Thermal effects on near-critically biased superconducting thin film particle detectors

Measurements of the phase transition characteristics, spontaneous voltage transients and effects of particle irradiation are reported for superconducting thin film patterns of Nb, Mo, and Ti near their critical temperatures and currents. The detailed behavior is examined and is found to reflect the presence of stable and/or metastable normal regions in the films, as well as the different temperature scaling laws for the intrinsic and thermal-runaway critical currents. Voltage transients (pulses) occur in the nominally superconducting samples when the current is below both the intrinsic and thermal runaway values. Thermal coupling between film and substrate is reported for a wide variety of samples fabricated by different methods. The coupling in all cases follows a thermal boundary resistance law with power flux proportional to difference of fourth powers of film and substrate temperatures. Coupling factors extracted from the data are found to be several times smaller than theoretical predictions. Resistive regions in the patterns produced by incident radiation are simulated. The simulations predict that a refractory superconductor such as Nb may be used as detector material which is sensitive to minimum-ionizing particles (MIP's), if operated at or above 0.995...