Master of Science (Honours)
Department of Engineering Physics - Faculty of Engineering
Hooper, Peter R, Development of a new silicon based detector module for PET, MSc HONS thesis, Department of Engineering Physics, University of Wollongong, 2003. http://ro.uow.edu.au/theses/362
In the quest to develop high spatial resolution Positron Emission Tomography (PET), the goal of this work is to characterise the innovative silicon detector array that forms the foundation of a new PET detector module design. The new detector module circumvents the current requirement for photomultiplier tubes by the implementation of its solidstate detector analogues. The detector module is based upon pixelated scintillation crystals optically coupled to custom designed silicon 8x8 photodiode arrays and read out by a 128 channel low noise charge sensitive preamp. Each element pad of the array has an active area of 3x3 mm2 and, for the work described in this thesis, is coupled via a fan-out connection system to the testing instrumentation. The testing instrumentation included I(V), optical excitation and nuclear spectroscopy (alpha particle and gamma ray spectroscopy). This work fits in as part of a larger project to provide proof of principle of this new PET detector module design. The dark current of individual pixel elements of an unbonded array was found to be 460 pA at an operating bias voltage of 40 V. To investigate uniformity of response across the array, three different methods have been used. Spectroscopy response on the photopeak of 137Cs in the case of optical coupling of each pixel with 3x3x3 mm3 CsI(Tl) showed uniformity of collected charge of ±28%. Investigation of direct response of pixels across the array with 59.9 keV X-rays from 241Am source demonstrated excellent response. To investigate the variation in uniformity of the detector pixel, the optical response of the array was measured using a laser diode (spot size of ~50 μm) and was found to be ±12%. The uniformity of a single pixel element response using scanning optical beam was measured to be ±1%. Initial characterisation of 3x3 mm2 diode taken from the same wafer used to create the array was optically coupled to a 4x4x10 mm3 LSO scintillator and then to a 3x3x3 mm3 CsI(Tl) scintillator and irradiated by 511 keV and 1.2 MeV gammas from 22Na and 662 keV gammas from 137Cs sources. The energy resolution for the LSO was shown to be 28.8% FWHM for 511 keV and 16.5% for the 662 keV gammas. And the energy resolution for the CsI(Tl) scintillator crystal was 9.5% FWHM for the 511 keV gammas and 7.7% FWHM for the 662 keV gammas. The time properties of this array were tested with coincidence techniques using current pulses from the front and back – side of the array. For modelling of charge induced in a pixel from LSO photons, in the case of 511 keV photopeak, we used low energy alpha particles generating the same amount of charge. The FWHM of the time spectra was found to be about 700 ps resulting mostly from noise and statistics of charge collection. This specially developed Si pixel array photodiode was demonstrated to satisfy the critical technical parameters required for high resolution PET scanners. Further work has been recommended for stabilisation the sensitive area of pixel to avoid increasing reverse current due to optical coupling. A special optical adhesive has been proposed to avoid this affect.
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