We have tested the high-energy imaging capabilities of detectors made of a Medipix1 pixel photon-counting chip hybridized with a 300 mm thick Si or a 200 mm thick GaAs substrate. The results presented in this paper show that an important part of the signal is due to secondary particles coming from g-ray diffusion in the detector surroundings. According to simulation studies realized with the Monte-Carlo code PENELOPE, the contribution of a 2 mm thick Pyrex screen placed between the source ( 60 Co) and the Si detector can reach 75% of the total number of counts. The main reason for this high relative sensitivity of the substrate to the secondary particles is its very poor detection efficiency at high g-ray energies. We also present results for imaging with a double-cone pinhole collimator, through comprehensive comparisons between experiment and simulation.
The first prototypes of a Medipix2 chip hybridized with a 1 mm thick pixel CdTe substrate have been carried out and we present experimental tests done with g-ray sources at high energies (up to 1.33 MeV). We also present results obtained with a dedicated pseudo-Monte-Carlo 3D simulation code. This simulation code, whose first stage is the single-event energy-deposition distribution, processes the charge-carrier transport taking into account trapping and diffusion and, at the same time, registers the induced signal on pixels. The first confrontations with experimental measurements are shown.