Scintillators and semiconductor detectors

Various processes occur during the detection of ionizing radiation within a scintillator, and proper detection designs are needed [1, 2, 3]. As a consequence of the interaction of radiation with the scintillation material, ionisation and excitation processes arise, and the energy (or part of it) of the incoming radiation is transferred to the atoms and molecules of the scintillator. Following deexcitation processes, photons originate in the ultraviolet/visible (UV/VIS) region of the electromagnetic spectrum, light that must be collected and converted in a suitable electric signal. In many cases, light collection simply may be obtained by coupling the scintillator directly with an optical detector, typically a photomultiplier tube (PMT). In other cases, depending on the particular application or measurement geometry, a light guide is required, which efficiently transmits the light emitted by the scintillator to the optical device. Finally, light photons are converted into electrons, and the resulting basic electric signal is amplified and properly processed. Let us consider in more detail the scintillation conversion mechanism in a wide band-gap material. This process may be explained by considering the energy band structure of an activated crystalline scintillator. An inorganic scintillator is indeed usually a crystalline solid containing a small amount of dopant, acting as a luminescent centre, which creates energy levels within the forbidden band between the valence band and the conduction band. Moreover, the natural impurities and defects present in the crystal are the origination of other energy levels, which may act as traps during the charge transport.