The effect of X-ray irradiating superconducting tunnel junctions on Al2O3 and LiNbO3 substrates

We have developed a phononmediated radiation detector using Nb-based superconducting tunnel junctions (STJs). The STJs are fabricated on a substrate as an absorber of the radiation, and a substantial amount of emergent phonons with an energy above the energy gap are detected. The performance of the detector depends on the collection efficiency of the phonons. A selection of an appropriate substrate is required for optimizing the detector performance. We fabricated Nb-based STJs on a Al₂O₃ and a LiNbO₃ substrates, and compared the pulse heights of substrate phonon events due to X-ray irradiation to the substrates. We found that the maximum pulse height of the phonon events for the LiNbO₃ substrate was about twice of that for the Al₂O₃ substrate, so that these spectra which was obtained from two STJs showed different shapes, especially the peak due to phonon events.