Validation of reference materials for uranium radiochronometry in the frame of nuclear forensic investigations

Applied Radiation and Isotopes 102 (2015) 81–86

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Validation of reference materials for uranium radiochronometry in the frame of nuclear forensic investigations Z. Varga a,n, K. Mayer a, C.E. Bonamici b, A. Hubert c, I. Hutcheon d, W. Kinman b, M. Kristo d, F. Pointurier c, K. Spencer b, F. Stanley b, R. Steiner b, L. Tandon b, R. Williams d a

European Commission, Joint Research Centre (JRC), Institute for Transuranium Elements (ITU), Postfach 2340, 76125 Karlsruhe, Germany Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 8745, USA c CEA, DAM, DIF, F91297 Arpajon, France d Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA b

H I G H L I G H T S


A methodology for preparing uranium age dating reference material was validated.
Detailed laboratory procedures for uranium age dating are summarized.
The difficulties and further developments of uranium age dating are demonstrated.

art ic l e i nf o

a b s t r a c t

Article history: Received 21 October 2014 Received in revised form 8 May 2015 Accepted 11 May 2015 Available online 12 May 2015

The results of a joint effort by expert nuclear forensic laboratories in the area of age dating of uranium, i.e. the elapsed time since the last chemical purification of the material are presented and discussed. Completely separated uranium materials of known production date were distributed among the laboratories, and the samples were dated according to routine laboratory procedures by the measurement of the 230Th/234U ratio. The measurement results were in good agreement with the known production date showing that the concept for preparing uranium age dating reference material based on complete separation is valid. Detailed knowledge of the laboratory procedures used for uranium age dating allows the identification of possible improvements in the current protocols and the development of improved practice in the future. The availability of age dating reference materials as well as the evolvement of the age dating best-practice protocol will increase the relevance and applicability of age dating as part of the tool-kit available for nuclear forensic investigations. & 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Nuclear forensics Age determination Uranium Thorium

1. Introduction If nuclear materials are diverted and afterwards interdicted, detailed investigation is required to identify the possible origin, intended use and hazard related to the material. Such analysis, which is now commonly referred to as nuclear forensics, involve the comprehensive physical, chemical and isotopic measurements (e.g. physical dimensions, crystal structure, radioactive and stable chemical impurities, classical forensic analysis) as well as the interpretation of these measured parameters (Kristo and Tumey, 2013; Mayer et al., 2013; Tandon et al., 2008). Based on this complex information, the assumed origin of the material can be verified or, for an unknown material, the provenance can be n

Corresponding author. Fax: þ 49 7247 951 99 491. E-mail address: [email protected] (Z. Varga).

identified with high reliability. Numerous characteristics (so-called signatures) of the material can be used for such purpose, such as the isotopic composition of U, Pb or Sr, elemental impurities, trace-level radionuclide content, crystal structure or anionic residues (Keegan et al., 2008; Lin et al., 2013; Mayer et al., 2005; Schwantes et al., 2009; Varga et al., 2009). Besides these parameters, the elapsed time since the last chemical purification of the material (commonly referred to as the “age” of the material) can also be measured for radioactive (nuclear) materials (Pointurier et al., 2013; Varga and Surányi, 2007; Wallenius et al., 2002; Williams and Gaffney, 2011). This unique possibility is based on exploiting the presence and decay of radioactive nuclides (usually isotopes of uranium or plutonium as the major component in case of nuclear materials): in the course of production, the radionuclide is chemically purified from impurities, including also its radioactive decay products. After

http://dx.doi.org/10.1016/j.apradiso.2015.05.005 0969-8043/& 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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production, the radioactive progenies start to grow-in again in the material. Assuming that the parent–daughter separation was complete, the elapsed time since the last separation, as well as the production date, can be calculated according to the decay equations after the measurement of the parent–daughter ratio in the sample. This radiochronometry age value, the so-called model age of the material, enables either the identification of the origin of the unknown sample or the verification of the source of the feed material. In contrast to most other characteristics used in nuclear forensics, the production date of the material is a predictive signature, thus it does not require comparison samples for origin assessment. This feature makes the production date one of the most prominent signatures for attribution (Kristo and Tumey, 2013; Mayer et al., 2013). For uranium materials the most frequently used chronometer is the 230Th/234U daughter/parent pair. As 230Th is present at trace levels in the investigated nuclear material (typically 10  10–10  7 g per gram sample depending on the enrichment and age), state-of-the-art analytical techniques are required to perform high precision analysis using as low a sample amount as possible. The special care required for the handling of sometimes highlyenriched nuclear or other associated radioactive materials also raises difficulties for the age dating measurements. This was also shown during the last Round Robin exercise organized by the Nuclear Forensics International Technical Working Group (ITWG), where the task was to analyze two different highly-enriched uranium metal samples to determine whether these materials originated from the same or different sources. Out of the nine participating Nuclear Forensic Laboratories worldwide only five reported age results, four of which reported model age values covering the known preparation (casting) dates given by the supplier, however, only two participating laboratories could achieve uncertainties low enough to verify the different production times of the two samples and, thus, the two different sources of the materials in question (Hanlen, 2011). Moreover, in order to put the obtained results on a more solid scientific and legally defensible basis, dedicated reference materials are required. In consequence, an emerging need for such materials has been recently expressed by the community involved in nuclear security programs (Inn et al., 2013; Leggitt et al., 2009). In practice, due to the lack of radiochronometry reference materials, already available reference materials certified only for major isotopic composition are used by nuclear forensic laboratories as a pragmatic solution to check the accuracy of their measurement results by comparing them with the reasonably well-known dates of final purification of these materials (usually referred to as assumed or archive ages) (Gaffney et al., 2009; Wallenius et al., 2002). However, as the 230Th/234U radiochronometry method is highly sensitive to the initial purity of the material, a very high degree of separation (more than 107) has to be achieved for this chronometer to eliminate the positive bias caused by the residual 230Th in the material (i.e. incomplete zeroing). A small discrepancy between the measured production dates and the respective archive values has already been reported for several enriched uranium

Fig. 1. Reported production dates for the natural uranium sample. Solid line: known production date (19 July 2011). Dashed line: average of the reported dates (21 June 2011) with twice the reported standard deviation of the reported dates (117 days, dotted lines).

Fig. 2. Reported production dates for the low-enriched uranium sample. Solid line: known production date (19 July 2011). Dashed line: average of the reported dates (23 July 2011) with twice the reported standard deviation of the reported dates (42 days, dotted lines).

reference materials by Gaffney et al. (2009) who attributed this effect to the incomplete separation during the production. Our major objective was the preparation and validation of a uranium-based reference material, which can be applied for the validation of age measurements based on the 230Th/234U chronometer. The material was prepared from high-purity uranium solutions of various uranium enrichments by completely separating the thorium decay product (Varga et al., 2012). By this means, the production date is well-established and precisely known. In contrast to other methods of producing age dating reference materials, this approach does not require relying on consensus or archive values, because, if all conditions are fulfilled (completeness of separation, long-term stability, closed system), the 230Th present in the material will solely be governed by the radioactive decay laws. Therefore, the material prepared can be used as a primary standard for age dating of uranium materials. The aim of the present collaboration is two-fold: firstly, to

Table 1 Reported production dates. Uncertainties are expressed as expanded uncertainties with a coverage factor of k ¼2. Laboratory

Production date-HEU

Production date-LEU

Production date – NU

Lab A Lab B Lab C Lab D Lab E AVERAGE Expected

27/08/20117 85 days 19/07/20117 26 days 24/06/2011 722 days 25/07/20117 57 days 21/07/20117 15 days 23/07/2011 723 days 19/07/2011

16/07/20117 22 days 19/09/20117 34 days 10/06/2011 771 days 17/07/20117 19 days 23/07/20117 42 days 19/07/2011

18/07/20117 23 days 07/01/20117 80 days 07/10/2011 746 days 24/07/2011 735 days 21/06/20117 117 days 19/07/2011

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the task is to determine the production date of the sample, the elapsed time (t) can be calculated from Eq. (1). after the measurement of NTh-230 and NU-234 in the sample. The final thorium purification of the material took place on 19 July 2011, which is also considered as the date of production in the absence of residual thorium. The purified uranium solutions were aliquoted into screw-capped PFA vials right after uranium purification and sample homogenization. Aliquots containing approximately 30 mg U were placed into each vial, and then they were evaporated to dryness immediately after aliquoting to avoid loss of Th by adsorption. The estimated uncertainty of the date of the final separation is less than 5 h taking into account the amount of residual 230Th in the sample, the duration of the final chemical separation and the time needed for the evaporation of the aliquoted samples.

Fig. 3. Reported production dates for the highly-enriched uranium sample. Solid line: known production date (19 July 2011). Dashed line: average of the reported dates (23 July 2011) with twice the reported standard deviation of the reported dates (23 days, dotted lines).

prove the applicability of this methodology for the preparation of a uranium age dating reference material by the independent measurement of expert laboratories. Since the validation requires the measurement of the 230Th decay product at very low levels from the freshly separated material, state-of-the-art instruments and well-established techniques are required. Secondly, this joint effort enables the identification of the best methodologies (best practices) for uranium age dating.

2. Preparation of the materials The investigated materials were prepared from uranium after complete separation of thorium decay products (zeroing the initial daughter nuclide concentration) at a well-known time and allowing the ingrowth of the daughter nuclides. The preparation of the material is described in detail elsewhere (Varga et al., 2012). The starting materials were high-purity uranium-oxide samples dissolved in nitric acid. Three uranium materials with different uranium enrichments were used: natural uranium (NU, 0.71% 235U abundance), low-enriched uranium (LEU, approximately 4% 235U abundance) and a highly-enriched uranium material (HEU, 235U abundance is about 70%). The dissolved uranium samples were purified with three consecutive extraction chromatographic separation steps in order to completely remove the 230Th decay product. The separation efficiency of Th was determined by gamma spectrometric measurement and by the addition of 232Th to the starting material and its re-measurement following the chemical separations. A total separation factor of approximately 3  107 was achieved, which corresponds to a 230Th/234U ratio in the final reference material of 10-11–10-13 at the time of preparation. Therefore, the residual 230Th is negligible compared to the ingrowth thereafter, and corresponds to less than a few hours, expressed as time. The amount of 230Th in the material is then solely the function of the 234U amount and the time elapsed since separation. The 230Th amount can be derived from the decay equations:

NTh − 230 = NU − 234

λU − 234 e−λU − 234 t − e−λTh − 230 t λTh − 230 − λU − 234

(

)

(1)

where NTh-230 and NU-234 are the amounts (numbers of atoms) of 230 Th and 234U in the sample, respectively, λTh-230 and λU-234 are the decay constants of 230Th and 234U, respectively, and t is the elapsed time since the separation of the material. Alternatively, if

3. Results and discussion 3.1. Comparison of reported results from the participating laboratories After the preparations, items of the prepared samples were then shipped to the expert nuclear forensic laboratories, where the production dates of the materials were determined according to their routine procedures. The participating laboratories in this study were Lawrence Livermore National Laboratory (USA), two laboratories from the Los Alamos National Laboratory (USA), the DIF center of CEA (France) and EC JRC Institute for Transuranium Elements (European Commission). The reported age results are summarized in Table 1 and shown in Figs. 1–3. The reported average production dates for the NU, LEU and HEU samples are 21 June 2011 (with an uncertainty of 117 days at k¼ 2), 23 July 2011 (with an uncertainty of 42 days at k¼ 2) and 23 July 2011 (with an uncertainty of 23 days at k¼2), respectively. For all samples the reported averages are in agreement with the known production date of 19 July 2011 within measurement uncertainty. The differences between the known production dates and the reported average production dates for the natural, LEU and HEU samples are 4.5 days, 4.7 days and 27.2 days, respectively. As no systematic and significant bias could be identified between the known and reported values of all three materials, the methodology for production of such uranium age dating reference material is expected to be applicable, which is an additional confirmation of the earlier study (Varga et al., 2012). All the reported production dates of the individual laboratories overlap with the average results even at 1-sigma level, thus the results are very consistent. However, if one compares the individual laboratory results, significant differences can be observed. While all reported individual HEU age results overlap with one another, significant differences are observed between some reported individual results in the case of the LEU and NU samples (Figs. 1–3). This difference is much higher for the NU sample than for the LEU material, which correlates with the 234U content (and therefore with the amount of 230 Th progeny). As the difference is probably not related to the reference material properties (e.g. inhomogeneity between the items), it is assumed to be the consequence of the difficulties in the measurement of the trace-level 230Th. In the case of the NU and LEU samples, the corrections for the trace-level 230Th measurement are more significant than for the HEU material, which are the most probable reasons for the differences in the reported results. 3.2. Detailed methodologies for age dating By sharing the details of the existing methodologies, possible

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Table 2 Analytical measurement methodologies of the participating laboratories. Sample preparation

Lab A

Lab B

Lab C

Sample taking, subsampling or sample pre-treatment

Sample was dissolved and aliquots were taken for U/Pa and U/Th analysis Ultrapure HCl,heated to 90 °C overnight

Sample dissolved in original sample vials and transferred with rinses into weighed vial 4 M HNO3 þ 0.05 M HF, into 30 mL PFA vial for primary solution, warmed on low temperature hotplate and ultrasonicated

Sample was dissolved in original Total sample was dissolved in the original vial prior to splitting Teflon vial without splitting.

TIMS (Triton)

Static multi-collection on NuPlasma MCICPMS. U-isotopic composition measured first on un-spiked aliquots. U IDMS on separate spiked aliquot from secondary dilution IDMS using 233U spike.

TE TIMS (VG Sector 54)

Multi-collector TIMS U isotopic analysis

TIMS, MTE-TIMS (Triton) and ICPMS (Element2)

Isotope dilution

Isotope dilution

1.2 mg done in triplicate

0.4–0.6 mg U

3 mg

Isotope dilution on Element 2 ICP-MS þ U isotopic composition by MC-TIMS 500 ng for U isotopic, 5 ng for U IDMS measurement

ICP-MS (Element XR)

MC-ICP-MS (NuPlasma)

Ion chromatography (AG 1-X8), single step Isotope dilution 229 Th (AEA Technology)

Three step: anion exchange, TEVA extraction, anion exchange Isotope dilution 229 Th calibrated against NIST 4342 A 230Th standard; NBL U010 used for mass bias correction. 2.25–4.52 mg U.

Ion counting TIMS (Isotopx Isoprobe T) Ion chromatography (Lewatit MP5080) Isotope dilution NBL 229Th

Dissolution conditions (e.g. type of acid, temperature, labware type)

234

U quantification method

Sample amount used for U analysis

Thorium analysis Measurement technique and instrument type Thorium chemical separation method 230 Th quantification method Standards (calibrants) applied, manufacturer Sample amount used for Th separation Mass bias/mass fractionation factor for Th measurement (if applied) Detector efficiency/gain measurement for Th Abundance sensitivity measurement (if applied)

Isotope dilution with

233

U

1.2 mg done in triplicate Exponential law correction with IRMM183 (U standard) in bracketing None (single collector ICPMS) None measurement of 229 Th/230Th; no significant amount of 232Th

Typical absolute method Below 1 fg blank Quality control sample used IRMM-184 (U standard)

230

229

0.99309 ( Th/ Th) determined from NBL U010 standards during analytical session N/A – peak jumping on same detector. None

1.5–1.9 fg

24 h closed vessel digestion on hotplate at Dissolved in heated acid (80– 90 °C in 8 M HNO3 (Optima HNO3 þ triple 90 °C) for  1 h and allowed to equilibrate overnight prior to use distilled H2O)

230

Th

Table Mountain Latite, secular equilibrium standard for spike calibration check

230

Th

U-630 U and Th Fractions

5 mg, done in duplicate

ICP-MS (Element2)

Anion exchange  3 (2–8 M HNO3 columns then 1- 9 M HCl column) Isotope dilution 229 Th calibrated against NIST 4342 A 230Th standard

Extraction chromatography (TEVA), single step Isotope dilution Custom-made natural 232Th (certified as mass fraction (Spex Certiprep Inc.) 1 mg, three replicates are done

1–3 mg, three replicates of each sample except natural U sample. None applied as not enough data NBL U010 measured by ICP-MS to quantify

85 fg of

Total sample was dissolved in the original PFA vial without splitting/ transfer. Subboiled ccHNO3 (3 ml to 30 mg U), heated to 90 °C for 1 h

ICP-MS (Element 2)

27 mg

Not applied. WARP used

Lab E

Using uranium with CRM U010

None

None (single collector ICP-MS)

Measured 236U in NIST U-960 (natural U) as a monitor of peak tailing. No correction applied. Measured 232Th signals o1e6 cps for all samples, natural U samples with comparable 238U signals exhibited negligible tailing 10–30 fg 230Th

Used on 230Th/232Th ratio with natural uranium with 236U abundance less than 10  9, linear correction

30 fg of

IRMM-035

IRMM-035

230

Th

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Uranium analysis Measurement technique and instrument type

Lab D

(1)

230

Th measurement (229Th spike calibration – Th standard uncertainty) (2) 234U measurement (233U spike calibration) (3) 234U half-life (4) Mass bias corrections

Measured 229Th/230Th ratio, 229Th and 233U concentration of the standard

Estimates of uncertainty are standard deviations BIPM Guide on internal independent observations propagated in quadrature with a coverage factor of two Measured 230Th/229Th ratio, 230Th and 233U con- Measured 230Th/230Th ratio, 232Th and 233U centration of isotope dilution standards, mass concentration of the standards, mass bias bias correction factors (NBL CRM U-010) correction factors BIPM Guide BIPM Guide

Excel based LANL developed software GUM Workbench Pro Excel

NBL CRM-125 A and NBL CRM-U630

75690 7 230 years at 2s Cheng et al. (2000)

245 500 7545 (k ¼ 1), from IAEA Live Chart 754007 300 (k ¼1), from IAEA Live Chart U 630 from NBL 245 250 7 490 years at 2s Cheng et al. (2000) 75690 7 230 years at 2s Cheng et al. (2000) None

245 250 7 490 years at 2s Cheng et al. (2000) Th half-life used 75690 7 230 years at 2s Cheng et al. (2000) Quality control sample NBS100 used Software used for Excel calculation Error propagation Approach used for uncertainty calculation 229 Major uncertainty Th concentration of the components tracer and counting statistic of 229Th and 230Th U half-life used

230

234

Age calculation

Table 3 Age dating calculation procedures of the participating laboratories.

245 250 7 490 years at 2s Cheng et al. (2000)

245 5007 600 (k ¼ 1), from Decay Data Evaluation Project DDEP (2014) 75380 7 300 (k¼ 1), from Decay Data Evaluation Project DDEP (2014) Self-prepared completely separated U with known production date GUM Workbench

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inappropriate steps in the procedures can be rectified (Tables 2 and 3). By this means, the approaches can be harmonized and best practices for age dating measurements can be developed. In all laboratories, mass spectrometry was used for age dating due to its lower detection limits and faster sample throughput compared to alpha spectrometry. The sample amount needed for uranium age dating is in the 1–27 mg range, except for Lab A, where the sample amount is limited to microgram level due to strict limitations in the amount of nuclear material authorized to be handled. As uranium is the major component, its measurement is generally wellestablished and easy to perform, often several independent techniques can be used for the measurement, which does not require chemical separation or pre-concentration. The measurement of trace-level 230Th, however, which requires the processing of higher sample amount and chemical separation, is the principal challenge in age dating. The applied chemical separation methods involve ion exchange chromatography and/or extraction chromatographic separations. Note that the number of purification steps (and therefore the time necessary) is highly different between the laboratories. This is generally related to the individual laboratory limitations on the uranium quantity that can be handled in the laboratory. The reported absolute detection limits, however, are relatively similar in spite of the different chemical separations, and are in the 1–85 fg 230Th range. In all laboratories isotope dilution is used to quantify the 230Th content of the sample. The tracers are either 229Th or 232Th. As the available 229Th standards (tracers) are often certified as activity concentration (e.g. Bq g  1), it has to be converted to amount (number of atoms) or mass fraction for the mass spectrometric application. This was accomplished converting the 229Th activity to 229Th amount content using the 229Th half-life by Lab A. The used 229Th half-life was 7932 755 years reported by Kikunaga et al. (2011). Lab B and Lab D measured the 229Th standard against 230Th radioactivity standard (NIST 4342 A), which was also converted to amount content using the 230Th half-life value given by Cheng et al. (2000). Note that in either case the conversion involves the use of half-life values, which highly affects the accuracy and precision. Lab C used 229Th standard based on mass fraction obtained from New Brunswick Laboratory, and cross-calibrated against natural Th standard (Spex Certiprep Inc.). Lab D applied custom-made natural Th standard (dominantly 232 Th) from Spex Certiprep Inc. certified by mass fraction. As no significant difference was found between the laboratory reported ages for the high-level HEU samples, no inconsistency could be demonstrated between the tracers used. The applied analytical instruments for the 230Th measurements are inductively coupled plasma mass spectrometry (ICP-MS) for four laboratories and thermal ionization mass spectrometry (TIMS) in one laboratory. The correction methods used in the laboratories are similar: mass bias correction for the measured thorium isotope ratios using certified uranium isotopic standards; no detector efficiency (gain) measurements are used, as also the multi-collector ICP-MS is operated at peak-jumping mode on the same detector; no abundance sensitivity correction is needed if 229 Th is used as standard for the isotope dilution quantification. The applied TIMS method was developed for small sample size with total evaporation, and is described in details elsewhere (Callis and Abernathey, 1991). Different quality control samples are used in the laboratories to validate the results, such as uranium isotopic standards, geological materials or thorium isotopic standards. Note that all quality control samples have completely different chemical composition and/or a different 230Th/234U or 230Th/232Th ratio than the analyzed uranium samples, and that the quality control value is not certified, but only based on repeated measurements. Thus, none of them are really ideal for the validation of the work. Using the measured 230Th and 234U amounts, the age of the

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materials have been calculated in each laboratory. There are notable differences in the 230Th and 234U half-life values used for this study. The calculations were performed using either self-prepared calculation software or by commercially available dedicated data evaluation software (Table 3). All laboratories reported combined standard uncertainty, calculated either based on the BIPM Guide (Metrology, 2008) or simply on rules of the propagation of uncertainty.

4. Conclusions Based on the results we can conclude that the methodology for preparing a uranium age dating reference material is valid, as the reported production date results are in agreement with the known production date. The reported individual laboratory values for the HEU sample (containing higher amounts of 230Th) are in agreement with one another within measurement uncertainty. However, for the NU and LEU samples, more pronounced differences could be observed. This reflects the enormous challenges associated with age dating of such young material, particularly when the sample size is fairly small, in the μg to mg range. Thus, further efforts are required to improve existing methodologies, especially for the less or no enriched and/or recently purified materials. The most important prerequisite is the availability of a (possibly certified) age dating reference material, prepared from low enriched uranium with low 230Th content (young material). The use of such material would help identify and overcome the current problems with the low-level thorium analysis. Another important field for improvement is the thorium tracer used for isotope dilution. The most suitable thorium tracer is 229 Th, which is currently available only as radioactivity standard. Therefore, if 229Th is to be used and measured (calibrated) against another thorium standard, one has to make sure that the traceability is guaranteed as well as all uncertainty components are taken into account. Besides the reference materials and standards, the half-life values of 229Th, 230Th and 234U are of utmost importance both in the measurement and the age calculation, thus care has to be taken when selecting a value. Re-evaluated half-life values, such as by the Decay Data Evaluation Project, (DDEP, 2014) generally have higher uncertainties than values reported in the literature, however, they can provide more reliable basic data and help avoid the underestimation of the final uncertainty (Pommé et al., 2014). The availability of age dating reference materials will help validate current and future age dating protocols, leading to an increased confidence in nuclear forensic conclusions and providing a legally defensible basis for the use of age dating results for prosecution purposes. Validation of these methods will increase their relevance and applicability as part of the tool-kit available for nuclear forensics investigations.

Acknowledgment Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U. S. Department of Energy under contract DE-AC52-06NA25396. This publication is LA-UR-14-28168. Work at Lawrence Livermore National Laboratory performed under the auspices of the U.S. Department of Energy under Contract DE-AC52-07NA27344. LANL

and LLNL thank the U.S. Department of Energy's National Nuclear Security Administration, Office of Non-proliferation and International Security, for financial support.

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