L. J. Hiller

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.

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.

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.

Multiple-tunneling noise in superconducting tunnel junctions from partial current integration

Superconducting tunnel junctions can be used as high-resolution particle or photon energy spectrometers. A photon absorbed in a superconductor breaks Cooper pairs into quasiparticles. These quasiparticles tunnel through the junction barrier and are detected as a pulse of excess current. Many junction designs allow the quasiparticles to tunnel more than once, an exponentially mixed Poisson process. However, multiple tunneling increases the fluctuation in the measured charge. We calculate the significance of these fluctuations algebraically as a function of time during the current pulse. We also calculate the finite integration window that minimizes the contribution of this noise. In addition, we calculate the effects of a low-pass amplifier and a Gaussian-shaping amplifier on the tunneling noise. With certain filtering time constants, the tunneling noise can be reduced while still providing some gain.

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.

Superconducting tunnel junction array development for high-resolution energy-dispersive x-ray spectroscopy

: Cryogenic energy-dispersive X-ray detectors are being developed because of their superior energy resolution (10 eV FWHM for keV X-rays) compared to that achieved in semiconductor energy-dispersive spectrometry (EDS) systems. So far, their range of application is limited because of their comparably small size and low count rate. We present data on the development of superconducting tunnel junction (STJ) detector arrays to address both of these issues. A single STJ detector has a resolution of around 10 eV below 1 keV and can be operated at count rates of the order 10,000 counts/sec. We show that the simultaneous operation of several STJ detectors does not dimish their energy resolution significantly, and it increases the detector area and the maximum count rate by a factor given by the total number of independent channels.

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.