Quasiparticle losses at the surface of superconducting tunnel junction detectors

Superconducting tunnel junctions (STJs) are promising as high energy resolution x- ray detectors. However, the theoretical limit of the energy resolution of STJs has not yet been reached for several reasons. In many cases quasiparticle losses limit the energy resolution. We have investigated STJs with different multilayer structures by means of low temperature scanning electron microscopy. By measuring the quasiparticle lifetime of Nb junctions with and without Ta passivation at the surface, we have identified quasiparticle losses at the surface of nonpassivated junctions as the dominant loss process. The temperature dependence of the quasiparticle lifetime gives information about the loss mechanism. The influence of quasiparticle traps on the effective quasiparticle lifetime is discussed. (C) 1999 American Institute of Physics. [S0021-8979(99)04801- X].