Superconducting absorbers for use in ultra-high resolution gamma-ray spectrometers based on low temperature microcalorimeter arrays

Higher resolution gamma-ray detectors are of significant interest to many scientific fields, including fundamental determinations of the Lamb shift in atoms with high atomic numbers, nuclear treaty verification, and environmental monitoring. High-purity germanium is presently the detector material of choice, but germanium detectors suffer from fundamental limitations on their resolution. Recent advances in the use of low temperature calorimeters can provide improvements of more than an order of magnitude. For instance, we report here a resolution of 27 eV FWHM at 103 keV. These calorimeters are made up of two components, a bulk absorber to stop incident gamma-rays and a thermometer made from a thin film electrically biased in the superconducting-to-normal phase transition, called a Transition Edge Sensor, or TES. We have now begun a systematic study of absorber behavior in order to increase efficiency and understand effects on resolution. This study leverages two recently developed capabilities: the ability to microfabricate highly uniform arrays of gamma-ray detectors and the ability to read out signals from many detectors in a single cool-down to 0.1 K using SQUID multiplexer circuits. Here, I will discuss two experiments we are currently conducting to identify better absorber materials. The first involves the use of tin (Sn) as the gamma-ray absorber and the effect of crystal grain size on the thermalization of absorbed photons. The second experiment examines the role of absorbed hydrogen in tantalum (Ta) absorbers. The presence of hydrogen is known to increase the heat capacity of Ta and may be a limiting factor in Ta's use as an improved absorber material.