Superconducting Tunnel Junction X-ray Detectors Research Articles

Current-voltage characteristics of 100-pixel 200-μm-square superconducting tunnel junction X-ray detector array

The superconducting tunnel junction (STJ) X-ray detector is an important tool for materials analysis because of its high energy resolution and high counting rate. In this work, the current-voltage (I-V) characteristics were investigated for a 100-pixel STJ array with a pixel size of 200 μm having a layer structure of Nb-Al/AlO x /Al-Nb and working at 0.3 K. The fabrication yield of the 200-μm STJ reached approximately 90%. A series of I-V curves for a single pixel in different cooling runs showed a discrete excess current, possibly due to the trapping of single fluxoids. With a reduction of the ambient magnetic field during cooling, larger STJ pixels can be used.

Improvement of Plug-in Wire for 200-pixel Superconducting Tunnel Junction X-ray Detector Array on Helium Three Cryostat

The design of plug-in wire system is improved for 200-pixel superconducting tunnel junction (STJ) array of X-ray detector on a helium three cryostat. The electrical connections between the STJ array to the room temperature electronics consists of plural wire modules, which have connectors at the both ends. A flexible coaxial cable with electrical conductors of stainless steel (SUS-FFC) is manufactured and attached to make the electrical connection between room temperature to the 3 K stage. After the installation of SUS-FFC, temperature of the cold stage is 302 mK, and the holding time is more than 115 hours. Compared with the previous setup, the temperature of 3He stage decreased 5 mK, and the holding time increased 30 hours.

Plug-in Wire for 200-pixel Superconducting Tunnel Junction X-ray Detector Array for Helium-3 Cryostat

A plug-in wire system is designed for a 200-pixel superconducting tunnel junction (STJ) X-ray detector array for a helium-3 cryostat. The STJ array and the room-temperature electronics are electrically connected by wire modules that have connectors at both ends. Two types of wire module are used to balance the electric conductance and thermal conductance under various temperature conditions. One type uses a copper-based miniature coaxial cable harness, and is utilized to electrically connect the room-temperature electronics to the 3 K stage. The other type uses woven wire looms of twisted-pairs of NbTi wire, and is used to electrically connect the 3 K stage to the helium-3 stage. With the installation of all wire modules, the temperature of the cold stage is less than 310 mK, and the holding time is more than 80 hours. The plug-in wire system is suitable for large-scale STJ X-ray detector arrays.

Performance of Tantalum STJ X-ray Detectors at Elevated Count Rates

We have developed an X-ray detector system based on Ta superconducting tunnel junctions (STJs) with an energy resolution below 10 eV under 1 keV. A principal advantage of STJs over other low-temperature detectors is their high count rate capability. In this work, we have optimized the operating conditions and digital pulse processing parameters for input count rates up to 25 kcps/pixel. Resolution below 10 eV is maintained up to 2.5 kcps/pixel, and at input rates of 25 kcps per pixel the resolution is 26 eV FWHM at 525 eV.

L-edge sum rule analysis on 3d transition metal sites: from d10 to d0 and towards application to extremely dilute metallo-enzymes.

According to L-edge sum rules, the number of 3d vacancies at a transition metal site is directly proportional to the integrated intensity of the L-edge X-ray absorption spectrum (XAS) for the corresponding metal complex. In this study, the numbers of 3d holes are characterized quantitatively or semi-quantitatively for a series of manganese (Mn) and nickel (Ni) complexes, including the electron configurations 3d10→ 3d0. In addition, extremely dilute (<0.1% wt/wt) Ni enzymes were examined by two different approaches: (1) by using a high resolution superconducting tunnel junction X-ray detector to obtain XAS spectra with a very high signal-to-noise ratio, especially in the non-variant edge jump region; and (2) by adding an inert tracer to the sample that provides a prominent spectral feature to replace the weak edge jump for intensity normalization. In this publication, we present for the first time: (1) L-edge sum rule analysis for a series of Mn and Ni complexes that include electron configurations from an open shell 3d0 to a closed shell 3d10; (2) a systematic analysis on the uncertainties, especially on that from the edge jump, which was missing in all previous reports; (3) a clearly-resolved edge jump between pre-L3 and post-L2 regions from an extremely dilute sample; (4) an evaluation of an alternative normalization standard for L-edge sum rule analysis. XAS from two copper (Cu) proteins measured using a conventional semiconductor X-ray detector are also repeated as bridges between Ni complexes and dilute Ni enzymes. The differences between measuring 1% Cu enzymes and measuring <0.1% Ni enzymes are compared and discussed. This study extends L-edge sum rule analysis to virtually any 3d metal complex and any dilute biological samples that contain 3d metals.

Development of Superconducting Tunnel Junction X-ray Detector with High Absorption Yields Utilizing Silicon Pixel Absorbers

A superconducting tunnel junction (STJ) array detector along with silicon pixel absorbers (STJ–SPA) is fabricated to achieve high detection efficiency at X-ray energies below 10 keV. The STJ pixels have dimensions of 100 \(\upmu \)m \(\times \) 100 \(\upmu \)m and are composed of Nb–Al/AlO\(_{X}\)/Al–Nb thin layers. The SPAs are also 100 \(\upmu \)m \(\times \) 100 \(\upmu \)m and have a depth of 400 \(\upmu \)m, and are isolated from each other by a deep trench with a depth of 350 \(\upmu \)m. The detection efficiency of the STJ–SPA exceeds 95 % at X-ray energies below 10 keV, and its energy resolution is 82 eV FWHM, as measured at the Si K\(\upalpha \) line at 1740 eV. By means of the STJ–SPA detector, the X-ray absorption spectrum of the light element sulfur with a concentration of less than 0.1 wt% in a soda-lime glass sample was successfully acquired.

Quasiparticle Self-Recombination in Superconducting Tunnel Junction X-ray Detectors

The mathematical modelling of the X-ray detectors based on superconducting tunnel junctions was performed taking into account diffusion of nonequilibrium quasiparticles, quasiparticle tunnelling and losses, self-recombination and exchange 2\(\Delta \)-phonons. The effects of recombination were examined in detail. The dependence of the signal on the photon energy and the energy resolution of the detectors were considered. A new analytical expression for a contribution of self-recombination to the signal was obtained.

Modification of Layer Structures of Superconducting Tunnel Junctions to Improve X-ray Energy Resolution

The layer structure of a Nb/Al-based superconducting tunnel junction (STJ) X-ray detector was modified to improve the energy resolution (\(\Delta E\)) of STJ X-ray detectors by suppressing the recombination of excited quasiparticles in the electrodes. A 100-pixel array of 100 \(\times \) 100 \(\upmu \)m STJs with a symmetric layer structure of Nb (300 nm)/Al-AlO\(_{x}\)(70 nm)/Al (70 nm)/Nb (300 nm) was fabricated. Fabrication yields of the 100 pixels were more than 90 %. The average leak current of the STJ array was 8 nA. The mean \(\Delta E\) of the STJ array was 6.7 \(\pm \) 1.0 eV for 400 eV X-rays. This \(\Delta E\) is the best value reported for Nb/Al STJs.

Characteristics of Nb/Al superconducting tunnel junctions fabricated using ozone gas

To improve the energy resolution (ΔE) of Nb/Al superconducting tunnel junctions (STJs), an ozone (O3) oxidation process has been developed to fabricate a thin defect-free tunnel barrier that simultaneously shows high critical current JC > 1000 A/cm2 and high normalized dynamic resistance RDA > 100 MΩ · μm2, where A is the size of the STJ. The 50-μm2 STJs produced by O3 exposure of 0.26 Pa· min with an indirect spray of O3 gas, which is a much lower level of exposure than the O2 exposure used in a conventional O2 oxidation process, exhibit a maximum JC = 800 A/cm2 and a high RDA = 372 MΩ · μm2. The 100-pixel array of the 100-μm2 STJs produced using the same O3 oxidation conditions exhibits a constant leak current Ileak = 14.9 ± 3.2 nA at a bias point around Δ /e (where e is half the energy gap of an STJ), and a high fabrication yield of 87%. Although the Ileak values are slightly larger than those of STJs produced using the conventional O2 oxidation process, the STJ produced using O3 oxidation shows a ΔE = 10 eV for the C-Kα line, which is the best value of our Nb/Al STJ x-ray detectors.

Temporal shape of the signals of superconducting tunnel junction X-ray detectors

Amplitudes and temporal shapes of the signals of X-ray detectors based on superconducting tunnel junctions are calculated in terms of a 2D diffusion model of the motion of quasiparticles in both electrodes of the detector. Special attention is given to quasiparticle recombination and the 2Δ-phonon exchange between electrodes. The results from calculations are compared to experimental data and it is shown that recombination leads to a nonlinear dependence of the detector response on the photon energy and distortion of the pulse shape.

High density processing electronics for superconducting tunnel junction x-ray detector arrays

Superconducting tunnel junctions (STJs) are excellent soft x-ray (100–2000eV) detectors, particularly for synchrotron applications, because of their ability to obtain energy resolutions below 10eV at count rates approaching 10kcps. In order to achieve useful solid detection angles with these very small detectors, they are typically deployed in large arrays – currently with 100+ elements, but with 1000 elements being contemplated. In this paper we review a 5-year effort to develop compact, computer controlled low-noise processing electronics for STJ detector arrays, focusing on the major issues encountered and our solutions to them. Of particular interest are our preamplifier design, which can set the STJ operating points under computer control and achieve 2.7eV energy resolution; our low noise power supply, which produces only 2nV/√Hz noise at the preamplifier׳s critical cascode node; our digital processing card that digitizes and digitally processes 32 channels; and an STJ I–V curve scanning algorithm that computes noise as a function of offset voltage, allowing an optimum operating point to be easily selected. With 32 preamplifiers laid out on a custom 3U EuroCard, and the 32 channel digital card in a 3U PXI card format, electronics for a 128 channel array occupy only two small chassis, each the size of a National Instruments 5-slot PXI crate, and allow full array control with simple extensions of existing beam line data collection packages.

Application of optimum digital filter based on genetic algorithms for transition edge sensor detector

We propose a new signal processing method to improve the energy resolution of superconducting tunnel junction X-ray detectors. An optimum FIR (Finite Impulse Response) filter design tool based on fast Fourier transform was developed using genetic algorithms. The fast Fourier transform is perhaps one of the most used algorithms today in the analysis and manipulation of digital or discrete data. The main feature of this tool is that it needs a minimum number of parameters to specify the FIR filter specifications. This tool was applied successfully on transition edge sensor detector.

Development of Ta-based STJ X-ray Detector Arrays for Synchrotron Science

We are developing a cryogen-free Ta-based superconducting tunnel junction (STJ) detector for soft X-ray spectroscopy at synchrotrons. With an energy resolution 10 times higher than conventional solid-state X-ray detectors and count-rate capabilities above 5 kHz/pixel, STJ detectors offer potentially increased sensitivity for fluorescence-yield X-ray absorption spectroscopy (FY-XAS). We have developed 36-pixel arrays of 208 \(\times \) 208 \(\upmu \)m Ta STJs with an energy resolution of \(\sim \)9 eV FWHM at the 525 eV oxygen K line. Compared to earlier Nb-based STJs, Ta-STJs offer improved energy resolution and absorption efficiency and extend the operating range to several keV. Here we describe the integration of the 36-pixel arrays into a cryogen-free, user-friendly X-ray spectrometer. A computer-controlled adiabatic demagnetization refrigerator coupled to a two-stage pulse tube refrigerator allows operation below 100 mK. The detector chip is located at the end of a 42 cm shielded snout for insertion into the analysis chamber. The system is currently being commissioned at the Advanced Light Source synchrotron.

Quasiparticle Freeze-Out in Superconducting Tunnel Junction X-ray Detectors with Killed Base Electrode

The current-voltage characteristics of superconducting tunnel junction (STJ) X-ray detectors were measured in the temperature range from 4.2 to 0.1 K. The freeze-out of the thermal tunneling current was compared between an STJ detector with a traditional Nb/Al/Al2O3/Al/Nb layer structure and a Ti/Nb/Al/Al2O3/Al/Nb/NbN detector whose low-gap Ti film kills the X-ray response of the base electrode. The current decrease and the linear low-temperature I(V) characteristics for the detector with the killed electrode can be qualitatively explained by tunneling current contribu- tions from the subgap states of the Ti film. The data are analyzed on the basis of the proximity theory in the dirty limit.

Development of Ta-Based Superconducting Tunnel Junction X-Ray Detector Arrays

We are developing new Ta-based superconducting tunnel junction (STJ) X-ray detectors for high-resolution soft X-ray spectroscopy at synchrotrons. STJ detectors combine the high-energy resolution of cryogenic detectors with the high count rate capabilities of athermal devices and the high efficiencies of solid state detectors, which increases the sensitivity for material analysis by fluorescence-detected X-ray absorption spectroscopy. Our STJ detectors are fabricated using thick, high-Z Ta absorber films that enhance quantum efficiency and spectral purity, and extend operational range to several keV compared with earlier Nb-based STJs. They offer an energy resolution of ~ 5 to 10 eV FWHM for soft X-rays up to ~1 keV, and count rates of several 1000 counts/s per detector pixel. For increased solid angle coverage, we have fabricated 36- and 112-pixel Ta-based STJ detector arrays with total areas of 1.4 and 4.5 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively. The 208 × 208 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> pixels have an energy resolution between 6.8 and 7.6 eV FWHM at 525 eV with a low-energy shoulder, and their responsivity is uniform to within 2% across the array. Here we discuss the performance of the array in the context of synchrotron science.

Soft X-ray absorption spectroscopy and resonant inelastic X-ray scattering spectroscopy below 100 eV: probing first-row transition-metal M-edges in chemical complexes.

X-ray absorption and scattering spectroscopies involving the 3d transition-metal K- and L-edges have a long history in studying inorganic and bioinorganic molecules. However, there have been very few studies using the M-edges, which are below 100 eV. Synchrotron-based X-ray sources can have higher energy resolution at M-edges. M-edge X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) could therefore provide complementary information to K- and L-edge spectroscopies. In this study, M2,3-edge XAS on several Co, Ni and Cu complexes are measured and their spectral information, such as chemical shifts and covalency effects, are analyzed and discussed. In addition, M2,3-edge RIXS on NiO, NiF2 and two other covalent complexes have been performed and different d-d transition patterns have been observed. Although still preliminary, this work on 3d metal complexes demonstrates the potential to use M-edge XAS and RIXS on more complicated 3d metal complexes in the future. The potential for using high-sensitivity and high-resolution superconducting tunnel junction X-ray detectors below 100 eV is also illustrated and discussed.

Energy Resolution of STJ X-Ray Detectors with Killed Electrode. Simple Model

Superconducting tunnel junctions X-ray detectors Ti/Nb/Al,AlOx/Al/Nb(2)/NbN with killed Ti/Nb electrode was studied as a function of bias voltage, energy of the absorbed quantum, and thickness of the Nb(2) layer. The data was compared with a simple diffusion model including the losses of excess quasiparticles due to self-recombination. It was shown that increasing of the electrode thickness reduces the self-recombination contribution and improves the linearity of the detector response.

Design of a 8×14 pixel Ta-based STJ X-ray detector array

We are developing high-resolution high-speed superconducting tunnel junction (STJ) detectors for synchrotron science to study metal speciation by fluorescence-detected X-ray absorption spectroscopy. The sensitivity of our current 36-pixel Nb–Al-based STJ spectrometer can be improved by replacing the Nb absorber film with higher-efficiency Ta and by increasing the array size. In addition, an improved device layout can reduce spectral artifacts that limit the peak-to-background ratio and thus the sensitivity for analyzing dilute samples. We present a detailed analysis of the spectral artifacts in our current STJs and discuss the considerations to reduce them in the next-generation 8×14 Ta-based STJ arrays, as well as the trade-offs between energy resolution, efficiency, speed and spectral purity.

The spectral response of superconducting tunnel junction X-ray detectors

The Advanced Detector Group at LLNL is developing superconducting Nb–Al–AlOx–Al–Nb tunnel junction detector arrays for high-resolution soft X-ray spectrometry. We have characterized the detector response function in the energy range between 50 and 2000 eV with monochromatic X-rays, in collaboration with the Physikalisch-Technische Bundesanstalt Laboratory at the BESSY II electron storage ring. The detectors have an energy resolution between 7 and 15 eV FWHM in that energy range, and a count rate capability of ∼10,000 counts/s per detector pixel. In addition, the detector response function exhibits several spectral artifacts, and although their magnitude is only on the order of 1% of the main line, they can reduce the sensitivity for X-ray analysis of dilute specimens if they overlap with the correspondingly weak emission lines. We have characterized these artifacts by scanning a collimated synchrotron radiation beam across the detector area. We identify their origin due to X-ray absorption at locations other than the main Nb absorber film, and outline future detector designs to reduce the artifacts and thereby extend the spectrometer sensitivity.

STJ X-ray detectors with titanium sublayer

Superconducting tunnel junctions (STJs) with the structure Ti/Nb/Al, AlOx/Al/Nb/NbN and corresponding layer thickness 30/100/8/13/150/30nm were investigated as X-ray detectors at T=1.35K. STJs with one active electrode in which the response of the other one is suppressed due to trapping layer on the surface opposite to the tunnel barrier have a number of potential advantages. The best line width (FWHM) is 78eV for 6400μm2 junction. Contribution of the electronic noise is about 50eV. The intrinsic detector line width is less than 60eV. The collected charge from inactive electrode is more than 8 times less than that from the active one. Titanium proved to be an appropriate material for a sublayer and a trap.