Factors of inhomogeneous spatial response of superconducting tunnel junction detectors

Spatial uniformity in the response of superconducting tunnel junction detectors (STJ) to photons is a main concern in the development of these devices. Low Temperature Scanning Synchrotron Microscopy (LTSSM) has been used to directly image spatial profiles of the response of Nb-based STJ to X-ray photons. Scanning an X-ray microbeam with a diameter of 5 - 10 mum enables visualization of the spectroscopic properties of STJ X-ray detectors at an actual working temperature of about 0.4 K. We have found that the inhomogencity of the junction response strongly depends on the junction size, the bias points, and the strength of applied magnetic field parallel to the insulation barrier. It is proposed that some resonance phenomena in the junction may influence signal creation processes.