Development of Superconducting Tunnel Junction Photon Detector using Hafnium

We present the development of a Superconducting Tunnel Junction (STJ) detector using Hafnium (Hf) as a photon detector which was designed to search for radiative decay of cosmic background neutrinos. The photon energy spectrum from neutrino radiative decay has a sharp edge at high energy end. To detect this sharp edge, we need a micro-calorimeter of infrared photons with high energy resolution. We have optimized the condition of producing a Hf-STJ detector and observed that a Hf-STJ detector had Josephson currents which disappeared by applying a magnetic field.