Cryogenic detection of 7Be with beryllium metal absorber for lithium solar neutrino experiment

A cryogenic micro-calorimeter to detect the 7Be electron capture decay has been developed. The detector absorber is a beryllium metal foil which has been previously irradiated by protons in order to produce the 7Be isotope. The 112 eV spectral line corresponding to the 7Be K-capture decay to the 7Li ground state (Auger-electron plus the nuclear recoil energy) has been successfully detected with an energy resolution of 53 eV FWHM. The observed decrease in the rate of the absorber activity corresponds to the expected 7Be half-life. The obtained results show that the 112 eV 7Be spectral line can be detected by a cryogenic μ-calorimeter with an efficiency close to 100%. This cryogenic method solves the principal problem in the development of a lithium radiochemical solar neutrino detector.