Gamma spectrometry in the ITWG CMX-4 exercise

Low enriched uranium samples of unknown origin were analyzed by 16 laboratories in the context of a Collaborative Materials Exercise (CMX), organized by the Nuclear Forensics International Technical Working Group (ITWG). The purpose was to compare and prioritize nuclear forensic methods and techniques, and to evaluate attribution capabilities among participants. This paper gives a snapshot of the gamma spectrometric capabilities of the participating laboratories and summarizes the results achieved by gamma spectrometry.


Introduction
This paper presents the state of practice in gamma spectrometry for nuclear forensics exercises. The Nuclear Forensics International Technical Working Group (ITWG) organized its fourth interlaboratory exercise in 2014, called Collaborative Materials Exercise (CMX-4) [1]. This paper documents the collective experience with gamma spectrometry during the CMX-4 exercise and it gives a snapshot of the applied approaches.
Nuclear forensics is the analysis of intercepted nuclear or other radioactive material to provide evidence for nuclear attribution in a legal context. The goal of the analysis is to identify the composition, origin, and intended use of interdicted nuclear or radiological samples, containers, and transport vehicles. Nuclear forensic analysis includes the characterization of the material and correlation with its production history [2]. The CMX-4 represents the second paired-comparison exercise organized to improve international cooperation and communication in case of a nuclear security event.
Three oxide samples of low enriched uranium (LEU) were selected as the materials to be characterized during the CMX-4 exercise. A scenario was included in which a seizure of nuclear material occurred and forensic analysis was requested. Laboratories were instructed to submit assessment reports in a 24 h, 1 week, and 2 month time frame. Participating laboratories categorized and characterized the exercise materials, and performed nuclear forensic evaluations. Each of the 16 participating laboratories was assigned a code name by the organizers to ensure anonymity and confidentiality of data.
Among the methods used in nuclear forensics, highresolution gamma spectrometry (HRGS) is a relatively rapid nondestructive analytical technique. Advantages include preservation of evidence and no, or a minimal, need for sample preparation. A disadvantage is that it suffers from higher uncertainty compared to destructive techniques, such as mass spectrometry (MS) [3]. This paper presents the isotopic composition, age, and signatures of the neutron irradiation history of the three LEU samples determined by gamma spectrometry. These values are compared to the community average values determined by mass spectrometry.

Sample description
Participants were provided three samples of similar size and mass: ES-1, ES-2, and ES-3. The ES-1 sample consisted of a physically homogenous, fine black U-oxide powder. ES-2 and ES-3 samples were dark gray, homogeneous, UO 2 pellets with smooth surfaces. Representative physical sample properties are shown in Table 1. Pellets are made of UO 2 , whereas ES-1 was a mixture of UO 2 and higher U-oxides. Further details on the exercise samples are provided in the introductory article of this Special Section [1].
Determining major U isotopic ( 234 U, 235 U, 238 U) abundances The samples were assayed first using HRGS for the 24 h and 1 week reports. Spectra were generally acquired by high purity germanium (HPGe) detectors for about 30-60 min in the case of the 24 h reports, whereas much longer measurement times were used for the 1 week reports.
Most participants of the exercise determined the major U isotopic ( 234 U, 235 U, and 238 U) abundances using computer codes [4,5] for automatic spectrum evaluation. These codes are based on so-called relative efficiency calibration (or intrinsic calibration) method [6]. The relative efficiency curve is obtained from the spectrum of the measured sample, thus the attenuation both in the sample (self-attenuation) and in absorbers (shielding) are taken into account. Therefore, the method does not depend on the sample size, geometry, physical, and chemical state. As all the information for determining the isotope ratios is present in the spectrum of the sample, no reference materials are required for calibration. However, for quality control purposes and for demonstrating laboratory performance, it is recommended to use a set reference materials.
Manual evaluation also occurred, after primary processing (measurement control, peak shape fitting, determining peak area, and deconvolution of overlapping peaks) by codes (FitzPeaks, PeakEasy, and Gammavision), followed by application of intrinsic self-calibration. For example, peaks of 214 Bi (for age dating) and descendants of 232 U were evaluated manually. Table 2 summarizes the gamma spectrometers and software used for determining the isotopic composition of the samples in the CMX-4 exercise. Only participants sharing their results for this paper are listed. High-efficiency coaxial HPGe detectors used for age dating and identifying reprocessed uranium are not included here.
For 234 U most participants obtained results that correlate well with the average of mass spectrometric results for all samples (Table 3). Biases fall within expanded uncertainty (k = 2) limits, amounting to 10-20% relative uncertainty. Older versions of MGAU tend to underestimate 234 U due to inaccuracies in the extrapolation of the intrinsic efficiency curve to 120.9 keV [7]. Evaluation by the GAMANAL code also resulted in unrealistically low 234 U abundance.
Concerning 235 U, all participants recognized the LEU character of the samples, regardless of the detector-type and software used. Participants found similar enrichment values of around 2.6-2.9% for samples ES-1 and ES-3, and 2.1-2.3% for the sample ES-2 ( Table 3). The majority of values reported by participants provided a means to differentiate ES-1 and ES-3 from ES-2, regardless of the method used.
MGAU v4.2 results from spectra taken by a planar detector compared very well to the average 235 U enrichment measured by mass spectrometry for all three samples. The slight underestimation of the 235 U content by MGAU may come from the coincidence summing losses in the peaks of 235 U [8]. Some evaluations of spectra taken by coaxial and broad-energy detectors resulted in significantly higher 235 U content. This could be due to coincidence summing losses in the high-energy peaks from 234m Pa, which leads to an underestimation of the activity ratio 238 U( 234m Pa)/ 235 U and therefore an overestimation of 235 U [9]. The 235 U enrichment estimates by the Identify software were significantly lower than values reported by mass spectrometry. One reason for the discrepancy could be wrong assumptions on the sample matrix, similar to what previously was observed for low resolution measurements [10]. Regarding 238 U, mass abundances of 97.2 ± 0.1% were reported for ES-1 and ES-3, whereas 97.8 ± 0.1% for ES-2, in agreement with mass spectrometric results.
It can be concluded that the inventory of the three major uranium isotopes established by HRGS generally agreed with mass spectrometric results within expanded uncertainties and confirmed the LEU character of the samples. The accuracy was generally enough for distinguishing ES-1 and ES-3 from ES-2. Exceptions for 235 U results came from some participants using coaxial germanium detector. Discrepancies for 234 U results took place mainly for participants using outdated computer routines [7], or efficiency transfer algorithms based on point-source efficiency calibrations instead of relative, intrinsic efficiency calibrations.
Results of uranium isotopic abundance measurements by gamma spectrometry and community average values by mass spectrometry for comparison are summarized in Table 3. The mass spectrometry average is calculated from the data given in graphical form in the CMX-4 After-Action report [1]. For this calculation the outliers were removed. A three-isotope plot of the relative biases with respect to mass spectrometry for the 234 U/ 238 U and 235 U/ 238 isotopic ratios is shown Fig. 1.

U age dating
The model age (time elapsed since the last chemical purification) of the material is important for determining of the origin of nuclear material outside of regulatory control. The daughter/parent ratio as a function of decay time is widely used for determining the age of radioactive samples [11,12]. Gamma spectrometric uranium age dating is nondestructive and suitable for relatively rapid assay. The method does not require the use of reference materials of known ages, nor radionuclide standards for method calibration. Usually there is no need to take subsamples or dismantle the investigated items, so preservation of evidence can easily be ensured. The method works particularly well for high-enriched and aged material. Its limits appear for low-enriched material and in sensitivity to background.
Uranium age dating by HRGS is based on the 234 U ? 230 Th ? 226 Ra chronometer [13][14][15][16][17][18][19]. 234 U can be Expanded uncertainties' coverage factor k = 2. The community average of mass spectrometric results is calculated without the outliers, and its expanded standard deviation (k = 2) is given detected by its 120.9 keV gamma line, whereas 230 Th has no measurable gamma rays. The next member of the 234 U decay series is 226 Ra, of which the only gamma-line at 186.2 keV overlaps with 185.7 keV line of 235 U. However, its short lived descendants 214 Pb and 214 Bi have measureable gamma lines.
The time needed for secular equilibrium between 226 Ra and 214 Bi is about 3 weeks, so it can be assumed that the activities of 226 Ra and 214 Bi are equal at the time of the measurement. Although we are not aware of any experimental evidence that 222 Rn would escape from the solid samples, it is useful practice to hermetically seal the samples in small containers.
For determining 214 Bi activity, the intensity of the 609.3 keV gamma line (and some other 214 Bi lines) can be recorded by a large coaxial HPGe spectrometer under lowbackground conditions. The same spectrum is used to determine 238 U peaks for relative efficiency. As 214 Bi is a cascade emitter, true coincidence summing losses can cause a bias for short sample-to-detector distances, and should preferably be corrected for. However, for larger sample-to-detector distances the bias due to true coincidence summing losses can often be neglected compared to other sources of uncertainty.
The line of 214 Pb at 352 keV does not suffer from true coincidence summing losses. However, it is usually difficult to quantify due to its low intensity and high background continuum. Furthermore, it is far away from the peaks which are used to construct the relative efficiency curve, so the uncertainty of the relative efficiency at 352 keV is very high.
The 226 Ra/ 238 U activity ratio determined through measuring 226 Ra descendants is divided by the 234 U/ 238 U ratio obtained during the measurement of the U isotopic composition. The age of the sample is then calculated from the 226 Ra/ 234 U ratio. 234 U is preferentially enriched along with 235 U in the enrichment process. Hence, for lower 235 U abundances, the amount of 234 U (and therefore of 214 Bi) is lower as well, so the corresponding activity is more difficult to measure.
To extend the capabilities of the method, a high-efficiency, 293 cm 3 well-type detector was acquired by a participant laboratory (Picasso). The first application of this kind of detector for uranium age dating was assaying the CMX-4 exercise samples [20]. The well-type HPGe detector (Canberra GCW6023) was in an iron chamber of 20 cm wall thickness. A spectrum of ES-3 acquired by the well-detector is shown in Fig. 2. Owing to their low enrichment and age, an upper limit of & 11 years was estimated uniformly for the three samples. This result was consistent with the results from destructive measurements.
Ten participant laboratories employed mass spectrometry and alpha spectrometry for determining 234 U and 230 Th in the samples [21]. One of those laboratories applied 231 Pa/ 235 U chronometer as well. Most of the measured ages (around 10 years for ES-1 and ES-3 and around 12 years for ES-2) were consistent with the known history of the material.

Identification of reprocessed material
Gamma spectroscopy is useful for screening uranium samples for fission products, and some of the actinide isotopes that would be produced by neutron activation. If these were detected, they would indicate reprocessed uranium, and also offer clues about the reprocessing technology by identifying deficiencies in the uranium purification chemistry. No fission or activation products were detected in the CMX-4 samples, and upper limits reported for representative nuclides were similar between the three samples.
The minor isotopes 236 U and 232 U are characteristic of reprocessed material (or blended/contaminated with reprocessed material) and their presence gives evidence of previous neutron irradiation (e.g., in a reactor) of the sample. The very low 232 U concentration can be measured by alpha- [22,23] or gamma spectrometry [18,[24][25][26].
• Vermeer identified 238, 583, and 727 keV gamma lines of 212 Pb, 208 Tl, and 212 Bi, respectively, (descendants of 232 Th and 232 U alike) in ES-1 and ES-3 samples, using a low-background system (15 ? 5 cm Pb; the inner 5 cm layer is of a low 210 Pb source), but the 911 keV line of 228 Ac (daughter of only 232 Th) was missing. Hence, it was concluded that the former did not derive from 232 Th, but rather from 232 U (decay scheme of the two nuclides is common starting from 228 Th). However, they could not find any of those lines in the ES-2 pellet. • Picasso evaluated 208 Tl, 212 Bi, and 212 Pb peaks in spectra of ES-1 and ES-3 samples, in absence of 228 Ac, but no such peaks for ES-2 above background. The 2614 keV 208 Tl peak was observed in the spectrum of ES-1 (also ES-3, Fig. 2) sample, corresponding to (6 ± 0.5)10 -11 % 232 U concentration, but difficult to exclude from background (& 10 -11 %) in the ES-2 sample. From the lack of the 911 keV line of 228 Ac, Picasso estimated a detection limit of about 10 -4 for the 232 Th/ 238 U mass ratio with a 12 h measurement time using the well detector. • Buonarroti performed analysis similar to that of Picasso, and obtained that ES-1 and ES-3 contain (1.40 ± 0.96)10 -10 % and (1.20 ± 0.70)10 -10 % of 232 U, respectively, whereas ES-2 contains less than 1 9 10 -10 %.
Presence of 232 U in ES-1 and ES-3 indicates that the samples were manufactured from material mixed or contaminated with reprocessed uranium. According to mass spectrometric results [1], the concentration of 236 U in both samples is about 0.0020 ± 0.0004% on average, but near the detection limit in ES-2 (B 10 -4 ). This confirms the conclusion from HRGS that ES-1 and ES-2 contained some reprocessed material, while ES-2 did not. 236 U cannot be analyzed in LEU by gamma spectrometry, only in extremely high-enriched (weapons-grade) material [13], because its peaks are masked by the much stronger peaks from the major U isotopes (e.g., the strongest, but still quite weak 49.369 keV line of 236 U lies very near the 49.55 keV line of 238 U.) Thus, 236 U abundances were only quantified by mass spectrometry.
A correlation between the 236 U and 232 U contents exists. According to Picasso's results, the 236 U/ 232 U abundance ratio was & 3 9 10 7 in the ES-1 and ES-3 samples. This value is in agreement with earlier results [18,24,26] on U samples over the full range of enrichments, from LEU to the highest enriched (90%) uranium, and is in accordance with theoretical predictions [27].
It is not clear where the reprocessed U in samples ES-1 and ES-3 comes from. All the three samples were made in the same factory (fictitious ''EA Fuel Products'' or virtually HIFAR [1]) and from natural U. If they got contaminated with reprocessed U in the enrichment plant, all three should contain traces of 236 U and 232 U, unless the facility became contaminated between processing the two source materials A and B.

Conclusions
For identifying the provenance of unknown nuclear material, information on the isotopic composition, the age, and previous neutron irradiation of the material is relevant. In addition to previous exercises when weapon grade materials were examined, this exercise confirmed that gamma spectrometry also plays a significant role for the analysis of LEU in the comprehensive response to these issues.
Results of this exercise confirmed that LEU can be categorized as such via gamma spectrometry within 24 h, regardless of the detector, software, and calibration methodology. For accurate determination of isotope ratios, the best results were acquired with planar HPGe detectors and current versions of the MGAU and U235 software. The utility of high-efficiency HPGe detectors in low-background setup was demonstrated for detecting trace 232 U, thus indicating contamination with reprocessed U. Challenges related to the age dating of low-enriched and young aged uranium (difficulty with determination of lower amounts of 214 Bi) were identified.
The combination of different analytical techniques increases the confidence in the results and can help further narrow down the set of possible origins and intended uses of the examined materials.