A particle detector based on a double barrier Josephson junction

We analyze the performance of a particle detector based on a double-barrier Nb/Al/AlOx/Al/AlOx/Al/Nb junction where the Al trapping layer is embedded between two insulating AlO/sub x/ barriers. Within this layer, a very sharp Andreev bound state (ABS) level E/sub 0/ is formed. Because of a very long recombination time, the charge amplification factor is expected to be 10-100 times larger in the double-barrier detector compared to ordinary single-barrier devices. The double barrier detector also has much higher idle-state subgap resistance at low temperatures. Because the middle Al quasiparticle trap layer is separated from the adjacent layers by two insulating tunneling barriers, the proximity effect is minimized. This provides much better alignment between the maximum of nonequilibrium quasiparticle distribution function and the maximum in the electron density of states. All the factors listed result in much higher sensitivity and the energy resolution of the double barrier Nb-based STJ as compared to their single-barrier analogs.

I. Introduction

Among various cryogenic, energy-dispersive, particle detectors [1]–[4], superconducting tunnel junctions (STJ's) combine high-count rates and excellent energy resolution. A conventional STJ has a layered structure consisting of two superconducting layers separated by a tunneling barrier [5], [6]. A particle (or an X-ray photon) absorbed in one of the electrodes breaks Cooper pairs and thereby generates excess charge carriers in proportion to its energy. These charges can then tunnel through the insulating barrier, thus producing an increased current proportional to the energy of the incoming particle/X-ray. The excess current (or the sub-gap tunneling conductance as it is sometimes called) serves as a measure of the particle/X-ray energy.