Use of a dc superconducting quantum interference device as read-out electronics for radiation detectors based on superconducting tunnel junctions

A new generation of astronomical detectors has recently been developed, on the basis of superconducting tunnel junctions (STJs). STJs are capable of detecting photons in a wide range of wavelengths, from X-rays to infrared; also the counting of single photons in the optical range has been demonstrated. Since these detectors operate at very low temperature (down to 100 mK), a natural candidate for the read-out electronics is the dc superconducting quantum interference device (SQUID), an extremely sensitive magnetometer that is fabricated with a similar technology to the STJ junctions. Biasing the STJ at a constant voltage, the current pulse caused by the incoming radiation is passed through the tightly coupled input coil of the SQUID and converted into a magnetic flux signal which can be detected. In this paper we present measurements on a home-made dc SQUID, showing the response of the device to current pulses, which simulate the real signals from a STJ. We show how the SQUID performance can be improved by using a superconducting fiux transformer, which allows us to detect signal amplitudes of fraction of nanoamperes, such as the signals coming from irradiation of STJs in the near-infrared range.