Dual Chronometer Measurements of Uranium Particles by LG-SIMS

Large geometry secondary ion mass spectrometry (LG-SIMS) has been widely used for isotopic measurements of uranium particles for many years. In recent years, it has been demonstrated it is possible to perform chronometry (a.k.a. age-dating) measurements of single uraniumcontaining, micrometer-sized particles using LG-SIMS. For these chronometry measurements, the analytes measured are the 234U – 230Th mother-daughter pair. This initial measurement protocol was developed using the mono-collector electron multiplier (EM) configuration on a LG-SIMS instrument with the preponderance of the counting time of an analysis cycle being on the 230Th to maximize measurement precision. In addition to the 234U – 230Th pair, the 235U – 231Pa chronometer pair has frequently been measured as a secondary chronometer for uraniumbased materials in several scientific fields. The 235U – 231Pa pair could have analytical value for some uranium particle sample types, such as highly-enriched uranium. Most LG-SIMS instruments have a multi-collector system configured with five EM detectors, which allows simultaneous measurement of up to five isotopes, typically improving measurement precision and detection limits over mono-collector peak hopping protocols. The multi-collector configuration allows the simultaneous counting of both 230Th and 234U, with the added advantage of allowing the collection of 231Pa, 232Th, and 235U isotopes as well. We present results on the development of our multi-collector system protocol for dual chronometer (234U – 230Th and 235U – 231Pa) measurements. Data presented will focus on results from a commonly available U-series Certified Reference Material (CRM) and from a natural uranium-containing mineral. We also discuss how limitations due to background from peak tailing — often referred to as abundance sensitivity — can negatively impact measurement detection limits for the multi-collector configuration. The reason for abundance sensitivity potentially having negative consequences for age-dating measurements is increased scattering from peak tails in multi-collector systems versus the mono-collector system for LG-SIMS.