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International decay data evaluation project
Nuclear Instruments and Methods in Physics Research A 422 (1999) 518—524
International decay data evaluation project R.G. Helmer Idaho National Engineering and Environmental Laboratory, Lockheed-Martin Idaho Technologies Co., Idaho Falls, ID 83415-2114, USA
Abstract Basic concepts of, and information from, radionuclide decay are used in many applications. Many of these applications require a knowledge of half-lives and radiation energies and emission probabilities. For over 50 years, people have compiled and evaluated measured data and combined these with theoretical results with the goal of obtaining the best values of these quantities. This has resulted in numerous sets of recommended values, many of which still have scientific, historical, or national reasons for existing. These sets show varying degrees of agreement and disagreement in the quoted values and varying time lags in incorporating new and improved experimental results. A new international group made up of six evaluators from four countries was formed in 1995 to carry out evaluations for radionuclides of importance in applications; it is expected that the results will become an authoritative and widely accepted set of decay data. This Decay Data Evaluation Project has selected nuclides to be evaluated, the methodology to be used, and a review and approval process. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction There are many fields that apply radionuclide decay data and the categories of data that are needed differ among these fields. The Decay Data Evaluation Project (DDEP) is primarily interested in the data needed for applied c-ray spectrometry; which includes applications such as nuclide identification and quantitative assay, but the evaluations are complete enough to provide the information needed for radiation dose calculations and many other applications. For almost any radionuclide one will find the related data in several compilations and the values
Corresponding author. Tel.: #1 208 526 4157; fax: #1 208 526 9267; e-mail: rhz@inel. gov.
of the quantities of interest will generally differ. Often these differences are insignificant for applied spectrometry and they can be ignored, but occasionally even small differences are important. Even if the differences are insignificant, they are a nuisance because one has to make a choice among them even though one may not have any basis for determining which is the best. There are several reasons for these differences between evaluations, including E different measurement results used as input data, — done at various times so newer references available to later evaluator, — some references were not available to an evaluator, — different judgements as to which references to use;
0168-9002/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 1 0 7 8 - X
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E different analytical techniques for combining data, — select only single ‘‘best’’ value, — average all values-weighted or unweighted, — analyze discrepant values in different ways; E different supporting data, — selection of internal-conversion coefficient tables. The quantities that are useful for applications involving c-ray spectrometry can be divided into two groups, with the first including those quantities that are always needed, namely, the E half-life, E c-ray energies (E ), and c E c-ray intensities or emission probabilities (P , in c c’s per 100 decays). This set of information is sufficient for most current c-ray spectral-analysis codes. However, future analysis programs should be able to make the corrections for coincidence summing between sequentially emitted c-rays. This often becomes important for Ge semiconductor detector measurements on small samples placed close to the detector. For this correction, one also needs a second set of quantities, namely, E for each nuclear level in the daughter nucleus, — its energy, — its a, b, or electron-capture feeding probability; E for each c ray, — its placement in the level scheme, — its K-shell and total internal-conversion coefficients. In order to test the evaluated results for internal consistency, it is also necessary to determine the energies and intensities of the particle decay modes, a, b\, b>, and EC as well as the electron and X-ray energies and intensities from internal conversion and the Auger process for c-ray transitions. These particle data are also needed for radiation dose calculations. In addition, any evaluation should include comments stating which sets of experimental data were used and what decisions were made, so that other evaluators can determine the quality of the evalu-
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ation and accept it as is, or build on it without redoing the evaluation from the beginning. 2. Overview of existing files A list of many of the collections of decay data from the last 20 years is given in Refs. [1—21]. This is not a comprehensive list; especially, since it includes only one non-English file. One might reasonably assume that the older data sets have been completely replaced by the newer ones; however, this has not happened. The 1979 set of decay data of Erdtmann and Soyka[3] and the 1977 set of Kocher [1] are still used. These files vary in that some are independent evaluations and some are collections of data taken from other files. Even the independent evaluations will often owe some credit to other files, if only that they start from the earlier files. Acknowledging the presence of more complex relationships, as well as a lack of information in some cases, the following is a crude division of the files into these groups: Independent evaluations [2—5,7,8,11,13,14,16, 19,21]; data extractions with data processed to get added quantities [1,6,12,20]; and data extractions [9,10,15,17,18]. A significant limitation of many of these sets of data is that there are no comments indicating the origin of the data for a particular radionuclide and especially what processing was done by the authors. This limitation is understandable since such documentation would require a great deal of effort and would take up a great deal of space. However, this makes it impossible for others to judge the quality of the evaluations and for subsequent evaluators to make good use of the results. It is also clear that if one is interested in, for example the placement of c-rays, most of these sets of decay data do not meet the need. 3. Comparison of data in files 3.1. Agreement among data For a large number of radionuclides the values in the different files are quite similar; this is illustrated
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Table 1 Comparison of values in evaluations for Cs Reference (year)
Half-life (years)
E c (keV)
P c (%)
1 (1977) 3 (1979) 5 (1983) 11 (1984) 6 (1985) 7 (1986) 10 (1988) 19 (1990) 13 (1991) 14 (1991) 16 (1993) 17 (1993) 19 (1994) 20 (1996) DDEP (1998)
30.17 (3) 30.1 30.14 30.15 (6) 30.0 (2) 30.0 (2) 30.25 (11) 30.1 (2) 30.18 (15) 30.21 (11) 30.17 30.17 (16) 30.07 (3) 30.07 (3) 30.018 (24)
661.645 (9) 661.62 661.6 (1) 661.660 (3) 661.660 (3) 661.660 (3) 661.660 (3) 661.660 (3) 661.660 (3) 661.660 (2) 661.66 661.66 661.660 (3) 661.660 (3) 661.657 (3)
85.1 (3) 84.62 85.1 85.2 (2) 85.1 (3) 85.21 (7) 85.20 (20) 85.21 (7) 85.1 (2) 85.22 (7) 85.1 (2) 85.1 (2) 85.1 (2) 84.99 (20)
in Table 1 where the quoted half-lives, and c-ray energies and emission probabilities are listed for the well-studied decay scheme of Cs. All of the halflives and all of the P values after 1980 agree very c well. Since 1986, the uncertainties quoted for the Pc values differ by a factor of 3 and are either 0.07 or 0.2, but these uncertainties are small enough that the difference is not significant for the applied user, even for the precise detector efficiency calibrations. However, the fact that only these two values occur suggests that the evaluators have used different methods of analysis, and this fact is of interest. 3.2. Differences among data Differences and time lags. There are cases, some well known, where there have been significant problems; two examples are given in Tables 2 and 3. The case of Pa (1.1 min) has been quite well known for many years. This case is especially interesting because the c ray from this nuclide is often used to determine the amount of U present; often a very important question. The P (1 0 0 1) values in the evaluations before 1993 in c Table 2 were all based on a single 1963 measurement. Researchers who were involved in the assay of U by means of c-ray spectrometry became aware of significant discrepancies between these
Table 2 Comparison of values for Pa (1.1 min) in evaluations and measured P values c Reference (year)
E c (keV)
P c (%)
1 (1977) 2 (1978) 3 (1979) 4 (1981) 5 (1983) 19 (1983) 7 (1986) 11 (1987) 17 (1993) 19 (1994) 20 (1996)
1001.025 (22) 1001.2 (2) 1001.03 1001.00 (2) 1001.0 (1) 1001.03 (3) 1001.00 (3) 1001.03 1001.03 1001.03 (3) 1001.03 (3)
0.589 0.59 0.59 0.59 (10) 0.59 0.59 (8) 0.65 (9) 0.59 0.839 (12) 0.837 (10) 0.837 (10)
Measurements after 1965 Year
P (%) c
Reference
1971 1986 1990 1992 1992 1992
0.83 0.834 (7) 0.839 (5) 0.79 (4) 0.845 (21) 0.82 (3)
[22] [23] [24] [25] [26] [27]
results and those from other measurement methods and this provided the impetus for the newer measurements given. The earliest of these newer measurements was published in 1986, but it took a long time for it to get into the widely circulated evaluations. The case of the P ratio for Ce is interesting c because it is a problem that was identified by a user of the decay data and has not been resolved yet. The assay of Ce is usually based on the 133 keV c-ray because it is the most intense line; the 80 keV line can then be used to verify the radionuclide assignment. But, if the P (80)/P (133) ratio used in c c the assay is too small, the analysis procedure may assign the residual area of the 80 keV peak to another radionuclide with a c-ray at this energy (e.g., Xe). The evaluations in Table 3 show a wide range of ratios, from 0.103 (10) to 0.137 (4) for the last five values. It is clear that the uniquely low measured value of Ref. [33] was adopted in four evaluations and the next lowest measured value of Ref. [35] strongly influenced several
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R.G. Helmer/Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 518—524 Table 3 Comparison of values for Ce in evaluations and measured P ratio c Reference (year)
P (80)/P (133) c c
1 (1977) 19 (1979) 3 (1979) 5 (1983) 11 (1983) 6 (1985) 7 (1986) 19 (1989) 14 (1991) 17 (1993) 20 (1996)
0.152 0.102 0.148 0.102 0.102 0.135 0.103 0.123 0.121 0.137 0.123
(16) (10)
(7) (10) (5) (7) (4) (5)
Measured Values Year
P (80)/P (133) c c
Reference
1969 1970 1970 1970 1976 1976 1977 1984
0.22 (2) 0.148 (12) 0.143 (14) 0.16 (1) 0.150 (4) 0.102 (10) 0.134 (8) 0.123 (5) 0.140 (7) 0.140 (4) 0.1379 (7)
[28] [29] [30] [31] [32] [33] [34] [35] Weighted average Priv. comm. Priv. comm.
1992 1992
For the evaluations and some measurements, the author computed the ratio from the individual P values. c This is not the entire uncertainty.
others. In the course of investigating this problem, the author found that two metrology laboratories had unpublished measurements, the 1992 entries. The best value of this ratio is clearly about 0.139; of the compilations of the last ten years only the Ref. [17] list has a value this large. Interestingly, the weighted average of the eight published values is 0.140 (7), but unfortunately this value was never used. Differences in evaluation methods. In spite of 50 years of experience in compiling and evaluating decay data, there are areas in which the methodology is not agreed upon. Over the past few years there have been discussions within the Non-Neutron Nuclear Data Working Group of the Interna-
tional Committee on Radionuclide Metrology on evaluation methods. And, various authors such as Zijp [37], Gray et al. [38], Woods [39], James et al. [40], and Rajput and MacMahon [41] have described alternative methods to deal with discrepant sets of data. Results from these methods are compared in Ref. [41]. The data in Table 4 show a comparison of the half-lives for three radionuclides from five different methods of ‘‘averaging’’ the measured values. The first two cases, Sr and Cs, illustrate the variation when the measured values are not consistent, while the last case, Eu, illustrates the results when the measured values are consistent. The first two methods make use of the original uncertainties assigned to the measured values, while the last three methods adjust these uncertainties, if it is found that the measured values are not consistent. The basic question is how to treat the discrepancies between the measured values. It would, of course, be desirable to be able to find errors or limitations in the original measurements so that one could knowingly modify the original uncertainties or even reject some of the measurements; however, most evaluators do not have sufficient knowledge about any particular set of measurements to do this. Therefore, one is left to use some adhoc data selection or some analytical method to adjust the weights of the individual measurements and/or the uncertainty in the evaluated value. The values of the half-lives deduced by the various methods are often in good agreement; it is the
Table 4 Half-life values from difference averaging methods all values are in years and are from MacMahon [36] Method
Sr
Weighted mean Bayesian [38] LRSW [37] Normalized residual [40] Rajeval [41]
28.56 28.56 28.60 28.83 28.80
Number of measurements 9
Cs (2) (14) (17) (5) (3)
30.10 30.10 29.93 30.06 30.10 18
Eu (1) (3) (21) (3) (2)
8.593 8.593 8.593 8.593 8.593
(4) (1) (4) (4) (4)
5
Internal uncertainty is given. This is equivalent to the weighted mean with the external uncertainty.
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uncertainties that are different. The Limitation of Relative Statistical Weight (LRSW) method [37] and the Rajeval method [41] give uncertainties that differ by factors of about 6 and 10 for the first two nuclides. This is a serious difference. For Sr, the external uncertainty of the weighted mean is 0.14, which agrees fairly well with the large uncertainty from the LRSW method, while for Cs, the external uncertainty of the weighted mean of 0.03 agrees with the small value from the Rajeval method. Whether the external uncertainty of the weighted mean agrees with the LRSW or Rajeval method is related to the relationship of the most precise measured value with the weighted mean.
4. International decay data evaluation project In the above discussion, the problems illustrated are: E many sets of decay data to choose from, E long time lag to get new measurements into files, and E little communication with users who have identified problems. In 1995, a new international collaboration was formed to address these problems; it is called the Decay Data Evaluation Project (DDEP). This group is based on an informal agreement and consists of M.-M. Be´, Laboratoire Primaire des Rayonnements Ionisants (LPRI) in France, E. Scho¨nfeld, Physikalisch-Technische Bundesanstalt (PTB) in Germany, T.D. MacMahon, formerly of the Centre for Analytical Research in the Environment (CARE), Imperial College in the United Kingdom; and in the United States E. Browne, Lawrence Berkeley National Laboratory (LBNL); J.K. Tuli, Brookhaven National Laboratory (BNL); and this author, who is the coordinator. LPRI and PTB also have established a formal agreement to cooperatively evaluate and publish decay data. Their new publication, Table of Radionuclides, will be an extension of the existing LPRI Table de Radionucle´ides [11]. Recently the DDEP has been expanded to include A. Nichols, AEA Technology, United Kingdom, and V.P. Chechev, Khlopin
Radium Institute, Russia, and J.M. Los Arcos, CIEMAT, Spain. One strength of the DDEP is that it brings together the expertise of several laboratories that are experienced in the precise measurement of the quantities involved in radioactive decay. This is illustrated by the fact that LPRI and PTB are the radionuclide standards laboratories of their respective countries and they have extensive experience in the precise measurement of half-lives and c-ray emission probabilities. This group established a list of &250 radionuclides that are of importance in the various application and should be evaluated. The group discussed, and agreed on, the methodology to be used in these evaluations. This includes: E account for (i.e., use or explicitly exclude) all measurements of a quantity, E generally use the Limitation of Relative Statistical Weight Method of computing the average of a set of values. This provides a procedure for treating a discrepant set of data; E use the Ro¨sel et al. internal-conversion coefficients [42] if theoretical values are used; E use the Scho¨nfeld data [43] for the electron-capture probabilities for various atomic shells and the Scho¨nfeld and Janben evaluation of the fluorescence yields [44]; E provide written documentation of all data used and all decisions and calculations; E review by other evaluators in the group. An example of the level of thoroughness that we are trying to establish is that in the evaluation of the decay of Se the evaluator used relative c-ray intensity data from 25 measurements in contrast to some other evaluations which might use only one or two measurements. The evaluations that have been prepared and are completed or in various stages of the review process include : H, Be, C, Na, Na Al, S, Cl, K, Ar, Sc, Cr, Mn, Co, Zn, Ge, Ga, Se Zr, Nb, Cd, In, Sn, Cs, Ba, Ce, Ba, La, Ce, Pr, Sm, Gd, Re, Ir, Ir, and Bi. The preparation of high-quality evaluations does not, in itself, address any of the three problems listed above. In addressing the problem of many
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different values in the various files, the DDEP has advertised the project. The participants in this project will assure that our results are used widely in France and Germany as well as their inclusion in ENSDF. The IAEA is now forming a new Coordinated Research Project, CRP, to provide an updated set of recommended data for nuclides used in detector calibration. This CRP will, in part, make use of the results of our DDEP. As this work progresses it will reduce the number of different values that are found in various sets of decay data. The fact that this is possible is illustrated by the c-ray energies in Table 2. Since the 1979 publication of Ref. [45], most evaluators have used the values from that paper and the consistency shown in the table has resulted. In the past, most of the decay-data files presented the data on a printed page which prevented updating values as new data became available and contributed to the second problem noted above. The ENSDF data are in a computer file from which the data for any desired decay scheme can be extracted at any time. Therefore, revised evaluations could be made available in a short time. In the future, other files should become available over Internet and the World Wide Web. Although these new methods of data delivery will not necessarily solve this problem, it is hoped that the owners of the electronic decay-data files will develop plans to, and methods of, up-dating the data in a more timely manner. The last problem listed above involves feedback from the users of the decay data when they identify potential problems. This will be considered at a later time, but with the Internet and WWW communications system, it is now technically possible to solicit comments and questions from users.
Acknowledgements The work of the members of this evaluation project is supported by many organizations. The research of the author is supported by the U.S. Department of Energy through the DOE Field Office, Idaho Contract DE-AC07-94ID13223 with Lockheed Martin Idaho Technologies Co.
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References [1] D.C. Kocher, Nuclear decay data for radionuclides occuring in routine releases from nuclear fuel cycle facilities, Report ORNL/NUREG/TM 102, 1977. [2] E. Browne, J.M. Dairiki, R.E. Doebler, (principal authors), in: C.M. Lederer, V.S. Shirley (Eds), A.A. Shihab-Eldin, L.J. Jardine, J.K. Tuli, A.B. Buyrn, (authors), Table of Isotopes, 7th ed., Wiley, New York, 1978. [3] G. Erdtmann, W. Soyka, The Gamma Rays of the Radionuclides, Tables for Applied Gamma Ray Spectrometry, Verlag Chemie, Weinheim, 1979. [4] A.L. Nichols, M.F. James, Radioactive heavy element decay data for reactor calculations, Report AEEW-R 1407, December 1981. [5] U. Reus, W. Westmeier, At. Data Nucl. Data Tables 29 (1983) 1, 205. [6] Table of Nuclear-Decay Data, Appendix A.3, in: A Handbook of Radioactivity Measurement Procedures, National Council of Radiation Protection and Measurement, 1985. [7] E. Browne, R.B. Firestone, in: V.S. Shirley (Ed.), Table of Radioactive Isotopes, Wiley, New York, 1986. [8] Decay data of the transactinium nuclides, Technical Reports Series 261, International Atomic Energy Agency, Vienna, 1986. [9] P. Frame, G. Gleason, M. Worthington, Nuclide Identification Catalog for Gamma Emitters and Alpha Emitters, Oak Ridge Associated Universities Report ORAU-88/ K-32, December 1988. [10] K. Debertin, R.G. Helmer, Gamma- and X-ray Spectrometry with Semiconductor Detectors, North-Holland, Amsterdam, 1988. [11] Table de Radionucle´ides, vols 1—4, Laboratoire de Me´trologie des Rayonnements Ionisants (now Laboratoire Primaire des Rayonnements Ionisants), France, 1983—1988. [12] C.W. Reich, Status of beta- and gamma-decay and spontaneous-fission data from tranactinium isotopes, in: Proc. IAEA Advisory Group Mtg. Transactinium Isotope Nucl. Data, Karlsruhe, Germany, IAEA-186. vol. III, International Atomic Energy, Agency, Vienna (1976) p. 265; C.W. Reich, R.G. Helmer, M.H. Putnam, Radioactive-nuclide decay data for ENDF/B, Report ANCR-1157 (ENDF210), Idaho National Engineering Laboratory, August, 1974. [13] X-ray and gamma-ray standards for detector calibration, Report IAEA-TECDOC-619, International Atomic Energy Agency, Vienna, September, 1991. [14] V.P. Chechev, F.E. Chukreev, Report INDC (CCP)-338, October 1991. [15] P. Ekstro¨m, L. Spanier, The ENSDF radioactivity data base for IBM-PC and computer network access, Report LUNFD6/(NFFR-3059)/1-11, 1989. [16] A.L. Nichols, Activation product decay data; UKPADD-2 Data Files, Report AEA-RS-5449, March 1993.
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[17] U. Scho¨tzig, H. Schrader, Halbwertszeiten and PhotonenEmissionswahrscheinlichkeiten von haufig verwendeten Radionukliden, 4. erweiterte Auflage, Report PTB-Ra16/4, July, 1993. [18] G. Rudstam, Absolute gamma branching ratios for fission products in the mass range 74—165, Report INDC (SWD)024, November 1993. [19] The computer file Evaluated Nuclear Structure Data File, ENSDF, prepared at National Nuclear Data Center, Brookhaven National Laboratory, and Nuclear Data Sheets, Academic Press, New York (1998 and earlier). [20] R.B. Firestone, in: V.S. Shirley (Ed), C.M. Baglin, S.Y. Frank Chu, J. Zipkin (assistant Ed), Table of Isotopes, 8th ed., Wiley, New York, 1996. [21] Table of Radionuclides, Laboratoire Primaire des Rayonnements Ionisants, France and Physikalisch-Technische Bundesanstalt, Germany, to be published. [22] R. Gunnink, R.J. Morrow, Report UCRL-51087, 1971. [23] C.E. Moss, Rad. Eff. 94 (1986) 81. [24] H.L. Scott, K.W. Marlow, Nucl. Instr and Meth. A 286 (1990) 549. [25] W.L. Lin, G. Harbottle, J. Radioanal. Nucl. Chem. 157 (1992) 367. [26] K. Siemon, R.A. Esterlund, J. Van Aarle, M. Knaack, W. Westmeier, P. Patzelt, Appl. Rad. Isotopes 43 (1992) 873. [27] P. Jagam, J.J. Simpson, J. Radioanal. Nucl. Chem. 166 (1992) 393. [28] P.C. Mangal, P.N. Trehan, J. Phys. Soc. Japan 27 (1969) 1.
[29] A. Anttila, M. Puparinen, Zeits. Phys. 237 (1970) 126. [30] J.L. Fasching, W.B. Walters, C.D. Coryell, Phys. Rev. C 1 (1970) 1126. [31] V.R. Potnis, G.P. Agin, C.E. Mandeville, J. Phys. Soc. Japan 29 (1970) 539. [32] B.V.N. Rao, G.N. Rao, J. Phys. Soc. Japan 40 (1976) 1. [33] J.M. Chatterjee-Das, R.K. Chattopadhyay, P. Bhattacharya, B. Sethi, S.K. Mukherjee, Radiochem. Radioanal. Lett. 27 (1976) 119. [34] R.J. Gehrke, R.G. Helmer, R.C. Greenwood, Nucl. Instr and Meth. 147 (1977) 405. [35] J. Dalmasso, H. Maria, A. Hachem , G. Ardisson, Nucl. Instr. and Meth. 221 (1984) 564. [36] T.D. Mac Mahon, Presented at ICRM Data Evaluation Workshop, Idaho Falls, Idaho, October 1992. [37] W.L. Zijp, report ECN-179, Petten, The Netherlands (1985). [38] P.W. Gray, T.D. MacMahon, M.U. Rajput, Nucl. Instr. and Meth. A 286 (1990) 569. [39] M.J. Woods, Nucl. Instr. and Meth. A 286 (1990) 576. [40] M.F. James, R.W. Mills, D.R. Weaver, Report AEA-RS1082, 1991. [41] M.U. Rajput, T.D. MacMahon, Nucl. Instr. and Meth. A 312 (1992) 289. [42] F. Ro¨sel, H.M. Fries, K. Alder, H.C. Pauli, At. Data Nucl. Data Tables 21 (1978) 92, 292. [43] E. Scho¨nfeld, Report PTB-6.33-95-2, 1995. [44] E. Scho¨nfled, H. Jan{en, Nucl. Instr and Meth. A 369 (1996) 527. [45] R.G. Helmer, P.H.M. Van Assche, C. Van der Leun, At. Data Nucl. Data Tables 24 (1979) 39.
International decay data evaluation project R.G. Helmer Idaho National Engineering and Environmental Laboratory, Lockheed-Martin Idaho Technologies Co., Idaho Falls, ID 83415-2114, USA
Abstract Basic concepts of, and information from, radionuclide decay are used in many applications. Many of these applications require a knowledge of half-lives and radiation energies and emission probabilities. For over 50 years, people have compiled and evaluated measured data and combined these with theoretical results with the goal of obtaining the best values of these quantities. This has resulted in numerous sets of recommended values, many of which still have scientific, historical, or national reasons for existing. These sets show varying degrees of agreement and disagreement in the quoted values and varying time lags in incorporating new and improved experimental results. A new international group made up of six evaluators from four countries was formed in 1995 to carry out evaluations for radionuclides of importance in applications; it is expected that the results will become an authoritative and widely accepted set of decay data. This Decay Data Evaluation Project has selected nuclides to be evaluated, the methodology to be used, and a review and approval process. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction There are many fields that apply radionuclide decay data and the categories of data that are needed differ among these fields. The Decay Data Evaluation Project (DDEP) is primarily interested in the data needed for applied c-ray spectrometry; which includes applications such as nuclide identification and quantitative assay, but the evaluations are complete enough to provide the information needed for radiation dose calculations and many other applications. For almost any radionuclide one will find the related data in several compilations and the values
Corresponding author. Tel.: #1 208 526 4157; fax: #1 208 526 9267; e-mail: rhz@inel. gov.
of the quantities of interest will generally differ. Often these differences are insignificant for applied spectrometry and they can be ignored, but occasionally even small differences are important. Even if the differences are insignificant, they are a nuisance because one has to make a choice among them even though one may not have any basis for determining which is the best. There are several reasons for these differences between evaluations, including E different measurement results used as input data, — done at various times so newer references available to later evaluator, — some references were not available to an evaluator, — different judgements as to which references to use;
0168-9002/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 1 0 7 8 - X
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E different analytical techniques for combining data, — select only single ‘‘best’’ value, — average all values-weighted or unweighted, — analyze discrepant values in different ways; E different supporting data, — selection of internal-conversion coefficient tables. The quantities that are useful for applications involving c-ray spectrometry can be divided into two groups, with the first including those quantities that are always needed, namely, the E half-life, E c-ray energies (E ), and c E c-ray intensities or emission probabilities (P , in c c’s per 100 decays). This set of information is sufficient for most current c-ray spectral-analysis codes. However, future analysis programs should be able to make the corrections for coincidence summing between sequentially emitted c-rays. This often becomes important for Ge semiconductor detector measurements on small samples placed close to the detector. For this correction, one also needs a second set of quantities, namely, E for each nuclear level in the daughter nucleus, — its energy, — its a, b, or electron-capture feeding probability; E for each c ray, — its placement in the level scheme, — its K-shell and total internal-conversion coefficients. In order to test the evaluated results for internal consistency, it is also necessary to determine the energies and intensities of the particle decay modes, a, b\, b>, and EC as well as the electron and X-ray energies and intensities from internal conversion and the Auger process for c-ray transitions. These particle data are also needed for radiation dose calculations. In addition, any evaluation should include comments stating which sets of experimental data were used and what decisions were made, so that other evaluators can determine the quality of the evalu-
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ation and accept it as is, or build on it without redoing the evaluation from the beginning. 2. Overview of existing files A list of many of the collections of decay data from the last 20 years is given in Refs. [1—21]. This is not a comprehensive list; especially, since it includes only one non-English file. One might reasonably assume that the older data sets have been completely replaced by the newer ones; however, this has not happened. The 1979 set of decay data of Erdtmann and Soyka[3] and the 1977 set of Kocher [1] are still used. These files vary in that some are independent evaluations and some are collections of data taken from other files. Even the independent evaluations will often owe some credit to other files, if only that they start from the earlier files. Acknowledging the presence of more complex relationships, as well as a lack of information in some cases, the following is a crude division of the files into these groups: Independent evaluations [2—5,7,8,11,13,14,16, 19,21]; data extractions with data processed to get added quantities [1,6,12,20]; and data extractions [9,10,15,17,18]. A significant limitation of many of these sets of data is that there are no comments indicating the origin of the data for a particular radionuclide and especially what processing was done by the authors. This limitation is understandable since such documentation would require a great deal of effort and would take up a great deal of space. However, this makes it impossible for others to judge the quality of the evaluations and for subsequent evaluators to make good use of the results. It is also clear that if one is interested in, for example the placement of c-rays, most of these sets of decay data do not meet the need. 3. Comparison of data in files 3.1. Agreement among data For a large number of radionuclides the values in the different files are quite similar; this is illustrated
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Table 1 Comparison of values in evaluations for Cs Reference (year)
Half-life (years)
E c (keV)
P c (%)
1 (1977) 3 (1979) 5 (1983) 11 (1984) 6 (1985) 7 (1986) 10 (1988) 19 (1990) 13 (1991) 14 (1991) 16 (1993) 17 (1993) 19 (1994) 20 (1996) DDEP (1998)
30.17 (3) 30.1 30.14 30.15 (6) 30.0 (2) 30.0 (2) 30.25 (11) 30.1 (2) 30.18 (15) 30.21 (11) 30.17 30.17 (16) 30.07 (3) 30.07 (3) 30.018 (24)
661.645 (9) 661.62 661.6 (1) 661.660 (3) 661.660 (3) 661.660 (3) 661.660 (3) 661.660 (3) 661.660 (3) 661.660 (2) 661.66 661.66 661.660 (3) 661.660 (3) 661.657 (3)
85.1 (3) 84.62 85.1 85.2 (2) 85.1 (3) 85.21 (7) 85.20 (20) 85.21 (7) 85.1 (2) 85.22 (7) 85.1 (2) 85.1 (2) 85.1 (2) 84.99 (20)
in Table 1 where the quoted half-lives, and c-ray energies and emission probabilities are listed for the well-studied decay scheme of Cs. All of the halflives and all of the P values after 1980 agree very c well. Since 1986, the uncertainties quoted for the Pc values differ by a factor of 3 and are either 0.07 or 0.2, but these uncertainties are small enough that the difference is not significant for the applied user, even for the precise detector efficiency calibrations. However, the fact that only these two values occur suggests that the evaluators have used different methods of analysis, and this fact is of interest. 3.2. Differences among data Differences and time lags. There are cases, some well known, where there have been significant problems; two examples are given in Tables 2 and 3. The case of Pa (1.1 min) has been quite well known for many years. This case is especially interesting because the c ray from this nuclide is often used to determine the amount of U present; often a very important question. The P (1 0 0 1) values in the evaluations before 1993 in c Table 2 were all based on a single 1963 measurement. Researchers who were involved in the assay of U by means of c-ray spectrometry became aware of significant discrepancies between these
Table 2 Comparison of values for Pa (1.1 min) in evaluations and measured P values c Reference (year)
E c (keV)
P c (%)
1 (1977) 2 (1978) 3 (1979) 4 (1981) 5 (1983) 19 (1983) 7 (1986) 11 (1987) 17 (1993) 19 (1994) 20 (1996)
1001.025 (22) 1001.2 (2) 1001.03 1001.00 (2) 1001.0 (1) 1001.03 (3) 1001.00 (3) 1001.03 1001.03 1001.03 (3) 1001.03 (3)
0.589 0.59 0.59 0.59 (10) 0.59 0.59 (8) 0.65 (9) 0.59 0.839 (12) 0.837 (10) 0.837 (10)
Measurements after 1965 Year
P (%) c
Reference
1971 1986 1990 1992 1992 1992
0.83 0.834 (7) 0.839 (5) 0.79 (4) 0.845 (21) 0.82 (3)
[22] [23] [24] [25] [26] [27]
results and those from other measurement methods and this provided the impetus for the newer measurements given. The earliest of these newer measurements was published in 1986, but it took a long time for it to get into the widely circulated evaluations. The case of the P ratio for Ce is interesting c because it is a problem that was identified by a user of the decay data and has not been resolved yet. The assay of Ce is usually based on the 133 keV c-ray because it is the most intense line; the 80 keV line can then be used to verify the radionuclide assignment. But, if the P (80)/P (133) ratio used in c c the assay is too small, the analysis procedure may assign the residual area of the 80 keV peak to another radionuclide with a c-ray at this energy (e.g., Xe). The evaluations in Table 3 show a wide range of ratios, from 0.103 (10) to 0.137 (4) for the last five values. It is clear that the uniquely low measured value of Ref. [33] was adopted in four evaluations and the next lowest measured value of Ref. [35] strongly influenced several
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R.G. Helmer/Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 518—524 Table 3 Comparison of values for Ce in evaluations and measured P ratio c Reference (year)
P (80)/P (133) c c
1 (1977) 19 (1979) 3 (1979) 5 (1983) 11 (1983) 6 (1985) 7 (1986) 19 (1989) 14 (1991) 17 (1993) 20 (1996)
0.152 0.102 0.148 0.102 0.102 0.135 0.103 0.123 0.121 0.137 0.123
(16) (10)
(7) (10) (5) (7) (4) (5)
Measured Values Year
P (80)/P (133) c c
Reference
1969 1970 1970 1970 1976 1976 1977 1984
0.22 (2) 0.148 (12) 0.143 (14) 0.16 (1) 0.150 (4) 0.102 (10) 0.134 (8) 0.123 (5) 0.140 (7) 0.140 (4) 0.1379 (7)
[28] [29] [30] [31] [32] [33] [34] [35] Weighted average Priv. comm. Priv. comm.
1992 1992
For the evaluations and some measurements, the author computed the ratio from the individual P values. c This is not the entire uncertainty.
others. In the course of investigating this problem, the author found that two metrology laboratories had unpublished measurements, the 1992 entries. The best value of this ratio is clearly about 0.139; of the compilations of the last ten years only the Ref. [17] list has a value this large. Interestingly, the weighted average of the eight published values is 0.140 (7), but unfortunately this value was never used. Differences in evaluation methods. In spite of 50 years of experience in compiling and evaluating decay data, there are areas in which the methodology is not agreed upon. Over the past few years there have been discussions within the Non-Neutron Nuclear Data Working Group of the Interna-
tional Committee on Radionuclide Metrology on evaluation methods. And, various authors such as Zijp [37], Gray et al. [38], Woods [39], James et al. [40], and Rajput and MacMahon [41] have described alternative methods to deal with discrepant sets of data. Results from these methods are compared in Ref. [41]. The data in Table 4 show a comparison of the half-lives for three radionuclides from five different methods of ‘‘averaging’’ the measured values. The first two cases, Sr and Cs, illustrate the variation when the measured values are not consistent, while the last case, Eu, illustrates the results when the measured values are consistent. The first two methods make use of the original uncertainties assigned to the measured values, while the last three methods adjust these uncertainties, if it is found that the measured values are not consistent. The basic question is how to treat the discrepancies between the measured values. It would, of course, be desirable to be able to find errors or limitations in the original measurements so that one could knowingly modify the original uncertainties or even reject some of the measurements; however, most evaluators do not have sufficient knowledge about any particular set of measurements to do this. Therefore, one is left to use some adhoc data selection or some analytical method to adjust the weights of the individual measurements and/or the uncertainty in the evaluated value. The values of the half-lives deduced by the various methods are often in good agreement; it is the
Table 4 Half-life values from difference averaging methods all values are in years and are from MacMahon [36] Method
Sr
Weighted mean Bayesian [38] LRSW [37] Normalized residual [40] Rajeval [41]
28.56 28.56 28.60 28.83 28.80
Number of measurements 9
Cs (2) (14) (17) (5) (3)
30.10 30.10 29.93 30.06 30.10 18
Eu (1) (3) (21) (3) (2)
8.593 8.593 8.593 8.593 8.593
(4) (1) (4) (4) (4)
5
Internal uncertainty is given. This is equivalent to the weighted mean with the external uncertainty.
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uncertainties that are different. The Limitation of Relative Statistical Weight (LRSW) method [37] and the Rajeval method [41] give uncertainties that differ by factors of about 6 and 10 for the first two nuclides. This is a serious difference. For Sr, the external uncertainty of the weighted mean is 0.14, which agrees fairly well with the large uncertainty from the LRSW method, while for Cs, the external uncertainty of the weighted mean of 0.03 agrees with the small value from the Rajeval method. Whether the external uncertainty of the weighted mean agrees with the LRSW or Rajeval method is related to the relationship of the most precise measured value with the weighted mean.
4. International decay data evaluation project In the above discussion, the problems illustrated are: E many sets of decay data to choose from, E long time lag to get new measurements into files, and E little communication with users who have identified problems. In 1995, a new international collaboration was formed to address these problems; it is called the Decay Data Evaluation Project (DDEP). This group is based on an informal agreement and consists of M.-M. Be´, Laboratoire Primaire des Rayonnements Ionisants (LPRI) in France, E. Scho¨nfeld, Physikalisch-Technische Bundesanstalt (PTB) in Germany, T.D. MacMahon, formerly of the Centre for Analytical Research in the Environment (CARE), Imperial College in the United Kingdom; and in the United States E. Browne, Lawrence Berkeley National Laboratory (LBNL); J.K. Tuli, Brookhaven National Laboratory (BNL); and this author, who is the coordinator. LPRI and PTB also have established a formal agreement to cooperatively evaluate and publish decay data. Their new publication, Table of Radionuclides, will be an extension of the existing LPRI Table de Radionucle´ides [11]. Recently the DDEP has been expanded to include A. Nichols, AEA Technology, United Kingdom, and V.P. Chechev, Khlopin
Radium Institute, Russia, and J.M. Los Arcos, CIEMAT, Spain. One strength of the DDEP is that it brings together the expertise of several laboratories that are experienced in the precise measurement of the quantities involved in radioactive decay. This is illustrated by the fact that LPRI and PTB are the radionuclide standards laboratories of their respective countries and they have extensive experience in the precise measurement of half-lives and c-ray emission probabilities. This group established a list of &250 radionuclides that are of importance in the various application and should be evaluated. The group discussed, and agreed on, the methodology to be used in these evaluations. This includes: E account for (i.e., use or explicitly exclude) all measurements of a quantity, E generally use the Limitation of Relative Statistical Weight Method of computing the average of a set of values. This provides a procedure for treating a discrepant set of data; E use the Ro¨sel et al. internal-conversion coefficients [42] if theoretical values are used; E use the Scho¨nfeld data [43] for the electron-capture probabilities for various atomic shells and the Scho¨nfeld and Janben evaluation of the fluorescence yields [44]; E provide written documentation of all data used and all decisions and calculations; E review by other evaluators in the group. An example of the level of thoroughness that we are trying to establish is that in the evaluation of the decay of Se the evaluator used relative c-ray intensity data from 25 measurements in contrast to some other evaluations which might use only one or two measurements. The evaluations that have been prepared and are completed or in various stages of the review process include : H, Be, C, Na, Na Al, S, Cl, K, Ar, Sc, Cr, Mn, Co, Zn, Ge, Ga, Se Zr, Nb, Cd, In, Sn, Cs, Ba, Ce, Ba, La, Ce, Pr, Sm, Gd, Re, Ir, Ir, and Bi. The preparation of high-quality evaluations does not, in itself, address any of the three problems listed above. In addressing the problem of many
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different values in the various files, the DDEP has advertised the project. The participants in this project will assure that our results are used widely in France and Germany as well as their inclusion in ENSDF. The IAEA is now forming a new Coordinated Research Project, CRP, to provide an updated set of recommended data for nuclides used in detector calibration. This CRP will, in part, make use of the results of our DDEP. As this work progresses it will reduce the number of different values that are found in various sets of decay data. The fact that this is possible is illustrated by the c-ray energies in Table 2. Since the 1979 publication of Ref. [45], most evaluators have used the values from that paper and the consistency shown in the table has resulted. In the past, most of the decay-data files presented the data on a printed page which prevented updating values as new data became available and contributed to the second problem noted above. The ENSDF data are in a computer file from which the data for any desired decay scheme can be extracted at any time. Therefore, revised evaluations could be made available in a short time. In the future, other files should become available over Internet and the World Wide Web. Although these new methods of data delivery will not necessarily solve this problem, it is hoped that the owners of the electronic decay-data files will develop plans to, and methods of, up-dating the data in a more timely manner. The last problem listed above involves feedback from the users of the decay data when they identify potential problems. This will be considered at a later time, but with the Internet and WWW communications system, it is now technically possible to solicit comments and questions from users.
Acknowledgements The work of the members of this evaluation project is supported by many organizations. The research of the author is supported by the U.S. Department of Energy through the DOE Field Office, Idaho Contract DE-AC07-94ID13223 with Lockheed Martin Idaho Technologies Co.
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