GRANULAR-ALUMINUM SUPERCONDUCTING DETECTOR FOR 6 KEV X-RAYS AND 2.2 MEV BETA SOURCES

A 2 μm wide superconducting strip of granular aluminum was used to detect the superconducting to normal transitions induced by the absorption of 55Fe, 6 keV X-rays or the passage of electrons from a 90Sr, 2.2 MeV beta source. The count-rate for X-rays reaches almost 70% efficiency over a wide range of bias currents, confirming the potential application for high-spatial-resolution X-ray detectors. We report the first evidence of switching by a 2.2 MeV beta source which emits electrons in the minimum-ionizing range. However, the inability to distinguish between transitions caused by minimum-ionizing electrons and those caused by lower energy, more heavily ionizing electrons emitted by the source, prevented us from demonstrating the full sensitivity of the granular aluminum detector to minimum-ionizing radiation. The switching threshold for X-rays depends on thermal propagation of a normal region which bridges the film width, and a numerical simulation is presented, the simple formulation of which allows extrapolation to other materials and temperatures. The very fast rise-time voltages are accurately described by a thermal propagation model. © 1990.