Characterization of an Nb/Al/AlO x/Al/Nb superconducting tunnel junction detector with a very high spatial resolution in the soft X-ray range

Abstract

A cryogenic superconducting tunnel junction (STJ) detector device was characterized and evaluated. It consists of an Nb/Al/AlO x /Al/Nb layered structure and contains four STJs with sizes between 70×70 μm 2 and 200×200 μm 2. For dedicated characterization experiments we employed monochromatized undulator radiation of high spectral purity, which is available at the PTB Radiometry laboratory at the electron storage ring BESSY II. We investigated the STJ energy resolution at various photon energies below 1500 eV with respect to the detector area, the illuminated area, the incident photon energy and the count rate while recording the detector response. Applying a 50 μm-aperture to collimate the incident beam, we achieved an energy resolution of less than 10 eV at a photon energy of up to 900 eV for the 141×141 μm 2-sized STJ. When a 5 μm-pinhole was employed approximately 25 mm in front of the STJ for the beam collimation, the energy resolution was independent of the detector size — between 5 and 9 eV for energies between 0.3 and 1.5 keV. Furthermore, we scanned both the effective detector area and the adjacent regions with a beam defined by a 5 μm-pinhole offering a beam width smaller than 18 μm. In this scan, both the energy resolution and the pulse height scale changed strongly within the central area of the STJ. These effects and those originating from the leads and edges are discussed. In addition, these findings contribute to understanding the decreasing energy resolution for increasing effective detector areas. We expect that such spatially resolved measurements will prove to be a proper method for detector evaluation and contribute to improving the detector layouts.