Asymmetric response of superconducting niobium-tunnel-junction x-ray detectors

The asymmetric response of the counter and base electrodes of x-ray detectors employing polycrystalline niobium tunnel junctions has been studied by low-temperature scanning electron microscopy (LTSEM). LTSEM has revealed that the base electrode produces a signal more than two times larger than the counter electrode, and effective quasiparticle lifetimes are 100 ns in the counter and 280 ns in the base. Based on the I-V characteristics and the measured effective quasiparticle lifetimes, we propose a model structure for a spatial variation of the gap parameter Delta, which involves proximity quasiparticle trapping layers. The small counter signal is caused by the shorter quasiparticle lifetime and the trapping effect. The LTSEM results are consistent with x-ray spectra for a radioactive Fe-55 source.