G. Brammertz

Generalized proximity effect model in superconducting bi- and trilayer films

This article presents a general model for calculating the density of states and the Cooper pair potential in proximity-coupled superconducting bi- and trilayer films. It is valid for any kind of bilayer S1-S2, whatever the quality of the materials S1 and S2, the quality of the S1-S2 interface, and the layer thicknesses. The trilayer model is valid for a thin S3 layer, whereas the other two layers have arbitrary thicknesses. Although the equations of the dirty limit are used, it is argued that the model stays valid in clean bi-and trilayer films. The typical example of superconducting tunnel junctions is used to show that existing models, which apply to very thin or very thick layers or to perfectly transparent S1-S2 interfaces, are too restrictive to apply to an arbitrary bilayer. The new model is applied to practical junctions, with layer thicknesses intermediate between the “thick” and the “thin” approximation.

The hard X-ray response of HgI2

Abstract We summarise the results of a number of X-ray measurements on a 7 mm 2 , 0.5 mm thick HgI2 detector carried out both in our laboratory and at two synchrotron radiation facilities in Germany. The detector energy response function was found to be linear over the energy range 2.3– 100 keV with an average rms non-linearity of 0.2%. Using a 50×50 μm 2 15 keV monoenergetic X-ray beam, the spatial uniformity of the detector response was determined by a raster scan technique to be better than 1%. At room temperature, the FWHM energy resolution was 600 eV at 5.9 keV rising to 6 keV at 100 keV with no statistically significant difference between pencil beam (50×50 μm 2 ) and full-area illumination. It was found that the FWHM energy resolution decreased with decreasing temperature reaching a minimum of 400 eV for 5.9 keV X-ray illumination at −19.2°C. Below this temperature, the FWHM begins to rise slowly. By considering the various contributions to the FWHM energy resolution, we estimate the charge collection efficiency to be (95.2±0.7)% at room temperature and (98±0.2)% at −19°C with a Fano factor of 0.30±0.24 at room temperature.

Critical temperature of superconducting bilayers: Theory and experiment

A generalized model for the critical temperature TC of superconducting bilayers is presented, which is valid with no restrictions to film thicknesses, TC of the layers, and interface resistivity. The model is verified experimentally on a series of Nb–Al and Ta–Al bilayers with Nb, Ta layer thicknesses of 100 nm and Al layer thicknesses ranging from 5 to 200 nm. Excellent agreement between theory and experiment was found for the energy gap and the TC of bilayers. The results are important for designing practical superconducting devices.

Local trap spectroscopy in superconducting tunnel junctions

We show that thermal activation of quasiparticles from local traps is responsible for the temperature variation of responsivity observed for some superconducting tunneling junction photon detectors. With this model, the depth of the local traps in two different proximized Ta structures was found to be the same, 0.20±0.02 meV.

Distributed read-out imaging devices for x-ray imaging spectroscopy

We present an experimental study of the performance of Distributed Read-Out Imaging Devices (DROIDs), 1- and 2-D photon-counting imaging spectrometers, based on Ta/Al-based STJs placed on a Ta absorber. Results obtained with highly collimated illumination with 10 keV X-ray photons clearly demonstrate the imaging capabilities of 2-D DROIDs. The derived spatial FWHM resolution is 7 micrometers for a 200 X 200 micrometers 2 absorber. With a 1-D DROID we have measured an intrinsic energy resolution of 15 eV FWHM for 6 keV photons. At high energies (E > 6 keV) the resolution is limited by spatial fluctuations in the qp recombination rate.

The effects of proton-induced radiation damage on compound-semiconductor x-ray detectors

We report the results of a series of experiments designed to assess the relative radiation hardness of a range of compound semiconductor X-ray detectors. The specific compounds tested were GaAs, InP, CdZnTe, HgI2 and TlBr, along with an elemental Si device. To allow meaningful comparisons, all devices were of a similar size and, with the exception of the InP detector, had sub-keV energy resolution at 5.9 keV. The irradiations were carried out using the University of Helsinki’s Cyclone 10/5 10 MeV proton cyclotron. Each detector was given six consecutive exposures - the integral fluences being; 2.66 x 109 p cm-2, 7.98 x 109 p cm-2, 2.65 x 1010 p cm-2, 7.97 x 1010 p cm-2, 1.59 x 1011 p cm-2, and 2.65 x 1011 p cm-2, respectively. In Si, these correspond to absorbed radiation doses of 2, 6, 20, 60, 120 and 200 krads. During the exposures, the detectors were kept unbiased and at room temperature. After each irradiation, the effects of the exposure were assessed, both at room temperature and at a reduced temperature using 55Fe, 109Cd and 241Am radioactive sources. It was found that with the exception of the HgI2 and TlBr detectors all materials showed varying degrees of damage effects.

Development of compound semiconductor arrays for x- and gamma-ray spectroscopy

We present preliminary results of X-ray measurements on three small format compound semiconductor arrays. The devices, a 4x4 pixel GaAs array fabricated on 325 micrometers epitaxial material, a 4x4 pixel CdZnTe array fabricated on a 4X4X1 mm3 mono crystal and a 3x3 TlBr array fabricated on a 2.7 x 2.7 x 1.0 mm3 mono crystal. The pixel size for all arrays is 350x350micrometers 2. Results are presented of 55Fe and 241Am measurements at 5.9 keV and 59.54 keV. For detector temperatures <+5 degree(s)C typical FWHM energy resolutions of 410 eV, and 600 eV at 5.9 keV and 640 eV and 1.4 keV at 59.54 keV were recorded for the GaAs, and CdZnTe arrays, respectively. Unlike the GaAs and CdZnTe arrays, the TlBr array showed a much wider variation in pixel performance and was difficult to operate with all pixels at a common bias. For example, biasing the detector so that all pixels worked within the operating envelope of the preamplifiers resulted in average energy resolutions of 20 keV at 59.54 keV. However, optimizing the operating conditions of individual pixels resulted in a marked improvement to ~2keV.

Optical photon detection in Al superconducting tunnel junctions

Abstract We report on the successful fabrication of low leakage aluminium superconducting tunnel junctions with very homogeneous and transparent insulating barriers. The junctions were tested in an adiabatic demagnetisation refrigerator with a base temperature of 35xa0mK. The normal resistance of the junctions is equal to ∼7xa0μΩxa0cm 2 with leakage currents in the bias voltage domain as low as 100xa0fA/μm 2 . Optical single photon counting experiments show a very high responsivity with charge amplification factors in excess of 100. The total resolving power λ /Δ λ (including electronic noise) for 500xa0nm photons is equal to 13 compared to a theoretical tunnel limited value of 34. The current devices are found to be limited spectroscopically by spatial inhomogeneities in the detectors response.

Large-format distributed readout imaging devices for x-ray imaging spectroscopy

We present an experimental study of the performance of one-dimensional Distributed Read-Out Imaging Devices (DROIDs), based on two Ta/Al-based STJs placed on either side of a Ta absorber strip. We focus our discussion on the prospects of building large-format photon-counting imaging spectrometers for applications at soft X-ray energies. Tunnel-limited spectroscopical resolutions have already been demonstrated for optical photons. With a 20 x 100 micrometers 2 absorber we have measured an intrinsic energy resolution of 2.1 eV FWHM for 500 eV photons. This demonstrates that at soft X-ray energies resolutions close to the tunnel limit are also feasible for these type of detectors. A detailed analysis of pulse-shapes with analytical models allows us to assess the main parameters that determine the performance of these detectors. In particular, we discuss the dependence of the quasiparticle diffusion constant on the temperature of the absorber. Extrapolation of these models indicates that it is possible to extend the length of the absorber to 1.5 mm, without a serious degradation of the detectors performance.

Development of practical soft X-ray spectrometers

Cryogenic soft X-ray imaging spectrometers are currently being developed for applications in the fields of astronomy and material sciences. In this paper we present experiments on optimized single devices, which show measured energy resolutions of 4.6 eV, 8.1 eV and 20.5 eV at 525 eV, 1.5 keV and 6 keV respectively. These energy resolutions combined with a quantum efficiency of more than 40% in the energy range from 0.5 to 2 keV together with a count rate capability of 15 kHz demonstrate the overall good performance of single Superconducting Tunnel junctions (STJs). Assembling these optimized single devices in a matrix read-out would provide the practical basis for a soft X-ray imaging spectrometer.