Localization of photon absorption with a long cylindrical scintillation detector: Dissertation

The subject of the study has been a long NaI(Tl) scintillation detector with a photomultiplier fitted to each end.The purpose was to define how precisely the absorption position and energy of a gamma quantum, which has hit it and been absorbed in it can be determined with this type of detector.The function P, which is proportional to the coordinate x of the absorption position, equals in (VA/V), where V and V are the voltages of the pulses obtained from the photomultipliers as the gamma quantum is being absorbed in some point in the detector.The function S (VA VB is almost constant regardless of the point of absorption.The precision of the measurement of the absorption position depends on: (a) the amount of light produced by the absorption of the gamma quantum (b) the light transmission coefficient of the scintillation crystal of the detector (c) the light reflection coefficient of the detector wall (d) the cathode sensitivity of the photomultipliers (e) the length and diameter of the detector (f) the homogeneity of the scintillation crystal and the optic homogeneity of the wall of the detector (g) the quality of the electronics in generating functions P and S .A good quality detector has been found to be one where S to 15 % of the light produced in one end is detected in the opposite end.The transmission coefficient of the scintillation substance must be the best possible and the same holds for the light reflection coefficient of the detector surfaces and the cathode sensitivity of the photomultipliers.The detector must also be of a homogeneous material and it optic characteristics must be homogeneous in all respects.The way in which the detector wall reflects light, and the detector length and diameter, essentially influence the way in which the light emitted in gamma absorption is distributed between the detector ends.With the best measured scintillation detector the absorption position has been determined by a precision of about 18 mm (FWHM).Two parallel long scintillation detectors with sufficient space for the target between them give the distribution of the positron annihilation quanta arriving from the target.This type of apparatus can then be used for positron scanning.Some characteristics of a constructed hybrid positron scanner have been determined and the device has been used to perform measurements with a phantom; the distribution of llC-ethanol in man has also been measured.