  Calorimetric decay energy spectroscopy of electron-capture-decaying isotopes
is a promising method to achieve the sensitivity required for electron neutrino
mass measurement. The very low total nuclear decay energy (QEC < 3 keV) and
short half-life (4570 y) of 163Ho make it attractive for high-precision
electron capture spectroscopy (ECS) near the kinematic endpoint, where the
neutrino momentum goes to zero. In the ECS approach, an
electron-capture-decaying isotope is embedded inside a microcalorimeter
designed to capture and measure the energy of all the decay radiation except
that of the escaping neutrino. We have developed a complete process for
proton-irradiation-based isotope production, isolation, and purification of
163Ho. We have developed transition-edge sensors for this measurement and
methods for incorporating 163Ho into high-resolution microcalorimeters, and
have measured the electron-capture spectrum of 163Ho. We present our work in
these areas and discuss the measured spectrum and its comparison to current
theory.
