Probability of quasiparticle self-trapping due to localized energy deposition in nonequilbrium tunnel-junction detectors

Abstract

Voltage-biased, superconducting tunnel junctions are investigated as x-ray detectors for applications requiring both high quantum efficiency and better than 1% energy resolution. The nonequilibrium quasiparticles, produced as the energy deposited degrades to the few-meV-per-excitation level, tunnel and are detected before they are lost to recombination. Previous event modeling ignored the energy cascade under the assumption that the equilibrium of the electrodes is minimally perturbed by the deposited energy. In this paper we demonstrate that that assumption is invalid. We calculate the local energy density as the average quasiparticle energy becomes of the proper magnitude to suppress the gap. The fraction of the nonequilibrium quasiparticles that become spatially trapped (never to tunnel) in the order-parameter well that their existence creates may vary between events. If so, the source of the observed non-Poisson-limited energy resolution of this class of detectors would be identified. The input parameters used were evaluated in equilibrium. Thus our conclusions need to be confirmed via a fully nonequilibrium calculation of the cascade.

Quasiparticle-phonon downconversion in nonequilibrium superconductors

“…It plays a central role in the process of particle or photon detection by a superconducting tunnel junction ͑STJ͒, determining the rate of production of mobile charges ͑quasielectrons and quasiholes͒ which tunnel in the biased STJ to produce the measured signal. [1][2][3] It is generally accepted that energy downconversion occurs in three distinct stages. The first stage starts as the particle energy E 0 is released in the form of a fast photoelectron.…”

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

“…The processes have been described in literature. 2,7,8 Here we mention them briefly. In niobium the photoabsorption of the 6-keV x ray creates a photoelectron with an energy of E-E L ͑3.2-3.5 keV͒, where E L is the binding energy of the L shell.…”

“…The quasiparticles relax to energies just above 2⌬ within 10 Ϫ12 -10 Ϫ9 s after the photoabsorption, and then recombine into Cooper pairs. 8 In order to obtain a reasonable signal, the excess quasiparticles should be counted within a quasiparticle lifetime. Hence, an important issue of the high energyresolution detectors is to obtain a long quasiparticle lifetime and a short tunneling time.…”

Energy Down-Conversion and Thermalization in Metal Absorbers

“…The energy down-conversion in a solid metal absorber following the absorption of an elementary particle or photon has been the focus of a number of studies [1][2][3][4][5]. It plays a central role in the process of energy deposition in any radiation or particle detector.…”