X -ray single photon imaging detectors using superconducting tunnel junctions

X-ray detectors based on Superconducting Tunnel Junctions (STJ) provide high energy resolution with good absorption efficiency and imaging capabilities. The energy of an x-ray photon is measured by detecting the excess quasiparticles generated by the absorption of the photon. The number of quasiparticles is measured by the increase in the tunneling current of the STJ. With two STJs attached to the absorber, the x-ray detector is capable of achieving 1-D imaging. We present ways to improve the performance of such 1-D imaging x-ray spectrometers. With better quasiparticle cooling in the tunnel junction, one of our devices shows an energy resolution of 13 eV FWHM for 6 keV x-rays with a spatial resolution of 0.2 mum over a 20 mum range. The noise sources are quantitatively studied for the guidance of future improvements. The measured energy resolution is about twice the theoretical intrinsic energy resolution these detectors can achieve. Three devices with different absorber lengths are measured to study the quasiparticle dynamics in a Ta absorber. By fitting a numerical model to the experimental data, the quasiparticle diffusion constant and lifetime are obtained. These parameters are critical in determining the upper limit of the absorber size. The spatial uniformity of the Ta absorber is also studied. The introduction of a Nb ground contact in the center of the Ta absorber causes spatial non-uniformity in the contact region and leads to extra energy broadening. Using Ta as a ground contact to the Al trap eliminates this problem. In searching for 2-D imaging detectors, we propose using four junction detectors. Simulations have been done on detectors that consist of a square absorber and four junctions on the sides or four junctions in the corners. Comparison shows that the detector with four junctions on the sides has better spatial performance.