Optimization of measurements by the HPGe detector with a very low background for achievement of the low MDA values

Abstract My thesis examines the measurement optimalization when the semiconductor HPGe detectors with a low background and a high relative efficiency are used for achieving low values of MDA (minimum detectable activity). I conducted my research in the Department of Spectrometry of SÚRO (National Radiation Protection Institute) in Prague. In this laboratory the standard geometries of detectors with lower efficiency are used. The aim of this thesis is to offer a proposal of new geometry, sizes and shapes of measurement cells which will better correspond with the demand for higher efficiency of updated detectors. The assignment was to find the best mathematical function to descibe the correlation between efficiency and the direction of the radiation point source towards the detector. This function is derived from experimentally obtained data. To conduct measurements, three kinds of detectors with high relative efficiency used for activity measurements at different geometries were selected. As the source of gamma radiation, the point sources with energies of 46 keV, 661 keV, 1332 keV in different distances and positions were used. The function was applied separatelly to each of the three detectors and to the areas above the detector and sidewards of the detector. The objective of the research was to project new measurement geometries or to optimalize the current ones which could be used also in other detectors with high efficiency. After integrating the function in the cylindrical area above the detector and the annular area sidewards of the detector, areas contributing substantially to the overall efficiency were determined. These areas may be considered to be the optimal sizes for new geometries in given positions. After combining all the optimal areas above the detector, sidewards of the detector and for selected energies and the detector, optimal sizes of a new geometry were found. The function can be used both in the front part and the side part of the detector but its validity is limited. The value of MDA for the particular geometry is in indirect proportion to the efficiency square root and thus this new geometry should lead to minimum values of MDA