Christian M. Castelli

Characteristics of the flight model optics for the JET-X telescope onboard the Spectrum-X-Gamma satellite

The joint European x-ray telescope (JET-X) is one of the core scientific instruments of the RUssian SPECTRUM X-(gamma) astrophysics mission. JET-X is designed to study the emission from x-ray sources in the band of 0.3-10 keV; in particular to meet primary scientific goals in cosmology and extragalactic astronomy. JET-X consists of two identical, coaligned x-ray telescopes, each with a spatial resolution of better than 30 arcsec half energy width. Focal plane imaging is provided by cooled x-ray sensitive CCD detectors which combine high spatial resolution with good spectral resolution, including coverage of the iron line complex around 7 keV at a resolution of (Delta) E/E approximately 1.5 percent. Each telescope is composed of a nested array of 12 mirror shells with an aperture of 300 mm and focal length of 3500 mm; the total effective area is 330 cm2 at 1.5 keV and 145 cm2 at 8.1 keV. The mirror shells have a Wolter I geometry and are manufactured by an electroforming replica process. The paper presents the characteristic of the flight model x-ray optics.

X-ray imaging performance of the flight model JET-X telescope

Construction of the flight model joint European X-ray telescope (JET-X) for the Russian spectrum-X mission has been completed and performance tests and calibration of the instrument have been carried out. Separate measurements of the responses of the x-ray mirrors, the CCD detectors and the optical filters already indicate that JET-X will achieve spatial resolutions of around 20 arcsec, an on-axis collecting area of 310 cm2 at 1.5 keV and an energy resolution of 130 eV at 6 keV. As a final step in the calibration of the telescope assembly, end-to-end x-ray tests on the complete instrument have been performed in the x-ray beam line facility at MPE Garching. Results from this calibration program are reported and the overall response of the two x-ray telescopes are compared with the previously measured responses of the mirror, the CCD detectors and the optical filters. In-orbit sensitivity responses are derived from these calibration data sets, for the normal operating modes of JET-X.

Soft X-ray response of charge coupled devices

Abstract The charge coupled device (CCD) is useful as an imaging detector in the X-ray waveband, and when used to detected single photon events can simultaneously provide spatial imaging and energy resolution. This capability is important in the field of X-ray astronomy. With radiation focussed onto the front electrodes, the CCD suffers from reduced response below 1 keV due to absorption in the electrode structure; the same process that limits the blue response in the optical band. Two alternative developments are described that extend the soft X-ray and blue responses of the CCD. Experimental data are presented for these two configurations of CCD and their performance characteristics as X-ray images are compared.

The X-Ray CCDs Developed for the Joint European X-Ray Telescope

The charge coupled devices (CCDs) developed for the Joint European X-ray Telescope (JET-X) are described in detail. A history of the development program and device performance is given. We present results from a comprehensive study to characterize the x-ray response of the flight model focal plane detectors. The goal of the program is to calibrate the efficiency, energy resolution, gain, etc. down to a precision of ∼1%. Final calibration data sets will be based on combinations of measurements and calculations. For example, the CCD quantum efficiency will be composed of discrete line measurements made at the University of Leicester test facility and calculation and synchrotron measurements from the Daresbury Synchrotron Radiation Source (SRS). The absolute normalizations will be provided by x-ray long beam pipe measurements at the Max Planck Institut fur Extraterrestrische Physik (MPE) Panter test facility in Munich. Using the available data, it is shown that it is possible to calibrate the quantum efficiency, the FWHM energy resolution, and the system gain of the flight devices to better than 1%.

A novel high energy X-ray detector concept using CCDs

Abstract The X-ray detection efficiency of charge coupled devices (CCDs) is dependent on the thickness of the active depletion layer in the silicon, typically less than 40 μm and limited by the purity of available epitaxial substrates. For X-ray imaging, therefore, the CCD detection efficiency drops sharply above 8 keV. In this Letter, we show that, using a novel illumination geometry, CCDs can detect high energy (up to 100 keV) X-rays with reasonable efficiency and good energy resolution. A novel 2-D imaging hard X-ray detector based on this geometry is discussed.

Further Development Of CCD X-Ray Detectors For Astronomy

A number of future space-borne X-ray astronomy missions have now been proposed, which will utilise CCD imaging devices in their focal planes. The CCDs will offer simultaneously high spatial resolution, excellent energy resolution and detection sensitivity. However, a number of technological developments are still to be demonstrated before the X-ray performance of CCDs is optimised. The development of CCDs fabricated on high resistivity silicon wafers, combined with back thinning and annealing processes, allows the production of fully depleted detectors. These devices are capable of detecting single X-ray photons , with high efficiency throughout the 0.1-15keV energy band. Efficient coverage of focal planes is facilitated with the introduction of new large area CCDs. The EEV P88000 series CCDs have a range of formats up to 1242 x 1152 pixels (6.5 sq cm), and have included process changes to optimise noise (5.5 electrons rms) and dark current. Results of a programme to investigate the tolerance of EEV CCDs to ionising radiation are also presented.

Soft x-ray performance of back-illuminated EEV CCDs

The European Space Agency X-ray Multi-Mirror mission will be devoted to X-ray imaging and spectroscopy with high throughput. Both the EPIC focal plane camera instrument, and the RGS dispersive spectrometer require detectors with high sensitivity in the soft X-ray waveband. A description of

X-ray and optical performance of the flight filters for the JET-X telescope

The optical filters on board the JET-X telescope comprise thin foils of aluminum coated Lexan. During ground calibration of the filters, narrow spectral regions of high UV leakage, with peak levels of up to a few percent, were observed in broad band optical measurements in the 1000 to 10,000 angstrom range. Furthermore, transmission values were typically up to two orders of magnitude higher than calculated for the aluminum thickness. Investigation showed that these effects were attributed to a combination of aluminum oxidation, which reduces the opacity, and the use of a double sided aluminum layer in the filter design which behaves as a Fabry-Perot interference filter. These effects were verified by a multi- layer model of the filter UV response. Recent redesign of the filters for the flight program eliminated the UV leakage by adopting a single aluminum layer configuration, thus eliminating interference effects, and increasing the thickness by 30% to compensate for oxidation levels. The integrated x- ray transmission below 1 keV was found to be only reduced by 3%. In parallel with the production of the new Lexan flight filters, a set of qualification model filters was produced by the Luxel Corporation in the USA. These filters use polyimide as a substrate material which has the advantage that it is optically opaque to wavelengths below 3000 angstroms, unlike Lexan which is transparent. These new filters were found to have superior mechanical strength, being able to survive extended qualification vibration without any visible degradation in performance, and had a higher cosmetic quality and attenuation levels. As a result, these filters have now been included in the JET-X flight program. We report on the optical tests results from both Lexan and polyimide filters along with high resolution x-ray transmission results carried out at the BESSY synchrotron facility in Germany. Results of the mapping of the filter edge structures, global transmission values and uniformity are presented.

Measurement of x-ray polarization with small-pixel charge-coupled devices

Measuring the polarization of x-rays emitted from cosmological objects yields explanations of the structure which characterize these sources. Polarization detection efficiencies of up to 18% have been measured for two, small pixel, charge coupled devices (CCDs) using an 80% polarized monochromatic synchrotron beam between energies of 7.5 keV and 35 keV. The device efficiencies at less than 15 keV are of particular interest for astronomical purposes where imaging, spectroscopy and polarization measurements can be carried out simultaneously. Polarization measurements using a CCD rely on the preferential direction of the ejected photoelectron along the E-field of the incident x-ray beam. The resultant charge cloud is sampled by the pixellated array of the CCD. It will be shown that the CCD polarization detection efficiency (modulation factor) is a function of the pixel size and the energy of the incident photons. The effect of depletion depth and impact of a field-free layer in the detector are reviewed. The two devices used were a commercial optical CCD, Kodak KAF1400, with 6.8 by 6.8 micrometer squared pixels and a specialized CCD, designed by EEV Ltd., deeply depleted with 4 by 9 micrometer squared pixels.

Use of x-ray CCDs in the calibration of the flight mirrors for the JET-X telescope on Spectrum-X

The x-ray mirror calibration program for the JET-X telescope on spectrum-X has recently been carried out at the 130 m long Panter x-ray beam line of the Max Plank Institute fur Extraterrestriche Physik. The excellent spatial resolution achieved with these mirrors, 15 arcsec half energy width (HEW) at 1.5 keV and 19 arcseconds at 8 keV, has proved to be difficult to measure precisely using previously established calibration methods (involving either slit detectors or the ROSAT PSPC imaging proportional counter). New diagnostic techniques have, therefore, been developed using a CCD imaging camera which utilized newly available x- ray CCD technology. Details of the calibration technique and the performance of the camera are provided and results are compared with those obtained from the slit and PSPC detectors.