Highly collimated photon detection using strongly coupled superconducting tunnel junctions

Preliminary results on the X-ray performance of Nb-based superconducting tunnel junctions (STJs) with a highly transmissive barrier are reported. The results show that the energy resolution of these detectors can be improved by collimating the X-ray photons onto the junction barrier area, thus reducing illumination of the surrounding substrate and leads. A charge output of about 50% of the theoretical maximum has been recorded for these STJs, with full width at half maximum resolution of about 200 eV at 6 keV. Several mechanisms which are believed to degrade the energy resolution are also discussed. X-ray events are also detected by other junctions on the same chip which are not illuminated. This may indicate the presence of a marked phonon transmission along the sapphire substrate which acts as a phonon waveguide analogous to the light transmission mechanism in fiber optics.<>