DETERMINATION OF PU ISOTOPE SYSTEMATICS OF TRINITITE POST-DETONATION MATERIAL

Effective forensic analysis of nuclear post-detonation materials (PDMs) relies heavily on the ability to identify the chemical and isotopic signature of the nuclear device (fuel) in a timely fashion. In-situ, spatially resolved analysis of PDMs using laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) offers this capability. This study reports in-situ isotope measurements of Pu within Trinitite PDM by LA-multi-collector-(MC)-ICP-MS, and these are compared to the recorded Pu ion signals (function of concentration) and trace element concentrations obtained by LA-ICP-MS.

Trinitite is the blast melt that resulted from the world’s first atomic test at the Trinity site, near Alamogordo, New Mexico (05:29:45AM on July 16^th, 1945). The Trinity device was an implosion-type device organized in several concentric shells and containing about 6 kg of supergrade Pu fuel (~99.1% of ²³⁹Pu, ~0.9% of ²⁴⁰Pu and trace amounts of ²⁴¹Pu). The lack of a commercially available Pu isotope standard for use as an external calibration standard to monitor instrumental mass bias during LA-MC-ICP-MS analyses is somewhat problematical. However, the latter was monitored using a ‘standard-sample’ bracketing technique measuring the ²³⁵U/²³⁸U ratio for the NIST SRM 610 standard glass wafer. Ion signals for ²³⁹Pu, ²⁴⁰Pu, and ²⁴¹Pu (+ ²⁴¹Am) were collected simultaneously by three ion counters. Prior to laser ablation analysis, the efficiency for each ion counter was assessed by measuring the Pb isotope ratios for solutions of varying concentrations (ppb level) of the NIST SRM 981 standard. The laser ablation data indicates that areas within Trinitite characterized by high ²³⁹Pu ion signals record ²⁴⁰Pu/²³⁹Pu ratios between 0.025 and 0.035, whereas regions that yield much lower ²³⁹Pu signals are associated with higher ²⁴⁰Pu/²³⁹Pu ratios. Similar trends were also recorded for laser ablation analyses conducted with a high-resolution-ICP-MS instrument. The combined Pu isotopic ratios obtained here can be explained by invoking 2-component mixing between remnants of the nuclear fuel used in the Trinity device and that present in the geological background (natural fallout).