Using phonon pulses to characterise superconducting tunnel junction (STJ) X-ray detectors

Nanosecond phonon pulse experiments have been used to determine fundamental parameters of STJs relevant to their use as X-ray photon detectors. A non- equilibrium distribution of phonons is used to generate an excess non-equil ibrium quasi-particle (qp) density in the STJ base electrode. The time depe ndence of the subsequent current signal is given by the sum of two exponent ial contributions which depend solely on the qp loss rates and tunnel rates for the top and base electrode of the device. Hence. four fundamental STJ parameters can be determined from measurements of the exponential time cons tants and pre-exponential current amplitudes. The technique outlined here i s demonstrated by data taken on a high-quality 50 mu m x 50 mu m niobium-ba sed STJ. (C) 2000 Elsevier Science B.V. All rights reserved.