Year
2013
Abstract
New Brunswick Laboratory (NBL) has characterized additional attributes for two currently available uranium Certified Reference Materials (CRMs). The additional characterization was performed as part of the DHS-sponsored Nuclear Forensics Reference Material program and will allow the CRMs to be used as standards for radiochronometric purification age determinations. The reference materials are a 63% 235U enriched U3O8 powder (CRM U630) and a 4% 235U enriched UO2 pellet (CRM 125-A). Age can be a key attribute for determining the provenance of a uranium material for which the accountancy trail has been lost (i.e., seized as contraband, unknown legacy material). Determining the time elapsed since the last chemical purification (colloquially called the \"age\") of uranium using the isotopic parent-daughter ratios of the 234U and 235U decay chains has been developed for a variety of analytical methods including alpha spectrometry and mass spectrometry. The efficacy of these methods has generally been demonstrated by measuring the ages of known samples and/or CRMs and comparing measured ages to available processing records. Previously, no CRMs with reference purification ages were available for method evaluation and verification. To address this issue, NBL evaluated two existing uranium CRMs to establish certified model purification dates. Requirements to certify a material for specific attributes include measurement traceability, material homogeneity, material stability, and GUM compliant measurement uncertainty. This CRM project was designed and implemented to measure the necessary variables for model age determination using traceable methods, to evaluate within-sample and/or sample-to-sample variability, and to provide sufficient information to make realistic GUMcompliant uncertainty estimates. The certified values determined for these materials are derived from the 234U- 230Th model purification date but ages determined using the 235U, 232U, and combined decay chains have been assessed to confirm the integrity of the model purification date. New and revised attribute values determined for the reference materials include U assay for U630, refined isotopic composition for CRM 125-A, and 230Th concentration and U purification dates for both CRMs (C125A: 08/18/1994 +/-116 days; U630: 06/06/1989 +/-190 days). These values will be provided in revised certificates available from NBL upon request.