Alpha coincidence detection for the assay of actinides

Interferences in both decay counting and mass counting techniques limit their application for some environmental monitoring applications. For example, ²³⁸U interferes with ²³⁸Pu in mass spectrometry measurements, while in conventional alpha spectroscopy measurements it is nearly impossible to separate ²³⁸Pu from ²⁴¹Am and ²³⁹Pu from ²⁴⁰Pu. These interferences are typically resolved by using chemical separation and/or different measurement techniques for different isotopes. We are investigating radiation detector concepts to simultaneously assay these four isotopes with minimal sample preparation by exploiting radiation signatures measured in coincidence with the predominate alpha decays of these isotopes. Particles in coincidence with the alpha decay include conversion electrons, gamma rays, x-rays, and Auger electrons. Each decay has a unique energy distribution enabling the separation of the isotopes. We are exploring two basic detector concepts to achieve these goals: a silicon-based design and a gas-detector design. The silicon system provides the potential for higher energy resolution at the cost of lower efficiency compared to a gas detector. In this paper, we will describe our evaluation of the different detector concepts, which will include estimations of potential detection efficiency, ability to resolve the isotopes, sample preparation and equipment requirements.