Toward a 256-Pixel Array of Gamma-Ray Microcalorimeters for Nuclear-Materials Analysis

We are developing a gamma-ray spectrometer for the analysis of nuclear materials based on an array of superconducting transition-edge-sensor microcalorimeters. The instrument will include eight columns of time-division-SQUID multiplexing circuitry capable of reading out 256 sensors. Our most recent sensors are bulk (1.5 mm square×0.25 mm thick) superconducting Sn absorbers glued to Mo/Cu bilayer thermometers. When fully populated, the active area of the spectrometer will be 5.76 cm2, and the maximum count rate of the array will approach 20 kHz. Thus, our spectrometer will be comparable to the state-of-the-art 100 keV high-purity-Ge detector in count rate and collecting area, but with an order of magnitude better energy resolution. Half the detectors will be optimized for operation up to 100 keV, and the other half for operation up to 200 keV. A version of the spectrometer with a partially populated detector array was delivered to Los Alamos National Laboratory in June, 2007. We describe the present status of that instrument. In addition, we review results from a prototype array of 14 detectors that achieved 47 eV average energy resolution (full width at half maximum at 103 keV) and 25 eV resolution in the best detector. An important application of this technology is determining the total Pu content in spent reactor fuel without detailed knowledge of the reactor’s operating history.