First results of a cryogenic optical photon counting imaging spectrometer using a DROID array

Context. In this paper we present the first system test in which we demonstrate the concept of using an array of Distributed Read Out Imaging Devices (DROIDs) for optical photon detection. Aims. After the successful S-Cam 3 detector the next step in the development of a cryogenic optical photon counting imaging spectrometer under the S-Cam project is to increase the field of view using DROIDs. With this modification the field of view of the camera has been increased by a factor of 5 in area, while keeping the number of readout channels the same. Methods. The test has been performed using the flexible S-Cam 3 system and exchanging the 10x12 Superconducting Tunnel Junction array for a 3x20 DROID array. The extra data reduction needed with DROIDs is performed offline. Results. We show that, although the responsivity (number of tunnelled quasiparticles per unit of absorbed photon energy, e- /eV) of the current array is too low for direct astronomical applications, the imaging quality is already good enough for pattern detection, and will improve further with increasing responsivity. Conclusions. The obtained knowledge can be used to optimise the system for the use of DROIDs.Comment: 7 pages, 9 figures, accepted for publicaiton in A&

Superconducting Tunnel Junctions as Photon Counting Detectors in the Infrared to the Ultraviolet

Photon counting experiments with Ta/Al superconducting tunnel junctions are presented. Single photon detection is demonstrated in the wavelength range l=200-2000 nm with a resolving power l/Dl=22-4. The response of the detector shows good linearity with photon energy. I. INTRODUCTION Superconducting tunnel junctions (STJs) are being investigated as photon counting detectors because of their predicted high energy resolving power. This quality arises from the low energy required to break Cooper pairs and to generate free charge carriers (quasiparticles) in a superconductor. Quasiparticles can be detected by the tunneling of electrons across the insulating barrier of the STJ. Calculations for Sn and Nb indicate that the initial number of quasiparticles N(E) generated by the absorption of a photon with energy E is given as [1], [2]: Here e »1.7D (with D the bandgap of the superconductor) (1) is the average energy required to generate one quasiparticle and F»0.2 is the Fano factor. Fano-li..