Marcos Bavdaz

Photoluminescence of Cd1—xZnxTe Crystals Grown by High‐Pressure Bridgman Technique

Photoluminescence (PL) spectra of detector-grade Cd 1-x Zn x Te crystals with x = 0.07 to 0.14 were measured as a function of the excitation laser power and over the temperature range from 12 to 295 K. Two bound exciton transitions at the energies of 13 and 24 meV below the band-gap E g and a number of phonon replicas dominate the low-temperature PL spectra. The transition at the energy of E g - 13 meV is attributed to a neutral donor-bound exciton D 0 X. The excitation power and temperature dependence of the peak at the energy of E g - 24 meV show that it is of excitonic origin with binding energy of 13 meV. The intensity of this A 0 X peak is found to depend on z, which suggests that it is related to structural defects. In some samples a donor-acceptor pair (DAP) transition at the energy of E g - 60 meV is observed. The PL spectra also exhibit a weak, defect-related broad peak at the energy of about E g - 200 meV. The PL of a large number of samples from different boules and locations in the boules is compared. The DAP-peak intensity was observed to depend strongly on the boule.

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.

X-Ray Pore Optics Technologies and Their Application in Space Telescopes

Silicon Pore Optics (SPO) is a new X-ray optics technology under development in Europe, forming the ESA baseline technology for the International X-ray Observatory candidate mission studied jointly by ESA, NASA, and JAXA. With its matrix-like structure, made of monocrystalline-bonded Silicon mirrors, it can achieve the required angular resolution and low mass density required for future large X-ray observatories. Glass-based Micro Pore Optics (MPO) achieve modest angular resolution compared to SPO, but are even lighter and have achieved sufficient maturity level to be accepted as the X-ray optic technology for instruments on board the Bepi-Colombo mission, due to visit the planet Mercury. Opportunities for technology transfer to ground-based applications include material science, security and scanning equipment, and medical diagnostics. Pore X-ray optics combine high performance with modularity and economic industrial production processes, ensuring cost effective implementation.

Gas scintillation proportional counters for x‐ray synchrotron applications

Gas scintillation proportional counters (GSPCs) as x‐ray detectors provide some advantages and disadvantages compared with proportional counters. In this paper the various configurations of xenon filled GSPC are described including both imaging and nonimaging devices. It is intended that this work be used to configure a GSPC for a particular application and predict its general performance characteristics. The general principles of operation are described and the performance characteristics are then separately considered. A high performance, imaging, driftless GSPC is described in which a single intermediate window is used between the PMT and gas cell.

X-ray focusing with Wolter microchannel plate optics

Abstract Square-pore microchannel plate (MCP) X-ray optics of the “lobster-eye” geometry have frequently been described in the literature. We have now investigated the use of a radial channel packing geometry which, in the context of an MCP pair slumped to the correct radii of curvature, can form a conic approximation to the Wolter Type I grazing incidence X-ray optic. Such an optic can provide a large effective area with very low mass and may be ideally suited for use in applications such as planetary imaging X-ray fluorescence. We present here the results of X-ray illumination of the first such optic, fabricated by Photonis SAS, France.

Light yield as a function of gas pressure and electric field in gas scintillation proportional counters

Abstract We have investigated the dependence of the scintillation light output for Xe on gas pressure in the range 0.14–1.4 bar, using a gas scintillation proportional counter, in different experimental configurations. We have compared our work with that of previous workers, and have shown that both our results and the results of previous authors are compatible with the intrinsic light output being independent of gas pressure, with any observed dependence being a pure experimental effect due to the spectral response of the various UV detectors used. We also use our experimental data for determining the ratio between the cross section of the Xe 2 ∗∗ + Xe → Xe 2 ∗ + Xe reaction and the rate of the Xe 2 ∗ → 2 Xe +γ UV reaction.

In focus measurements of IXO type optics using the new PANTER X-ray test facility extension

Future large X-ray observatories in space will require mirrors with large effective areas and long focal lengths to accomplish the proposed science. ESA programs for developing lightweight optics based on modules of silicon pore optics (SPO) and slumped glass optics (SGO) were put in place for the IXO mission (f=20m, r≈1m). To test such optics the MPE PANTER X-ray test facility has been upgraded / extended with support from ESA to accommodate in-focus measurements of such optics modules. We describe the extension to PANTER and the first results obtained from measuring such SPO and SGO modules during commissioning.

Noise analysis of gallium arsenide pixel X-ray detectors coupled to ultra-low noise electronics

The X-ray spectroscopic performance and the noise analysis of gallium arsenide pixel detectors coupled to ultra-low noise front-end electronics is presented. The pixel junctions have areas of 200 /spl times/ 200 /spl mu/m/sup 2/ and they are fabricated on an epitaxial gallium arsenide layer 40 /spl mu/m thick. The front-end electronics consists of a forward bias field effect transistor amplifier with an equivalent noise of 16 electrons root mean square at room temperature (139-eV full-width at half-maximum (FWHM) silicon equivalent, 159-eV FWHM GaAs equivalent). The whole system shows intrinsic energy resolutions of 242-eV FWHM at room temperature and 163-eV FWHM when cooled to -30 /spl deg/C. On the 59.5-keV line of /sup 241/Am, widths of 501-eV FWHM at room temperature and 465-eV FWHM at -30 /spl deg/C have been measured, close to the Fano limit. The different noise sources affecting the system have been disentangled and analyzed. It was found that the dielectric noise is the dominant component both at room temperature and when the system is cooled down. No excess line broadening due to signal charge loss was observed and a Fano factor of 0.124 /spl plusmn/ 0.004 has been derived from the width of the 59.5-keV spectral line of /sup 241/Am.

Accurate integration of segmented x-ray optics using interfacing ribs

Abstract. Future lightweight and long-focal-length x-ray telescopes must guarantee a good angular resolution (e.g., 5 arc sec HEW) and reach an unprecedented large effective area. This goal can be reached with the slumping of borosilicate glass sheets that allow the fabrication of lightweight and low-cost x-ray optical units (XOU). These XOUs, based on mirror segments, have to be assembled together to form complete multishell Wolter-I optics. The technology for the fabrication and the integration of these XOUs is under development in Europe, funded by European Space Agency, and led by the Brera Observatory (INAF-OAB). While the achievement of the required surface accuracy on the glass segments by means of a hot slumping technique is a challenging aspect, adequate attention must be given to the correct integration and coalignment of the mirror segments into the XOUs. To this aim, an innovative assembly concept has been investigated, based on glass reinforcing ribs. The ribs connect pairs of consecutive foils, stacked into a XOU, with both structural and functional roles, providing robust monolithic stacks of mirror plates. Moreover, this integration concept allows the correction of residual low-frequency errors still present on the mirror foil profile after slumping. We present the integration concept, the related error budget, and the results achieved so far with a semi-robotic integration machine especially designed and realized to assemble slumped glass foils into XOUs.

Silicon Pore Optics Development

Future X-ray astrophysics missions, such as the International X-ray Observatory, IXO, require the development of novel optics in order to deliver the missions large aperture, high angular resolution and low mass requirements. A series of activities have been pursued by ESA, leading a consortium of European industries to develop Silicon Pore Optics for use as an x-ray mirror technology. A novel process takes as the base mirror material commercially available silicon wafers, which have been shown to possess excellent x-ray reflecting qualities. These are ribbed, curved and stacked concentrically in layers that have the desired shape at a given radii of the x-ray aperture. Pairs of stacks are aligned and mounted into doubly reflecting mirror modules that can be aligned into the x-ray aperture without the very high angular and position alignment requirements that need to be achieved for mirror plates within the mirror module. The use of this silicon pore optics design substantially reduces mirror assembly time, equipment and costs in comparison to alternative IXO mirror designs. This paper will report the current technology development status of the silicon pore optics and the roadmap expected for developments to meet an IXO schedule. Test results from measurements performed at the PTB lab of the Bessy synchrotron facility and from full illumination at the Panter x-ray facility will be presented.