H. Netel

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 ...

High-resolution X-ray detectors with high-speed SQUID readout of superconducting tunnel junctions

Abstract We present our first results obtained using new high-speed SQUID systems for the readout of normal conductor/insulator/superconductor (NIS) and superconductor/insulator/superconductor (SIS) tunnel junctions. With an NIS device measured with a HYPRES SQUID we have achieved an energy resolution of 100 eV (FWHM) for 5.89 keV X-rays and an electronic noise of 40 eV at an operating temperature of 80 mK. With an SIS sensor at 200 mK and the same readout we have achieved an energy resolution of 29 eV (FWHM) at 5.89 keV and an electronic noise of 10 eV.

Analysis of pulse shape from a high-resolution superconducting tunnel junction X-ray spectrometer

Abstract Superconducting-insulating-superconducting (SIS) tunnel junctions coupled to superconducting absorbers may be used as high-resolution, high-efficiency X-ray spectrometers. Until recently, the X-ray-induced current pulse from such devices has been measured using FET-based negative-feedback charge or current amplifiers. The limited bandwidth and feed-back nature of these amplifiers have made it difficult to deduce the true shape of the X-ray induced current pulse. Recently, we have begun to use high-bandwidth amplifiers based on Superconducting Quantum Interference Devices (SQUIDS) to measure the current pulses from our tunnel junction X-ray spectrometers. We have measured pulses from devices with niobium X-ray absorbing layers coupled to aluminum layers that serve as quasiparticle traps. We present here a study of pulse shape as a function of bias voltage. In general, the X-ray induced pulses increase in amplitude and become longer as we increase the bias voltage. We found that it is possible to differentiate pulses produced by X-ray absorption in the top niobium film from those produced in the bottom niobium film by measuring the rise time of the current pulses. This allows us to produce a high resolution spectrum using only pulses produced in the bottom niobium film. The measured energy resolution of this spectrum is 29 eV FWHM at 5.89 keV, about 5 times better than that obtainable using semiconductor ionization detectors.

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 superconducting X-ray spectrometers with an active area of 282 /spl mu/m/spl times/282 /spl mu/m

Superconducting tunnel junctions coupled to superconducting absorbers may be used as high-resolution, high-efficiency X-ray spectrometers. We have tested devices with niobium X-ray absorbing layers coupled to aluminum layers that serve as quasiparticle traps. In this work we measure the current pulses from a large-area tunnel junction using an amplifier based on an array of 100 SQUIDs. Using this amplifier and a 282 /spl mu/m/spl times/282 /spl mu/m junction, we have measured an energy resolution of 19 eV FWHM for 1.5 keV X-rays and 21 eV for 2.6 keV X-rays. The area of this junction is eight times the area of any junction previously measured to have such high energy resolution.

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.

Development of a prototype superconducting X-ray spectrometer using a Ta crystal as an absorber

Abstract Superconducting tunnel junctions can be used as high resolution X-ray and γ-ray spectrometers. Until recently, most results were from detectors that consisted of niobium and aluminium thin films deposited on insulating substrates. Typically Nb films with thicknesses of several hundred nanometers are used as absorbers. These thin-film devices inherently suffer from poor quantum efficiency. To increase this efficiency a foil or a single crystal can be used as the supercounducting absorber. We are working on using ultra-pure, high- Z , superconducting crystals as the X-ray and γ-ray absorbers. We are developing a prototype detector with a 10 μm-thick Ta crystal as an absorber, which will have a quantum efficiency of greater than 99% at 6 keV. In this paper we present several of the design and fabrication issues involved in assembling the prototype superconducting crystal X-ray spectrometer.

High-resolution superconducting X-ray spectrometers with aluminum trapping layers of different thicknesses

Superconducting tunnel junctions coupled to superconducting absorbers may be used as high-resolution, high-efficiency X-ray spectrometers. We have tested devices with niobium X-ray absorbing layers coupled to aluminum layers that serve as quasiparticle traps. We present a study of device performance as a function of thickness of the trapping layers. We measured the best energy resolution using a device with a high-quality barrier and 200 nm-thick trapping layers on both sides of the tunnel barrier. This energy resolution was 36 eV full width at half maximum at 6 keV, about 4 times better than that obtainable using semiconductor ionization detectors.<<ETX>>

Proximity effect and hot-electron diffusion in Ag/Al/sub 2/O/sub 3//Al tunnel junctions

We have fabricated Ag/Al/sub 2/O/sub 3//Al tunnel junctions on Si substrates using a new process. This process was developed to fabricate superconducting tunnel junctions (STJs) on the surface of a superconductor. These junctions allow us to study the proximity effect of a superconducting Al film on a normal metal trapping layer. In addition, these devices allow us to measure the hot-electron diffusion constant using a single junction. Lastly these devices will help us optimize the design and fabrication of tunnel junctions on the surface of high-Z, ultra-pure superconducting crystals.

Temperature dependence of a superconducting tunnel junction x-ray detector

Superconducting tunnel junctions can be used as part of a high-resolution, energy-dispersive x- ray detector. The energy of the absorbed x ray is used to break superconducting electron pairs, producing on the order of 106 excitations, called quasiparticles. The number of quasiparticles produced is proportional to the energy of the absorbed x ray. When a bias voltage is maintained across the barrier, these quasiparticles produce a net tunneling current. Either the peak tunneling current or the total tunneled charge may be measured to determine the energy of the absorbed x ray. The tunneling rate, and therefore the signal, is enhanced by the use of a quasiparticle trap near the tunnel barrier. The trapping efficiency is improved by decreasing the energy gap, though this reduces the maximum temperature at which the device may operate. In our niobium/aluminum configuration, we can very the energy gap in the trapping layer by varying its thickness. This paper examines the performance of two devices with 50 nm aluminum traps at temperatures ranging from 100 mK to 700 mK. We found that this device has a very good energy resolution of about 12 eV FWHM at 1 keV. This energy resolution is independent of temperature for much of this temperature range.