Alpha spectrometric evaluation of SRM-995 as a potential uranium/thorium double tracer system for age-dating uranium mat
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Alpha spectrometric evaluation of SRM-995 as a potential uranium/thorium double tracer system for age-dating uranium materials F. E. Stanley • S. E. Glover • A. M. Stalcup H. B. Spitz • Donna Beals
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Received: 26 July 2012 / Published online: 26 August 2012 Ó Akade´miai Kiado´, Budapest, Hungary 2012
Abstract Uranium-233 (t1/2 * 1.59E5 years) is an artificial, fissile isotope of uranium that has significant importance in nuclear forensics. The isotope provides a unique signature in determining the origin and provenance of uraniumbearing materials and is valuable as a mass spectrometric tracer. Alpha spectrometry was employed in the critical evaluation of a 233U standard reference material (SRM-995) as a dual tracer system based on the in-growth of 229Th (t1/2 * 7.34E3 years) for *35 years following radiochemical purification. Preliminary investigations focused on the isotopic analysis of standards and unmodified fractions of SRM-995; all samples were separated and purified using a multi-column anion-exchange scheme. The 229Th/233U atom ratio for SRM-995 was found to be 1.598E-4 (±4.50 %) using recovery-corrected radiochemical methods. Using the Bateman equations and relevant half-lives, this ratio reflects a material that was purified *36.8 years prior to this analysis. The calculated age is discussed in contrast with both the date of certification and the recorded date of last purification. Keywords Age-dating Alpha spectrometry Nuclear forensics Uranium SRM-995 Thorium
Introduction Determining the age of nuclear materials, i.e. the time that has passed since last radiochemical purification, is of significant
F. E. Stanley (&) S. E. Glover A. M. Stalcup H. B. Spitz University of Cincinnati, Cincinnati, OH 45221, USA e-mail: [email protected] D. Beals Savannah River National Laboratory, Aiken, SC 29808, USA
importance to the field of nuclear forensics [1, 2]. Such information is invaluable in determining the provenance of the material. The ability to accurately gather age-dating information in a timely-manner is also a significant deterrent to nuclear terrorism and is of key importance in routine treaty verification as well as ongoing safeguards efforts [3]. Radioactive decay of actinides provides unique chronometers inherent to a given material. Several isotopic ratios have been exploited to determine nuclear material production ages. For example, enriched uranium can be age-dated based on the 230Th/234U and 231Pa/235U ratios [4, 5]. Additional nuclide ratios, such as the 229Th/233U ratio, may be exploited for special applications [2]. The 229 Th/233U ratio is distinct from the previous examples in that the 233U is a synthetic nuclide and its content in uranium materials is usually negligible. Uranium-233 is produced by neutron irradiation of 232 Th [2]. This is a multistep process, as shown: 232
Thðn; cÞ233 Th
!
ðb;22:3 minÞ
233
Pa
!
ðb;26:97 dÞ
233
U
The decay of 233U (t1/2 * 159,200 years) to 229Th (t1/2 * 7,340 years) [6] is useful as a special applications chronometer.
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