Output signal from Nb-based tunnel junctions by irradiation of 6 keV X-rays

Large-size tunnel junctions with a Nb-Al-AlO/sub x/-Nb structure have been fabricated in order to study their performance in X-ray detection. The junction detected 5.9-keV X-rays with an energy resolution of 1 keV. The output signal was analyzed by the use of coupled Boltzmann equations. The method based on these equations clarifies the basic behavior of the quasiparticles and phonons, and the average energy for producing a single electron is 3.2 Delta .