Quasiparticle diffusion, edge losses, and back-tunneling in superconducting tunnel junctions under x-ray irradiation

The efficiency of the charge collecting mechanism of a radiation detector b ased on superconducting tunnel junctions depends strongly on the competitio n between tunneling and losses of quasiparticles in the electrodes of the j unction. In this article we discuss the loss of quasiparticles, both in the bulk and at the edges of the electrodes. The quasiparticle loss taking pla ce during their diffusion in the junction limits the amount of charge carri ers tunneling and thereby the energy resolution of the detector. A charge a mplification process caused by multiple tunneling of quasiparticles is bene ficial to higher signal to noise ratios. Such multiple tunneling is always present in the case of junctions based on symmetrical electrodes. In this a rticle we present a theory for the detector response which takes into accou nt both multiple tunneling and the diffusion of quasiparticles. A model is presented for the spectral response of the detector based on four parameter s one of which is the degree of back-tunneling present in the device. Analy tical solutions of the model for rectangular and circular junctions are dis cussed. The predictions of the theory are compared with an experimental pul se height spectrum obtained on a Nb/Al2O3/Nb junction. (C) 1999 American In stitute of Physics. [S0021-8979(99)05620-0].