Nuclear Instruments and Methods in Physics Research A 339 (1994) 116-121 North-Holland

NUCLEAR INSTRUMENTS

& METHODS IN PHYSICS RESEARCH

The measurement of 24°pu/239pu and 238pu/239pu isotopic ratios by alpha-particle spectrometry

W. Raab *, J.L. Parus Safeguardz" Analytical Laboratory, IAEA, Wagramerstrasse 5, P.O. Boa" I00, A-1400 Vienna, Austria

The measurement of the alpha-activity ratio of 23~pu/(23'~Pu + 24°Pu is a routine practice in the determination of thc isotopic composition of plutonium. However, measurement of the atomic ratio of 24°Pu/23'~pu by alpha-particle spectrometry is hampered due to insufficient energy resolution for the set of closely spaced peaks of these two isotopes. Passivated and implanted, planar silicon (PIPS) detectors have recently become available with an energy resolution of 10 keV or better, which significantly improves the deconvolution of spectra from plutonium samples. A set of alpha sources was prepared on porcelain disks by ignition, and the spectra were accumulated at a gain of approximately 1 keV per channel. The GRPANL computer program as developed by Lawrence Livermore National Laboratory was used to analyze the spectra. The isotopic ratios were measured in parallel by mass spectrometry. It was found that the agreement on the ratios of 24°pu/239pu and 238pu/23'Jpu between mass spectrometry and measurements by PIPS detectors was within _+2%. Half-life values were obtained from the literaturc (M. Lammer and O. Schwerer, Handbook of Nuclear Data for Safeguards, Rep. INDC(NDS)-248, IAEA, Vienna, 1991; ref. [5]). Other factors were also studied to improve the accuracy of the data. The alpha-particle emission probabilities of highly enriched 23'~pu and z4°pu have been measured. The alpha-particle energies obtained in the fitting were in agreement with those in ref. [5]. The fitted energy values were used throughout this work.

1. Introduction

1.1. General

Plutonium concen t ra t ion de te rmina t ion is a very impor tan t task for safeguarding this e lement . Isotope dilut ion mass spect rometry (MS) is the generally ac- cepted analytical me thod for this purpose. Alpha spec- t rometry (AS) can be used as a complementa ry tech- nique, a l though the isotope ratio of 239pu/24Opu is

used from MS and an accuracy of +_0.3% on the p lu tonium concent ra t ion can be ob ta ined [1].

The measu remen t of the 24°pu/239pu isotope ratio

with acceptable accuracy would make a lpha spect rome- try a more comple te analytical technique for p lu ton ium concen t ra t ion de te rmina t ion by isotope dilution. The 239pu abundance in the analyzed sample can be calcu- lated, in most cases, from high resolut ion gamma-ray spect rometry with an accuracy of within + 1%. The main advantages would be a lower cost of ins t rumenta- tion, relative simplicity of opera t ion and a smaller amount of mater ia l used for analysis.

* Corresponding author.

On the o thc r hand, a direct measu remen t of the 24°pu/23qpu ratio by a lpha spect rometry can providc

quick informat ion about the quality of the measured p lu tonium because these two isotopes usually consti- tute at least 85% of the total p lutonium.

The alpha-part ic le spec t rum of a 23'~pu and 24°pu mixture presents a very' tight cluster of five peaks covering an energy range of about 63 keV. These peaks have a r a the r complicated shape with a significant low energy tail due to energy loss within the alpha-part ic le source. For successful deconvolut ion of this peak clus- ter the alpha-part ic le sources should be carrier-free, thin and uniform in thickness and the a lpha-par t ic le de tec tor should have the highest possible energy reso- lution. For rout ine measurements , however, the source prepara t ion procedure should bc relatively simple and reproducible .

1.2. Spectrum et~aluation program

G R P A N L ( G R o u P ANaLysis) is a package of pro- grams that analyzes gamma-ray, X-ray and a lpha-par- ticle spectra. The V A X compute r version can analyze a peak cluster in a window conta in ing up to 1024 chan- nels and the n u m b e r of variables being opt imized si-

0168-9002/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0 1 6 8 - 9 0 0 2 ( 9 3 ) E 0 6 5 6 - D

W.. Raab, J.L. Parus / Nucl. Instr. and Meth. in Phys. Res. A 339 (1994) 116-121 117

multaneously is 40 (peak positions, heights and shape parameters).

G R P A N L was developed at Lawrence Livermore National Laboratory and is described in detail else- where [2]. Only some features of this program will be presented here. The analytical expression for the peak shape function has the form

(1)

where Yi are the net counts in channel x i belonging to the single peak, Y0 the peak height, a = - 1 / (2o "2) = - 2 . 7 7 2 6 / ( F W H M ) 2 where F W H M is the peak-width

variable, x o is the peak position, and T ( x ) is the tailing function. The tailing function is given by

T( x ) = [A exp B ( x i - X o ) + C exp D( x i - x o ) ]

× [1 - exp 0 . 4 ( x i - x 0 ) 2 ] ~ , (2)

where A, C are the short- and long-term tailing ampli- tudes, B, D the tailing slopes, ,~ = 1 for x i < x o and t S = 0 f o r x , > x 0.

For alpha-particle spectra, long-term tailing is treated as background. Analysis of peak regions within a spectrum begins by preparing an analysis control file with the E D I G R P (EDIt GRouP) program. The file created by E D I G R P consists of two parts: the first part contains information about detector system labels, the names of the files that must be used at run-time, instructions on the type of calculations to be made and how they arc to be reported; the second part of the file contains the energy limits of the interval which covers the peak cluster, the peaks to be used for energy calibration, the energies and relationships of peaks in each cluster and values and options related to peak- shape parameters. Another file which is created by E D I S M P (EDIt SaMPle) contains information about measurement conditions, e.g. geometry, absorbers used

as well as a preliminary energy calibration. The only data that must be given directly to G R P A N L are the names of files created by E D I G R P and E D I S M P and the name of the spectral data file. G R P A N L can operate interactively or non-interactively and calcu- lates the alpha-particle energies, the intensities and the isotopic ratios.

2. Experimental

2.1. Preparation o f sources for alpha-particle spectrome- try

In most cases thc source preparat ion followed a routine procedure used at the Safeguards Analytical Laboratory [3]. An aliquot of 10 ILl of tetraethylene glycol (TEG) is pipetted onto a glazed porcelain disk having a circular recess of 8.5 mm diameter and 1 mm depth. The 10 ~tl of solution containing about 1 Ixg of plutonium is added and spread with a pipette tip over the entire surface of the recess. The disk is placed in a programmed electrical furnace where the liquid is evaporated and the solid residue is ignited at 850°C. The sources of pure 239pu and 24°pu were prepared by electrodeposit ion on polished stainless steel disks using a solution of NaHSO 4 and Na2SO 4 as a carrier elec- trolyte [4]. The ignition of thcse sources at about 400°C had no significant influence on the quality of spectra.

2.2. Analyzed materials

The materials used in this study were plutonium samples routinely analyzed at SAL. Before the alpha- source preparation, the solutions were purified from 241Am by partition chromatography. Additionally, four referencc materials (RMs) were used. Two of them

Table 1 Isotopic composition of materials measured

Material Isotopic composition [at.%] 238 Pu 239 Pu 240 Pu 241 Ptl 242 Pu

23~pu RM - 99.842 0.146 0.005 0.007 240pu RM - 0.43 99.40 0.I0 0.07 NBL 126 RM 0.0015 97.931 2.056 0.0109 0.0010 NBS 947 RM 0.260 78.031 18.822 1.670 1.217 3915 -01 1.137 61.991 23.767 9.454 3.651 3915-(12 1.359 59.319 25.673 9.141 4.5(18 3920-01 1.047 62.919 23.563 8.777 3.694 3920-(12 1.112 62.564 23.324 9.184 3.816 3922-01 1.215 62.392 24.089 8.267 4.037 3922-02 1.180 61.520 24.565 8.607 4.128 3922-03 1.029 62.999 23.631 8.668 3.673 4060-02 1.479 57.687 26.270 9.402 5.162 4060-03 1.497 57.500 26.323 9.460 5.220

il. RADIATION DETECTION

118 144 Raab, J.l.. Partt, / Nucl. Instr. and Meth. in Phys. Res. A 339 (1994) 116-121

were the 239pu and 24"Pu standard solutions containing over 99 at.% of the respective isotope. The sources prepared from these solutions were used for measure- ments of emission probabilities. Two other standards of very different isotopic composition as compared with thc routine samples were used for testing thc pertormancc of G R P A N L in a wider range of isotopic compositions. Tablc 1 shows the isotopic compositions of all materials measured in this work.

2.3. Measurements

The measurements were performed with the use of a PIPS detector of 50 mm 2 area and an energy resolu- tion of about 10 keV (FWHM) at 5.5 MeV alpha-par- ticle energy. The distance between the source and detector was about 13 mm. The pressure in the mea- suring chamber was about 5 Pa. The encrgy per chan- nel was 0.94 keV and the part of the spectrum below 3.5 MeV was offset. Two spectra were measured for each source prepared from the routine samples and the number of counts in the region of the 239pu + 24°pu peaks was 3 × 10 5.

3. Results and discuss ion

3.1. Spectrum ecaluation

Thc spectra of all routine samples and two RMs were evaluated in four different ways on a Micro Vax II computer. In all cases the 5168.19 keV peak of 2"mpu and the 5498.85 keV peak of 238pu wcrc not fixed, all other energies of 239pu and 24°pu were fixed to the first energy and of 23Spu to thc second energy, respec- tively.

In the first case all alpha-particle emission probabil- ities were not fixed (case AS I in Tables 3 and 4). In the second, third and fourth cases the peak intensities relative to the major peak of each isotope were fixed according to the emission probabilities for SET I, SET II and SET III shown in Table 2. From the single peak areas obtained, G R P A N L calculates, first, the alpha- activity ratios and, finally, the isotopic ratios.

in Fig. 1 a fit of individual peaks to the 23~pu + 2"mPu region of a spectrum is shown. Below the spectrum the residuals expressed as relative standard deviations of the number of counts in each channel are plotted.

EDIGRP F i l e Name: PU11.GRP EDISMP F i l e Mame: SI.5MP 18800 ~ , • , ,

D o u

1888

180

1 0

• I II WFI/I 1 i t II , , ....

4908 5008 510a -52@~ ....... ~3~ 5400 5500

ENERGY, keV

~ 51 ' ~ ~ . . . . . . • , , • • , %

;r _ • t I " I I * I e'* ' I ; ' 4900 5000 5100 ~20g , , . . , 5300 5 4 ~ 5500,,,

Spectral ¢ I I e : NBS947-83-1.2 Group 1 Energy range 4920.00 %o 5560.00 keV

Fig. 1. The fitting profiles of 2~SPu, 239pu and 24°pu peaks and the residuals.

5600

5&00

W. Raab, J.L. Parus / Nucl. Instr. and Meth. in Phys. Res. A 339 (1994) 116-121 119

Tablc 2 Emission probabilities for 238pu, 23'Jpu and 24°pu used for spectra evaluation (recommended, calculated and measured in this work)

Nuclide Energy [ 5 ]

[keV] Emission probabilities and standard deviations

IAEA [5] ~ Calculated b Measured c Recommended [6] SET 1 SET II SET III

238pu 5499.21 0.7104 (6) 0.7126 (7) 5456.5 0.2884 (6) 0.2866 (9) SET I IAEA 5357.7 0.00102 (2) 0.0008 (1)

2Y~Pu 5156.70 0.731 (7) 11.71311 (36) I).7160 (18) 0.7077 (14) 5143.8 0.150 (2) 0.1748 (29) 0.1664 (16) 0.1711 (14) 5105.5 0.118 (2) 0.1121 (16) 0.1176 (4) 0.1194 (7)

24°pu 5168.17 0.729 (5) 0.7330 (18) 0.7311 (8) 5123.68 0.270 (5) 0.2689 (17) 0.2682 (9) IAEA 5021.5 0.00089 (2) 0.01X183 (4) 0.01108 (1)

Given for reference only: not used in spectra evaluation. Mean values calculated from deconvolution of 14 spectra of series 3900 (Table 1).

c Measured in this work, mean values of six measurements for 239pu, five measurements for 24~)pu. Standard deviations in parentheses are expressed in units of the least significant figure.

Table 3 Comparison of MS and AS 24°pu/239pu isotopic ratios for different calculation modes

Material 240 pu/23~ Pu

MS ratio

(2"*°pu/239pu MS ratio)/(2"U)pu/239pu AS ratio)

AS I AS II AS III AS IV

3915-01 0.3834 1.0186 1.0200 1.0152 1.0181 1.0229 1.0233 1.11186 1.0212

3915-02 0.4328 1.0037 0.9989 0.9981 1.11030 1.0025 0.9978 0.9967 1 .DO 13

3920-01 0.3745 1.0086 1.0119 1.0074 1.0103 1.0111 1.0127 1.01184 1.0110

3920-02 0.3728 1.0166 1.0187 1.0136 1.0172 1 .DO 19 1.0039 0.9989 1.0021

3922-01 0.3861 1.0089 1.01115 1.0059 1.0087 1.0091 1.0106 1 .DO60 1.0088

3922-02 0.3993 1.0147 1.0161 1.0109 1.0141) 1.0147 1.0158 1.0105 1.0136

3922-03 0.3751 1.0111 1.0114 1.0055 1.0092 1.0165 1.0175 1.0118 1.0153

4060-02 0.4554 1.0254 1.0324 1.0279 1.0307 0.9999 1.0113 1.0066 1.0090

41160-03 (I.4578 1.0062 1.0159 1.0108 1.0136 1.0112 1.0171 1.0120 1.0151

mean 1.0113 1.0137 1.0092 1.0116 SD 0.0071 0.01182 0.0075 0.0075

NBS 947 0.2412 0.9917 1.11081 0.9967 1.01141 0.9918 0.9995 0.9882 0.9958 0.9865 1.0028 0.9926 0.9990 0.99116 1.0047 0.9942 1.01X15

mean 0.99t12 1.0038 0.9929 0.9999 NBL 126 0.0210 0.9356 1.0157 1.0155 1.0271

(I.9128 0.9912 0.9896 1 .DO 12 mean 0.9242 1.0035 1.{1026 1.0141

AS I - emission probabilities not fixed. AS II - emission probabilities SET I. AS III - emission probabilities SET II. AS IV - emission probabilities SET III. MS - mass spectrometry. AS - alpha-particle spectrometry.

11. R A D I A T I O N D E T E C T I O N

120 W. Raab, J.L. Parus /Nucl. Instr and Meth. in Phys. Res. A 339 (19941 116-121

3.2. Alpha-particle energies and emission probabilities

Preliminary runs of G R P A N L with the spectra of rout ine samples showed that be t t e r per formance was ob ta ined when the 239pu +24°pu and 238pu peak clus-

ters were processed simultaneously. The energy cali- bra t ion using the 5123.68 keV peak of 2'*°Pu and the 5499.21 keV peak of 238pu was easier to carry out.

The i terat ion process converged best when these two energies were not fixed and the o ther energies were fixed to them in both clusters. Thrce energies of bo th 239pu and 24°pu and four energies of 238pu were taken into account. The numerical values of cnergics were as r e commended in rcf. [5]. First, 14 spectra of series 3900 (Table 11 wcrc dcconvoluted. From the peak areas ob ta ined the a lpha-par t ic le emission proba- bilities were calculated for each of the three isotopes concerned. The means of these emission probabil i t ies are given in Table 2 (SET 1). G R P A N L also genera ted for each spect rum a new set of energies, differing slightly from the in t roduced ones. Thesc energies were also avcragcd and used in final evaluat ions of spectra.

Fivc spectra wcre measured from the 24°Pu elec- ( redeposi ted source; a total of 5 x 10 5 counts were accumula ted in the region of interest . The reference peak cnergics used for cal ibrat ion were 5123.68 and 5168.17 kcV (Table 2). The higher energy was not fixed for spectra evaluation. The peak energies derived by G R P A N L were 0.4 kcV lower than those introduced. Thc cont r ibut ion of 23'~pu to the spect rum was not more than 0.1% and was ignored. The means of rela- tive peak intensi t ies with thc s tandard deviat ion pro- vided the emission probabil i t ies given in Table 2 for SET II. The re is good agrcemenl with the I A E A r ecommended values [5] and those calculated from the test runs. For the ~-~'~Pu e lec t rodcposi tcd source, six spectra were taken and the n u m b e r of counts in the region of interest was also 5 x 10 5 . In this case thc n u m b e r of counts from 24°pu was significant and had to be taken into account. The count ratio of the 5168.19 keV peak of 24°Pu and the 5156.37 keV peak of 23'~pu was calculated from the certified isotopic composi t ion and, additionally, the intensity ratio for the two major a lpha-par t ic le energies of 24°Pu was fixed. In conse-

Table 4 Compari~n of MS and AS 23gpu/239pu isotopic ratios for different calculation modes

Material 238pu/239pu (23Spu/239pu MS a ratio)/(23Spu/239pu AS ratio)

MS ratio "~ AS I AS II AS III AS IV

3915-01 0.01834 1.0127 1.0116 1.0177 1.0121 1.0172 1.0166 1.0126 1.0167

3915 -02 0.02291 1 .f1026 (I.9996 0.9957 1.0017 0.99711 0.9948 0.9905 0.9961

3920-01 0.01664 1.11030 1.0048 1.0(06 1.01148 1.0(07 1.0073 1.0036 1.0079

3920-02 0.01777 1.0068 1.0068 1.0023 1.0068 0.9922 11.9927 0.9889 0.9928

3922-01 0.01947 1.0010 1.0010 0.9974 1.0010 1.0021 1.01121 0.9980 1.11021

3922-02 11.111918 1.0(131 1.0(131 0.99911 1.01131 1.0037 1.0037 0.9990 1.003 I

3922-03 0.111633 0.9945 0.9933 0.9885 0.9933 1.0012 1.110 12 0.9963 1.0006

4060-112 0.02564 1.0126 1.0158 1.0126 1.0158 0.9903 1.01104 0.9973 1.0000

4060-03 0.02603 0.9947 1.0023 11.9988 1.0019 1.0074 1.0062 1.01127 1.0(}66

mean 1.11027 1.11035 0.9995 1.0037 SD 0.0(373 0.0067 0.0069 0.11066

NBS 947 0.00333 0.9823 0.9940 0.9852 0.9911 11.9911 0.9911 0.9852 0.991 I 11.9823 11.9970 11.9881 11.99711 11.9823 0.99411 0.9881 0.9940

mean 11.9845 11.9940 11.9867 11.9933

AS I, AS II, AS 11I, AS IV - as in Table 3. " In the MS results the 238pu isotopic content was derived from the + 2,m Pu).

a-spectrometric measurements of a-activity ratio 238pu/(239pu

W. Raab, J.L. Pants / Nucl. Instr. and

quence, 0.55% of counts corresponding to 24°Pu alpha-particles was subtracted from each spectrum. Only three main alpha-particle groups of 239pu were taken into account, other minor groups were neglected duc to the low number of counts.

The SET III of alpha-particle emission probabilitics is a combination of I A E A values for 238pu and 24°pu and newly measurcd values by Garcia-Torafio et al. [6] for 239pu.

3.3. Isotopic ratios

The four sets of 2a°pu/239pu and Z~Spu/239pu iso-

tope ratios have been compared with the respective ratios measured by thermal ionization mass spectrome- try. Tables 3 and 4 contain the mass spectrometry ratios and the coefficients obtained by dividing these by the respective isotopic ratios from the alpha spec- trometry. The M S / A S ratio coefficients for all 18 spectra of the routine samples have been averaged and the standard deviation calculated for each group of results. For the 2'U)Pu/Z39pu ratio, as can be seen from Tablc 3, the results of alpha spectrometry are lower with a maximum difference of about 3%. The mean values arc very close for all modes of evaluation with a systcmatic error of about 1%. For two standard refer- ence materials shown at the bottom of Table 3 the results show a more random distribution, although the value of thc ratio coefficient for NBL 126 differs by more than 7% when emission probabilities were not fixed. The rcst of the coefficients are close to 1. It should be noted that the Z4°pu/Z~gPu isotope ratios in these two standards differ by more than a factor of 10 and NBS 947 differs by almost a factor of 2 from some routine samples. It is difficult to say which set of emission probabilitics is the most reliable. Since the difference of results between two spectra from the same samplc is in some cases about 2%, all results have been given the same weight.

Meth. in Phys. Res. A 339 (1994) 116-121 121

The 238pu/239pu isotope ratios determincd by alpha

spectrometry agree within a fraction of i% with com- bined a lpha /mass spectrometry values for all four modes of spectra evaluation. For NBS 947, thc agree- ment with the certified value is not so good, but the 238pu abundance in this matcrial is given with an accuracy no better than + 1%.

4. Conclusions

The G R P A N L program can bc used for measure- ment of 238pu/239pu and 24°pu/239pu isotopic ratios in

a very broad range of isotopic composition with an accuracy around ± 1 % . Present recommended emis- sion probabilities of 239pu should be updated in view of thc new published data. The peak-shape fitting func- tion which is used gives relatively high residuals and will require some adjustments.

References

[1] J. Parus, W. Raab, H. Swietly, J. Cappis and S. Deron, Nucl. Instr. and Meth. A 312 (1992) 278.

[21 R. Gunnink, W.D. Ruhter and J.B. Niday, GRPANL: A Suite of Computer Programs for Analyzing Complex Ge and Alpha-Particle Detector Spectra, Report UCRL- 53861, vols. 1 and 2, Lawrence Livermore National Labo- ratory, Livermore, California, 1988.

[3] J. Parus and W. Raab, Int. J. Radiat. Appl. Instr. A 39 (1988) 315.

[4] I.K. Kressin, Anal. Chem. 49 (1977) 842. [5] M. Lammer and O. Schwerer, Handbook of Nuclear Data

for Safeguards, Report INDC(NDS)-248, International Atomic Energy Agency, Vienna, Austria, 1991.

[6] E. Garcia-Torafio, M.L. Acefia, G. Bortels and D. Mouchel, Nucl. Instr. and Meth. A. (in press).

I1. RADIATION DETECTION