Standardization and decay data of 237Np

Applied Radiation and Isotopes 56 (2002) 415–420

Standardization and decay data of

237

Np

M.J. Woodsa,*, D.H. Woodsa, S.A. Woodsa, L.J. Husbanda, S.M. Jeromea, C. Michotteb, G. Ratelb, M. Crespoc, E. Garcia-Toranoc, L. Rodriguezc, A. Lucad, B. Deneckee, G. Sibbense, J. Morelf, M. Etcheverryf, D Santryg, H. Janssenh, h h . . E. Schonfeld , U. Schotzig a

Centre for Ionising Radiation Metrology, National Physical Laboratory, Teddington, Middlesex TW11 0LW, United Kingdom b " Bureau International des Poids et Mesures, Pavillon de Breteuil, F-92312 Sevres Cedex, France c CIEMAT Metrologia de Radiaciones Ionizantes, 28040 Madrid, Spain d National Institute of R&D in Physics and Nuclear Engineering ‘‘Horia Hulubei’’, IFIN-HH, RO 76917, Bucharest, Romania e EC-DG JRC Institute for Reference Materials and Measurements, B-2440 Geel, Belgium f Laboratoire National de Henri Becquerel, BNM-LNHB, F-91193 Gif-sur-Yvette Cedex, France g National Research Council Canada, M35 Ottawa K1A 0R6, Canada h Physikalisch Technische-Bundesanstalt, D-38116 Braunschweig, Germany

Abstract This paper reports contributions from participants in the EUROMET project (No. 416) which was entitled ‘‘237Np research into problems relating to purification, characterization and standardization’’. Primary standardizations were made by the defined low solid angle, coincidence, 4pa; 2pa and liquid scintillation counting methods. Secondary standardizations were made with calibrated gamma-ray spectrometers. Absolute X-ray, gamma-ray and alpha-particle emission probabilities were also determined. The results for the successful conclusion of both primary and secondary standardization are presented together with the values for alpha-particle and gamma-ray emission probabilities determined in this exercise. Several significant inconsistencies remain with the gamma-ray emission probabilities, and these are highlighted. r 2002 The National Physical Laboratory. Published by Elsevier Science Ltd. All rights reserved.

1. Introduction Because of its long half-life of 7.82
108 days, 237Np presents an increasingly important profile in the inventory of radioactive materials in the environment, for both long-term storage and environmental monitoring considerations. It is produced in the nuclear power cycle as a long lived waste product either by the action of thermal neutrons on 235U, or by the action of fast neutrons on 238U. Surprisingly, no standardizations using absolute methods of this radionuclide have been reported. In addition, previous studies of the decay data of 237Np have failed to produce a reliable and balanced decay scheme (Woods et al., 1988; Lowles et al., 1990). *Corresponding author.

The main problems are related to the emission probabilities associated with the low energy gammaray transitions (5.2, 8.0 and 9.8 keV). The capacity to harmonize emission probability data from different centres depends critically on the ability to demonstrate that there is good agreement among the primary standards used by those centres. A proposal to establish a collaborative project to study the actinide 237Np was agreed upon in May 1997, initially involving eight partner institutions. The project, EUROMET action No. 416, was entitled ‘‘237Np research into problems relating to purification, characterization and standardization. ’’ It was designed to demonstrate ‘‘equivalence’’ among national standards and to provide more accurate and consistent decay scheme data.

0969-8043/02/$ - see front matter r 2002 The National Physical Laboratory. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 1 ) 0 0 2 2 4 - X

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M.J. Woods et al. / Applied Radiation and Isotopes 56 (2002) 415–420

2. Source material The starting material, supplied by IRMM, was a solution of B2.5 g NpO2 in B36 g of 7 M HNO3, equivalent to an activity concentration of B1.6 MBq g1. Purification was undertaken by NPL and IRMM staff in order to minimize contaminants, particularly plutonium. Earlier work at NPL had also shown that transferring a solution from one ampoule to another sometimes disturbed the significantly equilibrium between 237Np and daughter 233Pa unless care is taken to use the appropriate chemistry. Therefore, the chemistry of the solution was adjusted to be 6 M HCl and 33 mg g1 oxalic acid. Weighed aliquots were dispensed into flame-sealed glass ampoules for distribution to the participants. This solution contained o0.12% of plutonium, by activity: this value had been determined by measurements at IRMM. No gamma-ray emitting contaminants were detected by PTB, using a Ge spectrometer, in the region 10 keV–2.7 MeV. Prior to shipment, measurements were performed at NPL by both ionization chambers and by gamma-ray spectrometry, to establish that secular equilibrium between 237Np and 233Pa had been reached. Individual participants also performed tests that confirmed this and generally prepared sources several months before the reference date to ensure that any disturbance to the equilibrium had disappeared before measurements began. Two ampoules from the batch were also dispatched to BIPM for measurement in the SIR system.

3. Measurements Twelve activity measurements were reported, using seven different methods, six of which can be regarded as ‘primary’ measurements, although some may be considered as more ‘absolute’ than others. Three laboratories performed secondary or relative activity determinations using calibrated gamma-ray spectrometers. Three laboratories measured absolute photon emission probabilities values and one reported absolute alpha-particle emission probabilities. Tests by the participants on the ampoules used for dispatch showed no evidence that activity had been adsorbed onto the glass walls.

described in other publications and in the final EUROMET project report (Gunther, 2000; Woods et al., 2000, 2001). 3.2. Secondary activity measurements Calibrated gamma-ray spectrometers were used by three laboratories to determine the 233Pa activity, using 233Pa gamma-ray emission probabilities from the literature. These measurements relied on the higher energy photon emissions from 233Pa and avoided any use of the 86 keV photons which arise from both the 237Np and the 233Pa decays. The 237Np activity was assumed to be equal to the measured 233Pa activity. In addition, NRCC measured the 237Np activity directly, based on the 86 keV peak. However, the results were rather low compared to all other activity results and were not included in the final analysis. 3.3. Summary of activity results from all participants Table 1 lists the 12 separate results, noting the method and laboratory concerned: overall weighted and unweighted means were determined. The two subgroups within Table 1 refer to the primary and secondary measurements separately, and mean values are given for these. Nine primary standardizations were carried out by five laboratories using a range of methods. The unweighted mean of these had a standard uncertainty of 0.21%. The reduced w2 value was 1.8, indicating that the scatter of the nine results was (only) slightly larger than would be expected from the reported uncertainties. For the relative measurements and for the mean of all results, the statistical analysis demonstrates that the scatter of the results is very consistent with the uncertainties supplied by the participants. The mean results of the primary and secondary measurements (149.50 and 149.1 kBq g1, respectively) were consistent with each other within the uncertainties. Analysis also indicates that there is no significant difference, within the quoted uncertainties, among the results obtained by different methods. 3.4. Measurement of photon emission probabilities by absolute methods

3.1. Primary activity measurements The six primary activity measurement methods were: defined low solid angle, 4pðabÞ–g-coincidence with efficiency extrapolation, 4pa–g-coincidence at one efficiency, 4pa; 2pa and liquid-scintillation using the CIEMAT–NIST method. Individual methods, equipment details and typical uncertainty budgets are

Three participants determined photon emission probabilities from the source emission rates, which were measured with calibrated Ge, Ge(Li) or Si(Li) detectors, and from the activity, as determined by the primary methods at each laboratory. The measured photon emission probabilities had been previously reported in publications by the individual

417

M.J. Woods et al. / Applied Radiation and Isotopes 56 (2002) 415–420 Table 1 Summary of the activity results. (1998-09-01, 12 h UT) Method/lab Primary measurements Defined solid angle Defined solid angle Defined solid angle 4pa–g coincidence 4pðabÞ–g coincidence Liquid scintillation Liquid scintillation 4pa 2pa 2 Weighted mean (w =df ¼ 1:8) Unweighted mean Relative measurements g-spectrometry, 233Pa g-spectrometry, 233Pa g-spectrometry, 233Pa 2 Weighted mean ðw =df ¼ 0:94) Unweighted mean All measurements 2 Weighted mean (w =df ¼ 1:5) Unweighted mean

Laboratory

(b)

(c)

(d)

Uncertainty (kBq g1)

Relative uncertainty (%)

BNM–LNHB PTB IRMM BNM–LNHB NPL PTB CIEMAT PTB CIEMAT

150.2 149.30 149.6 149.7 148.20 149.16 151.6 149.14 150.2 149.50 149.68

0.5 0.75 0.5 2.0 0.83 0.37 0.80 0.34 0.70 0.24 0.31

0.30 0.50 0.30 1.30 0.56 0.25 0.52 0.23 0.47 0.16 0.21

BNM–LNHB BIPM NRCC

148.8 150.8 147.9 149.1 149.2

1.0 1.5 1.7 0.75 0.86

0.7 1.0 1.2 0.50 0.58

149.48 149.55

0.21 0.30

0.14 0.20

participants but are summarized here in Table 2, alongside existing evaluated data, in order to highlight remaining discrepancies. The values in the table are restricted to those which (a) indicate unresolved discrepancies either amongst the participating laboratories or with existing evaluations, or (b) provide new data at low energies, or (c) have high emission probabilities and are normally used for secondary standardization purposes. Again, detailed uncertainty budgets and details of equipment and calibration nuclides are contained in the individual publications . and the final EUROMET project report (Schotzig et al., 2000; Luca et al., 2000; Woods et al., 2000). Some particular results are worth highlighting: (a)

Activity concentration (kBq g1)

There are a large number of newly identified lowenergy emissions below 30 keV. Three lines at 21.5, 27.7 and 288.3 keV found by PTB cannot be assigned to corresponding transitions in the existing 237Np or 233Pa decay schemes. Significant and unexplained discrepancies exist for the emissions at 28, 29, 40, 46, 57, 75, 86, 88, 107, 117, 169, 193, 195, 258, 279, 299 and 300 keV. The discrepancies at the 86 keV level are of particular concern since this emission is the one most often used for routine analysis of 237Np and is, indeed, the basis for all normalizations of relative emission probabilities.

3.5. Measurement of 237Np alpha-particle emission probabilities by absolute methods Measurements on the decay scheme of 237Np data were performed at IRMM in 1984 (Vaninbroukx et al., 1984) and in 1990 (Bortels et al., 1990) using a source prepared by vacuum deposition on a quartz disc in 1983. Since that time, improvements had been made to the detector and the electron deflection system enabling new measurements to start in 1998, using the same source from 1983. This source has a 237NpF4 layer of 8 mm diameter with a superficial density of about 5 mg cm2 and an impurity of 0.1% 238Pu found by alpha-particle spectrometry. The particular systems improvements included the provision of a strong magnetic field which prevented conversion electrons up to 190 keV from reaching the detector, thus suppressing their contribution to the detected alpha-particle spectra. The improvement to the spectra enabled six additional peaks to be identified. In total, 20 alpha-particle transitions with a FWHM of 9.28 keV were identified (Sibbens and Denecke, 2000) (see Table 3), including five transitions not found in the adopted 237Np decay scheme (Firestone et al., 1996). These alpha-particle emission measurements did not use the Np solution from the EUROMET project and are thus not directly linked to the activity values reported above. It was assumed,

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M.J. Woods et al. / Applied Radiation and Isotopes 56 (2002) 415–420

Table 2 Selected photon-emission probabilities in the decays of

237

Np and

233

Pa

Previous evaluations Eg (keV) 3.15 3.25 8.22 11.37 11.62 13.27 13.60 14.95 15.4 16.0 16.6 17.2 19.50 20.16 20.49 20.7 21.5 27.7 28.38 29.37 28.38+29.37 40.35 46.53 57.1 75.35 86.48 86.81 86.48+86.81 87.99 86.48 to 87.99 107.6 117.70 169.19 193.26 194.7 194.95 194.7+194.95 258.46 279.65 288.3 298.89 300.34 298.89+300.34 312.17 340.81 398.62 415.76

Akovali (1990)

This work IAEA (1986) (IAEA 261)

Reich (1990) (priv. com)

PTB

NPL

LNHB/IFIN

0.048(25) 0.015(8) 0.0012(5) 0.0155(8) 0.0118(7) 0.260(28) 0.157(17) 0.0064(6) 0.0066(5) 0.0791(29) 0.274(11) 0.115(5) 0.0465(17) 0.0349(13) 0.00806(29) 0.0103(4) 0.00356(13) 0.0084(7)

E0.090 Pgþce

0.0011(4) 0.150(10)

0.0015(1) 0.153(3)

0.00039(8) 0.0011(1) 0.0039(1) 0.0139(8) 0.124(4) 0.0197(12)

0.00106(6) 0.00382(11) 0.0132(4) 0.123(2) 0.0197(12)

0.0014(1)

0.00138(3)

0.142(3) 0.00140(2)

0.0034(2) 0.0016(1) 0.00073(7) 0.00049(5)

0.00173(3) 0.00071(1)

0.00171(2) 0.00071(1)

0.00184(10)

0.00185(2)

0.00185(2)

0.000039(16) 0.00002(2)

0.153(3)

0.1412(15)

0.132(4)

0.00110(4) 0.0037(1)

0.00104(4) 0.00354(8) 0.0138(4)

0.000215(16) 0.001067(19) 0.00360(5) 0.01401(25) 0.1286(21)

0.141(3)

0.1483(16)

0.123(2)

0.00249(8) 0.00169(4) 0.000633(19) 0.000437(10)

0.00188(3)

0.00177(5) 0.000274(6) 0.000109(4) 0.000164(5)

0.00161(4)

0.00035 0.0662(6)

0.0663(6)

0.0655(7)

0.0666(6)

0.386(4) 0.0447(4) 0.01390(12) 0.01745(16)

0.386(4) 0.0450(5) 0.0141(2) 0.0174(2)

0.385(4) 0.0450(5) 0.01406(15) 0.01765(18)

0.387(4) 0.0452(3) 0.01407(11) 0.01771(14)

however, that every 237Np decays by the emission of an alpha particle so that a normalization could be made to obtain absolute alpha-particle emission probabilities.

0.00034(10) 0.1350(16) 0.1354(12) 0.00028(4) 0.00150(5) 0.00366(3) 0.01270(8) 0.1140(24) 0.0261(22) 0.1401(6) 0.00167(4) 0.1417(6) 0.00347(8) 0.00182(12) 0.00092(11) 0.00030(5) 0.00033(8) 0.00163(7) 0.00196(6)

0.0015(3) 0.0639(6) 0.0654(7) 0.3780(23) 0.0441(2) 0.01390(13) 0.01740(7)

4. Conclusions This EUROMET project has stimulated a wide range of measurements of 237Np and 233Pa.

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M.J. Woods et al. / Applied Radiation and Isotopes 56 (2002) 415–420 Table 3 Alpha-particle emission probabilities of This work

237

Np Bortels (1990)

Firestone (1996)

Ea (keV)

Pa

Ea (keV)

Pa

Ea (keV)

Pa

4515.1(19) 4550.5(22)a 4572.6(26)

0.00035(4) 0.00011(3) 0.00048(23)

4513.7

0.00041(4)

4514.5(20)

0.0004(2)

4578.6(14) 4599.1(18) 4619.7(21)a 4640.0(10) 4665.0(9) 4680.5(18)a 4698.2(8)

0.00369(23) 0.00371(9) 0.00032(8) 0.0643(3) 0.03478(24) 0.00020(4) 0.00535(10)

4577.5 4598.5 4619 4639.5 4664.6

0.0041(2) 0.0039(2)

4698.0

0.0054(4)

4710.8(7)

0.01174(13)

4710.6

0.0120(5)

4766.5(8) 4771.4(8) 4788.0(9) 4803.5(10) 4816.8(10) 4824.5(36)a 4848.9(49)a 4866.4(14) 4872.7(14) a

0.0932(29) 0.2315(29) 0.4764(6) 0.02014(17) 0.02430(17) 0.00014(11) 0.00006(4) 0.0053(4) 0.0239(4)

4748 4766.4 4771.4 4788.1 4803.6 4817.0

4866.4 4873.0

0.0645(4) 0.0343(4)

4572.1 4573.8(20) 4581.0(20) 4598.6(20) 4620.0 4639.4(20) 4664.0(20) 4694.4(20) 4708.3(20)

0.00054 0.0040(4) 0.0034(4) 0.0618(12) 0.0332(10) 0.0048(20)

0.097(3) 0.2265(40) 0.4775(20) 0.0206(5) 0.0247(2)

4712.3(20) 4741.3(20) 4766.0(15) 4771.0(15) 4788.0(15) 4803.3(20) 4817.3(20)

0.00019 0.08(3) 0.25(6) 0.47(9) 0.0156 0.025(4)

0.0049(3) 0.00243(3)

4862.8(20) 4873.0(20)

0.0024 0.0044

The newly found transitions.

The 237Np activity was measured in nine primary standardizations by five laboratories, using a wide range of methods. The good agreement among the results for these methods should enable equivalence to be established between the participating laboratories. It will also allow more confidence to be placed in those standards which may be used in the future, for example, for photon emission probability studies. The similarly good agreement among the secondary standardizations, using calibrated gamma-ray spectrometers, confirms the accuracy of the emission probabilities for the higher energy photon emissions of 233Pa. The results for most of the absolute photon emission probabilities were self-consistent among the three participants and with earlier evaluated data. In addition, a significant number of new low-energy photon emissions were measured, which should assist in the establishment of a balanced decay scheme. However, there are several important photon emissions where inconsistencies remain and it is clear that further work is required in this area. Improvements to the detection systems have enabled additional alpha-particle transitions to be identified

along with significantly lower uncertainties than achieved in earlier work. Five of the newly found transitions are not in the presently adopted decay scheme of 237Np.

Acknowledgements The principal authors are indebted to all of those other colleagues who have contributed to the success of this project.

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