Producing 236U reference standards for Accelerator Mass Spectrometry at the University of Notre Dame

Abstract

²³⁶U is a rare isotope of uranium, naturally occurring in ores with an abundance of ²³⁶U/²³⁸U$<1\times 10^{-9}$. The ability to detect it and make isotopic ratio measurements has applications ranging from nuclear forensics and nonproliferation to energy production and environmental protection. Currently, Accelerator Mass Spectrometry (AMS) is the only technique sensitive enough to accurately measure ²³⁶U/²³⁸U isotopic ratios as they exist in naturally occurring ores in the range of ²³⁶U/²³⁸U$=10^{-12}-10^{-9}$. Some AMS facilities have demonstrated their capabilities to make these measurements. Historically, the lack of commercially available reference standards covering the range of naturally occurring ²³⁶U/²³⁸U abundances has necessitated the use of absolute measurements, notoriously difficult to do using AMS, resulting in increased uncertainties in measurements and a reliance on knowledge of systematic effects. To mitigate these issues, various AMS facilities have sought to develop their own reference standards. Using a reference standard prepared for other forms of mass spectrometry, a series of AMS suitable standards was created through dilution with low-background natural uranium. The techniques used to produce and characterize these materials as well as analysis of them using AMS will be discussed.