Effect of Shock Strength On Oblique Shock-Wave/vortex Interaction

An experimental study of the interaction between streamwise vortices and two-dimensional oblique shock waves has been conducted at Mach 2.5. The experiments involved positioning an instrumented two-dimensional wedge downstream of a semispan wing so that the trailing tip vortex from the wing interacted with the oblique shock wave formed over the wedge surface. The experiments were designed to simulate interaction of streamwise vortices with shock waves formed over aerodynamic surfaces or in supersonic inlets. The influence of oblique shock wave intensity on this inherently three-dimensional interaction was examined for vortices of variable strength. Results indicate that the interaction of a moderate strength vortex with an oblique shock wave can lead to the formation of a steady separated shock structure upstream of the oblique shock front. A significant expansion of the vortex core is observed in these cases, and the scale of the structure increases with shock wave intensity. In some instances the separated shock structure continues through the oblique shock front to strike the shock-generating wedge forming a three-dimensional shock-wave/boundary-layer interaction. The experiments indicate that significant distortion of streamwise vortices can be precipitated by oblique shock fronts with supersonic downstream conditions

Optimization of array design for TIBr imaging detectors.

Thallium bromide (TlBr) has attracted attention as an exceptional radiation detector material. Due to its high atomic number (81 for Tl, 35 for Br) it has excellent stopping power for hard X-ray and gamma rays and due to its high bandgap (2.7 eV) its operation requires no or only modest cooling. Promising energy resolutions have been demonstrated with detectors fabricated from high-purity samples (3.3 keV for 60 keV photons). These properties make TlBr the material of choice for hard X-ray imaging spectrometers in applications where small weight and/or size is important (e.g. space astrophysics and nuclear medicine). The charge response and spectroscopic performance of a semiconductor imaging array depend not only on material properties but on the pixel properties as well. It has been demonstrated, for instance, that the ratio between pixel size and thickness of the detector is an important factor for the charge response. This is known as the small-pixel or near-field effect. In this paper we investigate the optimization of TlBr pixel properties in a broader context, taking into account material properties (electron and hole mobility, diffusion and trapping), fabrication details and the specific energy range of application, with a view to optimizing both the response and energy resolution

The X-ray response of TlBr.

We present the results of a series of X-ray measurements on several prototype TlBr detectors. The devices were fabricated from mono-crystalline material and were typically of size 2.7×2.7×0.8 mm3. The material is extremely pure, having impurity concentrations <100 ppm. The measured electron and hole mobility–lifetime products were found to be 3×10−4 and 1×10−5 cm2 V−1, respectively, which are about an order of magnitude higher than previously reported values. Three detectors were fabricated and extensively tested over the energy range 2.3–100 keV at three synchrotron radiation facilities: the Physikalisch-Technische Bundesanstalt (PTB) laboratory at the Berliner Elektronenspeicherring für Synchrotronstrahlung (BESSY II), the European Synchrotron Radiation Research Facility (ESRF) and the Hamburger Synchrotron-strahlungslabor (HASYLAB) radiation facility. Room temperature energy resolutions under full-area illumination of 1.8 and 3.3 keV FWHM have been achieved at 5.9 and 59.95 keV, respectively. At reduced detector temperatures of −30°C, these fall to 800 eV and 2.6 keV FWHM, respectively. Under monochromatic pencil beam illumination, the measured energy resolutions at 6 and 60 keV were 664 eV and 3 keV FWHM at the same temperature. For energies <20 keV, the measured spectra display symmetric photopeaks. However, the peaks become increasingly tailed at higher energies. At the highest energies, the energy-losses due to the electrons and holes are clearly separated. Whilst the detectors gave reproducible results over 12 months of operation, it was observed that for synchrotron beam measurements above 45 keV, they were unstable, showing rate dependent gain shifts and polarization effects. These were not observed at lower energies. The spatial uniformity of the detectors was measured using a 50×50 μm2, 12 keV mono-energetic X-ray beam, raster scanned over the forward active area. Whilst two detectors were spatially uniform to a level commensurate with statistics, the third was not. In all cases, evidence was found for charge collection problems caused by field fringing