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

Abstract

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 at count rates approaching those of semiconductor detectors while still providing a significantly better energy resolution for soft x rays. Thus STJ detectors may prove very useful in microanalysis, synchrotron x-ray fluorescence (XRF) applications, and XRF analysis of light elements (K lines) and transition elements (L lines).

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

“…These devices have demonstrated good performance in the soft x-ray band at high count rates. 3,4 The absorption depth of x rays with energies less than 1 keV in Nb is less than a few hundred nm, so these detectors have a very high quantum efficiency in this band.…”

“…Since we used a count rate of ϳ200 counts/s, no pileup rejection was necessary. 3 The experiments presented here were performed at the Stanford Synchrotron Radiation Laboratory using beam line 1-1, which is equipped with a ''Grasshopper'' grazing-incidence, Rowland-circle geometry soft x-ray monochromator. A more detailed description of the experimental apparatus and measurement procedures is given in Ref.…”

Imaging of vortices in superconductors by electron beam scanning

“…1,2 Recently, superconducting tunnel junctions ͑STJ's͒ have attracted strong attention because of their application potential as particle and radiation detectors with high energy resolution. [3][4][5][6][7] Here the quasiparticles generated in the electrodes of the junction are detected by means of the tunneling conductivity. Simulating the irradiation event by electron beam scanning has provided useful information on the quasiparticle dynamics and for optimizing the performance of superconducting tunnel junction detectors, 8 because of the high spatial and temporal resolution achieved by this method.…”

Superconducting Tunnel Junction Array Development for High-Resolution Energy-Dispersive X-ray Spectroscopy

“…This allows STJ detector operation at count rates of several 1000 counts/sec with undiminished energy resolution. Operation at higher rates is possible with somewhat lower resolution due to pile-up (Frank et al, 1998b).…”

“…STJ detectors offer a slightly poorer resolution of 15.7 eV FWHM at 6 keV (Verhoeve et al, 1998), although this may be improved if future devices employ superconductors with extremely low critical temperatures (Peacock et al, 1996;LeGrand et al, 1998;Kraft et al, 1998). However, STJ detectors can be operated at significantly higher count rates of the order 10,000 counts/sec (Frank et al, 1998b).…”