The Effect of X-Ray Irradiating Superconducting Tunnel Junctions on and Substrates

We have developed a phononmediated radiation de- tector using Nb-based superconducting tunnel junctions (STJs). The STJs are fabricated on a substrate as an absorber of the ra- diation, 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 se- lection of an appropriate substrate is required for optimizing the detector performance. We fabricated Nb-based STJs on a and a substrates, and compared the pulse heights of sub- strate phonon events due to X-ray irradiation to the substrates. We found that the maximum pulse height of the phonon events for the substrate was about twice of that for the sub- strate, so that these spectra which was obtained from two STJs showed different shapes, especially the peak due to phonon events.