Alternative non-equilibrium superconducting X-ray detectors

Abstract

Non-equilibrium X-ray detectors based on isotropic superconductor absorbers offer the hope of energy resolution limited by the statistics of excitation production, i.e., δEE ∼ (ΔE)12, where Δ is the superconducting gap energy. This can be numerically equal to that of the best bolometers at significantly higher operating temperatures. Superconducting Tunnel Junction Detectors (STJDs) count the X-ray event induced excitations (quasiparticles (QP)) as an excursion of the subgap tunnel current. While STJDs are presently the best studied QP readout scheme, they have made slow progress toward the limiting energy resolution. Some of the difficulties are enumerated herein and are inherent in tunnel junctions. One alternative is to measure the conductivity excursion of a superconducting film that follows photon absorption. This excursion obeys exactly the same equation as the effective quasiparticle tunneling conductivity in a Josephson junction. Hence the two schemes should produce the same readout waveforms when the same superconducting absorbers are used. Seven specific approaches to realizing the measurement are presented. All versions of the conductivity readout offer simpler sample fabrication than STJD. Any superconducting material may be used. In commercial applications, the obvious benefit in operating temperature of using the HTSC materials may more than compensate for the penalty in energy resolution.