Alpha spectrometric evaluation of SRM-995 as a potential uranium/thorium double tracer system for age-dating uranium materials

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 uranium-bearing 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. (author)