N. Rando

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-speed energy-resolved STJ photometry of the eclipsing binary UZ For

We present high time-resolution optical photometry of the eclipsing binary UZ For using a superconducting tunnel junction (STJ) device, a photon-counting array detector with intrinsic energy resolution. Three eclipses of the �18 mag 126.5 min orbital binary were observed using a 6 × 6 array of Tantalum STJs at the 4.2-m William Herschel Telescope on La Palma. The detector presently provides individual photon arrival time accuracy to about 5 µs, and a wavelength resolution of about 60 nm at 500 nm, with each array element capable of counting up to �5000 photons s 1 . The data allow us to place accurate constraints on the accretion geometry from our timeand spectrally-resolved monitoring, especially of the eclipse ingress and egress. We find that there are two small accretion regions, located close to the poles of the white dwarf. The positions of these are accurately constrained, and show little movement from eclipse to eclipse, even over a number of years. The colour of the emission from the two regions appears similar, although their X-ray properties are known to be significantly different: we argue that the usual accretion shock may be absent at the non-X-ray emitting region, and instead the flow here interacts directly with the white dwarf surface; alternatively, a special grazing occultation of this region is required. There is no evidence for any quasi-periodic oscillations on time-scales of the order of seconds, consistent with relatively stable cyclotron cooling in each accretion region.

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.

Superconducting tunnel junctions as photon counting detectors in the infrared to the ultraviolet

Photon counting experiments with Ta/Al superconducting tunnel junctions are presented. Single photon detection is demonstrated in the wavelength range /spl lambda/=200-2000 nn with a resolving power /spl lambda///spl Delta//spl lambda/=22-4. The response of the detector shows good linearity with photon energy.

STJ observations of the eclipsing polar HU Aqr

We apply an eclipse mapping technique to observations of the eclipsing magnetic cataclysmic variable HU Aqr. The observations were made with the S-Cam2 superconducting tunnel junction detector at the William Herschel Telescope in 2000 October, providing high signal-to-noise ratio observations with simultaneous spectral and temporal resolution. HU Aqr was in a bright (high accretion) state (V = 14.7) and the stream contributes as much to the overall system brightness as the accretion region on the white dwarf. The stream is modelled assuming accretion is occurring on to only one pole of the white dwarf. We find enhanced brightness towards the accretion region from irradiation and interpret enhanced brightness in the threading region, where the ballistic stream is redirected to follow the magnetic field lines of the white dwarf, as magnetic heating from the stream-field interaction, which is consistent with recent theoretical results. Changes in the stream eclipse profile over one orbital period indicate that the magnetic heating process is unstable.

X‐ray characteristics of a niobium superconducting tunnel junction with a highly transmissive tunnel barrier

The results of an investigation into the x‐ray properties of a superconducting tunnel junction (STJ) are presented. The photoabsorption of an x‐ray photon by one of the thin superconducting films of the junction results in the production of quasiparticles, which may subsequently tunnel through the thin oxide barrier into the second superconducting film. The transfer of charge across the barrier is detected, and gives a measure of both the x‐ray photon energy and the effective energy gap e of the superconducting film in which the photoabsorption occurred. A charge output of 55% of the theoretical maximum has been obtained for a niobium‐based STJ. Such a charge output indicates a mean energy e of ≂4.7 meV is required to create a single charge carrier in the junction such that e/Δ≂3, where 2Δ is the junction energy gap. This is the lowest value of e/Δ obtained to date for x‐ray photoabsorption in STJs. The energy resolution of the device is, however, still poor, with a full width half maximum of ≂200 eV for ...

Direct determination of quasar redshifts

We present observations of 11 quasars, selected in the range z 2.2-4.1, obtained with ESAs Superconducting Tunnel Junction (STJ) camera on the WHT. Using a single template QSO spectrum, we show that we can determine theredshifts of these objects to about 1%. A follow-up spectroscopic observation of one QSO for which our best-fit redshift (z = 2.976) differs significantly from the tentative literature value (z 2.30) confirms that the latter was incorrect.

Superconducting tunnel junctions as photon-counting imaging spectrometers from the optical to the x-ray band

Superconducting tunnel junctions (STJs) have been exten- sively investigated as photon detectors. They combine intrinsic energy resolution with good detection efficiency and high responsivity, thus pro- viding energy-resolved photon-counting performance in a wide energy range, from the near IR (NIR) to the x rays. On this basis, STJs offer advantages with respect to alternative photon detection systems, such as an intrinsic energy resolution, higher speed, and when compared to wavelength dispersive systems, increased detection efficiency. We over- view the STJ development in the Astrophysics Division of the European Space Agency (ESA), including STJ fabrication and operation, spectro- scopic performance of single STJs, small arrays, and STJ-absorber structures in the UV-NIR and x-ray bands. As a first application we de- scribe S-Cam, a cryogenic camera for ground-based optical astronomy exploiting a 636 array of Ta STJs. This camera has undergone four campaigns at the 4.2-m William Herschel Telescope at La Palma (Spain), and future generations of the camera are under active develop- ment. For the soft x-ray band (50 to 3000 eV) a STJ-based instrument with an active area of 73 7m m 2 is proposed as part of the payload of the X-ray Evolving Universe Spectroscopy (XEUS) mission, which is cur- rently under study at ESA. Future developments include devices based on lower Tc superconductors for improved energy resolution as well as larger format detector arrays combined with alternative readout schemes.

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.