Superconducting particle detectors

The interaction of a nuclear particle or x-ray with a superconductor leads to the breaking of Cooper pairs and the creation of excess quasi-particles and phonons. Under certain conditions this can produce a transition from the superconducting state to the normal state which can be detected in a number of ways. In other cases the quasi-particle excitations are detected with a superconductor-insulator-superconductor (SIS) tunnel junction. The phonon excitations can also be detected with an SIS junction or arrays of SIS junctions, or with superconducting transition edge bolometers. In addition, these superconducting sensors can detect the phonons produced by individual particle interactions in dielectric absorbers. At very low temperatures individual particles can also be detected with normal metal-insulator-superconductor (NIS) tunnel junctions due to the increase in electron temperature on the N side. Because of the smallness of the superconducting energy gap and the sharpness of the normal-to-superconducting transition, better energy resolution and sensitivity to smaller energy depositions can be achieved using superconductors as compared to conventional detectors. Moreover, a wide range of materials can be made into detectors for specific physics applications. The basic physics of the interactions of nuclear particles and x-rays in superconductors and in dielectrics is outlined, and the present status of superconducting particle detectors is reviewed.