ABSTRACT
Detectors detect radiation by recording energy deposition into their active components. For most detectors this energy deposition is in the form of ionization produced in the detection medium (which may be solid, liquid, or gas) by charged particles. The choice of which detection medium to use depends to a large extent on what the detector will be used for. For example, in a tracking detector one wishes to detect the presence of a particle without affecting its trajectory, so the medium will be chosen to minimize energy loss and particle scattering (thus, low density). Conversely, if one wishes to measure the total energy deposition by calorimetry or spectroscopy, the absorber will be chosen to optimize energy loss, for example, by high density or high atomic number. Energy is then converted into an electrical signal, either directly or indirectly. In direct energy conversion, the incident radiation produces charge in the detector which is directly proportional to the energy absorbed and is collected by an electrode system. For example, in a gas counter the radiation ionizes the atoms/molecules of the gas, and the resulting charge is collected by electrodes. Similarly, in a semiconductor detector, the ionization produced by the radiation will create electron-hole pairs, which are swept toward the electrodes by an electric field. In indirect conversion, incident radiation excites atomic or molecular states that decay by the emission of light, as in the case of scintillation detectors. This light is then converted into an electrical signal using a photosensitive sensor, such as a photomultiplier tube.
