Spectral features of substrate phonon events obtained by illuminating superconducting tunnel junctions with X-rays

The pulse height spectra of the substrate phonon events obtained with superconducting tunnel junctions (STJs) were examined. We compared the spectra for STJs with and without a 50-nm-thick buffer layer of aluminum-oxide on the sapphire substrate under X-ray illumination. We found-that the spectral difference was concentrated in the high-energy tail of the component, corresponding to the events absorbed within a thin skin. The effective thickness of this skin was estimated to be about 3 /spl mu/m by using the amount of difference in the spectra. We explain that the spectral difference could be caused by the selective attenuation of high-frequency phonons in the substrate and buffer layer.