LARGE-AREA TUNNEL JUNCTION EXHIBITING 2 OPERATING MODES FOR X-RAY-DETECTION

High quality Nb-based tunnel junctions with surface areas of 100 x 100 mu m(2) and 200 x 200 mu m(2) were tested by irradiating x-rays. The 200 x 200 mu m(2) junction exhibited better x-ray response than that o f the 100 x 100 mu m(2) one, due to better collection of quasiparticle s. The junction detected 5.9-keV x-rays with an energy resolution (FWH M) of 93 eV and the energy required for single electron collection was 6.5 meV. In the operation of the 200 x 200 mu m(2) junction, two kind s of operation modes with different pulse heights were found by applyi ng a magnetic field with different ramping speeds. The individual oper ation modes gave different current-voltage characteristics. The Sine-G ordon equation was employed to calculate the current-voltage character istics for the large junction. From calculated results, it was confirm ed that the higher pulse height operation mode was related to a large reduction of de current through the junction.