Development of superconducting tunnel junction detectors as a far-infrared single photon detector for neutrino decay search

We present the development of Superconducting Tunnel Junction (STJ) detectors as a far-infrared single photon detector motivated by application to a search for the radiative decay of cosmic background neutrino. The photon energy spectrum from the neutrino radiative decays is expected to have a sharp edge at high energy end in a far-infrared region ranging from 14 meV to 25 meV (from 50 μm to 90 μm in wavelength). Thus, the detector is required to measure photon-by-photon energies with sufficiently high resolution to identify the edge structure. Detectors consisting of an array of niobium/aluminum STJ (Nb/Al-STJ) pixels with a diffraction grating and STJ using hafnium (Hf-STJ) are under consideration. Each Nb/Al-STJ pixel is capable of detecting single photons in the far-infrared region. The diffraction grating creates a variation in the photon wavelength across the pixel array. Hf-STJ is expected to have 2% energy resolution for single photon of 25 meV due to very small gap energy of hafnium.