Sensor and Method Development for Analysis of Alpha- and Beta-Decaying Radioisotopes Embedded InsideMicrocalorimeter Detectors

We discuss sensor and method development for the analysis of alphaand beta-decaying radioisotopes encapsulated within superconducting transition-edge sensor microcalorimeter absorbers. For alpha-decaying isotopes, e.g., ²³⁸Pu, ²⁴¹Am, and ²¹⁰Po, this is a measurement of the total nuclear reaction energy (Q) and the spectra consist of sharp, narrow peaks. The primary peak is at the Q value, with secondary peaks corresponding to gamma-ray-escape peaks. In contrast, for beta-decaying isotopes, e.g., ²⁴¹Pu, the spectrum is a low energy continuum ending at E=Q. We are developing transition edge-sensor (TES) microcalorimeters to measure these spectra simultaneously in a single sample, hence allowing quantitative analysis of all Pu isotopes from 238 to 242. We have developed and used TES microcalorimeter detectors for this purpose, and it represents a new quantitative analysis tool for nuclear forensics and safeguards. Due to the high efficiency of the embedded source geometry, measurement times can be minimized. The high dynamic range of our sensors creates the opportunity to measure the relatively low energy beta-decay spectrum of ²⁴¹Pu (Q = 20.78 keV) simultaneously with the Q ~ 5-6 MeV of alpha-decaying actinides. Finally, the technique could also be effective for determining the time since chemical purification of Pu using the ²⁴¹Pu/²⁴¹Am isotopic ratio via simultaneous measurement of the low-energy ²⁴¹Pu beta particles and the high-energy ²⁴¹Am Q-value.