Anthony J. Peacock

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.

Silicon pore optics: novel lightweight high-resolution x-ray optics developed for XEUS

The next generation astronomical X-ray telescopes (e.g. XEUS) require extremely large collecting area (10 m2) in combination with good angular resolution (5 arcsec). The existing technologies such as polished glass, nickel electroforming and foil optics would lead to excessively heavy and expensive optics, and/or are not able to produce the required large area or resolution. We have developed an entirely novel technology for producing X-ray optics which results in very light, stiff and modular optics which can be assembled into almost arbitrarily large apertures, and which are perfectly suited for XEUS. The technology makes use of commercially available silicon wafers from the semiconductor industry. The latest generation silicon wafers have a surface roughness that is sufficiently low for X-ray reflection, are planparallel to better than a micrometer, have almost perfect mechanical properties and are considerably cheaper than other high-quality optical materials. The wafers are bent into an accurate cone and assembled to form a light and stiff pore structure with pores of the order of a millimeter. The resulting modules form a small segment of a Wolter-I optic, and are easily assembled into an optic with large collecting area. We present the production principle of these silicon pore optics, the facilities that have been set up to produce these modules and experimental results showing the excellent performance of the first modules that have been produced. With further improvement we expect to be able to match the XEUS requirements for imaging resolution and mass.

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.

Lattice dynamics of a disordered solid-solid interface

Generic properties of elastic phonon transport at a disordered interface are studied. The results show that phonon transmittance is a strong function of frequency and the disorder correlation length. At frequencies lower than the van Hove singularity the transmittance at a given frequency increases as the correlation length decreases. At low frequencies, this is reflected by different power laws for phonon conductance across correlated and uncorrelated disordered interfaces which are in approximate agreement with the perturbation theory of an elastic continuum. These results can be understood in terms of simple mosaic and two-color models of the interface.

Optical photon counting using superconducting tunnel junctions

Abstract We demonstrate the feasibility of optical photon counting using a superconducting substrate in combination with an array of widely spaced superconducting tunnel junctions of lower energy gap. In the proposed device, a photon impinging on the substrate generates quasiparticles within it, and these are channelled towards, and are detected by, the nearest four elements of the junction array. We show that certain substrate/junction combinations generate sufficient numbers of quasiparticles per incident photon to permit pulse counting and positional encoding, whilst resulting in a quasiparticle number density which is low enough to prevent significant recombination within the relevant diffusion and tunnelling timescales. A combination of Al junctions with a Nb or Sn substrate provides the basis for a large area detector with very high sensitivity to individual optical and UV photons, high intrinsic time-resolution, and moderate energy resolution.

Fano factor implications from gas scintillation proportional counter measurements

Abstract The Fano factor for xenon may be determined using a gas scintillation proportional counter in which the measured energy resolution is extrapolated to the condition of infinite light yield. The true Fano factor is expected to be independent of X-ray energy, photomultiplier type and gas pressure. Using both drifted and driftless GSPCs we have measured the Fano factor over a large range of X-ray energies (0.27–44.23 keV), photomultiplier types and gas pressures (351–878 hPa). In all three cases, when making the assumptions usually adopted in these determinations we find that the extrapolated Fano factor is dependent on the conditions. The implication is that other processes are also contributing to the measured resolution and that the determined values of the Fano factor are in fact upper limits. The Fano factor for xenon is found to be

X-ray Multimirror Mission: an overview

The x-ray multi-mirror mission (XMM) is the second cornerstone mission of the ESA Horizon 2000 Science Programme. When launched in late 1999 it will provide a world-class observatory facility for x-tray astronomers. This paper describes the overall concept of the mission, including the spacecraft bus, the instruments and the innovative replicated optics that produce images of better than 20 arcsecs half energy width, and a total geometric collection area of more than 4500 cm2. The observatory is unique in that it will provide simultaneously, high throughput non-dispersive spectroscopic imaging (EPIC instrument), medium resolution dispersive spectroscopy (reflection grating spectrometer) and optical/UV imaging and timing from a co-aligned instrument (optical monitor). We describe some of the operational aspects and provide a brief description of the scientific potential of the payload.

Generalized proximity effect model in superconducting bi- and trilayer films

This article presents a general model for calculating the density of states and the Cooper pair potential in proximity-coupled superconducting bi- and trilayer films. It is valid for any kind of bilayer S1-S2, whatever the quality of the materials S1 and S2, the quality of the S1-S2 interface, and the layer thicknesses. The trilayer model is valid for a thin S3 layer, whereas the other two layers have arbitrary thicknesses. Although the equations of the dirty limit are used, it is argued that the model stays valid in clean bi-and trilayer films. The typical example of superconducting tunnel junctions is used to show that existing models, which apply to very thin or very thick layers or to perfectly transparent S1-S2 interfaces, are too restrictive to apply to an arbitrary bilayer. The new model is applied to practical junctions, with layer thicknesses intermediate between the “thick” and the “thin” approximation.

EXOSAT observations of the Cassiopeia A supernova remnant

The young supernova remnant Cas A has been observed with the European X-ray observatory satellite Exosat. The remnant was observed with all instruments covering the range from 0.5 to 25 KeV. It is shown here that the emission from the remnant cannot be described by a two-temperature isothermal plasma in collisional ionization equilibrium. A simple Sedov (1959) blast-wave model assuming equilibration between the electron and ion temperature but nonionization equilibrium also does not result in an acceptable fit due to strongly enhanced emission at low energies (not above 2.5 KeV). Observations made with the Exostat imaging proportional counter show that this extra emission comes from the interior regions of the remnant. The source of this low-energy flux is most probably supernova ejecta heated in a reverse shock. No evidence is found for a high temperature (not less than 20 KeV) component. 35 references.