New and revised half-life measurements results

Nuclear Instruments and Methaxl% in Physics Research A.112 41 North-Mulland

New and revised half-life measurements results M.P. Untcrwcgcr . D.D. Hoppes and F .J . Schima National Institute of Standards and Tec°Irno%W. Gaithemhurg, AID 208W. LISA The half-lives of many radionuclides have been measured in the Radioactivity Group (if NIST cnvr the last three decades. Tile Iz -""Bi. ''2 Eu . I Eu . I Eu. results of these measurements for many long-lived radionuclides such as "Co. "Cs. "'Kr. ~ 2 N.i. '"Ba . rwcr the last three decades and I°R Sh have been recently revised. The results for the half-lives of the many radionuclides measured are tabulated. Comparisons with the recommended values from the International Atomic Energy Agencl, Cmordinatcd Rc arch Program (IAEA-CRP) are given for the long-lived radionuclides. 1. Introduction

Irhrniteipt EMtp ~ a

One of the challenges facing evaluators of nuclear

data is to provide a single value from measurements that differ by considerably more than the uncertainties assigned to the measurements . Although there have been discussions as to the best selection process, the most satisfactory solution is for all experimenters to check their values and uncertainties in an attempt to reconcile previous differences. This has been done for several long-lived radionuclides of significance mcasured in this laboratory with pressurized ionization chambers over the past 25 years . It has become evident from observed discrepancies that the goodness of fit of an assumed exponential decay of data taken over a fractional half-life is an optimistic indicator of the uncertainty . although it has often been the only one considered . If the measurements extend over even a major fraction of the half lives considered here, changes in the electronics, analysis techniques, shielding, and even in the detector may have to be considered. Variations are compensated for by ratioing each measurement to an aged =='Ra source, but differences in relative response or changes in the sources themselves may make the compensation inexact. It is not possible to check for impurities after most of the major long-lived radionuclide has decayed, as it is for shorter-half-life radionuclides. Although for the last 18 years high-resolution Permanium spectrometers have been used to check for gamma-ray emitting impurities, with of the order of 10' counts collected in the most prominent peak, any bremsstrahlung from pure-beta-particle emitting radionuclides was not considered. Corrections for known impurities involve the response function of the ionization chamber as a function of energy if a calibration for the radionuclide is not available. These and other points are considered for ten ra-

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Fig . 1 . Electronic system for the 4r. -y ionization chamber . dionuclides in the sections that follow. where updated KIST values are compared with other recent ones. 2. Data collection and analysis Fig . 1 shows the schematic diagram of the electronic system for the 4r. y pressurized ionization chamber as operated at present . The ionization chamber is the prototype for the TPA Îvik. ii ionization châiiivei iii -an, ufactured by 20th-Century Electronics. Ltd . (Centronic). Croydon . England -' . It is a 2.5 cm-diameter reenMention of commercial products does not imply recommendation or endorsement by the National Institute of Standards and Technology. nor does it imply that the products identified are necessarily the best available for the purpose .

0168-9002/92/$05 .00 t 1992 - Elsevier Science Publishers B.V . All rights reserved

V. NUCLEAR DATA

350

M, p. Upitemxger et al. / New apad

traut-well ionization chamber, filled with 20 atm of :argon, in use since 1967. Measurements with newer 2.5 and 5 em diameter reentrant-well chambers are included in the results tabulated in table 1 for some of the short-lived radionuclides . All measurements for the long-lived radionuclides have been carried out on the older ionization chamber, A detailed description of the method of measurement is given in NBS Special Publication 250-10 [1]. Basically, each data point on the decay curve consists of the ratio of the sample to a radium reference source, with the sequence of datataking being the measurement of the background, radium reference source, sample, radium reference source and background . The basic analysis of the decay data and the chisquared fitting technique are described in NBS Special Publication 626 [2]. However, a recent review of the decay measurements for the long-lived radionuclides has led to the reanalysis of the data. Data acquired before January 1973 show a systematic tendency of giving a lower response than expected . This was dctcrmined by fitting the data measured after 1973 and extrapolating backwards through the older data points . The more active the source, the greater the deviation from the fitted data. It was at this time (1972-1974), that the old battery pack used for the power supply was replaced with a stable high-voltage supply, and the old vibrating reed electrometer and capacitor bank with the present multirange electrometer . These systematic trends indicate that current leakage was not compensated by the radium reference sources when the current due to the activity of the sample was significantly higher than that created by the radium reference source . Unexplained changes occurred periodically in the response of the ionization chamber to the radium reference sources before 1973 . Although measurements of various radionuclides did not indicate a real change in the system within the statistical uncertainties, this review of the half-life data would indicate otherwise. Therefore, all data for the long-lived radionuclides taken before January 1973 have been rejected . The response of the ionization chamber to the bremsstrahlung radiation from the ß-decay of 2 ' ° Bi which will grow in with the 22-year half-life of the parent 2 ' ° Pb had been considered to be negligible . However, a recent study by Christmas et al. [3] indicates a response of approximately 0.25% of that of the 226 Ra in equilibrium with the shorter-lived daughters. The decay data have now been corrected for this ingrowth and the uncertainty in the purification and sealing times of the radium reference sources considered in the overall errors assigned to the measured half-lives . All of the samples of long-lived radionuclides being measured for half-life determination have been re-

rettiswi half-lives

cently reexamined for gamma-ray emitting impurities and the value , . updated. The ionization chamber rcsponsc for all the impurities found are measured values and are not derived from the response curve of the ionization chamber. The uncertainties in the response, half-lives, and amounts of these impurities are incorporated in the fitting routine . 3. Long-lived radionuclides The half-life values from the present results are compared with those recommended for the 10 radionuclides in the evaluation done by Woods and Debertin (henceforth referred to as the IAEA evaluation) in the to-be-published IAEA Technical Report on X- and Gamma-ray Standards for Detector Calibration [4]. It should be noted that the IAEA evaluation includes the half-life values reported previously by us and would, by necessity, change slightly if our older values were replaced with the corrected values. When counting the number of sources followed, the same source referenced to two different radium reference sources is counted as two sources. The stated uncertainties are the sum in quadrature of the statistical uncertainty ("external standard deviation" in the weighted mean) and other uncertainties which include those from the Stevenson equation [2] and the correction for the 2 "'Bi ingrowth in the radium reference sources. '`Na Five sources were followed for 1 .9 to 4.7 half-lives. No impurities were observed in any of the sources. The estimated uncertainty due radium ingrowth is 0.01 d. Previous NIST result : 951 .71 ± 0.11 d. 950 .97 ± 0.12 d. Present NIST result : IAEA evaluation: 950.8 ± 0.9 d. Co Eight sources have been followed for 0.9 to 3.4 half-lives and are still being measured . Two of the sources have been found to contain 'S 2 Eu impurity and the data have been corrected. The estimated uncertainty due to radium ingrowth is 0.23 d. 6°

Previous NIST result : Present NIST result : IAEA evaluation :

g5Kr

1929.2 ± 2 .6 d . 1925 .12 ± 0 .46 d . 1925 .5 ± 0.5 d.

One source has been followed for 1.0 half-life and is still being measured . No impurities have been observed in the source . The estimated uncertainty due to radium ingrowth is 0.9 d. Present NIST result : 3934 .4 ± 1 .4 d.

MP. Untenn

r et ai. / New and rrr

halflit ,rk

351

Table 1 "alf-lives measured with ionization chambers Radionuclide

Number of source"%

1.1 .1 1.9- 4.7

1%,- .1

`2 Na h

24 Na

"Cr h 'Mn c7co h ` 2 `CO

"Fe

h

M'co

5

14 4 11 2 7 1

4fi sc h

8 1

13

"',Mo `" . Tc "'9Cd 1 lu'A

14 33 2

l'lln "3Sn

1 .1- 7.6 3 .6-10 .3 2 .3- 8 .9 3 .3- 7 .4 4.7-10.4 9.1 4.0- 9.3 0 .9- 3 .4 3 .2 1 .8- 8 .3 2.4- 8.7 1 .0 1 .1- 4.8 1 .3- 8 .1

6

"sZn 67Ga 7`Se

19 1 8 8

I" Kr ~' "Sr MY

R

1231 h

1251

125Sb h

127 Xe 1311 131M Xe 133B a h

133Xe 134 Cs b 137c s h 1;`'Ce 140 Ba 140 La 141 Ce 144 Ce t,

152 Eu 1'

1`3Gd h 153Sm a

3.6- 9 .5 2.1-12.o

1

11 11 3 18 1 5 21 2 4

3 5 6 9 10 2 1 1 4 2 1

'-'3-'Eu 1' 1ssEu b "'Ho a 169 Yb 181W 1s (' Re a "'tRe a 19 ~lr

3 2 1 14 3 2

201 TI 202 TI a 203 Hg 203 Pb 207 Bi n 22sTh h

12 1

"95Au 1"Au

a

Number of half-lives followed

1 5 4

14 7 2 6

Not previously tabulated in ref. [2]. Revised values .

3.4- 5 .3 9.3

1 .4- 9.3 2.3-11 .0 5.4-12.7 1 .4- 6.2 3.9 1.1-11.5 1 .0-10.9 1 .8 1 .2

4.8-11.2 1.7- 3.0 0.4- 0.6 1 .5- 6.4 1.8- 4.4 4.2 6.1 3.5 1 .0 7.3 7.3 1.1- 1.3 2.3- 2.4 7.2 3.4- 9.5 5.9- 6.6 5.7- 6.5 7.0- 7.1 2 .4 0.6- 6.0 4.5- 7.4 2.6-11 .5 1.4 1 .7- 6.6 1.8- 2.8 0.6 1.7- 8.3

half-life

C otistical uncc" r1ainq

Other uncc" rtain4-

(1 .; ~951 009114) h (950.97 0.15) d

0.t)(#)2.4 OA9

0.t9044 ().

(14 .9512 ±0.(032) h (83 .8.31 ± 0.(166) d (27.7010 0 .0012) d (312 .028 0.034) d (272 .11 ± 0 .26) d (70 .77 ± 0.1 D d (44 .5074 f 00)72) d (1925 .12t0 .46)d (244.164 0 .099) d (3 .26154 OOK)54) d (119 .809 0.066) d (39..34.4± 1 .4) d (64 .8530 t 0.0081) d (106.626 ± 0 .044) d (65 .939 ± 00158) h

().(X 0.030 0.(.xx)7 0 .034 0. 0 .11 O.(x 48 0.14 0 .099 () .(XX)15 0.014 1 .1 00)39 0.017

0.0(X)87) h

(l.0)31 00M15

(2 .80477 ± 0.00053) d 015.079 ± 0.080) d (13.2235 ± 0 .0019) h (59.49±0.13) d (1007.56±0.10)d (36.3446 ± 0.0028) d (8 .0197 ± 0.0022) d

MOW17 0.025 0.0019 0.03 0.10 0.0028 0.0005

(6.00718

(463 .26 ± 0 .63) d (249.950 ± 0.024) d

(11 .934±0.021) d

(3853.6±3 .6) d (5 .24747 ±0.00045) d (753 .88 ± 0.15) d 01015.0 ± 20.0) d (137 .734+0 .091) d (12.7527±0.0023) d (40.293 ± 0.012) h (32.510±0.024) d (284.558 ± 0.038) d (4945.5 ± 2.3) d (239 .472±0.069) d (46.2853 ±0.0014) h (3138.2±6 .1) d (1738.97 ± 0.49) d (26.7663 ± 0.0044) h (32.0147 ± 0.0093) d 021 .095 ±0.064) d (89.248 ± 0.069) h 07.021 ±0.025) h (73.810±0.019) d 086.098 ± 0.047) d (2.69517 ±0.00021) d (3 .0456 ± 0.0015) d 02 .466 ±0.081) d (46.619±0.027) d (51 .923 ± 0.037) h 01523.0± 19 .0) d (698.60±0.36) d

0.36 0.024

0 .014

1 .6 0.00045 0.11 6.0 0.029 0.0009 0.008 0.024 0.038 1 .0 0.041 0.0014 3.1 0.46 0 .0044 0.0026 0.042 O.Oti8 0.014 0.019 0.021 0.00021 0.0004 0.081 0.007 0 .013

10.0 0.14

0<0131 O .0= () .0tx)9 0.0 0.25 ().t) 0.0)53 0 .44 0 .0

t).txx)52 t).065 (1.9 0.0171 0 .041

0.(x)49 0.00086 ().51 ().0

1).Üx)51 0.076 0.0 0.12 0.01 0.() 0.0021 0 .016

3.2

0.11 19 .0 0.086 0.0022 0.009 0.0 0.0 2.1 0.055 0.0 5.2 0.18 0.0 0 .0089 0.048 0.067 0.021 0.0 0.042 0.0 0.0014 0.0 0.026 0.034 16.0 0.33 V. NUCLEAR DATA

352

M.P. Unterweger et al. / New and revised half-lives

12-'Sb One source has been followed for 3 .9 half-lives and is still being measured . No impurities have been observed in the source . The estimated uncertainty due to radium ingrowth is 0.01 d. Previous NIST result : Present NIST result : IAEA evaluation : L; .'

1004.0 f 8.0 v. 1007.56 t 0.10 d . 1007.7 ± 0.6 d.

Ba

155Eu

Two sources have been followed for 2.3 to 2.4 half-lives and are still being measured . Both the sources contain 152 Eu and 154 Eu impurities and the data have been subsequently corrected . The estimated uncertainty due to radium ingrowth is 0.18 d. Previous NIST result: Present NIST result: IAEA evaluation :

Four sources have been followed for 1.2 half-lives and are still being measured. All the sources contain i4CS 1 impurity and the data have been corrected . The estimated uncertainty due to radium ingrowth is 0.9 d.

2o7Bi

137Cs

Previous NIST result: Present NIST result: IAEA evaluation :

Previous NIST result : Present NIST result : IAEAevaluation :

3828.0 ± 11 .0 d . 3853.6 +_ 3 .6 d. 3862 ± 15 d.

Six sources have been followed for 0.4 to 0.6 halflives and are still being measured . No impurities have been observed in any of the sources . The estimated uncertainty due to radium ingrowth is 7.6 d. Previous NIST result : Present NIST result : IAEA evaluation:

11206 ± 8 d. 11015 ± 20 d. (1 .102 ± 0.006) x 104 d .

152E

u Four sources have been followed for 1 .0 half-lives and are sill being measured . All the sources contain 154Eu and 153Gd impurities and the data have been corrected accordingly . The estimated uncertainty due to radium ingrowth was 1 .5 d. Previous NIST result : 4956.0 ± 42.0 d. Present NIST result : 4945 .5 ± 2 .3 d . IAEA evaluation : 4933 ± 11 d .

1728 .0 ± 8 .0 d . 1738 .97 ± 0.49 d . (1 .77 ± 0 .005) X 10; d .

Two sources have been followed for 0.6 half-lives and are still being measured. No impurities have been observed in either of the sources . The estimated uncertainty due to radium ingrowth is 8.3 d. 11772 ± 329 d. 11523 + 19 d . (1 . :5 ± 0.07) X 104 d .

4. Revised and new half-lives Table 1 is the revised listing of half-lives determined at NIST since 1967. The data in this table supersede the values presented in NBS Spec-al Publication 626 [2] . The half-li °s of radionuclides not previously reported are noted _s well as those which have been significantly revised . Revised values for the half-lives of various "short-lived" radionuclides arise from improved impurity analysis, incorporation of addition data from new sources, and the re-evaluation of old data in the light of the problems discussed above concerning the long-lived radionuclides .

154EI1

Three sources have been followed for 1.1 to 1 .3 half-lives and are still being measured. All the sources contain 155Eu impurity and the data have been corrected . The estimated uncertainty due to radium ingrowth is 0.6 d. Previous NIST result : Present NIST result : IAEA evaluation :

3101 .0 ± 41 .0 d. 3138 .2 ± 6.1 d. 3136.8 ± 2.9 d.

References [11 J.M . Calhoun, NBS Special Publication 250-10 (1987) 13. [21 D.D. Hoppes and F.J. Schima (eds.), NBS Special Publication 626 (1982) 85. [31 P. Christmas, R .A. Mercer, M.J. Woods and S.M. Judge, Int . J. Appl. Radiat . Isot. 3401) (1983) 1555. [41 IAEA, X-ray and Gamma-ray Standards for Detector Calibration, IAEA TECDOC, to be published .