Development of X-ray detectors based on Nb/Al/AlOx/Al/Nb superconducting tunnel junctions

X-ray detectors based on Nb/Al/AlO x /Al/Nb superconducting tunnel junctions (STJ) have been fabricated. X-ray responses of the fabricated STJs were measured by illumination with X-rays from a 55 Fe electron capture source. The energy resolution of 41, 58 and 65 eV (FWHM) at 5.9 keV were obtained for 20×20, 100×100, 200×200 μm 2 , square-shaped devices, respectively. The X-ray measurements were performed at 0.35 K using a charge-sensitive preamplifier at room temperature. We also discussed the capability to suppress the substrate phonon events using Al 2 O 3 insulation layer. An advantage for the combined use of the Al 2 O 3 insulation layer and the sapphire substrate was demonstrated.