C. Patel

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.

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.

Interpretation of dc current-voltage characteristics of Ta(foil)AlOxAl tunnel junctions on the basis of a modified McMillan's model of the proximity effect

Abstract AlO x Al superconducting tunnel junctions (STJs) have been photolithographically fabricated on a 350 micron thick Ta foil to investigate the properties of the Al quasiparticle trap. The measured current-voltage ( I - V ) characteristics are shown to be reasonably well fitted by using modified densities of states generated by McMillans model of the proximity effect and by assuming a 24% normal metal component in the base Ta(foil)Al electrode.

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.

Superconducting indium detectors for solar neutrinos

Abstract We report on our progress in developing a superconducting indium neutrino detector. Two highlights are the quasiparticle trapping mechanism, and the merger of technologies to provide a viable route for fabricating stable detector elements.

An Indium Solar Neutrino Experiment

We report on our progress during the past three years in developing a superconducting indium neutrino detector. Two highlights are the quasiparticle trapping mechanism, and the merger of technologies to provide a viable route for fabricating stable detector elements.

An appreciation of quasiparticle trapping and the proximity effect

Experimental results on X-ray detection with NbAlAlOxAl superconducting tunnel junctions are presented. For the first time, we can report that the spectra from devices without a proximity effect at the NbAl interface are remarkably similar to those that do have a proximity effect. The similarity is due to the response of the Al junction being dominated by the detection of high energy phonons produced by the trapping, or recombination processes occurring in the Nb absorber after a particle interaction. These results indicate that the unequivocal demonstration of quasiparticle trapping requires a thorough understanding of the phonon response in these types of detector.