Developing Standard Reference Materials for ²³⁶U Accelerator Mass Spectrometry Capabilities at the University of Notre Dame

<p></p> <p>²³⁶U is a rare isotope of uranium, naturally occurring with an abundance below 1 part per billion. 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 \ratio isotopic ratios as they exist in naturally occurring ores and rock in the range of ²³⁶U /²³⁸U =1x10^-14-1x10^-9. To facilitate the measurements, standard reference materials are needed, however up to this point the only ones available for ²³⁶U AMS are either well above the natural range, or are natural samples the community has accepted to have a particular value. This thesis describes the synthesis procedure and successful characterization of a new series of ²³⁶U isotopic standards to be used to calibrate systems aimed at making these low ²³⁶U /²³⁸U measurements. This series, dubbed the USRM series, was measured against an existing community accepted standard and remarkable accuracy to the expected values of the USRM series was observed. Further measurements characterizing the ²³⁶U content in various specimens from the Ewing collection were made. In addition to these efforts, a series of upgrades to our AMS system will be described and their implications for improved uranium AMS measurements will be given. A new detection limit of ²³⁶U /²³⁸U =1x10^-12 was determined.</p> <p></p>