Positive utilization; Excess quasiparticle injection in superconducting tunnel junction X-ray detector with quasiparticle trapping layers

Nakamura, Tatsuya  ; Katagiri, Masaki; Okubo, Masataka*; Pressler, H.*; Takahashi, Hiroyuki*; Nakazawa, Masaharu*

Periodical distribution of charge output in superconducting tunnel junction (STJ) having a structure of Nb/Al/AlO/Al/Nb with size of 200 x 200 was first observed by Low Temperature Scanning Synchrotron Microscope experiments. Two swelled parts were appeared in the distribution of charge output and the period of the swelled parts was about 100. It is found that this distribution was occurred under the conditions; (1) the size of the STJ is more than twice of Josephson characteristic length and (2) a bias voltage applied to the device is more than 0.2mV of about the energy gap of the trapping layers. It was confirmed that this anomalous distribution of charge output significantly distorts the shape of the photo peak and degrades the energy resolution of the STJ detector.