D.J. Goldie

Statistical noise due to tunneling in superconducting tunnel junction detectors

We discuss the statistical noise associated with tunneling which is intrinsic to superconducting tunnel junctions. We deduce a simple expression for the noise in the general case where the tunneling probabilities may be dissimilar. The statistical tunneling noise exceeds the Fano‐factor limited statistical noise of the quasiparticle creation for most cases.

Tunnel junction arrays on InSb: A high resolution cryogenic detector for X- and γ-rays

Abstract We have measured the response of a series array of superconducting tunnel junctions to the non-equilibrium phonons generated in single crystals of InSb by nuclear particle interactions at a temperature of 100 mK. The results demonstrate an energy resolution below 210±60 eV (FWHM) for photons of energy 22.1 keV after subtracting the electronic noise. The detector response has high linearity for the detection of photons with energies in the range 3 to 662 keV.

Superconducting tunnel junctions and quasiparticle trapping

The interaction of a nuclear particle or X-ray with a superconductor leads to the breaking of Cooper pairs and the creation of excess phonons and quasiparticles. The basic physics and the use of superconducting tunnel junctions as detectors of excess quasiparticles are reviewed. For superconducting absorbers of appreciable mass intrinsic limitations require the use of the phenomenon of quasiparticle trapping. The relaxation phonons released in the trapping process can also be detected and they can lead to amplification via further pair breaking. Superconducting tunnel junctions can also detect phonons produced by particle interactions in a substrate absorber. The use of series-connected arrays of junctions to achieve high sensitivity and good energy resolution is discussed.

Cryogenic detectors for experiments in elementary particle physics

Abstract The development of cryogenic detectors is largely motivated by the need for very good energy resolution in a number of particle physics experiments, and by the need to detect very small energy depositions. Good energy resolution can be obtained by utilizing the smallness of the superconducting energy gap or detecting the phonons which are produced by particle interactions. These detection schemes require low temperatures, where in addition the thermal fluctuations are small compared to the minute energies expected to be deposited in some experiments. Moreover, cryogenic detectors permit the tailoring of the target or absorber materials to match the particle physics goals. The basic physics behind the detection of excitations induced by particle interactions in bulk single crystal materials is reviewed, and recent results on the efficient detection of these excitations with series arrays of superconducting tunnel junctions are presented. Progress towards the implementation of particle physics experiments, such as the detection of low energy solar neutrinos, search for dark matter particles, search for neutrinoless double beta-decay and precision observation of a 17 keV neutrino in beta-decay, using cryogenic detectors is reviewed.

Energy resolved X-ray detection in Al-Nb proximity layers

The authors report on their initial investigation of the response to 6 keV X-rays of composite aluminium/niobium films forming one side of a superconducting tunnel junction. The non-equilibrium state generated by the X-ray interactions that occur in the thick Nb layer is detected with high-resolution detectors. The choice of a smaller gap superconductor as a probe film permits detection of the phonons generated by quasiparticle trapping from the Nb and enhancement of the initial quasiparticle number.

The effect of hotspot formation on quasiparticle yields in superconducting aluminium films

Existing models show that any hotspot created in an Al film following a particle interaction is rapidly diluted. Quasiparticles and phonons decouple at high energies so that on short time-scales quasiparticle diffusion determines the energy transport. Few quasiparticles have energies near the energy gap and there is little gap suppression. We have measured the initial quasiparticle yields in very thin Al films and find somewhat surprisingly that this yield is reduced. It appears that a short time-scale quasiparticle loss mechanism, probably associated with localised over-injection, exists as quasiparticles scatter to low energies within the phonon hotspot.

Phonon emission in quasiparticle trapping

Abstract Using energy selective S-I-S′ tunnel junctions, we have studied the phonons produced by quasiparticle trapping into copper films from single crystal superconducting indium. Excitations in the crystal can be detected both by measuring the increase in the quasiparticle population in the trap and by detecting the relaxation phonons emitted in the trapping process.

Particle Detection with Superconducting Tunnel Junctions—Modelling the Non-Equilibrium State Generated by Particle Interactions

Superconducting tunnel junctions (STJ’s) have attracted much interest as high resolution detectors for nuclear particles. Detection schemes have been implemented where the STJ provides the read-out for particle interactions in the superconducting films which comprise the junction. Alternatively the STJ may provide read-out for particle absorption in a dielectric or superconducting absorber crystal. We give here an overview of current work directed towards understanding the details of the phonon and quasiparticle yields in superconductors, and thus establishing a theoretical basis for the ultimate energy resolution and how it varies from one material to another.

Transport phenomena in single crystal superconducting niobium

Abstract Following recent interest in niobium as a superconducting detector for possible astronomical dark matter, we have extended previous measurements of phonon propagation in high purity single crystal niobium below 1K. We have also, for the first time, observed the propagation of quasiparticles in bulk niobium, with a characteristic diffusion time an order of magnitude slower than that of the phonons.

Development of a high resolution cryogenic detector with applications in neutrino physics

Abstract Cryogenic detectors have been developed comprising series arrays of superconducting tunnel junctions (SASTJ) fabricated on single crystals of InSb. Experimental results, at an operating temperature of 100 mK, demonstrate excellent linearity of the response to gamma-rays with energies in the range 3 to 662 keV. The detection threshold is limited by the electronics to 0.3 keV. The intrinsic energy resolution of the device corresponds to 210 eV (full width at half maximum) for photons of energy 22.1 keV. Preliminary results are presented from an investigation of the response of the detector to an enclosed 63 Ni β source. Extension to a solar ν detector is discussed.