Two dimensional imaging by series-connected STJs with sapphire absorber

The position determination capability of the series-connected superconducting tunnel junctions (STJs) with Nb/Al/AlOx/Al/Nb junctions as a radiation detector is reported. Four groups of series-connected STJs are fabricated on a single crystal of sapphire substrate surrounding 3.5×3.5 mm2 detection area. The signals detected by two STJs with 1.5 mm separation give a time delay caused by the propagation of phonons, and this enables us to measure the irradiated position. Using α-particles of 5.5 MeV, we clearly distinguished the incident positions with a potential resolution of 0.16±0.07 mm (FWHM). Directional dependence in the phonon propagation in the sapphire substrate is not detected.