Magnetic Microcalorimeter Gamma Detectors for High-Accuracy Nuclear Decay Data

Magnetic microcalorimeters (MMCs) are low-energy gamma-ray detectors with ultra-high energy resolution that are operated at temperatures below 50 mK. They use metallic Au absorbers and paramagnetic Ag:Er sensors to measure gamma-ray energies from an increase in detector temperature. Ultra-low temperatures are made accessible to non-expert users with “dry” dilution refrigerators that replace the traditional liquid N2 and He pre-cooling with a mechanically cooled pulse-tube refrigerator. Our MMC gamma detectors have achieved an energy resolution below 40 eV FWHM at 60 keV, and they have a uniform and reproducible response function that allows adding spectra from different detector pixels without loss in energy resolution. These properties make MMCs well-suited to re-measure nuclear decay data with increased precision and smaller systematic errors to increase the accuracy of non-destructive assays (NDA). We present recent MMC gamma-ray spectra of Th-234 to show how MMCs can resolve lines that are relevant for NDA analysis and that overlap in Ge spectra. We also discuss the accuracy that MMCs can achieve for nuclear decay data with an emphasis on gamma-ray energies and intensity ratios that current NDA codes are based on.