Carl Budtz-Joergensen

X-ray calibration of the SODART flight telescopes

The on- and off-axis imaging properties and effective area of the two SODART flight telescopes have been measured using the expanded beam x-ray facility at the Daresbury synchrotron. From on-axis measurements the encircled power distribution and the point spread function at three energies 6.627 keV, 8.837 keV and 11.046 keV have been measured using a one-dimensional position sensitive detector. We found that the point spread function can be presented well by a function which is a sum of a Gaussian divided by the radius and two exponential terms where the first has a 1/e value close to 2 arcmin and the other a 1/e value of ca. 15 arcmin. The data have been used to calculate the half power diameter (HPD) for three different SODART focal plane detectors, the high energy proportional counter (HEPC) with a field of view (FOV) of 65 arcmin, the low energy proportional counter (LEPC) with a FOV of 33 arcmin and the 19 element solid state detector array (SIXA) with a FOV of 18 arcmin. We found that the HPD decreases with increasing energy due to poorer figure of the outermost mirrors. The HPD falls in the range from 2.4 to 3.8 arcmin depending on energy and FOV. Data have also been obtained on the on- and off-axis effective area at all three energies and compared to that obtained from a raytracing of an ideal telescope configuration. We found that the measured on-axis effective area integrated over a FOV of 105 arcmin is ca. 65% of the area predicted from an ideal geometry. Finally the one- dimensional detector data has been used to obtain the radial dependence of the on-axis HPD and the on-axis effective area and the data from the two-dimensional position sensitive detector has been used to obtain the azimuthal dependence of the on-axis HPD and the on-axis effective area.

Development of large-area CZT detector systems

DSRI has initiated a development program of CZT x-ray and gamma ray detectors employing strip readout techniques. A dramatic improvement of the energy response was found operating the detectors as so-called drift detectors. For the electronic readout, modern ASIC chips were investigated. Modular design and the low power electronics will make large area detectors using the drift strip method feasible. The performance of a prototype CZT system will be presented and discussed.

Joint European x-ray monitor (JEM-X): x-ray monitor for ESA's INTEGRAL mission

JEM-X will extend the energy range of the gamma ray instruments on ESAs INTEGRAL mission (SPI, IBIS) to include the x-ray band. JEM-X will provide images with arcminute angular resolution in the 2 - 60 keV band. The baseline photon detection system consists of two identical, high pressure, imaging microstrip gas chambers, each with a collecting area of 500 cm2. They view the sky through a coded aperture mask (0.5 mm tungsten) at a separation of 3.4 m. The two detector boxes are formed from 2 mm thick stainless steel plate and are filled with 5 bar Xe. The field of view is defined by the collimator mounted on top of the detector. Each collimator consists of an array of bonded square tubes of Mo. The internal surface of these tubes is covered by a graded shield. The collimator provide also the support for the detector windows which are made out of 250 micrometer thick beryllium foils. The detector sensor elements consists of microstrip plates shaped as regular octagons with a diameter of 292 mm. The basic microstrip pattern is similar to the one chosen for the HEPC/LEPC detector system on SRG. The detector position resolution will be sufficient to ensure an angular resolution for JEM-X of better than 3 arcmin throughout the 2 - 60 keV band.

High-Energy/Low-Energy Proportional Counter (HEPC/LEPC) detector systems on the SRG

The low energy proportional counter LEPC (0.2 - 8 keV) and the high energy proportional counter HEPC (2 - 25 keV) for the Danish-Russian X-ray telescopes XSPECT/SODART are presently being tested at DSRI. The sensor principle of these detectors is based on the novel micro strip gas counter (MSGC) where the strip electrodes are deposited by photolithography onto a rigid substrate. The MSGC offers many advantages: A uniform gas gain, an excellent energy (approximately 13% at 6 keV) and position-resolutions (>> 1 mm), a fast charge collection and a low operating voltage. The energy response, imaging and background rejection performance of LEPC (82 X 82 mm) and HEPC ((sigma) equals 160 mm) will be discussed.

High energy x-ray reflectivity and scattering study from spectrum-x-gamma flight mirrors

Line radiation from Fe K-alpha(1), Cu K-alpha(1), and Ag K-alpha(1) is used to study the high energy X-ray reflectivity and scattering behavior of flight-quality X-ray mirrors having various Al substrates. When both the specular and the scattered radiation are integrated, near theoretical reflectivities are found for all mirrors. Results of scattering studies show that scattering is strongly correlated with the Al foil type. Mirrors based on new 0.4 mm Al foil are found to have a typical scattering FWHM of about 1.1 arcmin, whereas mirrors based on 0.3 mm Al foil have an FWHM of greater than 1.5 arcmin. For all mirrors and for all energies, the scattering is found to exhibit the characteristic asymmetries predicted by a first order vector scattering theory.

X-ray imaging with the XSPECT/SODART microstrip proportional counters

The Danish Space Research Institute will provide a set of four imaging microstrip proportional counters (MSPC) as part of XSPECT, the Danish contribution to the SODART telescopes. A high- and a low-energy detector (HEPC and LEPC) will be provided for each of the SODART telescopes. The design is fixed and a prototype of LEPC is under construction. The present paper describes the imaging properties of this detector. The inherent position resolution is < 0.2 mm around 6 keV, and positional nonlinearities are < 0.1 mm. The gas gain is independent of position to better than 0.5%. Finally, the point spread function, important for studying weak objects in the vicinity of strong ones, has been measured.

Microstrip proportional counters for SODART telescope on Spectrum Rontgen Gamma

Four position sensitive proportional counters will be delivered by the Danish Space Research Institute as focal plane instruments for the Soviet Danish Roentgen Telescope SODART. These detectors will incorporate the novel microstrip electrode design. This design has been tested with Xenon gas and an Fe-55 X-ray source. The energy resolution is better than 14 percent FWHM up to gas gains of 10,000, the rise time of pulses from X-ray events is fast, 100 ns, allowing for efficient background rejection by rise time analysis. A position resolution of 1 mm FWHM is easily obtained by resistive charge division of the signals from the cathodes and from a wire grid placed in front of the microstrip plate. The detector performs without significant degradation of gain or energy resolution at fluxes exceeding 10 exp 6/s sq cm and a total dose of 10 exp 11 photons.

Radiation damage effects in CZT drift strip detectors

At DSRI, in collaboration with the cyclotron facility at Copenhagen University Hospital, we have performed a study of radiation effects exposing a 2.7 mm thick CZT drift strip detector to 30 MeV protons. The detector characteristics were evaluated after exposure to a number of dose loads in the range from 2*108 to 60*108 p+/cm2. Even for the highest dose loads, which had a dramatic effect on the spectroscopic performance, we were able to recover the detectors after an appropriate annealing procedure. The radiation damage was studied as function of depth inside the detector material. A numerical model that emulates the physical processes of the charge transport in the CZT detector was used to derive the charge trapping parameter , μτe (the product of charge mobility and trapping time) as function of dose. The analysis showed that the electron trapping increased proportional with the proton dose. The radiation contribution to the electron trapping was found to obey the following relation: (μτe)-1rad =(2.5±0.2)*10-7*Φ (V/cm2) with the proton fluence, Φ in p+/cm2. The trapping depth dependence, however, did not agree well the damage profile calculated using the standard Monte Carlo simulations, TRIM for the proton induced radiation effects. The present results suggest that proton induced nuclear reactions contribute significantly to the radiation damage. Further work will elaborate on these effects. The detector energy resolution was investigated as function of proton dose. It was found that the observed degradation is well explained by the decrease of μτe when the fluctuations of the electron path length are taken into account. The proton irradiation produced In meta stable isotopes in the CZT material. Their decay and production yield as function of depth were analyzed.

Dual-purpose camera for terrestrial x- and gamma-ray observations

This paper describes the x-ray camera for the Atmospheric X- ray Observatory (AXO) proposed for the Danish Small Satellite Program, which is under evaluation for the next mission in 2003. AXO is aimed at localizing the origin of the Terrestrial Gamma Flashes (TGF) that have been observed with BATSE. An additional objective is a detailed mapping of the auroral x-ray and optical emission. The x-ray camera to be used must be capable of detecting quite weak and pointlike, short-duration emission from TGF, and also to handle with the rather intense and extended radiation from auroral activity. The x-ray energy range is 5-200 keV and the angular resolution about 2 degrees. The requested satellite orbit is polar with an altitude of 500 km so that the phenomena can be seen from a close range. The design of a coded mask camera matching these requirements is discussed in terms of energy and angular resolution, sensitivity, count rates, and time resolution. Detailed simulations of the camera imaging capabilities are presented.

Preliminary background prediction for the INTEGRAL x-ray monitor JEM-X

The JEM-X (joint European x-ray monitor) experiment will be flown onboard the ESAs INTEGRAL satellite. The instrumental background level of the two JEM-X twin detectors will depend on several parameters, among which the satellite orbit and mass distribution, and the detectors materials play a major role. Based on the information available at the present stage of the emission design, we have computed the instrumental background to be expected because of two main background components: direct diffuse x-ray background and secondary photons originated by the interactions of the primary cosmic rays with the spacecraft structures. This calculation has been carried out by means of a Monte Carlo simulation using the code MCNP. The background due to on- orbit material activation and to the primary cosmic rays direct interactions with the detecting medium has not been considered. The INTEGRAL satellite structure is only now being completely defined and the details of the instrument design are still under evolution. The present background estimation can therefore be only preliminary and based on some reasonable approximations on the radiation environment in which the INTEGRAL experiments will operate.