H.J. Besch

Study and application of hole structures as gas gain devices for two dimensional high rate X-ray detectors

Abstract Recent developments in hole structures seem to be a promising enrichment for gas-filled photon counters. Up to now, only few investigations of the performance and limitations of these structures have been carried out. In this work a micro-hole structure is presented together with investigations focused on applications with position sensitive pressurized X-ray detectors for synchrotron radiation. In particular, position resolution, gas gain, rate capability, drift field influence and pressure behavior have been studied. In addition, first two dimensional X-ray images have been measured using a gas amplifying micro-hole structure in combination with a resistive position encoding structure. Since different types of hole structures were tested, predictions of structure geometries adapted for specific applications were carried out.

GAS AMPLIFYING HOLE STRUCTURES WITH RESISTIVE POSITION ENCODING : A NEW CONCEPT FOR A HIGH RATE IMAGING PIXEL DETECTOR

Abstract Promising recent developments in gas amplifying hole structures (e.g. CAT, MICROMEGAS, GEM) were accompanied by a lack of appropriate readout structures. Here, a new hole structure is presented which is combined, for the first time, with a truely two-dimensional resistive position encoding device. The investigations with this prototype detector are focused on applications with high rate X-ray sources, including general studies of gas gain, energy resolution and rate capability. In addition, latest results of the imaging performance (uniformity, spatial resolution, etc.) will be reported.

Gas gain and signal length measurements with a triple-GEM at different pressures of Ar-, Kr- and Xe-based gas mixtures☆

Abstract We investigate the gas gain behaviour of a triple-GEM configuration in gas mixtures of argon, krypton and xenon with 10% and 30% of carbon dioxide at pressures between 1 and 3 bar . Since the signal widths affect the dead time behaviour of the detector we present signal length measurements to evaluate the use of the triple-GEM in time-resolved X-ray imaging.

Silicon drift detector readout electronics for a Compton camera

A prototype detector for Compton camera imaging is under development. A monolithic array of 19 channel silicon drift detector with on-chip electronics is going to be used as a scatter detector for the prototype system. Custom designed analog and digital readout electronics for this detector was first tested by using a single cell Silicon drift detector. This paper describes the readout architecture and presents the results of the measurement.

Development of a two-dimensional virtual-pixel X-ray imaging detector for time-resolved structure research.

An interpolating two-dimensional X-ray imaging detector based on a single-photon counter with gas amplification by GEM (gas electron multiplier) structures is presented. The detector system can be used for time-resolved structure research down to the micro s time domain. The prototype detector has been tested at the SAXS (small-angle X-ray scattering) beamline at ELETTRA synchrotron light source with a beam energy of 8 keV. The imaging performance is examined with apertures and standard diffraction targets. Finally, the application in a time-resolved lipid temperature-jump experiment is presented.

On the dynamic two-dimensional charge diffusion of the interpolating readout structure employed in the MicroCAT detector

An essential part of the MicroCAT detector is its interpolating readout concept. A theoretical description of the charge diffusion process on the two-dimensional resistive readout structure and its influence on the position reconstruction is carried out. The corresponding inhomogeneous, time-dependent diffusion equation, with the geometric boundary conditions of the readout cells taken properly into account, is solved numerically. Signals realistically distributed in space and time are used as input to simulate the detector response. The time development of two-dimensional charge distributions is investigated. Charge losses to neighbouring cells and distortions due to the linear four-node-encoding algorithm used for reconstructing the impact position of a photon are considered. Since the thermal noise of the resistive layer leads to a limited position accuracy, the spatial resolution is calculated as a function of the integration time, the event position and the gas gain for different resistivities of the readout structure. The time dependence of the simulated signals allows an estimation of the expected rate capability of a given readout structure. In order to verify the theoretical predictions a measurement using an injection of charge pulses in known positions was performed.

On the possibility of utilizing scattering-based contrast agents in combination with diffraction-enhanced imaging

Preliminary experiments have been carried out in order to evaluate the potential of the Diffraction Enhanced Imaging (DEI) technique in combination with contrast agents not based on X-ray absorption properties, but that provide strong scattering signals. The contrast agents tested in this study are microbubble echo-enhancing agents, usually used in ultrasound examinations, which are completely invisible with conventional X-ray absorption techniques. A DEI set-up has been implemented at the Medical Beamline at the synchrotron radiation facility ELETTRA (Trieste, Italy). The analyzer crystal is a single flat silicon crystal utilized in the [111] reflection. By shifting the analyzer crystal to different positions of the rocking curve it is possible to detect the scattered photons; in particular, if the sample consists of a large number of particles with size smaller than the pixel size of the detector, an overall effect can be visualized. Phantoms containing ultrasound contrast agents have been built and imaged at different angular positions of the analyzer crystal at 17 keV and 25 keV. For all the phantoms a much stronger contrast has been measured in comparison to the contrast evaluated from the images produced with normal absorption methods.

Gas gain operations with single photon resolution using an integrating ionization chamber in small-angle X-ray scattering experiments ☆

In this work a combination of an ionization chamber with one-dimensional spatial resolution and a MicroCAT structure will be presented. Initially, MicroCAT was thought of as a shielding grid (Frisch-grid) but later was used as an active electron amplification device that enables single X-ray photon resolution measurements at low fluxes even with integrating readout electronics. Moreover, the adjustable gas gain that continuously covers the entire range from pure ionization chamber mode up to high gas gains (30 000 and more) provides stable operation yielding a huge dynamic range of about 108 and more. First measurements on biological samples using small angle X-ray scattering techniques with synchrotron radiation will be presented.

On image reconstruction with the two-dimensional interpolating resistive readout structure of the Virtual-Pixel detector ☆

The two-dimensional interpolating readout concept of the Virtual-Pixel detector (ViP detector) goes along with an enormous reduction of electronic channels compared to pure pixel devices. However, the special concept of the readout structure demands for adequate position reconstruction methods. Theoretical considerations reflecting the choice and the combination of linear algorithms with emphasis on correct position reconstruction and optimised spatial resolution are presented. A subsequent two-dimensional coordinate transformation further improves the image response of the system. Measured images show how far the theoretical predictions of the simulations can be verified and to what extent they can be used to improve the position reconstruction.

Investigation of the performance of an optimised MicroCAT, a GEM and their combination by simulations and current measurements ☆

A Micro compteur a Trous (MicroCAT) structure which is used for avalanche charge multiplication in gas filled radiation detectors has been optimised with respect to maximum electron transparency and minimum ion feedback. We report on the charge transfer behaviour and the achievable gas gain of this device. A three-dimensional electron and ion transfer simulation is compared to results derived from electric current measurements. Similarly, we present studies of the charge transfer behaviour of a Gas Electron Multiplier (GEM) by current measurements and simulations. Finally, we investigate the combination of the MicroCAT and the GEM by measurements with respect to the performance at different voltage settings, gas mixtures and gas pressures.