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Theoretical and experimental studies of a superconductive detector of energetic particles May, George Anthony

Abstract

In nuclear-particle energy-loss spectrometers, it is desirable to have the signal charge per unit energy loss as large as possible, because the fractional resolution due to statistical fluctuations in the signal charge is inversely proportional to the square root of the number of charge carriers generated. Thus superconductors with an inherently narrow energy-gap 2Δ would be of interest if the quasiparticles generated from the energy of the incident particle could be distinguished from the Cooper-pairs. A superconductive tunnelling junction (STJ) satisfies this condition because the current flow consists essentially of quasiparticle tunnelling current provided that the Josephson-supercurrent is suppressed by a steady magnetic field. Thus if the particle energy is used to create quasiparticles in a STJ the increase in quasiparticle density causes a measurable increase in the tunnelling current, which constitutes the signal. The mechanism proposed for the transformation of the particle energy to quasiparticles involves the conversion of energy to heat when the particle penetrates the STJ and enters the substrate. The transient increase in the temperature of the junction films increases the thermally generated quasiparticle density. This thermal model gave numerical results in good agreement with experimental results obtained with 5.13 MeV α-particles incident on thin film lead and tin STJ's deposited on microscope glass slide substrate. The SJT's used for the experiments consisted of crossed 2000 A thick metal films, separated by an oxide barrier of approximately 12 A thick produced by glow-discharge anodization. Reproducible fabrication of arrays of STJ's was achieved by this method. Measurements were made on the junctions at different temperatures between 1.2 K and about half the critical temperature, and with different junction bias voltages. A steady magnetic field of 15 gauss was used to suppress the Josephson supercurrent. 5.13 MeV α-particles were directed at the junctions, and voltage signals caused by the particle impacts were observed across the junction. The signal amplitudes were temperature and bias-voltage dependent. The best signal to noise ratios (peak signal/rms noise) observed were 20 and 40 respectively for lead and tin junctions, using a transformer-input N-channel metal oxide semiconductor preamplifier operated at liquid helium temperatures. The pulse amplitude distributions were analysed and found to consist of an initially decreasing pulse-density with increasing pulse amplitude, then a nearly flat plateau region followed by a rapid drop off. This type of distribution curve was theoretically predicted using the thermal model mentioned above. The form of the distribution curve is a consequence of the distribution of the position of the particle impacts on the STJ, and of the angle of the particle impact. Thus superconductive particle detectors with this type of geonetry and with uncollimated particle sources do. not give rise to line spectra. Based on the physical understanding of the nature of signal-pulses from the a-bombarded STJ's on glass substrates, a heat-sink chip type detector is proposed. This is expected to be a superior and practical particle energy spectrometer. Theoretical investigations were made into the relative merits of superconductor-insulator-superconductor (SS) and normal metal-insulator- superconductor (NS) tunnelling junctions as fast response thermometers. For ω< 10⁹ Hz, SS junctions were shown to be theoretically superior in sensitivity and signal to noise ratio. It was also found theoretically that for the SS junctions, there is a temperature which for a specified bandwidth, junction capacitance, and superconductor type optimises the signal amplitude. Moreover, the inherent junction electrical noise, essentially shot noise, was shown to be inversely proportional to Δ.

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