Pixel Detectors

Results of the characterisation of three different pixel detectors are presented. The first is an energy resolving detector (ERD1) which has been characterised using laboratory sources and the synchrotron radiation source (SRS) at Daresbury. The ERD1 is a 16 by 16 array of 300 mum by 300 mum square pixels, the detector is 300 mum thick Si and is bump-bonded using gold studs to the RAL PAC5 read out. Energy spectra and diffraction lines acquired at the SRS are presented and show the imaging and simultaneous spectroscopic capabilities of the ERD1. The energy resolution was investigated using X- rays of energies between 6 and 60 keV from laboratory sources. The achieved full-width-at-half-maximum (FWHM) of the photo peaks is in the range of 300 eV to 500 eV with an electronic noise of 227 +/-43 eV. Charge sharing was investigated for different energy X-rays and is shown to be significant with up to around 10% of events sharing some charge. The second detector is a large area detector (LAD1). It is based on single photon counting and is designed for imaging in synchrotron radiation applications. Results of tests performed with a single chip module at the Daresbury SRS are presented. The detector is 300 mum thick Si with 150 mum by 150 mum pixels bump-bonded to an (RAL ALADIN) array of 64 by 64 read out channels. The spatial resolution was determined using the modulation transfer function (MTF) with a result of (5.1 +/- 0.1) 1p/mm at an MTF value of 0.3. Theoretical studies of the spatial resolution predict a value of 5.3 1p/mm. The image noise in photon counting systems is investigated theoretically and experimentally and is shown to be given by Poisson statistics. The rate capability of the LAD1 was measured to be 250 kHz per pixel. Theoretical and experimental studies of the difference in contrast for ideal charge integrating and photon counting imaging systems were carried out. It is shown that the contrast differs and that for the conventional definition (contrast = (background - signal)/background) the photon counting device will, in some cases, always give a better contrast than the integrating system. Simulations in MEDICI are combined with analytical calculations to investigate charge collection efficiencies (CCE) in semiconductor detectors. Different pixel sizes and biasing conditions are considered. The results show charge sharing due to the limited mean free drift lengths of the charge carriers, the improvement of the CCE in unipolar detectors with decreasing pixel size and the "small pixel effect" which shows the improved CCE of the photo peak with smaller pixels. The third detector is a graphite pixel detector for ion beam profiling. The system was tested in the ion implanters at the University of Salford and Surrey. Results are presented showing real time profiling of the ion beam and the measurement of the beam current. The secondary electron emission was qualitatively measured for different beam energies and different positions on the detector. The loss of secondary electrons follows the profile of the detector and increases with higher energy ions