Beta spectrum studies with cryogenic micro-calorimeters

Low energy beta spectra are studied with sources encapsulated in a superconducting Sn absorber. An NTD germanium thermistor measures the temperature transient (∼ 5 msec) caused by individual beta decays, and the total energy not given to the neutrino is determined with a resolution of ∼ 0.5 keV (FWHM) and a threshold of ∼ 5 keV. The response function is determined from the mono-energetic electron capture and internal conversion source 109Cd, both separately and simultaneously with the beta spectrum. The endpoint of the unique first-forbidden decay 107Pd → 107Ag+e-+bar νe is found to be 37.5 ± 0.1 keV. High statistics results are compared to the unique first-forbidden shape and the atomic exchange correction. The latter is seen for the first time. Preliminary results of a search for heavy neutrino admixtures are 0.15 ± 0.33%