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

An energy resolution of DeltaE = 12 eV [full width at half maximum (FWHM)] has been measured for the Mn-55 K-alpha1 line (E = 5.9 keV) using a superconducting lead absorber (90x 90x 1.3 mum(3)) readout by a single aluminum superconducting tunnel junction (100x 100 mum(2)). The total detector area has been illuminated, while a slit mask of 150 mum width was used to partially shadow the detector leads and substrate area from impinging x rays. The total electronic noise contribution was measured as DeltaE(elec) = 4 eV (FWHM). The superconducting tunnel junction is located on a Si3N4 membrane of thickness 0.3 mum, 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. (C) 2001 American Institute of Physics.