Energy resolution of 12 eV at 5.9 keV from Al-superconducting tunnel junction detectors

Abstract

An energy resolution of ΔE=12 eV [full width at half maximum (FWHM)] has been measured for the Mn55 Kα1 line (E=5.9 keV) using a superconducting lead absorber (90×90×1.3 μm3) readout by a single aluminum superconducting tunnel junction (100×100 μm2). The total detector area has been illuminated, while a slit mask of 150 μm width was used to partially shadow the detector leads and substrate area from impinging x rays. The total electronic noise contribution was measured as ΔEelec=4 eV (FWHM). The superconducting tunnel junction is located on a Si3N4 membrane of thickness 0.3 μm, the lead absorber is separated from the superconducting tunnel junction’s top layer by a thin layer of natural aluminum oxide. No deviations from linear energy response were observed in the energy range between E=1.74 keV and E=6.49 keV. The same resolution was obtained for a single aluminum superconducting tunnel junction, where the influence of substrate events was suppressed by a metallic buffer layer between tunnel junction and substrate. The escape of recombination phonons into the substrate causes small nonlinearities in the tunnel junction’s energy response.