Squid Readout of Cryogenic Particle Detectors

Cryogenic radiation and particle detectors are widely investigated nowadays. Their high intrinsic energy sensitivity makes them suitable for a wide range of applications. In order to take full advantage of the sensitivity of these newly developed cryogenic detectors, in general current sensors are needed with a white noise level of the order of 10-12 A/√HZ and a bandwidth larger than 1 MHz. We applied a Double Relaxation Oscillation SQUID (DROS) as a low-noise current preamplifier for measuring the current-voltage characteristics of a superconducting Nb/Al-AlOx/Al/Nb Josephson tunnel junction. A white current noise level smaller than 2 pA/√Hz was obtained. Thanks to the large flux-to-voltage transfer of a DROS (typically 1 mV/Ø o), no modulation techniques are required, allowing simple room-temperature flux locked loop electronics. Since the Josephson junction was voltage biased, no magnetic field had to be applied to suppress the Josephson supercurrent, which eliminates distortion of the subgap I-V characteristics by Fiske resonances. The measurements show that a DROS is a serious alternative for the high resolution readout of an X-ray spectrometer based on a superconducting tunnel junction