [23, 26]; geodesic configuration of 180 hexagonal and 12 pentagonal segmented HPGe detectors). Identification of the interaction points and quantification of the energy deposited at these locations can be achieved with highly-segmented Ge detectors and pulse-shape analyses. However, the sequence of a γ -ray scattering process is too fast for measurement compared with the time resolution of a Ge detector, and therefore tracking algorithms based on the underlying interaction processes have to be used. While GRETA and AGATA will permit access to the farthest reaches of the Chart of the Nuclides, other areas of nuclear application will also benefit from the operational advances in Ge detector technology. Thus, the development of position-sensitive γ -ray detectors for nuclear structure studies will have important repercussions in astrophysics, medical imaging and nuclear safeguards. GRETINA represents a significant stepping-stone towards GRETA, and consists of coaxial Ge crystals of tapered hexagonal shape that make up seven modules, each containing four 36-fold segmented crystals (Fig. 4). This set of 28 Ge crystals covers a quarter of the 4π solid angle envisaged for GRETA, and will be extensively tested in a series of nuclear structure, reaction and symmetry studies [27]. Similarly, an AGATA sub-array consisting of five triple clusters of highly-segmented Ge detectors has been assembled at Laboratori Nazionali di Legnaro, Padova (total of 15 Ge Fig. 4. Quarter-based layout of the GRETINA detectors (foreground) compared with the full 4π solid-angle system proposed for GRETA (background). A. L. Nichols crystals compares with the full AGATA 4π geometry of 60 triple clusters (180 Ge detectors)). Complementary analysis systems are available to operate in conjunction with the AGATA sub-array, and so extend the measurement capabilities to a broader range of nuclear physics topics [28]. Assembled stacks of planar double-sided Ge strip detectors (DSSD) possess sufficient pixelation to achieve γ -ray positional resolution [22]. Possible drawbacks are envisaged, such as the existence of dead layers at the edge of each planar crystal and the need for mechanical structure to ensure the provision of sufficient cooling to the HPGe detectors. Nevertheless, such systems are being studied for their γ -ray tracking potential and possible application in highresolution photon imaging. The ability to discriminate between γ rays of slightly different energies by means of a scintillator is of importance in medical imaging, γ -ray spectroscopy and X-ray astronomy. Both NaI(Tl) and CsI(Tl) have been the scintillators of choice for over 50 years because of their reasonable energy resolution, while other scintillators such as Bi4 Ge3 O12 in positron emission tomography (PET) and PbWO4 in highenergy nuclear physics have found appropriate application. However, lanthanum halides doped with Ce3+ have also been shown to exhibit significant promise based on good energy resolution in combination with fast luminescence decay [29–31]. The superior energy resolution of LaBr3 (Ce) scintillator has successfully been used with Gammasphere to quantify with good precision the lifetimes of nuclear levels between 50 ps and 1 ns, as populated in the IT and β − decay of 177m Lu [32]. Potentially new and improved scintillators are constantly being assessed for adoption in high-resolution γ -ray spectroscopy, and studies of their performance provide essential guidance as to their suitability and application [33]. Another noteworthy technique for accurate X- and low-energy γ -ray detection is the X-ray microcalorimeter [34, 35]. Effectively, the detector system consists of 36 microcalorimeters in 6 × 6 array, although only 32 are electronically 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 229 Th, 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 4π γ -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 to achieve an approximately 97% 4π solid-angle geometry. Various other ancillary γ -ray detectors may also be used to study coincidence gating. More recent assemblies have Unauthenticated Download Date | 6/12/17 1:06 PM Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications involved the assessment, design and construction of other suitable detector materials and arrays. Good overall detection efficiency and energy resolution are required, along with a sound spectral understanding in order to disentangle the decay components from each other and any background activity. TAGS has proved to be of importance in the development of a more correct and comprehensive understanding of the nucleus, reaching beyond some of the specific limitations of γ singles and coincidence measurements [39, 40]. Furthermore, TAGS studies have been used to determine the average β − and γ energies of particular fission produces, aiding considerably in calculations of the decay heat emitted by irradiated reactor fuel [38, 41] – see also Sect. 4.2. 2.3 Electron spectroscopy Appropriate studies of β, Auger-electron and internal conversion-electron emissions have yielded important data in the resolution of difficulties associated with the population and depopulation of daughter nuclear levels and the derivation of a normalisation factor to convert relative γ -ray emission probabilities to absolute values. Various types of detector system have been used, e.g. permanent-magnet 180◦ spectrograph, permanent-magnet pre-accelerating √ spectrograph, six-gap spectrometer and iron-free π 2 doublefocusing spectrometer [42–44]. While sound quantification of electron emissions has proved important input to the evolution of fully consistent decay schemes and well-defined decay data, this type of complex spectrometric study has been seriously neglected in recent years. A major problem is the bulky nature of the magnet systems, coupled with the delicate nature of the multi-element detector assemblies – their maintenance and operation have historically proved to be costly. β − intensity distributions are normally determined from any calculated imbalances in the γ population and depopulation of nuclear levels of the decay schemes assembled and quantified by means of γ -ray spectroscopy. However, such experimental studies can be fraught with difficulties resulting from the failure to detect all of the weak γ transitions. Faced with this basic problem of omission in the study of complex decay schemes, Total Absorption Gamma-ray Spectroscopy (TAGS) has been used to determine unambiguous β − feeding data, as outlined above in Sect. 2.2. 2.4 Specific activity and mass spectrometry Accelerator mass spectrometry (AMS) and thermal ionization mass spectrometry (TIMS) have been successfully used to determine the half-lives of long-lived radionuclides by counting the number of ions of interest after acceleration, and relating these data to the specific activity of the radionuclide in the ion source. Normal procedures involve irradiation processes to generate the radionuclide of interest, followed by purification through the removal of undesirable elements/species by chemical separation. Small fractions of the resulting purified solutions are used to prepare targets for mass spectrometric studies in order to determine radionuclide/element ratios, and measurements are also made on carefully weighed bulk samples to determine their specific γ -ray activities by means of standard 619 γ -counting techniques. The half-life of the radionuclide of interest is determined by substituting the results of the two forms of measurement into the basic equation of radioactive decay. Half-lives of approximately 20 to 50 years and above can prove awkward to measure by standard spectroscopic techniques, and have been shown to benefit from quantification via ultrasensitive mass-spectrometric measurements of isotopic concentrations. Examples of the successful adoption of this approach include 32 Si (half-life of 133 ± 9 a [45]), 44 Ti (half-life of 54.2 ± 2.1 a [46]), 60 Fe (half-life of (1.5 ± 0.3) ×106 a [47]), 126 Sn (half-life 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 spectrometry (ICP-MS) and specific activity measurements to determine the half-lives of 79 Se (half-life of (3.77 ± 0.19) × 10+5 a [51]) and 126 Sn (half-life of (2.33 ± 0.10) × 10+5 a [52] and (1.98 ± 0.06) × 10+5 a [53]). Further mass-spectrometric studies should be encouraged to address existing anomalies and uncertainties that remain in half-life values for long-lived radionuclides of recognised importance (i.e. 32 Si, 60 Fe, 59 Ni, 79 Se and others). 3. Recommended decay data Compilations and evaluations of nuclear data have been produced since the early 1930s to inform and assist nuclear physicists, α and γ spectroscopists, and basic nuclear science researchers. More specifically, extended libraries of reaction cross sections, fission yields, nuclear structure and decay data have evolved in well-defined formats for application by nuclear engineers and plant operators in power generation, fuel reprocessing and waste management. As outlined below, such libraries have historically been updated over agreed time intervals either by means of international consensus, or more localised efforts based on immediate national needs. Regular improvements to and subsequent controlled studies of the contents of a decay-data library also provide a rapid means of assessing their applicability and suitability – such activities assist greatly in defining further requirements to improve the existing decay data. 3.1 Decay-data files Consideration has been given to the most comprehensive databases that are judged to be most relevant to defining the status of and existing requirements for improved decay data: 1) ENSDF (Evaluated Nuclear Structure Data File) – consists of nuclear structure and decay data for all known radionuclides (www.nndc.bnl.gov/ensdf/). Complete mass chains are evaluated at regular intervals (nominally every seven to ten years) by members of the International Network of Nuclear Structure and Decay Data Evaluators, and published in Nuclear Data Sheets [10]. The strength of ENSDF is to be found within the completeness of this comprehensive database (e.g. see Fig. 5 for the IT decay data of 99m Tc in ENSDF). NuDat represents an electronic chart of the nuclides based primarily on the contents of ENSDF and constitutes a user-friendly means of accessing decay data (www.nndc.bnl.gov/nudat2/), while MIRD Unauthenticated Download Date | 6/12/17 1:06 PM 620 A. L. Nichols Fig. 5. Nuclear data sheet for the IT decay of 99m Tc (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. Unauthenticated Download Date | 6/12/17 1:06 PM 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 nuclides a Comments 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 processing ENSDF decay data through the RADLST code to give the energies and intensities of all emitted radiation, along with their dose rates (www-nds.iaea.org/mird/). NSR (Nuclear Science References) is an ancillary bibliographic database through which mass-chain evaluators and other users can accesses all publications of interest (www-nds.iaea.org/nsr/index.jsp). 2) DDEP (Decay Data Evaluation Project) – contains comprehensive atomic and nuclear decay data evaluations of selected radionuclides, with emphasis placed on the completeness of each decay scheme and the derivation of X-ray and electron decay data [54–60]. Multinational evaluations are submitted via a coordinator to the custodian of both the database and associated comments files at the Laboratoire National Henri Becquerel, CEA Saclay, France. Comprehensive decay data for 194 radionuclides are contained within the DDEP files (www.nucleide.org/DDEP_WG/ DDEPdata.htm), as observed on 4 June 2012. 3) Relevant IAEA databases – a number of nuclear data initiatives organised as Coordinated Research Projects (CRPs) under the auspices of the International Atomic Energy Agency (IAEA) have resulted in the generation of highquality decay data. a) Update of X ray and gamma ray decay data standards for detector calibration and other applications, 1998– 2005 [61–63], www-nds.iaea.org/xgamma_standards/, www-nds.iaea.org/publications/tecdocs/sti-pub-1287_Vol1. pdf, www-nds.iaea.org/publications/tecdocs/sti-pub-1287_ Vol2.pdf. Nuclides within this particular database of high relevance to nuclear medicine include 60 Co, 64 Cu, 66 Ga, 67 Ga, 99m Tc, 111 In, 123 I, 125 I, 131 I, 137 Cs, 153 Sm, 166 Ho, 169 Yb, 192 Ir and 201 Tl [62, 63]. b) Updated actinide decay data library, 2005–2011 [64–66] – data also forwarded for inclusion in the DDEP database, and listed on the web site www.nucleide.org/ DDEP_WG/DDEPdata.htm. Nuclides within this particular decay database address requirements in (i) nuclear power generation with the inclusion of all important Th, Pa, U, Np, Pu, Am and Cm radionuclides and their natural decay products, and (ii) nuclear medicine applications identified with 212 Bi (228 Th decay chain), 213 Bi (229 Th/225 Ac decay chain), 211 At/211 Po, 223 Ra (within 227 Ac decay chain), 241 Am and Cf [66]. Other nuclear data libraries have been assembled and subsequently updated in various ways, based on the guidance and requirements of the nuclear power industry and related fuel-cycle activities. These decay-data sub-libraries have been prepared via a combination of evaluation and direct adoption of recommendations from respected data sources (e.g. data extraction from ENSDF, DDEP and other evaluated atomic and nuclear data files). These recognised nuclear applications databases are maintained in the internationally-accepted ENDF-6 format [67], and are briefly described in Table 1. JENDL-FP files have been assembled by staff working at the Japan Atomic Energy Research Institute [68], and now part of the Japan Atomic Energy Agency; JEFF3.1.1 constitutes a combination of multinational assessments and evaluations carried out under the auspices of the Nuclear Energy Agency of the OECD [69]; ENDF/B-VII.1 is a predominantly USA initiative with a number of significant international inputs (latter are mainly files dedicated to nuclear reaction excitation functions) and retention of various data from the earlier ENDF/B-VII.0 library [70, 71]. The decay data include half-lives, decay modes and Q-values, branching ratios, average decay energies, α, β, EC/β + , Auger-electron, conversion-electron, γ , X-ray, etc. energies and emission probabilities, and various other nuclear parameters (e.g. internal conversion coefficients), along with comprehensive uncertainty data. Communication links with many electronic decay-data files are rapid and comparatively easy to implement in the age of the PC, Internet, CD and DVD. As mentioned above, NuDat provides a user-friendly means of extracting decay data from ENSDF (www.nndc.bnl.gov/nudat2/); LiveChart also accesses ENSDF, and offers the user a wide range of procedures to interrogate this database and display the nuclear parameters of interest in various ways (www-nds.iaea.org/relnsd/vchart/index.html). JANIS constitutes an equivalent software platform for the rapid inspection and display of numerical data within the JEFF nuclear data library (www.oecd-nea.org/janis/). Dedicated catalogues of γ -ray spectra have been assembled to assist greatly in the characterisation and quan252 Unauthenticated Download Date | 6/12/17 1:06 PM 622 tification of the radioactive content of materials [77, 78]. Systematic studies of the γ -ray emissions of individual radionuclides have been carried out by means of NaI(Tl) scintillation, Si(Li), Ge(Li) and HPGe detectors, and these “fingerprint” spectra are of immense value in spectroscopic analyses. With the advent of the Internet, these γ -ray spectrum catalogues and enhancements have conveniently been made available in electronic form through the Web site http://www.inl.gov/gammaray/catalogs/catalogs.shtml, for NaI – http://www.inl.gov/gammaray/catalogs/pdf/naicat.pdf, and for Ge and Si(Li) detectors – http://www.inl.gov/ gammaray/catalogs/pdf/gecat.pdf. 3.2 Status of decay data Important work continues or is being planned to explore further structural facets of the nucleus, particularly through the use of radioactive ion beams to study specific areas of the chart of the nuclides in a highly focussed manner. Furthermore, significant efforts are constantly being made to respond fully to the demands of decay-data users in support of their energy and non-energy applications. Accurate measurements and sound evaluations of decay data have been proposed and are underway to provide information to assist in basic nuclear physics research and ensure the efficacy and validity of nuclear data as applied to fission and fusion reactor physics, nuclear medicine and analytical science. Some of the sub-libraries of recommended decay data are primarily dedicated to reactor operations and fissionbased fuel cycles (JENDL-FP, JEFF3.1.1 and ENDF/BVII.1), although they can be used further afield for fusion research studies and various non-energy applications, if desired. Updating of these ENDF-6 decay-data files is predominantly reliant on the regular evaluation efforts within the ENSDF initiative, although some sub-libraries do contain a limited number of recommended decay-data files that have been individually evaluated and assembled in a more dedicated manner (e.g. within JEFF3.1.1). DDEP evaluations are based on the implementation of a series of agreed evaluation procedures, with the aim of assembling high-quality files of recommended decay data of immediate value as standards in radionuclide metrology; these particular data sets have also been assessed and are now supported by the Bureau International des Poids et Mesures (BIPM) as a means of encouraging worldwide adoption. Specific inadequacies and uncertainties remain to be addressed with respect to the decay-scheme data for a number of radionuclides, whether considering either a particular radionuclidic application (e.g. nuclear medicine), or encompassing a relatively large number of radionuclides (e.g. shielding and decay heat calculations). These envisaged needs are discussed in Sects. 4 and 5, below, covering such disparate issues as power reactor control and safety, shielding/storage of irradiated fuel, radiation dosimetry, nuclear medicine, radioactive dating, and astrophysics. Further work is required to build on previous initiatives such as IAEA CRPs through consideration of possible decay-data needs over the next 5 to 15 years. Improvements to some of the existing data are required, along with new additions to ensure that some pre-emptive measures are in place to address our future needs. A. L. Nichols 4. Applications in reactor technology Along with various neutron-induced cross sections as a function of neutron energy, the decay data of actinide fuels, their (n, γ ) reaction products and heavy-element decay chains, fission products and activation products are important inputs to the calculation of the radionuclide inventories generated by irradiated fuel bundles within the reactor core of a power reactor. Such inventory data are essential in the plant design (particularly the provision of adequate shielding), operational safety measures that include annual shutdown procedures, decommissioning, fuel handling, storage and reprocessing, and waste disposal. Furthermore, studies of efficient transmutation by means of the recycling of high burn-up fuel need to be assessed from the points of view of power production and reducing the quantities of long-term actinide and fission-product waste inventories. Stable and radioactive nuclides introduced into inventory calculations are of the order of 120 actinides and heavyelement nuclides within their decay chains, 1050 fission products, and 700 possible activation products. While the decay data of the important actinides (identified as specific Th to Cf radionuclides), their heavy-element decay-chain products and the commonly-occurring activation products are judged to be sufficiently well quantified for purpose, a number of notable fission products merit improvement and further experimental study (see below). 4.1 Normal operational characteristics of reactor systems Many technical features of reactor operations such as criticality and power distributions are basically dependent on neutron physics (neutron-induced cross sections, angular distributions, secondary energy distributions, and ν̄ (average prompt neutron emissions)). The importance of well-defined decay data arises when consideration is given to the following: (a) shielding requirements, (b) proposed utilisation of recycled and high burn-up fuel, and (c) irradiated fuel storage and reprocessing, along with the subsequent needs of waste management. Power, rod worth and structural damage are regularly assessed during plant operation, and in this regard the decay of short-lived fission and activation products need to be known. Furthermore, changes occur in the fuel, structure and absorber materials under neutron irradiation, and the half-lives and emissions of all major fission products (of the order of 90 radionuclides) and the actinide decay chains are required as inputs in such analyses. Modelling codes have been developed to determine neutron and gamma transport within and in close association with the reactor core. The gamma decay data are normally grouped into bins for ease of calculation, covering the energy range from zero to 10 MeV. Half-lives and branching fractions are also required as input and, together with inventory calculations, provide the means of performing time-dependent studies of the necessary shielding requirements. Equivalent shielding studies are also undertaken for irradiated fuel storage, transport, reprocessing and disposal. All of the required decay data originate from ENDF-6 files [68, 69, 71], and are judged to be accurately known. Unauthenticated Download Date | 6/12/17 1:06 PM Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications 4.2 Decay heat A sound knowledge of the time-dependent energy release from radioactive nuclides is extremely important in formulating safe operational procedures for nuclear power reactors, identified primarily with the need to maintain cooling to 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 intermediateterm 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 heavyparticle, light-particle and electromagnetic radiation characteristics to determine the total energy release rates for heavy-particles, light-particles and electromagnetic radiations [79–81]: M i λTi Ni (t)E HP HHP (t) = 623 of γ -ray singles data to calculate β − transitions by means of gamma population-depopulation balances of the proposed nuclear levels [82]. TAGS (Total Absorption Gamma-ray Spectroscopy) measurements can overcome these difficulties, and provide the necessary mean beta and gamma decay energies for sound decay heat calculations [37, 38, 41, 83]. i=1 HLP (t) = M i λTi Ni (t)E LP i=1 HEM (t) = M i λTi Ni (t)E EM i=1 where HHP (t), HLP(t) and HEM (t) are the total decay heat of the heavy-particle, light-particle and electromagnetic radiations, respectively, at time t after reactor shutdown; λTi is the total decay constant of radionuclide i; Ni (t) is the number of i i i , E LP and E EM are atoms of radionuclide i at time t; and E HP 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 conversion coefficients can be used to derive β − feeding to daughter nuclear levels, all forms of singles-based Ge detector possess low intrinsic efficiency for the detection of high-energy γ rays above ∼ 1.5 MeV that undermines such an exercise. Furthermore, the determination of direct β − decay to the ground state of the daughter nucleus can pose even more serious problems for various other reasons. Hardy et al. have demonstrated that ∼ 20% of the true γ -ray intensity above 1.7 MeV for a fictional radionuclide (Pandemonium) may remain undetected, impacting significantly on the use Fig. 6. Decay-heat calculations for irradiated 239 Pu as a function of cooling time in terms of (a) total energy release rate per fission second, (b) light-particle/electron energy release rate per fission second, constituting the β − , β + , Auger and conversion electron components, and (c) electromagnetic energy release rate per fission second, constituting the γ , X-rays, annihilation radiation and internal bremsstrahlung components. Tobias (1989) denotes benchmark data for 239 Pu as determined by a least squares fit to all available measured data [84]; JEFF311 defines the calculated decay heat when the JEFF3.1.1 decay-data sublibrary was adopted [69] in which TAGS data from Greenwood et al. had been included [38]; JEFF311 + TAGS are equivalent decay-heat calculations in which TAGS data from Algora et al. had also been added [41]. Unauthenticated Download Date | 6/12/17 1:06 PM 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 Br, 87 Br(β − n), 88 Br(β − n), 89 Kr, Kr, 92 Rb, 96 Y, 99 Zr, 100 Zr, 98 Nb, 99 Nb, 100 Nb, 101 Nb, 102 Nb, 103 Mo, 102 Tc, 103 Tc, 104 Tc, 132 Sb, 135 Te, 136 I, 136m I, 137 I(β − n), 137 Xe, 139 Xe, 140 Xe Additional for U-Pu fuel 86 90m 90 105 Rb a , 89 Sr, 97 Sr, 105 Mo, Tc, 106 Tc, 107 Tc, 142 Cs(β − n), 145 Ba a , 143 La a , 145 La a Additional for Th-U fuel 85 Se, 86 Se, 84 Br, 89 Br, 87 Kr, 91 Kr, 88 Rb, Rb, 92 Sr, 96m Y, 97 Y, 98 Zr(?), 99m Nb, 100m Nb, 102m Nb, 101 Mo, 128m Sb, 129m Sb, 130m Sb, 133 Sb, 138 Xe, 139 Ba, 141 La, 146m La 94 a: TAGS measurement recommended in repeat of equivalent studies by Greenwood et al. [38]. As determined from the TAGS measurements of Greenwood et al. [38], total mean β − and γ energies ( Ē β− and Ē γ ) for twenty-nine fission products of importance in decayheat calculations have been incorporated into the JEFF3.1.1 decay-data sub-library [69] to replace inadequate values that arose from the evaluation of gamma-ray spectroscopic studies. However, even with these more reliable data in place, significant disagreements are still observed when the reformulated decay-heat calculations are compared with available benchmark data [84]. As shown in Fig. 6, calculations of the total and component energy release rate per fission second for thermal-neutron irradiated 239 Pu, in which the adjusted JEFF3.1.1 decay data were used, differ significantly from the decay-heat benchmark at cooling times between 5 and 8000 seconds (i.e. up to 2.2 h). Under these circumstances, WPEC Sub-group 25 (SG-25) of the NEA-OECD addressed the requirements for additional experimentallyderived fission-product decay data of relevance to 235 U and 239 Pu reactor systems, and emphasised the need for further TAGS measurements [85]. A similar assessment exercise has been undertaken for Th-U fuel in which 233 U(n,f) occurs – while U-Pu and Th-U fuels exhibit much in common, there are also a number of notable differences [86]. The two sets of proposals for new TAGS studies are brought together in Table 2. Some of the listed radionuclides represent repeat measurements of Greenwood et al. as a sensible means of cross checking different sets of TAGS data [38]. A consequence of these particular assessments is that recent dedicated TAGS studies have focussed on the β − decay of 101 Nb, 105 Mo and 102,104,105,106,107 Tc [41]. As depicted in Fig. 6, these additional measurements have clearly addressed much of the long-standing discrepancy in the γ component of the decay heat for 239 Pu between cooling times of 5 and 8000 s [87]. However, the same cannot be said for the equivalent 235 U decay-heat calculations – very little change arises from the introduction of the new decay-energy data, and significant disagreements with the decay-heat benchmark remain for cooling times up to 4000 secs. This significantly differing impact of thermal-neutron fission on the release of decay heat from 235 U and 239 Pu can be attributed to the marked differences between the light-peak fission yields of the two systems. While further TAGS measurements are underway to study the β − decay of 87,88 Br, 94 Rb and 137 I [88], much remains to be done as specified above. 4.3 Other reactor-related issues Delayed-neutron decay is an important process in the control of nuclear fission, as well as providing insights into uncertain features of nuclear structure and the population of un- bound levels of nuclei. Extensive studies of delayed-neutron and related gamma-ray emissions have been undertaken by means of singles and coincidence counting techniques in order to provide level-density information and well-defined evidence of complex β − n decay for many important fissionproduct nuclides [89–93]. However, various short-lived fission products that undergo β − n decay still remain to be characterised in a similar manner, along with other delayedneutron emitting nuclides of both lower and higher mass to assist in astrophysical and basic nuclear structure investigations [94]. Antineutrinos are emitted as part of the β − -decay process of fission products generated in the reactor core: A Z X → Z+1A Y + e− + ν̄, and undergo virtually no attenuation in their flux over long distances. Therefore, their unchanged spectral signature could be used to monitor the efficacy of reactor operations based on the nature of the irradiation, particularly with respect to the detection of clandestine procedures and unannounced fuel movements. Various collaborations have aided in the development of antineutrino detection techniques to probe reactor operations non-invasively [95, 96]. Reliable fission-product databases are also required that contain radionuclides from which the uncertainties identified with Pandemonium have been removed by measurement (see Sect. 4.2), allowing β − -decay data to be used with confidence in the derivation of predicted antineutrino spectra for comparison with and interpretation of the measured antineutrinos [88]. 5. Non-energy based applications The nature and properties of radioactivity can be successfully used in a significant number of non-energy based applications. Some applications are highly significant because there are no satisfactory alternatives, while others represent important supportive procedures in various forms of integrated multi-method approach (e.g. combinations of medical treatment: surgery-chemotherapy-radiotherapy). 5.1 Radiation dosimetry Quantification of the absorption of atomic and nuclear radiation is important in defining the dose received and damage inflicted on structural materials and human tissues, and hence the consequences of exposure to all forms of radioactive emission. Radiation dose refers to the amount of energy deposited in matter and the resulting biological effects that Unauthenticated Download Date | 6/12/17 1:06 PM 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: E γ (keV), Pγ (%) 67 Ga 3.2613(5) d 81 Rb/81m Kr generator 81m 93.307(12), 38.1(7)%; 184.577(17), 20.96(44)%; 300.232(21), 16.60(37)%; 393.528(20), 4.59(10)% 190.46(16), 67.66(32)% Kr: 13.10 s Rb: 4.572(4) h 82 Rb: 1.2575(2) min 82 Sr: 25.34(2) d 99m Tc: 6.0067(5) h 99 Mo: 2.7479(6) d 2.8049(4) d 13.2234(37) h 8.0233(19) d 81 82 Sr/82 Rb generator 99 Mo/99m Tc generator 111 In I 131 I 123 133 201 Xe Tl 5.2474(5) d 3.0421(17) d 511 annihilation, 190.9(4)%; 776.52(1), 15.08(16)% 140.511(1), 88.5(2)% 171.28(3), 90.61(20)%; 245.35(4), 94.12(6)% 158.97(5), 83.25(21)% 80.1850(19), 2.607(27)%; 284.305(5), 6.06(6)%; 364.489(5), 81.2(8)%; 636.989(4), 7.26(8)% 80.9979(11), 37.0(3)% 135.312(34), 2.604(22)%; 167.45(3), 10.0(1)% are dependent on such parameters as radionuclidic activity, energy of the radiation, time of exposure, and distance from the source. Data requirements for dosimetry studies are as follows: 1. Isotopic abundances for all target elements, 2. Cross-section data for perceived nuclear reactions, 3. Radiation damage cross sections, and 4. Decay data of all reaction product nuclides. Overall nuclear data requirements need to cover the operation of accelerators, reactor cores and their fuel cycles, and all relevant features of nuclear medicine involving diagnosis, therapy, magnetic resonance imaging and associated research studies. Safe operations of reactor power plant, related fuel-cycle processes and non-energy-based nuclear procedures are determined from the perspective of the envisaged radiation dose rates and the need to undertake reputable calculations in the design and introduction of mitigating features such as appropriate shielding. Dedicated libraries of nuclear data have been assembled to assist in such analyses, and would normally include sub-sets of reaction-product decay data. The International Reactor Dosimetry File IRDF-2002 was released in 2005/2006 [97], and has become internationallyrecognised as a validated source of data assembled in the desired format for safety studies of fission-reactor systems. This file contains decay data for 85 radionuclides (58 ground states, 25 first isomeric states, and two second isomeric states), including half-lives, decay modes and intensities as taken from ENSDF (see Sect. 3.1, and Ref. [10]) and reformatted for nuclear applications. Proposals have been formulated to extend IRDF-2002 further as IRDFF-1.0 (International Reactor Dosimetry and Fusion File) in order to encompass fusion systems [98, 99] – good progress has been made in this respect with the introduction of newlyevaluated excitation functions and related decay data [100]. 5.2 Nuclear medicine Both diagnostic and therapeutic nuclear procedures extend across a wide range of medical activities to address and benefit human health in a safe and efficacious manner [101]. Specific radionuclides have been identified as potentially suitable for various life-saving applications. While their production routes and decay properties need to be defined with confidence, some deficiencies remain, especially with regard to the optimum generation of specific radionuclides, either the minimization or elimination of impurities, and the accurate quantification of various decay parameters [102–104]. The latter requirements include a sound knowledge of the half-life and α, β − , Auger-electron, conversion-electron, β + , γ and X-ray energies and emission probabilities as appropriate for patient dose-rate calculations. Various radionuclides of the desired purity and decay characteristics have been adopted for the diagnostic imaging of physiological and biochemical processes within the human body. Nuclear techniques such as Single-Photon Emission Computer Tomography (SPECT) and Positron Emission Tomography (PET) complement other imaging methods (e.g. magnetic resonance and ultrasound) to furnish an extremely powerful means of detecting functional abnormalities and disease. A number of γ -emitting radionuclides possess decay characteristics that are highly suitable for diagnostic studies by means of gamma cameras (Table 3) – their chemical form is particularly important in ensuring efficient and precise delivery to the body function(s) of interest. Charged-particle cyclotrons and linear accelerators are increasingly being used to generate radionuclides for both diagnostic and therapeutic purposes [105, 106], along with nuclear reactors employed to produce various activation and fission products for similar applications. Sealed sources of 60 Co and 137 Cs provide external γ beams that penetrate the body and constitute the means of controlled therapeutic treatment to internal tumours. Studies of heavy-ion beam therapy have provided striking evidence for the accurate 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 placed in or close to the tumour, while attempting to ensure minimal damage to healthy tissue – 103 Pd, 125 I (as Xray emitters), 137 Cs and 192 Ir (as γ emitters) with effective ranges of a few centimetres are commonly used, while β − Unauthenticated Download Date | 6/12/17 1:06 PM 626 emitters include 32 P, 90 Y and 188 Re with effective ranges of a few millimetres. Radio-immunotherapy involves the covalent bonding and labelling of monoclonal antibodies and peptides with radionuclides such as 90 Y, 131 I, 153 Sm and 213 Bi for injection into the bloodstream, followed by transport and attachment to tumours. PET has become an extremely noteworthy and successful technique for the diagnosis of cancer – the most popular radionuclide for such studies has been 18 F (half-life of 1.83 h), which requires the radionuclide production and patient treatment facilities to be close to each other geographically. The proposed use of radionuclides in nuclear medicine necessitates a sound knowledge of their production cross sections, half-lives and decay schemes, and much effort has been expended to derive such data through measurement and evaluation. IAEA initiatives have included a Coordinated Research Project (CRP) on “Charged particle cross-section database for medical radioisotope production: diagnostic radioisotopes and monitor reactions” to consider the nuclear data requirements for diagnostic radionuclides [107], followed by an equivalent study for therapeutic radionuclides defined as “Nuclear data for the production of therapeutic radionuclides” [108]. While both projects were primarily dedicated to measurements and evaluations of relevant cross sections for product nuclides and impurities, limited amounts of recommended decay data were tabulated from ENSDF [10]. An IAEA consultants’ meeting was held from 3 to 5 September 2008 in Vienna, Austria, dedicated to “Highprecision beta-intensity measurements and evaluations for specific PET radioisotopes”. Participants assessed the decay data for approximately 50 positron-emitting radionuclides, and recommended a series of measurements and evaluations to improve the known decay characteristics of existing and potential PET nuclides [109]. Furthermore, an IAEA consultants’ meeting was held at the same venue from 21 to 24 June 2011 to discuss “Improvements in chargedparticle monitor reactions and nuclear data for medical isotope production”, and formulate a work programme for a Coordinated Research Project aimed at improving the excitation functions of charged-particle monitor reactions and the nuclear decay data for a number of medical radionuclides [110]. Continued developments in medical imaging and therapy will involve the utilization of improved diagnostic and therapeutic techniques, along with the production of potentially more effective and suitable radionuclides. Such an envisaged set of circumstances merits the consideration of future needs to expand and improve the content of nuclear databases for medical applications. Thus, a further technical meeting was organised by the IAEA from 22 to 26 August 2011 in which invited specialists were asked to assess “Intermediate-term nuclear data needs for medical applications: cross sections and decay data” [111]. A summary of their recommendations for decay data is given below, based on the envisaged requirements for (a) diagnostic γ -ray emitters, (b) positron emitters, (c) therapeutic β − , X-ray and γ -ray emitters, (d) therapeutic Augerelectron emitters, and (e) therapeutic α emitters, and covering an estimated timescale of approximately fifteen years. The reader is also referred to the original IAEA report for A. L. Nichols the recommended measurements and evaluations of excitation functions in order to optimize the production of these radionuclides [111]. 5.2.1 Diagnostic γ -ray emitters Reactor-produced 99m Tc is the most commonly used γ -ray emitting radionuclide for diagnostic purposes, and both the cross-section and decay data are well known. Decay data for 67 Ga, 97 Ru, 111 In, 123 I, 147 Gd, 201 Tl and 203 Pb are also reasonably well defined. Both 67 Ga and 111 In possess relatively long half-lives and are rarely used anymore for diagnostic purposes, although they have been adopted as therapeutic radionuclides (see Sect. 5.2.4); 99m Tc: Auger-electron and other low-energy electron decay data are required to assess possible application in microdosimetry; 123 I: Auger emissions may become an issue, if therapeutic applications arise in the future. 5.2.2 Positron emitters Excitation functions for the four most commonly used shortlived positron emitters in PET studies have been recently evaluated and well quantified (11 C, 13 N, 15 O and 18 F [107]). Noteworthy decay data needs are as follows: 57 Ni, 66 Ga, 72 As, 73 Se, 75 Br, 76 Br, 77 Kr, 81 Rb, 82m Rb, 83 Sr, 86 Y, 89 Zr, 94m Tc, 120 I and 121 I: decay-data measurements and evaluations are required; 44 Ti/44 Sc, 52 Fe/52m Mn, 62 Zn/62 Cu, 72 Se/72 As and 140 Nd/140 Pr generators: decay-data measurements and evaluations are required. 5.2.3 Therapeutic β −, X-ray and γ -ray emitters Comprehensive production cross sections and relevant decay data for β − -emitting radionuclides used to perform internal radiotherapy have undergone study in a recent IAEA CRP (see IAEA Technical Reports Series No. 473 for the production of 32 P, 89 Sr, 90 Y, 131 I, 186 Re, 188 Re and others [108]). The decay data of sources used commonly in external radiation therapy (e.g. 60 Co and 137 Cs) and brachytherapy (e.g. 192 Ir) are also well defined. Focus is best placed on β − emitters with known discrepant data, and radionuclides believed to possess future therapeutic potential (specifically low-energy β − and X-ray emitters): 67 Cu, 103 Pd, 161 Tb, 169 Er and 175 Yb: decay-data measurements and evaluations are required; 191 Pt/191m Ir generator: require decay-data measurements and evaluation of 191 Pt parent. 5.2.4 Therapeutic Auger-electron emitters 125 I is the most commonly used Auger-electron emitter for internal radiotherapy – both reactor-production and decay data are well characterized. The following radionuclides are also identified as potentially suitable for application with respect to microdosimetry at the molecular and cellular levels, and therefore would require much improved Auger-electron decay data: 67 Ga, 71 Ge, 77 Br, 99m Tc, 103 Pd, 111 In, 123 I, 140 Nd, 178 Ta, 193m Pt, 195m Pt and 197 Hg. 5.2.5 Therapeutic α emitters 230 U/226 Th: decay-data measurements and evaluations are required. Unauthenticated Download Date | 6/12/17 1:06 PM Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications 5.3 Neutron activation analysis When nuclear research reactors became more accessible in an increasing number of countries during the 1960s, neutron activation analysis (NAA) evolved as a common method of determining low-level concentrations of the elements. Thus, a flux of 2 × 1013 n cm−2 s−1 provided the means of achieving 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. The introduction of solid-state Ge crystals as gammaray detectors refined simultaneous detection considerably, and instrumental neutron activation analysis (INAA) and prompt gamma-ray neutron activation analysis (PGAA) became extremely powerful, non-destructive techniques. These neutron-activation procedures are capable of simultaneous, in-situ multi-element assay in a rapid, quantitative manner across almost all of the Periodic Table from hydrogen to uranium. Elemental coverage of PGAA complements that of conventional, delayed INAA, and the list of measurable elements includes the low Z and high abundance elements in organic and geological materials, along with high cross-section elements such as B, Cd, Sm and Gd. Analyses of hydrogen and boron are especially important because of the paucity of other reliable analytical techniques for trace levels of these two elements. Together, PGAA and INAA can measure all elements except oxygen in most common materials. Nearly every neutron capture involves the (n, γ ) reaction, and therefore the yield of prompt γ rays per neutron is greater than that of delayed γ rays. Unfortunately, PGAA usually has a poorer sensitivity compared with that of INAA because the neutron flux is some five orders of magnitude lower in an external reactor beam than for irradiation adjacent to the core. Over the previous 20 years, the adoption of PGAA has increased because of the growing availability of high-flux thermal and cold beams from neutron guides [112]. Such guided beams can be made free of fast neutrons and superfluous γ rays to give improved signalto-background ratios and spectral quality. Comprehensive review articles and books dedicated to PGAA have also been regularly published [113–115]. A major advance in PGAA was the assembly of a comprehensive compilation of more than 10,000 neutron-capture γ rays of the elements [116], quantified in terms of their energies, abundances and cross sections. Furthermore, a substantial computer readable sub-set of these data was made available on diskette (International Nuclear Geophysics Database-90 (INGD-90)), along with an IAEA Technical Report [117]. This work continued through an IAEA initiative from 1999 to 2005, designed to improve the quality and quantity of the nuclear data in order to apply PGAA reliably and with much greater confidence in such fields as materials science, chemistry, geology, mining, archaeology, environmental monitoring, food analysis and medicine [118]. Despite what has been written above, there are recognised to be two drawbacks to the use of all forms of neutron activation analysis: (a) even though the technique is essentially non-destructive, the neutron-irradiated sample may remain radioactive for years after analysis, requiring the need to implement strict handling and disposal protocols; (b) 627 the number of suitable nuclear reactors is declining – with a lack of irradiation facilities, the technique has declined in popularity, become more expensive, and has been replaced to a significant degree by ICP-MS/AES (inductivelycoupled plasma mass spectrometry and atomic emission spectroscopy) and PIXE (proton-induced X-ray emission spectroscopy). 5.4 Radioactive dating Natural radioactive decay can be used to determine the age of samples that contain a rather select number of specific radionuclides and/or their related in-growth radioactive or stable daughter nuclides. Atomic bomb tests in the atmosphere over the course of the 1950s and early 1960s, along with other nuclear activities and accidents, have also generated discrete radioactive markers which provide the means of dating certain environmental materials and samples accurately. Such changes in nuclidic content are described by the standard radioactive decay laws, and provide very accurate dating mechanisms based entirely on the half-lives of the radionuclides involved [119–122]. 5.4.1 Time markers Atmospheric testing of large nuclear devices at specific times has deposited discrete amounts of radioactivity in the environment. Under certain circumstances, some of these radionuclides will remain firmly at their original location, and can be subsequently used as time markers. Thus, 3 H, 14 C, 137 Cs and 239 Pu are known to mark the years 1962–63 accurately, and are firmly absorbed onto soils, sediments and other materials, with the possibility of being buried by sedimentation and soil-forming processes, as well as by snow precipitation. Cosmogenic radionuclides are produced in the earth’s atmosphere by the interactions of high-energy cosmic radiation with various available atoms such as nitrogen and oxygen. An important nuclide in this respect is 14 C within the natural CO2 bio-cycle, for which the sensitivity of detection has increased dramatically with the advent of accelerator mass spectrometry (AMS) and the potential to analyse very small specimens. The most popular radioactive time markers based on direct measurements are listed in Table 4, along with their recommended half-lives. 5.4.2 Radioactive parent and stable decay product Specific radionuclides possess half-lives sufficiently long for a significant fraction of their atoms to be present today, many years after their formation and incorporation into the solar system. Identified as geochronometers, their existence provides an accurate means of quantifying the absolute ages of events that go back to the formation of the solar system, with even the potential to date events prior to this particularly important process. Several such long-lived radionuclides are listed in Table 5, and their suitability is determined by a number of critical factors: – Maintenance of a closed system during the course of radioactive decay to ensure no escape of parent and daughter products from the geological feature/sample. Unauthenticated Download Date | 6/12/17 1:06 PM 628 A. L. Nichols Table 4. Radioactive time markers and their recommended half-lives [10, 54–59]. Parent radionuclide Half-life 3 12.312(25) y 1.387(12) × 106 a 5.70(3) × 103 a 7.17(24) × 105 a 153(19) y H Be 14 C 26 Al 32 Si Time marker in glaciology and hydrology Lunar, meteor and marine geosciences, and glaciology Archaeology, geology, glaciology, hydrology, etc. Lunar, meteor and marine geosciences Half-life covers time span inaccessible by 14 C and 210 Pb studies 3.01(3) × 105 a Cosmology, geoscience and hydrology 1.61(7) × 107 a Hydrology, and other possible applications 30.05(8) a Recent sedimentation rates in aquatic systems, and accumulation rates of glaciers 210 Pb 22.23(12) a Dating over the previous 100 to 150 years (e.g. lake (226 Ra 1.600 (7) × 103 a) sediments and glacier ice) 2.4100 (11) × 104 a Recent sedimentation rates in aquatic systems, and accumulation rates of glaciers 10 36 Cl I 137 Cs 129 226 Ra-222 Rn-210 Pb 239 Pu Comments Table 5. Long-lived radionuclidic chronometers applicable to geological and cosmological dating [10, 54–59]. Parentdaughter 40 40 Parent half-life 1.2504(30) ×109 a K(EC)-40 Ar K(β − )-40 Ca 4.81(9) ×1010 a 1.02(1) ×1011 a Rb(β − )-87 Sr La(EC)-138 Ba 138 La(β − )-138 Ce 147 Sm(α)-143 Nd 176 Lu(β − )-176 Hf 187 Re(β − )-187 Os 190 Pt(α)-186 Os 87 138 1.060(11) ×1011 a 3.76(7) ×1010 a 4.33(7) ×1010 a 6.5(3) ×1011 a Th and U decay chains 232 Th-208 Pb Noteworthy half-lives 232 Th Ra 228 Th 235 U 231 Pa 227 Ac 238 U 234 U 230 Th 226 Ra 210 Pb 228 235 238 U-207 Pb U-206 Pb 1.402(6) ×1010 a 5.75(4) a 1.9126(9) a 7.04(1) ×108 a 3.267(26) ×104 a 21.772(3) a 4.468(5) ×109 a 2.455(6) ×105 a 7.54(3) ×104 a 1.600(7) ×103 a 22.23(12) a Measured isotope ratio 40 40 Ar/38 Ar Ca/42 Ca Sr/86 Sr Ba/137 Ba 138 Ce/142 Ce 143 Nd/144 Nd 176 Hf /177 Hf 187 Os/188 Os 186 Os/188 Os Comments EC branch of 10.75(15)%, and β − branch of 89.25(15)%; tendency towards preference for 40 Ar/39 Ar ratio method (see Sect. 5.4.2) 87 138 EC branch of 65.6(5)%, and β − branch of 34.4(5)%; not commonly used T1/2 (186 Os(α)) of 2.0(11) ×1015 a; not commonly used Measured isotope ratio Comments 208 Dating based on secular equilibrium Pb/232 Th 207 235 206 234 226 Pb/235 U U/231 Pa Dating based on secular equilibrium; disequilibrium studies Pb/238 U U/230 Th Dating based on secular equilibrium; various disequilibrium studies Ra/230 Th – No daughter material incorporated into the geological feature/sample during or after formation. – Able to quantify the parent and daughter products to the desired accuracy. – Well-defined parent half-lives. These requirements present significant technical challenges to geologists, analysts and measurers of decay data (latter in their determination of the half-lives of such longlived radionuclides). However, developments in analytical techniques such as high-precision mass spectrometry and inductively-coupled plasma optical emission spectroscopy have assisted greatly in improving the accuracy of geochronology. Furthermore, given the difficulty in measuring the absolute abundance of a specific nuclide, researchers have benefitted from determining particular isotopic ratios of the daughter element (Table 5), and using these data to calculate the date of sample formation based on the assumption that the system has been closed to both parent and daughter mobility from the time of interest to the present. Major natural phenomena prevent the retention of inert argon (and 40 Ar) within the crystalline structure of many forms of geological feature, and also result in argon absorption from the atmosphere. Faced with such serious uncertainties in the long-term stability of argon content for sample analysis and the need to undertake two separate analytical procedures in order to measure parent potassium and daughter argon, research scientists have increasingly preferred to date their samples of interest by the more sophisticated Unauthenticated Download Date | 6/12/17 1:06 PM Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications quantification of 40 Ar/39 Ar ratios that arise within samples irradiated in a well-defined fast-neutron flux. Within the carefully controlled conditions of a suitable nuclear reactor, 39 Ar can only be produced via the 39 K(n f , p)39 Ar reaction, which is dependent upon the amount of 39 K within the irradiated sample, length of irradiation, neutron-flux density and neutron-capture cross section of 39 K, whereas the 40 Ar content arises from the natural EC decay of 40 K. Massspectrometric measurements can be made more precisely on isotopes of the same element (argon), although corrections may still be required to allow for the presence of atmospheric argon and interference from other reactor-produced isotopes. Under these better considered circumstances, studies of 40 Ar/39 Ar ratios are more commonly undertaken in preference to 40 Ar/38 Ar ratio measurements. Other debatable assumptions and various analytical difficulties also exist within the other systems listed in Table 5 [120–122], although they will not be considered further in this particular review. When deemed to be in secular equilibrium, the three naturally-occurring decay chains of 232 Th, 235 U and 238 U to particular stable Pb nuclides constitute independent geochronometers, based on the parent radionuclides possessing much longer half-lives than their respective daughters (Table 5). Measurements of sample 208 Pb/232 Th, 207 Pb/ 235 U and 206 Pb/238 U ratios in conjunction with their 208 Pb/204 Pb, 207 Pb/204 Pb and 206 Pb/204 Pb ratios, respectively, provide a means of standardising and dating closed geological systems accurately for ages greater than 30 million years [120, 122]. Non-radiogenic 204 Pb is used as the reference isotope within such samples, possessing a very weak radioactive half-life for α decay of ≥ 1.4 × 1017 a, which effectively permits the nuclide to be defined as stable. Externally-driven geological events can significantly disturb secular equilibrium. If measurements can be made of (a) subsequent decay of a radionuclide that has been separated from precursors, or (b) ingrowth of a radioactive daughter, the resulting data can be used to establish the date of the original geological disturbance (e.g. time at which parent separated from daughter nuclei during a sedimentation process, and initiated radioactive disequilibrium). Important information on the geological and geochemical history of geographical features and deposits can be determined in this manner [123]. The decay data of importance in radioactive dating are the half-lives and relevant branching fractions (e.g. for the EC and β − decay modes of 40 K and 138 La). Geologists, geochemists and other specialists in the field of geochronometry believe that these parameters need to be known with greater accuracy in many cases. Uncertainties in the parent half-life range from 1.0% for 138 La and 147 Sm, 1.6% for 187 Re, 1.9% for 87 Rb and 176 Lu, to as much as 4.6% for 190 Pt – half-lives for these long-lived radionuclides need to be determined with improved accuracy in order to derive more precise ages. The equivalent data for both 40 K and 87 Rb, and the Th-UPb decay chains are known to higher precision, and therefore can be adopted in age calculations with greater confidence – however, there are other more substantial issues to consider in such studies that relate to analytical difficulties and the extremely important prerequisite for sample stability over many years. 629 5.5 Other industrial and research applications Extremely unfortunate incidents and terrorist activities over recent years have underlined the essential need to detect, identify, localise and minimise/eliminate nuclear and radiological threats at both the national and international levels. Thus, there exists the need to develop increasingly userfriendly nuclear detection and identification systems in order to improve support towards ensuring the safety and security of the populace. Recommended nuclear data are a very important aid in addressing criticality safety and sound stockpile stewardship, and preventing the illicit trafficking of nuclear materials. Such onerous requirements include the need to adopt agreed sub-sets of recommended decay data from a respected database such as ENSDF [10]. Thus, a particularly specific set of well-defined data have been adopted with a reasonable degree of confidence in their correctness and permanency [124], hopefully to ensure a consistently correct approach through these parameters towards the maintenance of the desired level of national and international security. Decay data adopted in such circumstances include the radionuclide half-life, major radiations, and energies of the main γ -ray emissions to be effectively used as fingerprints for any radioactivity detected. Radioactive tracers are often used to measure the rates of reaction of chemical processes for (a) basic research purposes, (b) optimisation of industrial production routes, and (c) determination of the movement of molecules through biological systems and tissue to obtain important physiological data and understanding for application in human health, livestock control and plant husbandry. A number of radionuclides have been commonly applied to studies of complex biochemical, biological and metabolic behaviour to resolve existing uncertainties, particularly the more suitable radionuclides of H, C, P, S and I. Research studies into reaction kinetics and mechanisms have also benefited immensely from these labelling techniques. Obvious care has to be taken to adopt radiotracers that permit the phenomena of interest to be successfully monitored (e.g. by means of γ -ray spectroscopy), with decay-data parameters defined sufficiently well that they do not hinder accurate studies of the chemical reactions and transport mechanisms of interest [125–127]. Accurate information describing the spatial distribution of a wide range of minerals in the Earth is an essential requirement for the development of economic routes in the exploration, extraction and processing of oil, coal, metalliferous and non-metalliferous materials, including such properties as total clay content, grain density and porosity. Rapid on-line elemental analysis of natural products such as oil and coal (H, C, N, O, Al, Si, S, Ca, Fe), raw minerals for construction (O, F, Na, Mg, Al, Si, P, S, K, Ca, Ti, Mn, Fe), and mixed ores for manufacture (Al, Mn, Fe, Ni, Cu, Ag, Au and others) can be achieved by means of nuclear borehole logging [117, 127]. The popular neutron activation sources are 252 Cf, 241 Am-Be and 14-MeV neutron tubes based on d(t, n)4 He. Decay-data needs that arise from such remote irradiations can be readily adopted from ENSDF [10]: natural abundances, half-lives, decay modes and branching fractions, and the energies, intensities and uncertainties of the γ rays emitted following neutron capture were collected toUnauthenticated Download Date | 6/12/17 1:06 PM 630 gether in the early 1990s to create the International Nuclear Geophysics Database (INGD-90) [117]. Nuclear astrophysics constitutes productive collaboration between astrophysicists and nuclear physicists to study the nuclear reactions of relevance to stellar modelling, γ -ray, optical and X-ray astronomy, and the formation and evolutionary existence of the cosmos. A primary aim is to improve our understanding of the origins of the universe through the concept of nucleosynthesis, which can be defined in terms of the formation mechanisms of the elements and the commensurate generation of energy in the stars. Various proposed cosmological phenomena need to be satisfactorily addressed through nuclear physics (helium fusion; s, r and rp processes; X-ray processes; etc.) in order to improve our understanding of the driver mechanisms for active galaxies, core collapse, supernova neutrinos, neutron stars and black holes. Spectral studies of complex nuclear structure play an important role in addressing the fundamental nature of the universe, and will continue to do so for many years to come. Recent and future advances in our ability (a) to generate unstable radionuclides close to the proton and neutron drip lines, and (b) to detect and quantify their decay characteristics will aid considerably in the advance of basic research within both nuclear physics and astrophysics. 6. Concluding remarks and future perspectives An extremely important requirement in the adoption and application of radioactive decay data is that such numerical data and their uncertainties should be sound and sufficiently accurate. These needs have been satisfactorily addressed with respect to the main fission products and actinides produced in power reactors, although highly-specific decaydata measurements and evaluations remain outstanding to address particular needs in decay-heat calculations and the potential future implementation of various high burn-up fuel strategies. Other significant issues that have arisen in recent years include the provision of improved delayed-neutron data, and the beneficial requirement for well-defined β − spectra to assist in non-invasive studies of the antineutrino emission from operational reactors. A wide range of activation products are identified with various proposed designs of fusion reactor. Their decay properties also need to be sufficiently well known to address standard operational requirements, and study radiation dosimetry, longer-term radiotoxicity, and the bulk handling of activated structural materials as radioactive waste. While much of the decay data would appear to be readily available from various well-established databases, the quantity of such data would suggest a need for careful assessment to ensure all envisaged requirements are met. Significant diagnostic and therapeutic developments continue to occur in both organ imaging and radiotherapy. Thus, efforts are underway to couple positron-emission tomography (PET) with X-ray computed tomography (CT) and magnetic resonance imaging (MRI), with the aim of achieving comprehensive and detailed organ assessment. New possibilities for improved internal radiotherapy are also being explored on the basis of the following procedures: A. L. Nichols 1. combination of PET and radiotherapy derived from radioimmuno reactions, and 2. Auger-electron and α-particle therapy at the cellular and molecular levels. Long-term decay-data needs in nuclear medicine will depend upon the relative success of such proposed developments and the commensurate identification of the most suitable radionuclides in these studies and medical treatments. Overall, demands are expected to gravitate towards positron emitters and appropriate therapeutic radionuclides, as outlined in Sect. 5.2. An increased adoption of metallic-based positron emitters can be envisaged to occur as a consequence of improved coordination chemistry (especially for Ti, Ga and Cu radionuclides) arising from advances in the preparation of organometallic compounds. The consequences of adopting highly-focused therapeutic treatment through microdosimetry techniques will also require improved characterisation and quantification of the atomic and nuclear decay data of the most suitable low-energy Auger-electron emitters. Another noteworthy decay-data issue is the requirement for more accurately known half-lives of specific long-lived radionuclides that are used as geochronometers in the dating of localised geological events (see Sect. 5.4) and nucleosynthesis within the solar system. The half-lives and associated uncertainties of 87 Rb, 176 Lu, 187 Re and 190 Pt may be significant in this respect, although other features of such work are of considerable importance, particularly the requirement for sample stability over many hundred-million years and potentially serious inadequacies in the adoption of a particular analytical technique to quantify multi-element and multiisotope content. Requests for new nuclear data measurements are driven primarily by the results of methodical evaluations to identify inconsistencies and unforeseen gaps in the desired data. Thus, a healthy synergy exists in which αβγ spectroscopy laboratories undertake decay-data studies driven by data inadequacies highlighted by decay-scheme evaluations of direct relevance to basic nuclear physics research and application in the industrial, medical and environmental sectors. Under these circumstances, the communication and transfer of such data and information has improved considerably, and large quantities of nuclear data can be rapidly accessed within seconds, thanks to the work undertaken over many years by staff at the various national and international nuclear data centres. 7. Data uncertainties Data and their uncertainties are presented throughout the text and tables in the form 1234(x), where x is the uncertainty in the last digit or digits quoted in the measured or evaluated number. This uncertainty is generally expressed at the 1σ confidence level. Examples: 1739(8) means 1739 ± 8; 0.0171(22) means 0.0171 ± 0.0022; 1.13(17) × 10+6 means (1.13 ± 0.17) × 10+6 . Acknowledgment. Advice and information has been gratefully received from Roberto Capote Noy (IAEA, Vienna, Austria), Christopher J. Dean (Serco, Winfrith Newburgh, Dorset, UK), Filip G. Kondev (Argonne National Laboratory, USA), Robert W. Mills (National Nuclear Unauthenticated Download Date | 6/12/17 1:06 PM Radioactive decay data: powerful aids in medical diagnosis and therapy, analytical science and other applications Laboratory, Seascale, Cumbria, UK), Alexander R.L. Nichols (JAMSTEC, Yokosuka, Japan), Syed M. Qaim (Forschungszentrum Jülich, Germany) and Balraj Singh (McMaster University, Hamilton, Canada). References 1. Magill, J., Galy, J.: Radioactivity Radionuclides Radiation. Springer, Berlin, Heidelberg and New York (2005). 2. Nichols, A. L., Decay data: review of measurements, evaluations and compilations. Appl. Radiat. Isot. 55, 23–70 (2001). 3. Nichols, A. L., Nuclear decay data: on-going studies to address and improve radionuclide decay characteristics. In: AIP Conf. Proc. – Int. Conf. on Nuclear Data for Science and Technology. (Haight, R. C., Chadwick, M. B., Kawano, T., Talou, P., eds.) Vol. 769, Part 1, AIP, Melville, New York, USA, ISBN 0-7354-0254X, ISSN 0094-243X (2005), pp. 242–251. 4. 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