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Semiconductor pixel detectors were originally developed for particle physics experiments as a logical development from silicon microstrip detectors. offering the potential for high spatial precision, two dimensional location of ionising charged particle trajectories. The development became practical, as with microstrip detectors. only with the availability of suitable VLSI read-out electronics and reliable (and affordable) interconnect technology, ("flip-chip" bonding). Pixel detectors have also been studied more recently as imaging devices, particularly for X-rays in medical and non-destructive testing applications, and in synchrotron radiation beams. In the following, a description is given of the evolution and current state of development of pixel detectors for all of these applications. Reference is made to both monolith ic and hyb...
New techniques of transient microwave absorption and their application for the extraction of recombination parameters of SI GaAs with different doping concentrations and layered n(+)- epi-GaAs structures are presented. Experimentally obtained decay kinetic shape and lifetime variations indicate multi- centre recombination and trapping effects. In highly doped material the extracted absolute carrier lifetime values of 10- 50 ns in the substrate and 300-500 ns in the epi-layer allow the evaluation of the higher quality of the epi-layer. A depth scan of the excess carrier decay in the epi-layer revealed that carrier lifetime Values of 1-3 mus measured in the undoped material are nearly homogeneous in the epi-layer, while they decrease towards the epi-substrate boundary.
A discussion is given of two types of ionizing particle detectors, which are based on radiation-generated charge carrier capture into quantum wells of semiconductors. The principle of operation of the two detector types depends on the strength of the coupling between the quantum wells. The first, isolated quantum well detector, is based on fast electron-hole pair recombination into 2D excitons and their decay into light quanta. The second, coupled quantum well detector, is based on electron and hole hopping between quantum wells. The advantages and limitations of the detectors are discussed.
We present room temperature measurements of inhomogeneities and instabilities in a GaAs pixel detector array at high bias voltage. The evolution of the inhomogeneity structure and its change due to increase of bias voltage was investigated. The dark current image was analyzed. It is proposed that a cellular structure of higher and lower conductivity regions is responsible for the inhomogeneities and their spatial and temporal instability. The electrical inhomogeneity in LEC SI- GaAs and its dependence on electric field was qualitatively investigated by using a hybridized GaAs pixel detector and a model of the influence of crystal inhomogeneity on the measured parameters of the detector array is proposed.
We used a pixellated hybrid silicon X-ray detector to study the effect of the sharing of generated charge between neighbouring pixels over a range of incident X-ray energies, 13-36keV. The system is a room temperature, energy resolving detector with a Gaussian FWHM of 265 eV at 5.9 keV. Each pixel is 300 mum square, 300 mum deep and is bump bonded to matching read out electronics. The modelling packages MEDICI and MCNP were used to model the complete X-ray interaction and the subsequent charge transport. Using this software a model is developed which reproduces well the experimental results. The simulations are then altered to explore smaller pixel sizes and different X-ray energies. Charge sharing was observed experimentally to be 2% at 13 keV rising to 4.5% at 36 keV, for an energy threshold of 4 keV. The models predict that up to 50...
Detectors fabricated with SI-GaAs and Si bulk material were bonded to Photon Counting Chips (PCC), developed in the framework of the MEDIPIX Collaboration. The PCC consists of a matrix of 64 x 64 identical square pixels (170 mum x 170 mum) with a 15-bit counter in each cell. We investigated the imaging properties of these detector systems under exposure of a dental X-ray tube at room temperature. The image homogeneity and the mean count rate were determined via hood exposure images and compared. Exposures for GaAs detectors exhibit a 3 times larger spread in count rate per image in comparison to Si detectors. This also results in a 3 times worse signal to noise ratio. IV- characteristics and X-ray images at different values of the detectors bias voltage were also taken and show a 30 times higher leakage current for GaAs, The Si detecto...
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