Development of Superconducting Tunnel Junction Detector Using Hafnium for COBAND Experiment

Abstract

We present the development of a Superconducting Tunnel Junction detector using hafnium (Hf-STJ) as a far infrared single photon detector for COsmic BAckground Neutrino Decay search (COBAND) experiment. The photon energy spectrum from the decay of cosmic background neutrino is expected to have a sharp edge at the high energy end in a far-infrared region ranging from 14–25 meV in the cosmic infrared background and the overwhelming infrared foreground from the zodiacal emission. We are developing a Hf-STJ which is expected to have 2% energy resolution for a single photon of 25 meV. We have successfully produced a superconductor-insulator-superconductor structure using Hf. However, it is found to suffer from a large leakage current and needs modification of the Hf-STJ to reduce it. We have developed two new types of Hf-STJ: Hf-STJ with an Al layer and Hf-STJ with a new sputtering condition. The leakage current density of two new types of Hf-STJ becomes 16 times smaller than the old Hf-STJ and obtained a response to the visible light. Because of its large leakage current, further optimization is underway.