Quantitative evaluation of the intrinsic X-ray peak probability of a superconducting tunnel junction with the cascade process model of primary photoelectrons

The electron collision cascade process of primary photoelectrons generated by absorbed X-rays was studied to develop a clear model of the signal producing process of a superconducting tunnel junction (STJ), and the intrinsic peak probability PI of the STJ was calculated as a function of X-ray energy. X-rays of 5–18 keV were measured by a 178 μm×178 μm Nb/AlAlO2/Al/Nb STJ with the Nb electrode thickness of 200 nm and PI was experimentally obtained. The calculated PI agreed well with the measured one for the range of X-ray energies of 5–12 keV. The numerical evaluation of PI with this model reveals that the increases of the electrode thickness is effective to increase PI.