International program to improve decay data for transactinium nuclides

Nuclear Instruments and Methods in Physics Research A242 (1986) 475 479 North-Holland, Amsterdam

INTERNATIONAL R.G. HELMER

PROGRAM

TO IMPROVE

475

DECAY DATA FOR TRANSACTINIUM

NUCLIDES

and C.W. REICH

Idaho National Engineering Laboratory, EG&G Idaho, Inc., P.O. Box 1625, Idaho Falls, Idaho 83415 USA

To help meet an identified need for precise decay data, in 1977 the IAEA organized an international Coordinated Research Program (CRP) to measure and evaluate half-lives and 2/- and c~-emission probabilities for selected transactinium nuclides of importance for reactor technology. The CRP goals were (1) to determine a list of data that needed improvement, (2) to encourage new measurements, and (3) to evaluate the available data. All three phases of this work are now complete. Our participation in this effort has involved the measurement of ),-ray emission probabilities for 232"233'2Z'5U, 238.239.240.241 Pu, 229Th and 23~pa, as well as participating in the data evaluation, The ~,-emission probabilities were determined from the measurement of y-emission rates with the goal of obtaining uncertainties of _<1%. y-measurements were made on calibrated Ge detectors. These calibrations were done by standard methods, generally involving measurements at - 60 y-ray energies from 14 to 2700 keV. The efficiency calibration functions were assigned uncertainties ranging from 2% below 50 keV to 0.50% from 400 to 1400 keV. The determination of the decay rates of the various sources involved several techniques. The 238pu, 239pu and 24°pu samples were calibrated by gross c~-emission-rate measurements at NBS. The 235U sample was taken from an NBS-calibrated spike solution. The 241pu and 233U samples were calibrated by isotope-dilution mass spectrometry based on spikes of the calibrated 239pu, 24°Pu and 235U materials. Some of our results are given, together with a comparison of some present and previous results.

1. Introduction In recent years the assay of radioactive materials has b e c o m e of increasing importance, especially in the areas of e n v i r o n m e n t a l m o n i t o r i n g and reactor technology. This increase in i m p o r t a n c e has b r o u g h t with it a need for more precise data on the decay properties of radioactive nuclides. Historically, most m e a s u r e m e n t s of decay data have been done as parts of studies whose primary motivation has been for other purposes, such as nuclear-structure studies. As a consequence, the quality of certain categories of the decay data is less than might be obtained, and the data sets may be incomplete. These limitations have been addressed in a systematic way for the decay data of selected transactinium nuclides by the study described here. In 1975, the I n t e r n a t i o n a l Atomic Energy Agency (IAEA), in cooperation with the O E C D Nuclear Energy Agency, convened an Advisory G r o u p Meeting on T r a n s a c t i n i u m Isotope Nuclear D a t a [1]. At this meeting users a n d measurers of these data surveyed the requirements for, a n d the status of, the decay data for t r a n s a c t i n i u m nuclei relevant to fission-reactor technology. The categories of decay data that were addressed were half-lives (total and partial), T1/2, c~-particle emission probabilities, P~, a n d ,/-emission probabilities, Pv. This group found that the accuracy of m a n y of these data was not adequate to satisfy some identified needs, a n d they r e c o m m e n d e d the formation of an international p r o g r a m of m e a s u r e m e n t and evaluation to improve the situation. 0 1 6 8 - 9 0 0 2 / 8 6 / $ 0 3 . 5 0 65 Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

In response to this r e c o m m e n d a t i o n , the IAEA Nuclear Data Section in 1977 organized a C o o r d i n a t e d Research Program (CRP) on the m e a s u r e m e n t and evaluation of these decay data. Five laboratories agreed to participate in this CRP; namely A E R E Harwell, U n i t e d Kingdom; C B N M , Geel, Belgium; INEL, Idaho, USA; J A E R I , Tokai-Mura, Japan; and L M R I , Gif-surYvette, France. Individuals from other laboratories have a t t e n d e d C R P meetings as observers. In its initial p l a n n i n g the C R P developed a list of i m p o r t a n t nuclides and the required accuracy for each quantity [1]. These required accuracies spanned the ranges 0.2 0.5% for TI/2, 1.0 3.0% for P,~ and 0.5-5.0% for Pv" Most of the requirements for Pv values were 1.0%. The nuclides included in the C R P list were 22~-2a0. 23~Th ' 231.233pa' 232 236,23g.239U,236-239Np ' 236,238 242pu ' 241 243Am" 242 246Cm and 252Cf. The main objective of the C R P was to arrive at a consistent set of decay data satisfying the required accuracies. To achieve this objective, the group has met annually from 1978 to 1984 and reviewed the status of the existing data in order to agree on priorities, to coordinate the on-going measurements, to initiate new measurements, and to intercompare the results obtained. Our participation in the m e a s u r e m e n t portion of this C R P is described in sect. 2 and an example of the evaluation work is given in sect. 3.

IV. GAMMA RAY SPECTROSCOPY

476

R.G. Helmer, C. [~A Reich / Improving decay data for transaetinium nuclides

2. Experimental measurements Of the nuclides of interest to this CRP, we have carried out and published [2-10] measurements of Py for some p r o m i n e n t -/-rays from the decay of 238-24o Pu, 241pu including its d a u g h t e r 237U, 232U and its decay chain members, 233'235U, 233pa, a n d 2297h and its decay chain members. The measurements for 233pa, 232U and 229Th were carried out [9,10] with source calibration methods other than those described here (e.g., 4v/3-y calibration methods were used for 233pa) a n d will not be discussed here. The other nuclides were done by the methods described below. 2.1. Source-activity calibrations All of the source-activity calibrations of the Pu and U isotopes were based, directly or indirectly, on a-particle emission-rate m e a s u r e m e n t s at the N a t i o n a l Bureau of Standards (NBS). For 235U the material was an NBS calibrated spike solution. For 238 240pu ' samples were sent to NBS for c~-particle emission-rate measurements. The isotopic purities of these materials were good enough that only 24°pu needed any correction for c~-emissions from other nuclides. For 233U a prior solution calibration existed. This calibration was confirmed a n d that for 24~Pu was determined by isotope-dilution mass spectrometry based on spikes of the calibrated solutions of 235U for 233U and 239pu and 24°pu for 241pu. The source purities are given in table 1, along with the uncertainties in the activities. 2.2. y-ray emission-rate measurements 2.2.1. Detector-efficiency calibration F o r all of these nuclides the y-ray emission rates were measured on G e or Ge(Li) detectors. The latest

Table 1 Source calibration information Nuclide

Isotopic abundance (%) ~}

Uncertainty in solution activity (%) h}

233U 235U 238pu 239Pu 24°pu 241Pu

99.99 + 99.82 99.90(1) 99.9985 99.930 d) 98.7

0.4 ':) 0.06 0.18 0.23 0.3 0.5

~o In at.%. h} At 1o level. c} Arbitrarily assigned by experimenters. d) Sample includes 0.014 at.% 238pu which yields 1.0% of the c~-particle activity.

m e a s u r e m e n t s were made on two G e semiconductor detectors with volumes of - 8 and 114 cm 3. The efficiency calibrations of these detectors have been d o c u m e n t e d [11-13]. These calibrations have used a b o u t 20 sources, a b o u t 20 nuclides and a b o u t 60 y-rays with energies from 14 to 2754 keV. Although some of these sources were calibrated in our laboratory by 4rrB y techniques a n d one came from L M R I , most were obtained from the metrology laboratories NBS and PTB. In column 3 of table 2 the uncertainties in the activities are listed for a few calibration sources used in the last complete efficiency calibration of these detectors. In the d e t e r m i n a t i o n of the efficiency values from the spectral data, the following steps are involved and each contributes to the final uncertainty. The y-ray peak areas must be measured in a reproducible manner. In the present case, this was done by doing the analysis with an interactive c o m p u t e r system that allowed the user to j u d g e the quality of each peak fit and to adjust the fitting parameters to o b t a i n a suitable fit. For the conversion of the peak areas to detector efficiencies one needs the best available values of the half-life and y-emission probabilities for the calibration nuclides. Our values of these parameters were based on our review [1 1,12] of the existing evaluations and the m e a s u r e m e n t s available at that time. The uncertainties related to these data are given in columns 4 and 5 of table 2 for the sample cases. The experimental data require two corrections that d e p e n d on the m e a s u r e m e n t conditions, namely those for the r a n d o m - s u m m i n g and coincidence-summing losses. The r a n d o m - s u m m i n g correction depends on the c o u n t rate of the m e a s u r e m e n t and may depend on the y-ray energy. This correction typically is a b o u t 1% at 1000 c o u n t s / s a n d has been measured for our systems. The coincidence-summing correction depends on the s o u r c e - d e t e c t o r distance (or the solid angle s u b t e n d e d by the detector from the source) and the decay scheme of the particular nuclide. In the efficiency-calibration nuclides, this correction is typically 1%, but is - 0 % in m a n y cases. In table 2, the uncertainties from these two corrections are shown in columns 6 and 7 for the sample cases. G i v e n the i n f o r m a t i o n discussed above, one can c o m p u t e an efficiency function. A n example of part of such a curve is shown in fig. 1, where the efficiency is multiplied by E 16 (E is the energy) to expand the visibility of the uncertainties in the values. In this particular case, an uncertainty o f 0.75% has been assigned to the interpolated efficiency values in this energy range. In addition to measuring the efficiency function at a particular time, one must m o n i t o r its change with time. Several careful experiments [14 15] have indicated that changes with time should be expected a n d may be energy dependent. Such changes have been monitored

Table 2 Uncertainties in decay data, measurements and measurement corrections for a few of the nuclides used in a detector efficiency calibration Nuclide

Ev

Uncertainties (%)

(keV)

24Na 54Mn 57Co

6°Co 65Zn 85Sr 8SY 94Nb 133Ba

152Eu

1368 2754 834 14 122 136 1173 1332 1115 514 898 1836 702 871 53 79 + 80 276 302 356 383 121 244 344 411 433 778 867 964 1085 1112 1408

16.4

I

Initial

Decay

activity

correction

0.18

0.00

0.5 0.33

0.02 0.11

0.47

0.01

0.5 0.5 0.77

0,04 0,05 0,12

0.5

0,00

0.5

0,05

0.33

0.08

I

Py

0.002 0.006 0.0 3.0 0.22 1.7 0.02 0.005 0.2 0.4 0.4 0.07 0.1 0.1 0.59 0.94 0.35 0.35 0.33 0.34 0.77 0.64 0.61 0.48 0.47 0.44 0.95 0.27 0.34 0.28 0.27

I

I

Summing

Time

Peak

Random

Coincidence

dependence

area

0.08 0.09 0.02 0.18 0.16 0.16 0.06 0.06 0.03 0.03 0.00 0.00 0.00 0.00 0.04 0.04 0.03 0.03 0.03 0.03 0,06 0.05 0.05 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04

0.03 0,03 0.00 0,10 0,14 0,23 0.03 0.03 0.01 0.02 0.05 0.05 0.04 0.04 0.11 0.08 0.14 0.10 0.06 0.06 0.11 0.17 0.03 0.09 0.13 0.06 0.20 0.14 0.02 0.12 0.13

0.11 0.11 0.04 0.00 0.00 0.00 0.00 0.00 0.04 0.04 0.06 0.06 0.00 0.00 0.01 0.01 0.04 0.04 0.04 0.04 0.03 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08

0.20 0.32 0.12 0.38 0.10 0.24 0.10 0.10 0.17 0.29 0.32 0.50 0.18 0.21 0.29 0.10 0.31 0.18 0.10 0.38 0.10 0.10 0.10 0.22 0.20 0.15 0.28 0.16 0.21 0.18 0.17

I

I

Total

0.30 0,40 0.52 3.0 0.49 1.8 0.49 0.49 0.52 0.70 1.00 0.94 0.54 0.55 0.82 1.1 0.69 0.64 0.61 0.69 0.87 0.77 0.72 0.65 0.65 0.60 1.08 0.52 0.55 0.52 0.52

I

16.0 15.6 15.2

2'- 144 14 °I,' I 13 6"~.

"l'~,,

-....

132t-

_

12a I_ 12.41 120

" I 160

1

I

I

I

I

I

I

200

240

280

320

360

400

440

Energy (keV) Fig. 1. Detector efficiency data, interpolated (solid) curve and estimated lo uncertainty (dashed curves) for a - 8 cm 3 planar Ge detector. The efficiency has been multiplied by E 16 to aid in the visual resolution. The different data symbols represent sources from different suppliers.

R.G. Helmer, C. IV. Reich / lmprot, ing decay data for transactinium nuclides

478

Table 3 Results from INEL measurements of y-ray emission probabilities and energies for isotopes of U and Pu as part of the IAEA decay data CRP Nuclide

Ev (keV)

233U [4]

29.192 42.4 54.699 118.968 120.816 135.3 146.345 164.522 208.171 245.345 291.354 317.2 320.541 84.2 143.8 163.4 185.7 205.3

235 U [5]

23Spu [6]

Pv (y per 105 decays) (l)

12.0 (3) 86.2 (t3) 18.2 (3) 4.06 (4) 3.32 (3) 2.32 (2) 6.57 (6) 6.23 (5) 2.29 (3) 3.62 (3) 5.37 (5) 7.76 (7) 2.90(3) 6.84(10)× 103 11.01 (8)×103 5.12 (4)× 103 57.2 (5)×103 4.96 (5)xlO 3

(1) (2) (1) (2) (2) (2) (2) (4) (5)

43.498 (1) 99.853 (3) 152.720 (2)

Nuclide

Ev (keV)

239pu [3]

51.624 129.296 203.550 332.845 375.054 413.713

24°pu [2]

45.244 (2) 104234 (6) 160.308 (3)

241Pu [71

77.0 103.7 160.0 164.6 ~ 208.0 a

0.0211 (5) 0.102 (3) 0.00654 (9) 0.0457 (4) 0.520 (5)

241pu [8]

64.8 77.1 103.7 148.6 160.0 164.6 208.0 267.5 332.4 368.6 370.9

0.0314 (3) 0.0203 (4) 0.1032 (12) 0.1855 (16) 0.00651 (14) 0.0454 (4) 0.520 (5) 0.01741 (17) 0.00233 (5) 0.00096 (4) 0.00263 (4)

38.2 (8) 7.43 (8) 0.936 (10)

Pr (y per 105 decays) (1) (1) (5) (5) (3) (5)

~')

a~ ~ a) a~ a) ~)

27.1(5) 6.41 (5) 0.568 (4) 0.492 (4) 1.547 (12) 1.455 (9) 43.5 (9) 7.18 (7) 0.402 (4)

,,7 y-ray follows the decay of the 237U daughter.

o f the efficiency function. T h e s a m e c o r r e c t i o n s are m a d e a n d the half-lives of the U a n d Pu nuclides are n e e d e d for d e c a y corrections. W h e n possible the U a n d Pu sources were m a d e a n d m e a s u r e d in a m a n n e r similar to that for the efficiencyc a l i b r a t i o n sources. H o w e v e r , the long half-lives a n d small Pv values for the U a n d Pu i s o t o p e s o f t e n m a d e it

a n d taken into a c c o u n t in our analyses; the resulting u n c e r t a i n t i e s are given in c o l u m n 8 of table 2.

2.2.2. y-emission rates T h e m e a s u r e m e n t s with the U a n d Pu i s o t o p e s involve the s a m e c o n c e r n s as those for the m e a s u r e m e n t Table 4 y-ray emission probabilities in the decay of 235U Ev

Pv (v per decay)

(keV)

Previously recommended ref. [16]

143.8 163.4 185.7 205.3

0.105 (8) 0.047 (4) 0.54 0.047 (4)

Recently measured Measurements by CRP participants C B N M - 1982 ref. [17]

A E R E - 1983 ref. [18]

INEL ref. [5]

0.109 0.050 0.575 0.050

0.107 (2) 0.0497 (10) 0.573 (6) 0.0505 (5)

0.1101 (8) 0.0512 (4) 0.572 (5) 0.0496 (5)

(2) (1) (9) (2)

1984

Other measurements 1983 ref. [19]

Mean values from the 1982 1984 measurements ref. [20]

0.1093 (15) 0.0507 (8) 0.561 (8) 0.0503 (9)

0.1096 (8) 0.0508 (4) 0.572 (5) 0.0501 (5)

R. G. Helrner, C. W. Reich / Improving decay data for transactinium nuclides

necessary to use sources with diameters of 1 or 2 cm (to reduce the effect of "t-ray self-absorption to an acceptable level) rather than the 1 or 2 m m of the calibration sources. This fact required an additional correction [13] for the source size. The results of our Pv m e a s u r e m e n t s are summarized in table 3. Of the 47 values given, 28 have uncertainties of < 1.0%. W h e r e we have also measured the T-ray energies these values are included with their uncertainties; otherwise n o m i n a l energy values are given.

479

a n d other measurements. This will provide an excellent set of evaluated decay data for inclusion in various national and international decay-data files.

Acknowledgment This work was supported by the US D e p a r t m e n t of Energy under D O E C o n t r a c t No. DE-AC07-76IDO1570.

References 3. Evaluation and intercomparison of measurements With the completion of the m e a s u r e m e n t efforts, the C R P u n d e r t o o k the intercomparison and evaluation of the available results. A l t h o u g h new data do exist for m a n y other nuclides, the detailed evaluations by the C R P are for the half-lives of 22STh, 234.239U, 237,239 241pu ' 241-243Am ' 242Cm and 252Cf and the T-emission probabilities for 228229Th, 231'233pa, 232-235"237"239U, 237'239Np, 238-241pu, 241'243Am a n d 2aaCm as well as some P~ data. A n interesting example of the i m p r o v e m e n t in the P~ values is the decay of 235U. Before the C R P measurements, the r e c o m m e n d e d values [16] were based on relative m e a s u r e m e n t s and a value of Py = 0.54 for the strongest y-ray, at 185 keV. No uncertainty was available for this Pv value. Given the i m p o r t a n c e of 23SU to reactor technology, the poor quality of these data is surprising. The four new m e a s u r e m e n t s given in table 4, including three from the C R P group, show excellent agreement a n d at 185 keV the average value, 57.2(5)%, is 5.6% above the old value. Therefore, it is clear that, at least in this case, the C R P effort has provided more accurate data.

4. Summary and comments The I A E A C o o r d i n a t e d Research Program on transactinium nuclide decay-data m e a s u r e m e n t and evaluation has been successful in stimulating interest on the part of metrology groups in several countries to measure with high precision a large quantity of decay data for selected actinide nuclides in response to an identified need. This group has also evaluated the data from these

[1] Proc. Advisory Group Meeting on Transactinium Isotope Nuclear Data, Karlsruhe, FRG (November 3-7. 1975) Vols. I-lII, IAEA-186 (IAEA, Vienna, 1976) see especially, Vol. I, pp. 25-28. [2] R.G. Helmet and C.W. Reich, Int. J. Appl. Radiat. lsot. 32 (1981) 829. [3] R.G. Helmer, C.W. Reich, R.J. Gehrke and J.D. Baker, lnt. J. Appl. Radiat. Isot. 33 (1982) 23. [4] C.W. Reich, R.G. Helmet J.D. Baker and R.J, Gehrke. Int. J. Appl. Radiat. Isot. 35 (1984) 185. [5] R.G. Helmet and C.W. Reich, Int. J. Appl. Radiat. Isot. 35 (1984) 783. [6] R.G. Helmer and C.W. Reich, Int. J. Appl. Radiat. Isot. 35 (1984) 1067. [7] R.G. Helmer and C.W. Reich, Int. J. Appl. Radiat. lsot. 36 (1985) 117. [8] H. Willmes, T. Ando and R.J. Gehrke, Int. J. Appl. Radiat. Isot. 36 (1985) 123. [9] R.J. Gehrke, R.G. Helmer and C.W. Reich, Nucl. Sci. Eng. 70 (1979) 298. [10] R.J. Gehrke, V.J. Novick and J.D. Baker. Int. J. Appl. Radiat. Isot. 35 (1984) 581. [11] R.G. Helmer, Nucl. Instr. and Meth. 199 (1982) 521. [12] R.G. Helmet, US DOE Report EGG-PHYS-5735 (1983). [13] R.G. Helmer, Int. J. Appl. Radiat. lsot. 34 (1983) 1105. [14] K. Debertin, PTB report PTB-Ra-12 (1980). [15] D.D. Hoppes, private communication (1981). [16] M.R. Schmorak, Nucl. Data Sheets 21 (1977) 91. [17] R. Vaninbroukx and B. Denecke, Nucl. Instr. and Meth. 193 (1982) 191. [18] M.F. Banham and R. Jones, Int. J. Appl. Radiat. lsot. 34 (1983) 1225. [19] D.G. Olson, Nucl. Instr. and Meth. 206 (1983) 313. [20] C.W. Reich and R. Vaninbroukx, Proc. 3rd IAEA Advisory Group Meeting on Transactinium Isotope Nuclear Data, Uppsala, Sweden, (May 21 25, 1984), report IAEA-TECDOC-336 (Vienna, 1985).

IV. GAMMA RAY SPECTROSCOPY