Axel van Dordrecht

X‐ray characteristics of a niobium superconducting tunnel junction with a highly transmissive tunnel barrier

The results of an investigation into the x‐ray properties of a superconducting tunnel junction (STJ) are presented. The photoabsorption of an x‐ray photon by one of the thin superconducting films of the junction results in the production of quasiparticles, which may subsequently tunnel through the thin oxide barrier into the second superconducting film. The transfer of charge across the barrier is detected, and gives a measure of both the x‐ray photon energy and the effective energy gap e of the superconducting film in which the photoabsorption occurred. A charge output of 55% of the theoretical maximum has been obtained for a niobium‐based STJ. Such a charge output indicates a mean energy e of ≂4.7 meV is required to create a single charge carrier in the junction such that e/Δ≂3, where 2Δ is the junction energy gap. This is the lowest value of e/Δ obtained to date for x‐ray photoabsorption in STJs. The energy resolution of the device is, however, still poor, with a full width half maximum of ≂200 eV for ...

High-resolution x-ray detection at 1.2 K with niobium superconducting tunnel junctions

X-ray spectra at 6 keV from niobium based superconducting tunnel junctions with highly transmissive tunnel barriers are presented. Signals from the two films can clearly by discriminated by their different temporal and pulse height characteristics. Levels of tunneled charge as high as 2.7 X 106 electrons at 5.9 keV are observed. The best energy resolution obtained at T equals 1.2 K is 53 eV FWHM including electronic noise, for a 50 X 50 micrometers 2 device in a configuration where the x-ray source is collimated to selectively illuminate the center part of the device. Non-linearity is observed which appears dependent on film volume, implying that self recombination may play an important role in these devices. The energy resolution is found to degrade with increasing magnetic field. The spectra from the polycrystalline top film appear significantly degraded if magnetic flux is deliberately trapped in the device.

Soft x-ray (50-1800 eV) measurements of niobium superconducting tunnel junctions

Measurements are presented on the x ray response of superconducting tunnel junction (STJ) detectors, over the energy range of 50 - 1800 eV. This includes the measurement of the lowest x-ray energies published to date. Energy resolution and response linearity is measured as a function of device geometry. It is shown that self-recombination of quasi-particles leads to an energy non-linearity which depends on junction volume. The effect of count rate limitations on energy resolution is established for rates up to 10 kHz.

Nb tunnel junctions as x-ray spectrometers

The properties of niobium superconducting tunnel junctions as x-ray detectors are investigated. The charge output, which depends on the geometry of the system, is severely reduced by self recombination in small size junctions and the specific characteristics of the barrier. These loss mechanisms, coupled with another energy loss mechanism due to phonon propagation into the substrate, cause additional variances on the charge output, which degrade the energy resolution.

Soft x-ray and EUV response of superconducting tunnel junctions: strong deviations from linearity

Nb-Al-AlOx-Al-Nb Superconducting Tunnel Junctions (SJTs) have been extensively investigated by a number of groups as potential next generation high resolution photon detectors for x-ray astronomy. The response of such devices has been studied from EUV to soft x-rays, using highly monochromatic synchrotron radiation. Based on the current understanding of the charge production and tunneling mechanisms in STJs, it would be expected that at lower energies, the responsivity of the detector would increase as the role of self-recombination of charge carriers into Cooper pairs declines in importance. Here responsivity is simply defined as the measured charge per eV of photon energy deposited in the junction. This trend however was not observed till the lowest energies. Below a threshold energy the responsivity fell, reaching a minimum level, after which it became constant. This minimum level is lower than the responsivity at 6 keV, by a factor up to 5, leading to a clear mismatch between x-ray and EUV performance. This paper summarizes the observations, and presents quantitative explanations for the feature, based on the existence of local traps in the STJ.

X-ray response of Nb-based superconducting tunnel junctions with Al trapping layers

Measurements of the x ray response of niobium-based superconducting tunnel junctions with Al trapping layers are presented. Signal amplitudes equivalent to as many as 2.3 multiplied by 108 tunneled electrons for a 5.9 keV photon are observed, corresponding to an amplification factor of approximately 100 per initially created quasiparticle in niobium. The detectors, however, exhibit a significant non-linearity in their energy response. The energy resolution is approximately 140 eV FWHM at 5.9 keV.

Superconducting tunnel junctions as photon-counting detectors

An experimental analysis of Niobium based superconducting tunnel junctions is presented, evaluating their performance as photon counting detectors. Several mechanism are found to be responsible for the degradation of the energy resolution. In particular, the high magnetic fields necessary to suppress the Josephson current in square junctions are shown to smear the energy bandgap. It is experimentally verified that in junctions with special geometries the Josephson current can be sufficiently suppressed by much lower fields. Several loss and contamination mechanisms are also discussed. Experimental results on new developments, such as quasiparticle trapping blocks, source collimation and substrate buffering, are presented, with a view to demonstrating significant improvements in energy resolution.

Development of a superconducting-tunnel-junction array for x-ray astronomy

Arrays of superconducting tunnel junctions (STJs) provide the possibility to perform high-resolution imaging spectrophotometry at x-ray wavelengths. We describe the applications of STJ arrays to x-ray astronomy, and present measurements on a 3 by 3 test array of Nb/Al-based STJs, operated at a temperature of 1.2 K, which illustrate the current photometric and spectroscopic capabilities of such devices. These results demonstrate the basic experimental feasibility of STJ arrays and indicate that there are no fundamental problems to be expected in the development of large-format x-ray detector arrays based on STJs.

Integration time dependence of tunnel noise and energy resolution of superconducting tunnel junctions.

Superconducting tunnel junctions (STJs) offer the capability of photon counting with intrinsic energy resolving power. This resolving power is ultimately limited by the variance on the number of charge carriers generated in the photon absorption process (Fano limit) and the variance on the number of tunnelled charge carriers (tunnel limit). In addition, the performance can be degraded by electronic noise related to the read-out of the devices and by spatial non-uniformities in the response across the detector area. The present generation of our Ta-Al STJs is such that their spectroscopic performance in the UV/visible is limited by tunnel noise. This noise contribution is usually considered a device constant, (which may only vary marginally with bias conditions) and evaluated for infinite integration time. It can be shown, however, that the tunnel noise contribution is strongly time dependent and can be reduced by almost an order of magnitude for a properly chosen integration time. In this paper we present the experimental demonstration and numerical simulations of this time dependence on a series of Ta-Al STJs with different pulse decay times. The experimental results are in qualitative agreement with the simulations, but do not quite achieve the predicted performance. For the optimum configuration, an effective tunnel noise contribution of ~70% of the conventional tunnel limit is found.