Superconducting Thin-Film Detector of Nuclear Particles

Abstract

Superconducting films of tin and indium on supporting substrates have functioned successfully as detectors of individual nuclear particles. The films are sufficiently thin and narrow that individual α-particle impacts initiate superconducting to normal transitions which spread, in the presence of a transport current, to span a full film cross section. The transitions are observed by means of the ir drop produced by a transport current. For low current densities self-terminating voltage pulses of a few nsec duration are observed. At higher current densities the boundaries of a normal region initiated by an α particle propagate by Joule heating to the ends of the film. The range of the 5.3-MeV α particles utilized for these experiments greatly exceeds the 0.1 μm thickness of the films and the resulting deposition of energy in the substrate affects the response of any film which is in direct contact with its substrate. The introduction of a thin thermally insulating layer of varnish between the film and its substrate, which in these experiments was either glass or crystalline quartz, increases the thermal decay constant to the extent that the film is thermally isolated from its substrate for a period comparable to that of pulse formation. The variation of count rate with film current has been studied and is shown to be consistent with the variation of critical current density along the length of the film. This model, coupled with classical heat diffusion within the film, accounts for the observed behavior of the thermally isolated films.