Line-splitting in high-resolution superconducting tunnel junction EUV detectors

We have developed high-resolution Nb-Al-AlOx-Al-Nb tunnel junction extreme ultra-violet (EUV) detectors. In the energy range between 25 and 70 eV, we have measured an energy resolution of 2.2 eV full-width at half maximum (FWHM). The energy resolution degrades significantly in the energy range between {approx}80 and {approx}230 eV where the Nb absorber is partially transparent and some of the photons are absorbed in the Al trap layers. We have for the first time observed a distinctly different response for photons absorbed in the Nb and the Al layer of the same junction electrode. We have modeled this effect with Monte-Carlo simulations of the charge generation process in superconducting multilayers.