A superconducting tunnel junction x-ray detector with performance limited by statistical effects

We have characterized a thin-film Nb/Al/AlOx/Al/Nb superconducting tunnel j unction (STJ) optimized for low electronic noise as an x-ray detector in th e 0.2-1 keV photon energy range. The spectra measured with this junction ha ve high spectral purity with, to the best of our knowledge, the best energy resolution ever achieved with this type of detector in this energy band. T he discrepancy between the theoretical and experimental energy resolution i s only about 15%. Part of this small discrepancy may be explained by the fa ct that our junction has electrodes made from niobium/aluminum bilayers, wh ile the theoretical result is for electrodes made from only one material. T o the best of our knowledge, this is the first time that resolution achieve d with a STJ x-ray detector is in agreement with the resolution predicted f rom statistical fluctuations in the creation and tunneling of quasiparticle s. (C) 1998 American Institute of Physics. [S0003-6951(98)03435-4].