Clare Louise Foden

The properties of niobium superconducting tunneling junctions as X-ray detectors

The properties of superconducting tunnel junctions based on niobium are investigated. The limiting resolution of such junctions should be ⋍ 4 eV for 6 keV X-rays. Currently only between 2 to 25% of the theoretical charge is detected. The principal loss mechanisms, which not only reduce charge but seriously degrade resolution, are found to be phonon loss to the substrate, and recombination of the excess quasi-particle population in both films. The phonon loss is probably due to relaxation phonons from quasi-particles relaxing towards the bandgap. The quasi-particle self recombination is a direct result of the very large excursion from equilibrium produced during the X-ray photoabsorption process. Finally 6 keV X-rays have been detected directly in sapphire crystals by using the niobium junction only as a detector of beamed ballistic phonons. The use of a suitable crystal as the X-ray absorber and phonon source opens up interesting possibilities for position sensitive spectrometers based on high quality niobium junctions.

Optical photon counting using superconducting tunnel junctions

Abstract We demonstrate the feasibility of optical photon counting using a superconducting substrate in combination with an array of widely spaced superconducting tunnel junctions of lower energy gap. In the proposed device, a photon impinging on the substrate generates quasiparticles within it, and these are channelled towards, and are detected by, the nearest four elements of the junction array. We show that certain substrate/junction combinations generate sufficient numbers of quasiparticles per incident photon to permit pulse counting and positional encoding, whilst resulting in a quasiparticle number density which is low enough to prevent significant recombination within the relevant diffusion and tunnelling timescales. A combination of Al junctions with a Nb or Sn substrate provides the basis for a large area detector with very high sensitivity to individual optical and UV photons, high intrinsic time-resolution, and moderate energy resolution.

X‐ray characteristics of a niobium superconducting tunnel junction with a highly transmissive tunnel barrier

The results of an investigation into the x‐ray properties of a superconducting tunnel junction (STJ) are presented. The photoabsorption of an x‐ray photon by one of the thin superconducting films of the junction results in the production of quasiparticles, which may subsequently tunnel through the thin oxide barrier into the second superconducting film. The transfer of charge across the barrier is detected, and gives a measure of both the x‐ray photon energy and the effective energy gap e of the superconducting film in which the photoabsorption occurred. A charge output of 55% of the theoretical maximum has been obtained for a niobium‐based STJ. Such a charge output indicates a mean energy e of ≂4.7 meV is required to create a single charge carrier in the junction such that e/Δ≂3, where 2Δ is the junction energy gap. This is the lowest value of e/Δ obtained to date for x‐ray photoabsorption in STJs. The energy resolution of the device is, however, still poor, with a full width half maximum of ≂200 eV for ...

Nb tunnel junctions as x-ray spectrometers

The properties of niobium superconducting tunnel junctions as x-ray detectors are investigated. The charge output, which depends on the geometry of the system, is severely reduced by self recombination in small size junctions and the specific characteristics of the barrier. These loss mechanisms, coupled with another energy loss mechanism due to phonon propagation into the substrate, cause additional variances on the charge output, which degrade the energy resolution.

Optimization of superconducting tunnel junction based x‐ray detectors

Current research into x‐ray detection using superconducting tunnel junctions indicates that the poor spectral resolution obtained so far, in comparison with theoretical expectations, is partly due to the excellent acoustic coupling of the junction and substrate. The substrate acts both as a source of noise and as a heat sink for the nonequilibrium junction, thus masking the intrinsic response of the superconducting electrodes to photoexcitation. A new design for a superconducting tunnel junction based on an x‐ray detector is presented. The design effectively decouples the substrate and junction and should therefore eliminate many causes of spectral degradation, bringing resolution closer to that predicted theoretically, and thus allowing experimental investigation of the intrinsic superconducting film response to x‐ray photoexcitation. An outline of the way in which the design can be optimized geometrically to achieve the decoupling is given. Further optimization of the intrinsic film response to x‐ray ph...