Half-life data a critical review of TECDOC-619 update

Transcription

1 ARTICLE IN PRESS Applied Radiation and Isotopes 60 (2004) Half-life data a critical review of TECDOC-619 update M.J. Woods a, *, S.M. Collins b a Ionising Radiation Metrology Consultants Ltd, 152 Broom Road, Teddington, Middlesex TW11 9PQ, UK b Centre for Acoustics and Ionising Radiation, National Physical Laboratory, Teddington, Middlesex TW11 0LW, UK Abstract An accurate knowledge of selected nuclear decay data is critical to a wide range of processes involving radionuclides. An IAEA publication in 1991 was dedicated to the evaluation of half-lives and specific gamma-ray emission probabilities for 39 selected radionuclides considered to be important for detector efficiency calibrations. A new exercise was initiated in December 1998 to update this previous evaluation and to include a number of new radionuclides of interest; 62 radionuclides were considered along with specific parent daughter combinations (to give a total of 64 radionuclides). That work is now being completed and a new set of recommended data is being prepared for publication. A critical comparison of the 1991 and 2003 half-life data suggests that there has not been any significant improvement in the accuracy of the recommended data. The possible reasons for this situation are discussed together with the evaluation procedure and the quality of the available data. Proposals are made for concerted actions that could lead to a significant improvement in these recommended half-life data. r 2003 Elsevier Ltd. All rights reserved. Keywords: Half-lives; Evaluations; TECDOC-619 update; Radioactivity; 64 radionuclides 1. Introduction An accurate knowledge of selected nuclear decay data is critical to a wide range of processes involving radionuclides, including radiopharmaceutical production, medical administrations, safeguards investigations and radioactive waste disposal. A Coordinated Research Project (CRP) was organised by the International Atomic Energy Agency (IAEA) to evaluate half-lives and appropriate gamma-ray emission probabilities for 39 selected radionuclides that were considered to be important for detector efficiency calibrations (IAEA, 1991). Since the publication of those data in 1991, further experimental data has entered the public domain, and an increased number of radionuclides were deemed to be important not just for calibration purposes but also for other user *Corresponding author. Tel.: ; fax: address: mike.woods@blueyonder.co.uk (M.J. Woods). applications. Some 64 radionuclides and two decay chains were identified, and a new evaluation exercise was initiated in December 1998 (Nichols, 2004). 2. Selection of radionuclides While the earlier CRP concentrated on those radionuclides employed for the efficiency calibration of detectors used in X- and gamma-ray spectrometry, the CRP that began in 1998 broadened the scope to include additional radionuclides that play major roles in a number of different applications. The categories that were considered include: X- and gamma-ray calibration standards, dosimetry standards, medical applications, environmental monitoring, waste management and safeguards. Sixty-four radionuclides were selected from a large number of candidates. The evaluation effort for the photon emission probabilities was shared amongst a number of participants, whilst that for half-lives was allocated to the National Physical Laboratory in the UK /$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi: /j.apradiso

2 258 ARTICLE IN PRESS M.J. Woods, S.M. Collins / Applied Radiation and Isotopes 60 (2004) Source data The initial sources of half-life data were the original reports (IAEA, 1991; Woods and Munster, 1988) and Nuclear Data Sheets (both specific nuclide evaluation publications and Recent References). Hard copies of the original publications were retrieved, perused and used as additional sources of references. The data retrieval processes used by the evaluators of the photon emission probabilities also provided an additional data source. Perusal of the data was necessary for a number of reasons. First, it was important to determine whether the values quoted in the original publications were in fact the results of valid measurements by the authors. On several occasions, the decay of a radionuclide was monitored in order to validate the stability of some measurement device. Although a measured half-life value may have been published in this manner, such data cannot always be regarded as entirely valid. Some authors may only have quoted and referenced values that they noted from the publications of other authors. Second, the validity and accuracy of the transcription of data from the original author to the referencing paper needed to be confirmed. Third, the confidence level of the quoted uncertainties needed to be determined. 4. Selection of data in the evaluation A number of problems arise when seeking to set criteria which ensure maximum objectivity and minimum subjective intervention by the evaluator. These difficulties can be simplified if all measurements are made to the same degree of scientific robustness; all uncertainties can be estimated comprehensively and in accordance with the same rules; all relevant correction factors, particularly for impurities, can be determined accurately, corrected for and taken into account in the uncertainty budget; all measurement results can be shown to be from the same population; and results from sufficiently independent measurements are available to allow a satisfactory statistical evaluation of the mean and associated uncertainty to be made. Measurements covering a period of about 40 years had normally to be considered in order to obtain a meaningful number of data points for any radionuclide, and the majority of these data were published between 25 and 40 years ago. There have been significant changes in measurement capabilities, detection of impurities and uncertainty estimation procedures during this time and (as would be expected) in none of the cases considered in this evaluation exercise could all of the required criteria be satisfied. A very significant step change relates to the general availability of germanium for high-resolution gamma-ray spectrometers in about 1968, which resulted in accurate determinations of the levels of radioactive impurities. Different selection criteria can be developed for each radionuclide, but this ad hoc approach would lead to an unacceptable level of subjectivity. Therefore, a set of selection criteria were developed that, although not totally satisfactory, would allow the adoption of a common and transparent procedure. These criteria were: (a) only data published after 1967 should be included (could be relaxed at the discretion of the evaluator if insufficient data were available); (b) only the latest published value from any institution should be used; (c) published values without an associated uncertainty estimate should not be used; (d) unless otherwise stated in the text, the published uncertainty should be regarded as being at the 68% confidence level. 5. Evaluation and reporting criteria The same evaluation procedure was followed as in the previous CRP, namely: (a) all evaluations to be expressed in days, using a conversion factor of days per year; (b) principle of limitation of relative statistical weights to be invoked so that no individual value would have a statistical weight of greater than 50% when calculating the mean value; (c) when evaluating a set of data, the external (SW1) and internal (SW2) uncertainties of the weighted mean (WM) were calculated together with the standard deviation (SU) of the unweighted mean (UWM). If the sum of SU and the larger of SW1 and SW2 were larger than the difference between WM and UWM, WM would be used as the best estimate of the mean value together with a standard uncertainty equal to the larger of SW1 and SW2. Failing this criterion, the best estimate would be represented by UWM and a standard uncertainty of SU; (d) when necessary, the uncertainty on the best estimate would be increased to ensure that the published value with the smallest uncertainty estimate would be within the interval represented by the best estimate plus or minus the associated standard uncertainty; (e) evaluators would be allowed to reject values that are clearly outliers; (f) the uncertainty of the evaluated half-life value would be reported using the shorthand notation whereby, for example, the value is

3 ARTICLE IN PRESS M.J. Woods, S.M. Collins / Applied Radiation and Isotopes 60 (2004) written as (14). Each final uncertainty is expressed as the standard uncertainty, with a coverage factor of k ¼ 1; and is given as an integer between 3 and 29 inclusive. The half-life value is rounded to the nearest significant digit; all uncertainties are rounded upwards. 6. Results The results of the present evaluation are presently being collated and will be published by the IAEA in the near future and full details of the analysis of data for each radionuclide are given by Woods et al. (2003). A summary of the results is given in column 2 of Table 1. Additional information is contained in columns 3 7 and informs the discussion below on the outstanding issues which still need to be addressed if a sufficiently accurate and robust data set is to be provided for the relevant user communities. 7. Discussion 7.1. Required accuracy The required accuracy of the half-life depends on the specific application and, for any particular radionuclide, the requirements may differ between the different uses. In 1991, it was decided that the maximum source-in-use period for a calibration nuclide would be deemed to be 15 years or 5 half-lives, whichever is the shorter, and that the uncertainty contribution to a calibration arising from the half-life should not be greater than 0.1%. This approach allowed a decision to be made as to whether the evaluated uncertainty was sufficiently small or whether further measurements should be recommended. That assumption has not been challenged and, in the absence of any other recommendations, the same specification has been applied here to derive a required uncertainty; column 3 of Table 1 gives the ratio of the evaluated to required uncertainty. A ratio greater than 1 indicates that further measurements need to be performed to reduce the uncertainty. Table 1 indicates that 60% of the half-life values have an evaluated uncertainty which is still too large for highly confident adoption Reduced w 2 value The evaluation of a half-life can only be regarded as satisfactory if all of the data can be shown to be from the same statistical population. A simple indicator of this requirement is the conventional reduced w 2 value which is determined as the ratio of (SW1) 2 to (SW2) 2. A satisfactory situation is indicated if the observed value is less than the corresponding theoretical value. Column 4 in Table 1 indicates the ratio of the observed to the theoretical value: again, a number greater than 1 indicates a problem. Table 1 indicates that over half (56%) of the radionuclides have unsatisfactory reduced w 2 values Number and age of data points The minimum number of independent data points that are required for a satisfactory determination of a mean value is considered to be of the order of 6 or greater. If this value is used as well as the preferred requirement that only values published after 1968 should be used, 70% of the radionuclides fail to meet these requirements. There are no values published after 1967 in 4 cases ( 66 Ga, 94 Nb, 166 Ho m and 226 Ra); three of these cases deserve further comment Ga Although 66 Ga is regarded as being of particular importance for the provision of calibration photons with energies above 3600 kev, the latest published half-life data originate from 1939, 1952, 1959 and The reduced w 2 value indicates that these four values are from the same population. However, the evaluated uncertainty is only 0.64%, and is a factor of 22 too large if the 0.1% uncertainty contribution over 5 half-lives criterion for calibration nuclides is invoked Ho m and 226 Ra 226 Ra has long been used as a reference source of choice because this radionuclide has a long half-life. However, the published half-life data shows a similar pattern to that for 66 Ga, with the most recent measurements published in 1949, 1956, 1959 and The reduced w 2 value and the ratio of the observed to required uncertainty indicate that the situation is satisfactory, but it should be noted that the latter only uses the 15-year period-of-use which may be insufficient for waste management purposes. 166 Ho m has been suggested recently as an alternative to 226 Ra for reference source purposes, partly because of the similarity in their half-lives. However, only one measurement of the half-life has been published and that from almost 40 years ago (1964). Furthermore, the uncertainty is 15%, about 40 times greater than that of 226 Ra Progress since 1991 Since the publication of TECDOC-619 in 1991, only about 70 new half-life values have been published for the 64 radionuclides in this evaluation. If these new data are examined in more detail, almost all of these publications are in fact updates of data that have been previously

4 260 ARTICLE IN PRESS M.J. Woods, S.M. Collins / Applied Radiation and Isotopes 60 (2004) Table 1 Evaluated half-lives and analysis Nuclide Evaluated half-life (days) Evaluated/required standard uncertainty Observed/theoretical w 2 /(n 1) Number of data points Used Since 1968 More measurements required? 22 Na (23) Yes 24 Na (9) Yes 40 K (13) o Yes 46 Sc (4) Yes 51 Cr (20) No 54 Mn (26) Yes 55 Fe (23) Yes 56 Co (8) d Yes 56 Mn (19) Yes 57 Co (5) Yes 58 Co (6) Yes 59 Fe (13) Yes 60 Co (27) No 64 Cu (18) Yes 65 Zn (20) Yes 66 Ga (25) Yes 67 Ga (4) No 68 Ga (7) Yes 75 Se (29) No 85 Kr 3927 (8) Yes 85 Sr (5) No 88 Y (24) No 93 Nb m 5.74 (22) Yes 94 Nb 7.3 (9) Yes 95 Nb (12) Yes 99 Mo (10) Yes 99 Tc m (22) No 103 Ru (13) Yes 106 Rh (4) Yes 106 Ru (18) Yes 109 Cd (12) Yes 110 Ag m (10) Yes 111 In (6) Yes 113 Sn (4) Yes 123 I (15) No 123 Te m (25) Yes 125 I (14) No 125 Sb (21) No 129 I 5.87 (22) 10 9 o Yes 131 I (22) Yes 133 Ba (12) Yes 134 Cs (10) Yes 137 Cs (4) Yes 139 Ce (20) No 141 Ce (14) Yes 144 Ce (6) Yes 152 Eu 4941 (5) Yes 153 Sm (10) Yes 154 Eu (14) Yes 155 Eu 1736 (6) Yes 166 Ho (13) Yes 166 Ho m 4.4 (7) Yes 169 Yb (6) No 170 Tm (8) Yes 192 Ir (10) No

5 ARTICLE IN PRESS M.J. Woods, S.M. Collins / Applied Radiation and Isotopes 60 (2004) Table 1 (continued) Nuclide Evaluated half-life (days) Evaluated/required standard uncertainty Observed/theoretical w 2 /(n 1) Number of data points Used Since 1968 More measurements required? 198 Au (7) Yes 201 Tl (17) Yes 203 Hg (12) No 207 Bi 1.18 (3) Yes 226 Ra (22) Yes 228 Th (23) Yes 234 Pa m (11) Yes 241 Am (4) No 243 Am (8) 10 6 o No published, and principally from only three institutes, namely Atomic Energy of Canada Ltd, the National Institute for Standards and Technology, and Physikalisch-Technische Bundesanstalt. These new data are generally replacements and do not increase the number of data points available for evaluation. Therefore, the number of new, independent data is only of the order of 20. Again, a closer examination of these new data is interesting. For many of the replacement data, the uncertainty quoted is larger than that previously published. This no doubt indicates that the relevant uncertainty budgets have been considered in more depth and more robustly than before. Some of these new data have uncertainties that are significantly larger than the rest of the data in the evaluation set for the radionuclide of concern. When the evaluation is conducted, the resulting weighting factor for these data mean that they have little or no significant impact on the final result. In essence, these values add little or nothing to the improvement in evaluated half-lives Achievable and required uncertainties As mentioned above, the required uncertainty levels used in Table 1 were calculated from the criteria that were originally laid down for the 1991 evaluation which dealt solely with those radionuclides used for the efficiency calibration of X- and gamma-ray spectrometers. As the selection of radionuclides in the present evaluation now covers a wider range of uses, these blanket criteria should probably be revisited and specific requirements determined for each radionuclide. Whatever the outcome of such a re-examination, it is useful to consider what levels of uncertainty can be achieved. The majority of half-life measurements are made using an ionisation chamber which has many advantages over other systems. Sealed, high-pressure ionisation chambers can exhibit extremely good long-term stability and have a wide dynamic measurement range, where signal losses due to recombination effects can be Number of values < Range of published uncertainty (%) Fig. 1. Frequency of uncertainty range for published half-life values. negligible. This degree of stability allows measurements to be made on a continuous basis over many half-life periods. However, a recent consideration of this technique (Schrader, 2004) argues that the achievable accuracy is limited to about 5 parts in 10,000 (0.05%). A total of 373 individual data points have been used in this evaluation exercise and a breakdown in terms of uncertainty is given in Fig. 1. Of these, over one-third (1 2 8) of the total quote an uncertainty which is less than 0.05%. Indeed, 14 of these originate from the laboratory of the authors who have suggested the 0.05% limit, and begs the question as to whether these 14 uncertainties may now be revised upwards. We are not convinced of the validity of the 0.05% conclusion, but this is the first time that such an evaluation has been made and the results represent a reasonable starting point for discussion. The accurate measurement of half-lives requires very stable equipment: generally this would comprise highsensitivity ionisation chambers with a large dynamic range, current measurement systems which have good long- and short-term stability and linearity of response, stable long-lived reference sources, and facilities to determine and assess the effects of radioactive impurities. There also needs to be an availability of resources to ensure that measurements can be performed over a

6 262 ARTICLE IN PRESS M.J. Woods, S.M. Collins / Applied Radiation and Isotopes 60 (2004) significant number of half-life periods, backed-up by personnel who are expert in this technique and have the capability to identify and accurately estimate all the significant components of uncertainty. While there are a sufficient number of expert institutes, there has been no concerted effort in the past to co-ordinate the activities of these groups. When evaluations are conducted, there is obvious merit in discussions of the observed differences between published values between the various experimenters; perhaps the differences between them could be resolved by discussion and additional measurements. In practice, the various published values have been determined independently of other experimenters, often years or even decades apart, and the opportunities to resolve differences have been lost. There is a need to improve half-life data and these experts groups should seek to develop coordinated measurement programmes with agreed approaches to the estimation of uncertainties. Specialised and standards institutes should seek to collaborate and co-ordinate a simultaneous measurement programme for a selected range of radionuclides. That programme should also consider in depth the individual uncertainty components. Acknowledgements This work was developed and undertaken with the assistance of the Nuclear Data Section of the International Atomic Energy Agency (IAEA), and under the auspices of the IAEA-CRP Update of X-ray and Gamma-ray Decay Data Standards for Detector Calibration and Other Applications. The work was also supported by the National Measurement System Policy Unit of the UK Department of Trade and Industry. 8. Conclusions and recommendations A number of new half-life values have been published since the previous IAEA CRP on X-ray and Gammaray Standards for Detector Calibration, but there has been no significant improvement in the quality of the mean values. However, there are indications that more robust and in-depth uncertainty estimations are being made in some institutes. There is a requirement for more half-life measurements to be made to improve the half-life accuracy for the majority of radionuclides considered in this exercise. Furthermore, the accuracies required by the various user communities need to be examined in greater depth and on a radionuclide-specific basis. References IAEA, In X-ray and gamma-ray standards for detector calibration. IAEA-TECDOC-619, International Atomic Energy Agency, Vienna, Austria, pp Nichols, A.L., IAEA co-ordinated research project: Update of X-ray and gamma-ray decay data standards for detector calibration and other applications. Appl. Radiat. Isot. this issue, doi: /j.apradiso Schrader, H., Half-life measurements with ionization chambers a study of systematic effects and results. Appl. Radiat. Isot. this issue, doi: /j.apradiso Woods, M.J., Munster, A.S., Evaluation of half-life data. NPL Report RS (EXT) 95. Woods, M.J., Collins, S.M., Woods, S.A., Evaluation of half-life data. NPL Report, submitted for publication.