Large-Area Tunnel Junction Exhibiting Two Operating Modes for X-ray Detection

Abstract
High quality Nb-based tunnel junctions with surface areas of 100×100 µm2 and 200×200 µm2 were tested by irradiating x-rays. The 200×200 µm2 junction exhibited better x-ray response than that of the 100×100 µm2 one, due to better collection of quasiparticles. The junction detected 5.9-keV x-rays with an energy resolution (FWHM) of 93 eV and the energy required for single electron collection was 6.5 meV. In the operation of the 200×200 µm2 junction, two kinds of operation modes with different pulse heights were found by applying a magnetic field with different ramping speeds. The individual operation modes gave different current-voltage characteristics. The Sine-Gordon equation was employed to calculate the current-volt-age characteristics for the large junction. From calculated results, it was confirmed that the higher pulse height operation mode was related to a large reduction of dc current through the junction.

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