Nb-Based Superconducting Tunnel Junctions With (beta)-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 Tc 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.