Studies of Nuclear Decay with Transition-Edge Sensor Microcalorimeters

Microcalorimeters are thermal sensors that measure deposited energy by transducing it to a temperature rise. The use of a high-quality thermometer in combination with operation at sub-Kelvin temperatures to suppress electrical and thermal noise sources allows the energy of individual photons and particles to be measured with great precision. One particularly effective thermometer is the superconducting Transition-Edge Sensor (TES): a thin film that is electrically biased into the superconducting-to-normal transition where resistance is a strong function of temperature. TES microcalorimeters have demonstrated energy resolutions as good as 22 eV FWHM for 97 keV gamma-rays and approximately 1 keV FWHM for both alpha particles and nuclear decays with MeV-scale energies. Microcalorimeter arrays and multiplexed readout techniques can overcome the area and count rate limitations of individual devices. In this presentation I review the current status of TES devices and describe their use for measurement of nuclear decay. Recent work includes measurements of the decay spectrum of ¹⁶³Ho to constrain the mass of the electron neutrino, accurate determination of the massic activity of a solution containing ²⁴¹Am, and precision gamma-ray spectroscopy to characterize material from the nuclear fuel cycle. I also review the fabrication and readout technology behind TES arrays and discuss ongoing efforts to scale to arrays with 10⁴ to 10⁶ elements.