GRAIN-SIZE EFFECTS IN CVD DIAMOND DETECTORS

CVD diamond nuclear detectors have been investigated with 241Am alpha particles and with Sr-90 beta particles in various conditions of bias polarization and irradiation geometry. It is proved that the parameter indicated as collection distance in the literature is an average value defined over a broad distribution which should be related to the grain size distribution inside the detector and, likely, also to fluctuations of the electrical field inside the grains. The sampling techniques used to detect the pulses normally select the largest values of the collection distances, as underscored by results obtained from our measurements with beta particles by using these techniques; results have been found to be in good agreement with literature data. The collection length distribution has been found to be compatible with a superposition of two log-normal distributions, which are typical of grain dimension distributions in polycrystalline materials. It can be argued that grain size is a limiting factor for the collection distance and contributes with its fluctuations to the broadening of the pulses height spectrum of a minimum ionizing particle (MIP) in CVD diamond detectors. It can also be argued that sampling techniques suffer from pulse counting losses, which should be partly due to grain boundary effects.