A. van Dordrecht

The properties of niobium superconducting tunneling junctions as X-ray detectors

The properties of superconducting tunnel junctions based on niobium are investigated. The limiting resolution of such junctions should be ⋍ 4 eV for 6 keV X-rays. Currently only between 2 to 25% of the theoretical charge is detected. The principal loss mechanisms, which not only reduce charge but seriously degrade resolution, are found to be phonon loss to the substrate, and recombination of the excess quasi-particle population in both films. The phonon loss is probably due to relaxation phonons from quasi-particles relaxing towards the bandgap. The quasi-particle self recombination is a direct result of the very large excursion from equilibrium produced during the X-ray photoabsorption process. Finally 6 keV X-rays have been detected directly in sapphire crystals by using the niobium junction only as a detector of beamed ballistic phonons. The use of a suitable crystal as the X-ray absorber and phonon source opens up interesting possibilities for position sensitive spectrometers based on high quality niobium junctions.

RESPONSE LINEARITY OF NB TUNNEL JUNCTION DETECTORS FOR PHOTON ENERGIES FROM 1.5 TO 6.4 KEV

Recent experimental results show a linear energy response in high quality Nb‐Al‐AlOx‐Nb superconducting tunnel junction detectors for photon energies between 1.5 and 6.4 keV. The experimental data are based on both direct x‐ray illumination and on the escape and re‐absorption of fluorescent photons created in the junction electrodes and in the silicon substrate. The observed linearity of the energy response raises questions on the validity of some theoretical models which describe the relaxation process occurring in a superconducting thin film after x‐ray photoabsorption. Such models generally predict nonlinear effects due to large quasiparticle number densities and short recombination times.Recent experimental results show a linear energy response in high quality Nb‐Al‐AlOx‐Nb superconducting tunnel junction detectors for photon energies between 1.5 and 6.4 keV. The experimental data are based on both direct x‐ray illumination and on the escape and re‐absorption of fluorescent photons created in the junction electrodes and in the silicon substrate. The observed linearity of the energy response raises questions on the validity of some theoretical models which describe the relaxation process occurring in a superconducting thin film after x‐ray photoabsorption. Such models generally predict nonlinear effects due to large quasiparticle number densities and short recombination times.

High-resolution x-ray spectra measured using tantalum superconducting tunnel junctions

The spectral response of a 100×100 μm2 tantalum based superconducting tunnel junction to 5.9 keV x-ray photons from a 55Fe source has been studied. In full illumination the energy resolution for the Mn Kα line complex is 56 eV, dominated by spatial nonuniformity in the response of the detector. When illuminating selectively a 5–10 μm diam spot in the center of the detector, the energy resolution improves to 22 eV, corresponding to 15.7 eV for the individual Mn Kα1 and Mn Kα2 lines. This exceeds the predicted theoretical energy resolution of 7.3 eV for this type of device by only a factor of ∼2.

SINGLE PHOTON DETECTION AT VISIBLE AND X-RAY WAVELENGTHS WITH NB-AL SUPERCONDUCTING TUNNEL JUNCTIONS

Photon counting experiments at wavelengths ranging from near infrared to x-ray with niobium based superconducting tunnel junctions with aluminum trapping layers are presented. Single photons can be detected up to a wavelength of 1 μm. The response in the ultraviolet to near-infrared region is characterized by a good energy linearity (<2.5%), a capability to handle event rates up to ∼3 kHz, and moderate energy resolving power (E/ΔE≈7 for E=4 eV). The x-ray response at 6 keV is characterized by anomalously high signals compared to the low energy response, a severe energy nonlinearity and a relatively poor energy resolution of ∼140 eV, full width at half maximum.

A burst length discrimination system for a gas scintillation proportional counter

Abstract A burst length discrimination system has been developed for the rejection of background events in a gas scintillation proportional counter intended for use in X-ray astronomy. The system utilised a dual constant fraction trigger (CFT) which could determine risetimes from 0.5 to 12 μs. The principle of the dual constant fraction trigger is discussed.

ON THE DETECTION OF SINGLE OPTICAL PHOTONS WITH SUPERCONDUCTING TUNNEL JUNCTION

We report the detection of individual optical and ultraviolet photons using a different approach to photon detection based on a superconducting tunnel junction. A 20×20 μm2 junction, employing a 100 nm niobium film and operated at a temperature of ∼0.4 K, has been used to detect individual photons with inherently high quantum efficiency (>45%) over a broad wavelength range (between 200 and 500 nm), yielding high temporal (sub-ms) resolution, spatial resolution determined by the junction size, under conditions of minimal dark current, and in the absence of read noise. The quantum efficiency is limited by surface reflection, and could be improved by the deposition of antireflection coatings. The theoretical wavelength response range continues into the far UV and soft x-ray region, and is presently limited beyond 500 nm largely by the available signal processing electronics. The device intrinsically functions at very high incident photon rates—with count rates of order ∼10 kHz or higher being feasible and ag...

Transmission electron microscopy and atomic force microscopy analysis of Nb‐Al‐AlOx‐Nb superconducting tunnel junction detectors

The performance of photon detectors based on superconducting tunnel junctions are related to their current ‐ voltage (I‐V) curve characteristics and, ultimately, to the quality of the thin tunnel barriers (of order 1 nm) which separate the two superconducting thin films. Both the optimization of the spectroscopic performance of these detectors and the development of a reproducible and high yield fabrication route, require a better understanding of barrier quality and growth techniques. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) provide valuable tools for the investigation of the barrier region and for the control of the quality of the different thin films and related interfaces. In this paper, the results of a TEM and AFM evaluation of Nb‐Al‐AlOx‐Nb tunnel junctions are reported, together with their interpretation on the basis of the I‐V curve performance at low temperature (T≥0.3 K). Thickness disuniformities of the Al plus AlOx overlayer and evidence of barrier defects have...

Soft X-ray performance of superconducting tunnel junction arrays

A number of 6/spl times/6 element arrays of Ta-based superconducting tunnel junctions have been manufactured for photon counting applications with moderate energy resolution in ground-based optical astronomy. The individual array elements show low leakage, uniform responsivity across the array, good simultaneous Josephson current suppression and minor crosstalk between adjacent pixels. The same arrays have been characterized in the soft X-ray range (E=270-1500 eV). The base electrode response shows good energy resolving power (E//spl Delta/E/spl ap/140). Unwanted spectral features originating from other parts of the detector can be largely eliminated by rise-time filtering. Modifications in the layering are necessary in order to improve the soft X-ray detection efficiency.

ON THE DEVELOPMENT OF SUPERCONDUCTING TUNNEL JUNCTIONS FOR USE IN ASTRONOMY

Abstract Superconducting Tunnel Junctions (STJ) have now been under development for a number of years for a wide range of astronomical applications. Devices based on niobium–aluminium or tantalum–aluminium have been shown to be efficient photon counting energy dispersive spectrometers from the near-infra-red to X-ray region of the spectrum. The basic performance characteristics of tantalum-based devices, in terms of detection efficiency, spectral resolution and signal linearity with photon energy, are provided for each wave band of astronomical interest, namely: the near-infra-red (NIR [1–5 μm]), optical [400–1000 nm], ultraviolet (UV [100–400 nm]), extreme ultraviolet (EUV [10–100 nm]), soft X-ray (SXR [100–2000 eV]) and medium X-ray (MXR [2–10 keV]) regions of the spectrum. Although design issues which allow the optimization of the performance for a specific waveband are of importance, we show that current generations of tantalum-based STJs have already very good performance over the whole spectral range. A particular improvement in the resolving power and a description of the role played by various mechanisms in degrading the energy resolution from the theoretical tunnel limited value is given based on specific measurements at relevant photon wavelengths. Additional astronomical characteristics for future applications, such as the time resolution and imaging capability are also presented, based on experimental data and the performance of the first small format 6×6 pixel STJ array described. Finally an assessment is provided on the possibility for further improvements, particularly in the area of higher spectral resolution, through the use of lower band gap superconductors.

Photon counting from visible to X-ray wavelengths with NbAl superconducting tunnel junctions

Niobium based superconducting tunnel junctions with 120 nm thick aluminium trapping layers have been used in photon counting experiments at visible and X-ray wavelengths. Due to the efficient trapping of quasiparticles in the Al layers and a highly transmissive tunnel barrier, signals as high as 2.3 × 108 electrons of detected charge at an X-ray photon energy of 5.9 keV are observed. The high signal levels together with the low noise levels of these devices have allowed the detection of signals from single photons in the UV/optical wavelength range 220–500 nm at an operating temperature of 370 mK. The typical resolution is 100 nm (FWHM) at λ = 300 nm, which is dominated by the electronic noise.