Nb-based superconducting tunnel junctions with β-Ta trapping layers for photon detectors

We have successfully fabricated Nb-based superconducting tunnel junctions (STJs) with beta-Ta trapping layers. In an Nb/beta-Ta stacked structure, beta-Ta film showed the thickness dependence of critical temperature T-c in current-voltage (I-V) characteristics due to the proximity effect from Nb films. We fabricated Nb-based STJs with beta-Ta trapping layers for X-ray detector working at 0.4 K. We obtained an energy resolution of 52 eV for 5.9 keV X-ray irradiation at 0.4 K. We expect a high energy resolution by optimizing the thickness of beta-Ta trapping layers and improving the fabrication process.