Hans Kraus

Abstract:

The development of an imaging cryogenic X-ray spectrometer based upon X-ray absorption in sapphire and readout by two tungsten superconducting phase transition thermometers (SPT) is reported. The resistance changes of the SPTs are measured by a superconducting quantum interference device (SQUID) readout system and eventually result in electrical signals which are proportional to the energies carried into the SPTs through phonons. 1D imaging over an absorber length of 10 mm with a position resolution of ≈20 μm was achieved.

Abstract:

A theoretical model to describe the spectral response of SIS tunnel junctions under irradiation in the presence of diffusion and edge losses of non-equilibrium quasiparticles has been developed. The model takes into account the back-tunnelling effect, in the general case of asymmetrical rates of tunnelling, γ12≠γ21. It provides a quantitative estimate of parameters like the edge reflectivity, the diffusion length and the diffusion coefficient, which are important parameters for a diagnostic of STJ-detectors. The predictions of the theory are compared with an experimental spectrum obtained from a Nb-based STJ.

Abstract:

We have developed a fast current readout system for STJ detectors with a fast current preamplifier by direct signal observation. The preamplifier has frequency bandwidth of 110 MHz with stable voltage bias operation. By the various experiments using a Ta STJ and Nb STJ, it is confirmed that the readout system is effective for the evaluation of STJs, research of the mechanism for signal production process and the X-ray detection with high counting rate.

Abstract:

The efficiency of the charge collecting mechanism of a radiation detector based on superconducting tunnel junctions depends strongly on the competition between tunneling and losses of quasiparticles in the electrodes of the junction. In this article we discuss the loss of quasiparticles, both in the bulk and at the edges of the electrodes. The quasiparticle loss taking place during their diffusion in the junction limits the amount of charge carriers tunneling and thereby the energy resolution of the detector. A charge amplification process caused by multiple tunneling of quasiparticles is beneficial to higher signal to noise ratios. Such multiple tunneling is always present in the case of junctions based on symmetrical electrodes. In this article we present a theory for the detector response which takes into account both multiple tunneling and the diffusion of quasiparticles. A model is presented for the spectral response of the detector based on four parameters one of which is the degree of back-tunneling present in the device. Analytical solutions of the model for rectangular and circular junctions are discussed. The predictions of the theory are compared with an experimental pulse height spectrum obtained on a Nb/Al2O3/Nb junction. © 1999 American Institute of Physics.

Abstract:

An energy resolution of Delta E = 12 eV (FWHM) was measured for the Mn-55 K alpha(1) line (E = 5.90 keV) using a single superconducting Al/AlxOy/Al tunnel junction. The total detector area of 100 x 100 mu m was illuminated, while a slit mask of about 150 mu m width was used to shadow the detector leads and substrate area from the impinging x-rays. The total electronic noise contribution to the energy resolution was measured as Delta E-elec = 7 eV (FWHM). The process of tunnel junction fabrication was optimized towards a reproducible and high-quality growth of the tunnel barrier. High-quality aluminium films promote long quasi-particle lifetimes and ensure reproducible tunnel barrier growth. The residual resistance ratio of a 1 mu m thick aluminium test film was about 100, Copyright (C) 1999 John Wiley & Sons, Ltd.