Physical properties of the superconducting Ta film absorber of an X-ray photon detector

We have developed single-photon 1-D imaging detectors based on superconducting tunnel junctions. The devices have a Ta film with an Al/AlO_x/Al tunnel junction on each end and a Nb contact in the center. The best energy resolution of this kind of detector is 13 eV for 5.9 keV X-ray photons. Two devices with different lengths: 500 and 1000 μm are measured to study the nonequilibrium quasiparticle dynamics in the superconducting Ta film. The diffusion constant and lifetime of quasiparticles in the Ta films have been derived by fitting the measured current pulses to the model. The comparison of the simulation and measurement results proves that the quasiparticle loss is not primarily due to the Nb ground contact in the center of the Ta absorber, but is due to the uniform nonthermal loss in the Ta film. The Nb ground contact does contribute to the broadening of the energy width in the center of the Ta film.