Measurement of production date (age) of nanogram amount of uranium
- 270 Downloads
JRC-Karlsruhe obtained a swipe sample from a highly enriched uranium seizure, which had taken place in 2011. Due to the very low amount of uranium (nanograms) a new method needed to be developed to determine the U production date (age). The particles on the swipe were collected on a pyrolytic graphite planchet using a vacuum impactor and they were subsequently leached with ccHNO3. The “bulk” U isotopic composition (235U: 72.51 ± 0.03 wt%) and the production date (December 1992 ± 1 year) determined by MC-ICP-MS indicated that the material showed similarity with two other HEU cases seized earlier in Europe.
KeywordsNuclear forensics Production date Uranium Inductively coupled plasma mass spectrometry Swipe sample
The nuclear forensics methods aim at providing hints on the intended use, origin, production time and history of nuclear and other radioactive materials [1, 2, 3]. The scientific results, obtained in a timely manner, support the nuclear forensic findings and may serve law enforcement as investigative leads or as evidence. Several characteristic parameters (signatures) such as physical dimensions of the material, isotopic composition of U and Pu, impurities or production date can be used to re-establish the material history, hence link the material in question to a production process or even a facility. Moreover, traditional forensic evidence associated with the material may help to identify individuals who handled the material [1, 4, 5, 6, 7, 8, 9]. Determination of the production date (age dating) is based on the radioactive decay of the material and the measurement of the formed daughter products relative to the parent nuclides. The (model) age of the material is a prominent signature as it is a so-called predictive signature and does not require any reference information [10, 11]. Uranium age dating is typically carried out using the decay of 234U to 230Th, achieving the quantification of both nuclides by isotope dilution mass spectrometry. Age dating measurements, however, require typically milligram amounts of material and a tedious separation due to the low abundance of the decay products [3, 12]. Using lower sample amount can result in high uncertainties due to the small quantity of the daughter nuclides and lower measurement precision, e.g. due to the higher contribution from the background. Age dating of U particles by secondary ion mass spectrometry (SIMS) would require relatively large particles (micrometer-sized) of an old material .
As the sample amount was very low (the material was not even visible on the swipe) and the particles were small (based on the scanning electron microscopy their size was between 100 and 200 nm), only a limited number of analysis could be performed. It was decided to look into the production date (age) of the material besides the determination of the isotopic composition of U, which was performed by Large-Geometry SIMS (LG-SIMS) as a primary technique for particle analysis . Due to the small size of the particles production date measurement could not be performed by LG-SIMS. Thus, a new method had to be developed, which combined the measurement of “bulk” (i.e. the average of all the particles) U isotopic composition and the production date determination of nanogram amount of a uranium sample by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS).
Theory of direct production date measurement
The developed method is applicable for highly enriched uranium (i.e. containing relatively high amount of 234U), which is reasonably pure from other elements. For instance, “dirty” swipe samples can contain high amounts of Pb, e.g. from lead shielding, and due to the possible molecular interferences (e.g. 204Pb12C14N+) erroneous results can be obtained. This analysed sample contained HEU particles on a piece of paper, which had been in touch only with the bulk HEU powder. Therefore the blank level was expected to be low.
In addition to that, the method is obviously applicable only for HEU with single U composition and not for mixtures containing e.g. natural uranium. This sample was analysed prior to the age determination by SIMS, which showed that the HEU particles had all the same isotopic composition and no other uranium particles, with different composition, were detected.
Reagents and materials
All labware was thoroughly cleaned before use. Suprapur grade nitric acid (Merck, Darmstadt, Germany) was used for the sample preparation, which was further purified by sub-boiling distillation (AHF analysentechnik AG, Tübingen, Germany). For dilutions ultrapure water was used (Elga LabWater, Celle, Germany). Perfluoralkoxy (PFA) vials with a diameter of 22 mm (volume: 3 mL) were purchased from AHF analysentechnik AG (Tübingen, Germany). Polished pyrolytic graphite planchets were obtained from ANAME Instrumentación Científica (Madrid, Spain).
MC-ICP-MS operating parameters
MC-ICP-MS instrument settings
Forward power (W)
Cooling gas flow rate (L min−1)
Auxiliary gas flow rate (L min−1)
Nebulizer gas flow rate (L min−1)
Sample introduction conditions
Solution uptake rate (µL min−1)
Spray chamber temperature (°C)
Membrane temperature (°C)
Sweep gas flow rate (L min−1)
Number of spectra acquired
6 × 5
Magnet delay between blocks (s)
Cup configuration—U isotopic measurementsa
L1: 234U, Axial: 235U, H1: 236U, H3: 238U
Cup configuration—age dating measurementsa
IC0: 230Th, H1: 234U
The ICP-MS was optimised daily (torch position, gas flows, voltages) using a 50 ng g−1 multi-elemental solution (Inorganic Ventures, Christiansburg, USA). The optimisation aimed at achieving highest sensitivity and stability of the acquired U signal. The 234U signal was aimed to about 1 V intensity for the age dating measurement and only the 230Th and 234U signals were measured.
For the U isotopic composition determination the isotopes 234U, 235U, 236U and 238U were measured on the Faraday detectors, while for the age dating 230Th was measured on the ion counter equipped with a retardation filter and 234U was measured simultaneously on a Faraday detector. The retardation filter improves the abundance sensitivity on m/z = 230 by a factor of ~ 10. The measurement of the age dating was done in one sequence to make sure that Th and U sensitivities, detector efficiencies and mass bias (i.e. f′ in Eq. 5) were constant. Calibrant to calculate the f′ was measured before each sample. The MC-ICP-MS measurement was performed in the following order: Calibrant—Quality control—Calibrant—Sample—Calibrant—Quality control. Dilute HNO3/HF mixture between the samples was used to remove the Th and U traces.
Measured U samples
The used certified reference materials (CRMs), U500, U850 and CRM U630, are U standard reference materials in the form of U3O8 (New Brunswick Laboratory Argonne, IL, USA) and they have 50, 85 and 63% nominal enrichment of 235U, respectively. All CRMs are certified for their U isotopic composition, while the CRM U630 is certified for model purification date as well (certified model date: 6 June, 1989 with an uncertainty of 190 days). For the U isotopic measurements the mass bias was determined using the U500 CRM whereas for the quality control (to check the U isotopic abundances) U850 CRM was used. For the production date measurement the 230Th/234U amount ratio and the age was calculated after calibrating with the CRM U630 radiochronometric standard. For quality control purpose a 70% highly enriched uranium (HEU-70) was used (known production date: 19 July, 2011), which has been prepared in the JRC-Karlsruhe, afterwards analysed and validated by several international laboratories . The concentration of the samples for age measurement was approximately 250 ng g−1 U.
The same procedure was applied to the “real” sample. It was evaporated and taken up in 1.5 mL 4% HNO3. About 390 ng U could be recovered from the sample. The sample was split in two fractions in order to measure the “bulk” (i.e. the average of the particles) U isotopic composition (about 10% of the aliquot) and the production date (about 90% of the aliquot) by MC-ICP-MS.
Results and discussion
Minimum U amount for age dating
Using about 300 ng of U (with about 0.15 pg 230Th) one can get an age result, which agrees with the model purification date and which is precise enough. Above this U amount the precision of the measurement, which is about 6% relative to the measured value, will not increase significantly.
Results of the swipe sample
“Bulk” U isotopic composition
“Bulk” U isotopic composition of the swipe sample by MC-ICP-MS
U (k = 2)
Relative U (%)
Age dating result
Age dating measurement sequence and obtained results
f′ (1/a) of CRM U630
Model age (a)
0.53 ± 0.01
6.8 ± 0.2
September 2011 ± 0.2 years
0.53 ± 0.01
25.6 ± 1.0
December 1992 ± 1.0 years
0.52 ± 0.01
7.0 ± 0.2
June 2011 ± 0.2 years
The age dating result for the swipe sample is December 1992 with an uncertainty of 1.0 years. The uncertainty is mainly determined by the 230Th/234U ratio measurements and CRM U630 model purification date uncertainty making up > 95% of the overall uncertainty, other contributions (e.g. from the half-life uncertainties) are minimal.
A novel sample preparation and measurement procedure was developed and validated for the “bulk” U isotopic composition and age determination from a swipe sample containing only nanogram amounts of U by MC-ICP-MS. The U isotopic information could be used to complement the LG-SIMS results performed on individual U particles. Using these two main characteristics, i.e. isotopic composition and age of uranium, one could link the HEU seized in Moldova to two other similar illicit trafficking cases, which had taken place around 10 years earlier in Bulgaria  and in France . First of all, the packaging of the sample in all cases was very similar: cylindrical lead container lined with yellow paraffin wax and the HEU powder was inside a flamed-sealed glass ampoule. The U isotopic composition in the Bulgarian case was 234U: 1.2%, 235U: 72.7%, 236U: 12.1% and 238U: 14.0% (all values are atom- %, uncertainties are less than 0.5%) , while in the French seizure the U isotopic composition was 234U: 1.17 ± 0.02, 235U: 72.57 ± 0.86, 236U: 12.15 ± 0.14 and 238U: 14.11 ± 0.08 . Therefore, both the Bulgarian and French HEU cases agree well with the HEU seized in Moldova. The age dating results differed about one year between the Bulgarian and French cases being 30 October, 1993 ± 50 days, and November 1994 ± 100 days, respectively . Thus, the age dating result for the HEU found in Moldova is in agreement with the Bulgarian seizure. In conclusion, the HEU material found in three different European countries within the time span of 12 years is very likely coming from the same source material. Whether the HEU in different cases originates from the same batch, i.e. was purified at the same time, can be questioned. However, the link between the cases is evident.
The LG-SIMS laboratory of JRC-Karlsruhe (M. Hedberg, C. Vincent, N. Albert) are gratefully acknowledged for their indispensable contribution.
- 5.Tandon L, Hastings E, Banar J, Barnes J, Beddingfield D, Decker D, Dyke J, Farr D, FitzPatrick J, Gallimore D, Garner S, Gritzo R, Hahn T, Havrilla G, Johnson B, Kuhn K, LaMont S, Langner D, Lewis C, Majidi V, Martinez P, McCabe R, Mecklenburg S, Mercer D, Meyers S, Montoya V, Patterson B, Pereyra RA, Porterfield D, Poths J, Rademacher D, Ruggiero C, Schwartz D, Scott M, Spencer K, Steiner R, Villarreal R, Volz H, Walker L, Wong A, Worley C (2008) Nuclear, chemical, and physical characterization of nuclear materials. J Radioanal Nucl Chem 276:467–473CrossRefGoogle Scholar
- 8.Varga Z, Wallenius M, Mayer K, Meppen M (2011) Analysis of uranium ore concentrates for origin assessment. Radiochim Acta 1:1–4Google Scholar
- 10.Varga Z, Mayer K, Bonamici CE, Hubert A, Hutcheon I, Kinman W, Kristo M, Pointurier F, Spencer K, Stanley F, Steiner R, Tandon L, Williams R (2015) Validation of reference materials for uranium radiochronometry in the frame of nuclear forensic investigations. Appl Radiat Isot 102:81–86. https://doi.org/10.1016/j.apradiso.2015.05.005 CrossRefPubMedGoogle Scholar
- 12.Wallenius M, Morgenstern A, Apostolidis C, Mayer K (2002) Determination of the age of highly enriched Uranium. Anal Bioanal Chem 374:379–384Google Scholar
- 15.Esaka F, Watanabe K, Fukuyama H, Onodera T, Esaka KT, Magara M, Sakurai S, Usuda S (2004) Efficient isotope ratio analysis of uranium particles in swipe samples by total-reflection X-ray fluorescence spectrometry and secondary ion mass spectrometry. J Nucl Sci Technol 41(11):1027–1032. https://doi.org/10.1080/18811248.2004.9726327 CrossRefGoogle Scholar
- 16.Hedberg PML, Peres P, Cliff JB, Rabemananjara F, Littmann S, Thiele H, Vincent C, Albert N (2011) Improved particle location and isotopic screening measurements of sub-micron sized particles by secondary ion mass spectrometry. J Anal At Spectrom 26(2):406–413. https://doi.org/10.1039/c0ja00181c CrossRefGoogle Scholar
- 18.Baude S, Chartier B, Kimmel D, Mariotte F, Masse D, Peron H, Tilly D (2007) The French response in cases of illicit nuclear trafficking lessons learned from a real case (IAEA-CN-154/062). In: International conference on illicit nuclear trafficking: collective experience and the way forward, Edinburgh, Scotland, 2007. International Atomic energy Agency, ViennaGoogle Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.