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

Abstract

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.

Cryogenic particle detectors

“…This amplification effect, referred to as the 'Gray effect', has first been observed by Gray [Gra78,Gra81] who also proposed a transistor based on this amplification mechanism. The Gray effect has also been observed experimentally in the Sn-junction detectors [Twe86b, Twe86c] and more recently by the detector of Labov et al [Mea93,Mea95].…”

“…Much effort was switched to more robust junctions fabricated from Nb, and the first X-ray detector signals were seen by the ESTEC group [Gar89]. The high energy resolution of 30 eV at 6 keV with Nb-technology junctions obtained recently by the Lawrence Livermore group [Mea93,Mea95] and the energy resolution of 50 eV at a higher operating temperature of 1.2 K by the ESTEC group [Ver94] indicate promising prospects for future applications of superconducting tunnel junction detectors. Just recently, even the counting of single UV photons with Nb junction detectors has been reported by the ESTEC group [Ver95,Per93].…”

“…In another experiment, the Oxford group observed the emission of phonons of quasiparticles relaxing in the trapping process, by sweeping the bias current of the sensing superconducting tunnel junction [Gol91]. Two other experiments based on quasiparticle trapping will be discussed in the following section: the TU-Munich 6 keV Sn-film X-ray detector experiment of 1989 [Kra89] and the recent high energy resolution result of Labov et al obtained with a Nb/Al junction [Mea93,Mea95].…”

Superconducting tunnel junction x-ray detectors for high resolution spectroscopy

“…presumably due to significant pulse pile up These results could probably be improved further by applying additional signal processing techniques such as pile-up rejection The high count rate measurements arc discussed in more detail in Frank ef al [21] STJ Performance as a Function of Al Thickness and Junction size To better understand the behavior of these devices, we fabricated a series of detectors with similar tunnel barrier characteristics but with different aluminum "trapping" layer thicknesses Previous measurements of this type have been performed with 6 keV X rays and similar detectors [22], and with detectors that have thinner Al layers [23] Here we describe measurements of detector response to soft X-rays from 0 2 to 1 keV, using detectors with both a range of aluminum trapping layer thickness, and a range of junction sizes We measured detectors with four different Al trapping layer thicknesses 35 nm, 50 nm, 100 nm and 200 nm For each Al trap thickness, we measured detectors of four different sizes 20 X 20 wm*, 50 x 50 pm*, 70 x 70 pm2 and 141 x 141 pm* making up for a total of 16 junctions We illuminated each of these detectors with X rays mnging from 200 eV to 1000 eV in 50 eV steps For all measurements similar conditions were used as much as possible The X-ray pulses were filtered using the Ithaco 4302 with a 1 MHz low pass filter and a 3 15 !&z high pass filter These settings appear to work rcasonably well for all junctions even though the pulse length varied from 1 2 to 9 fls We did not optimize the filter settings for each detector Instead, we chose. a relatively large band pass of 3 15 kHz to 1 MHz for the pulse shaping in order to not distort the pulse shape too much While this way of pulse shaping is not optimal for achieving best energy resolution it allows us to compare the pulses from different STJs with decay times ranging from 1 2 to 9 ps…”

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

“…The Al layers, with lower superconducting gap energy than the Nb layers, help increase the signal by concentrating quasiparticles near the tunnel barrier by means of`quasiparticle trapping ' (Booth, 1987). [For more details on the operating principle, see Mears et al (1995) and Frank et al (1998).] The theoretical limit for the FWHM energy resolution of an X-ray spectrometer based on the measurement of the X-ray induced quasiparticle excitations in pure Nb has been calculated to be about 5 eV at 6 keV incident X-ray energy (Rando et al, 1992).…”