256-pixel microcalorimeter array for high-resolution γ-ray spectroscopy of mixed-actinide materials

The application of cryogenic microcalorimeter detectors to γ-ray spectroscopy allows for measurements with unprecedented energy resolution. These detectors are ideally suited for γ-ray spectroscopy applications for which the measurement quality is limited by the spectral overlap of many closely spaced transitions using conventional detector technologies. The non-destructive analysis of mixed-isotope Pu materials is one such application where the precision can be potentially improved utilizing microcalorimeter detectors compared to current state-of-the-art high-purity Ge detectors (HPGe). The LANL-NIST γ-ray spectrometer, a 256-pixel microcalorimeter array based on transition-edge sensors (TESs), was recently commissioned and used to collect data on a variety of Pu isotopic standards to characterize the instrument performance. These measurements represent the first time the simultaneous readout of all 256 pixels for measurements of mixed-isotope Pu materials has been achieved. The LANL-NIST γ-ray spectrometer has demonstrated an average pixel resolution of 55 eV full-width-at-half-maximum at 100 keV, nearly an order of magnitude better than HPGe detectors. Some challenges of the analysis of many-channel ultra-high resolution data and the techniques used to produce quality spectra for isotopic analysis will be presented. The LANL-NIST γ-ray spectrometer has also demonstrated stable operation and obtained high resolution measurements at total array event rates beyond 1 kHz. For a total event rate of 1.25 kHz, approximately 5.6 cps/pixel, a 72.2 eV average FWHM for the 103 keV photopeak of 153Gd was achieved.