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 super conducting tunnel junctions (STJs) were examined. We compared the spectra f or STJs with and without a 50-nm-thick buffer layer of aluminum-oxide on th e sapphire substrate under X-ray illumination, We found that the spectral d ifference was concentrated in the high-energy tail of the component, corres ponding to the events absorbed within a thin skin. The effective thickness of this skin was estimated to be about 3 mum by using the amount of differe nce in the spectra. We explain that the spectral difference could be caused by the selective attenuation of high-frequency phonons in the substrate an d buffer layer.