J.B. Le grand

Energy resolution and high count rate performance of superconducting tunnel junction x-ray spectrometers

We present experimental results obtained with a cryogenically cooled, high-resolution x-ray spectrometer based on a 141 μm×141 μm Nb-Al-Al2O3-Al-Nb superconducting tunnel junction (STJ) detector in a demonstration experiment. Using monochromatized synchrotron radiation we studied the energy resolution of this energy-dispersive spectrometer for soft x rays with energies between 70 and 700 eV and investigated its performance at count rates up to nearly 60 000 cps. At count rates of several 100 cps we achieved an energy resolution of 5.9 eV (FWHM) and an electronic noise of 4.5 eV for 277 eV x rays (the energy corresponding to C K). Increasing the count rate, the resolution 277 eV remained below 10 eV for count rates up to ∼10 000 cps and then degraded to 13 eV at 23 000 cps and 20 eV at 50 000 cps. These results were achieved using a commercially available spectroscopy amplifier with a baseline restorer. No pile-up rejection was applied in these measurements. Our results show that STJ detectors can operate ...

A superconducting tunnel junction x-ray detector with performance limited by statistical effects

We have characterized a thin-film Nb/Al/AlOx/Al/Nb superconducting tunnel junction (STJ) optimized for low electronic noise as an x-ray detector in the 0.2–1 keV photon energy range. The spectra measured with this junction have high spectral purity with, to the best of our knowledge, the best energy resolution ever achieved with this type of detector in this energy band. The discrepancy between the theoretical and experimental energy resolution is only about 15%. Part of this small discrepancy may be explained by the fact that our junction has electrodes made from niobium/aluminum bilayers, while the theoretical result is for electrodes made from only one material. To the best of our knowledge, this is the first time that resolution achieved with a STJ x-ray detector is in agreement with the resolution predicted from statistical fluctuations in the creation and tunneling of quasiparticles.

High resolution tunnel junction extreme ultraviolet detectors limited by quasiparticle counting statistics

Superconducting tunnel junctions (STJs) can be used as high-resolution high-count rate photon detectors. They are based on the measurement of the excess quasiparticle tunneling current caused by the absorption of a photon in one of the junction electrodes. We have fabricated Nb-Al-AlO/sub x/-Al-Nb tunnel junction detectors with different sizes and characterized them in synchrotron experiments. We present a study of the detector performance in the energy band between 50 and 1000 eV. For photon energies below 70 eV, the intrinsic device resolution of the best STJ devices agrees with the theoretical limit set by the statistics of the charge generation and tunneling processes.

Inhomogeneous response of superconducting tunnel junctions with a killed electrode for X-ray spectroscopy

Nb-based superconducting tunnel junctions are being developed as high energy resolution X-ray detectors. Unfortunately, loss of excess quasiparticles at the edges, combined with lateral diffusion, results in an inhomogeneous response. To study this degradation of energy resolution, we manufactured detectors with a Ta trap in the top or bottom electrode away from the tunneling barrier. Excess quasiparticles in this so-called killed electrode will be trapped effectively and thus removed from the tunneling process. The X-ray spectra of the active electrode can be fitted with a model based on classical diffusion of quasiparticles. On junctions with a killed bottom electrode also Low Temperature Scanning Electron Microscopy (LTSEM) measurements have been performed. The X-ray spectra and the LTSEM scans are consistent with each other and with the model. The energy resolution of the junctions presented here is limited by loss of quasiparticles at the edges.

Cryogenic high-resolution X-ray spectrometers for SR-XRF and microanalysis

Experimental results are presented obtained with a cryogenically cooled high-resolution X-ray spectrometer based on a 141 x 141 micro m Nb-Al-Al(2)O(3)-Al-Nb superconducting tunnel junction (STJ) detector in an SR-XRF demonstration experiment. STJ detectors can operate at count rates approaching those of semiconductor detectors while still providing a significantly better energy resolution for soft X-rays. By measuring fluorescence X-rays from samples containing transition metals and low-Z elements, an FWHM energy resolution of 6-15 eV for X-rays in the energy range 180-1100 eV has been obtained. The results show that, in the near future, STJ detectors may prove very useful in XRF and microanalysis applications.

Fabrication of Nb/Ta-based Superconducting Tunnel Junctions for X-ray detectors

In this work Nb/Ta/TaOx/Ta/Nb Superconducting Tunnel Junctions [STJs] are being investigated instead of conventional Nb/Al STJs. We obtained Ta films in the superconducting bcc-phase with a TC>4 K and have fabricated high quality STJs with thermally limited subgap currents downto 1.15 K. Process descriptions are given.

Superconductive tunnel junctions for X-ray spectroscopy

In order to investigate the influence of quasi-particle trapping on the performance of superconductive tunnel junctions as X-ray detectors, a series of Nb/Al/Al/sub 2/O/sub 3//Al/Nb junctions have been produced with different Al-layer thicknesses in the bottom electrode. The proximity effect between the Nb absorber and the Al trapping layer plays a dominant role, because it greatly influences the trapping time of excess quasi-particles from the Nb electrode to the Al trapping layer. A study of the influence of Al layer thickness on the operation of tunnel junctions as X-ray detectors is presented. Quasi-particle trapping is shown to be quite an efficient process in Nb junctions with Al-layers of 10 nm and thicker. The proximity model of A.A. Golubov and E.P. Houwman can explain the present data quite well. Nb junctions suffer from the bandgap reduction at oxidized and anodized surfaces. Without any special attention, extremely fast loss processes, about 30 ns, are present.<<ETX>>

Superconducting high-resolution X-ray detectors for metalloprotein L-edge spectroscopy

Abstract Superconducting tunnel junctions (STJs) can be used as high-resolution energy-dispersive X-ray detectors. STJ detectors are based on the measurement of an increased tunneling current from excess charge carriers that are excited above the superconducting energy gap by the absorption of an X-ray. Nb-based STJ detectors have a theoretical energy resolution limit below 5 eV FWHM for X-ray energies below 1 keV at count rates up to 10 000 counts/s. We have developed Nb–Al–AlOx–Al–Nb X-ray detectors and operated them in a cryostat below 0.4 K in synchrotron experiments. Their resolution varies between 4.6 and 8.9 eV FWHM for X-ray energies between 0.2 and 1 keV. We present fluorescence spectra of metalloproteins and discuss the potential of STJ detectors for fluorescence-detected L-edge absorption spectroscopy of dilute samples.

Imaging of vortices in superconductors by electron beam scanning

Abrikosov vortices trapped in a superconducting tunnel junction and oriented perpendicular to the barrier plane were imaged by electron beam scanning at 1.6 K. We have used NbAlOxNb junctions. As an important feature, the top Nb electrode was covered with a SiO2 film of 300 nm thickness, absorbing most of the 5 keV beam energy. The signal generating the image is explained by a model, assuming that the beam-induced electronic excitations in the SiO2 overlay film are trapped in the local magnetic field protruding from a vortex, resulting in an increased recombination rate. In addition to providing a novel approach to the imaging of the vortices in superconductors, our results are important for understanding quasiparticle losses in tunnel junction detectors.

Field dependence of Fiske resonances in Nb-AlO/sub x/ based Josephson junctions

Fiske resonances have been measured in a rectangular Nb-Al, AlO/sub x/, Al-Nb Josephson junction as a function of the magnetic field applied parallel to a junction side. Due to the high quality factor of the junction, many resonant modes could be measured, using a special measuring technique. Each mode shows more lobes than reported before. The measured curves are in very good agreement with the high Q theory of I.O. Kulik (1967), and Q-values ranging from 40 to 450 have been obtained. The surface resistance of the Al/Nb barrier probably dominates the microwave losses in the junction barrier at 4.2 K. Losses due to the quasi-particle tunneling current can be neglected. Fiske resonances in a square junction at a field angle of pi /4 rad were also measured. Two-dimensional resonant modes have been found. The field dependence of the