Radiation Detectors

Radiation detectors can generally be divided into three broad categories: proportional (gas) chambers, semiconductor detectors, and scintillation detectors.

The proportional chamber works on the principle of detecting the ionisation produced by radiation as it passes through a gas chamber. A high electric field is applied within this chamber that results in an acceleration of the ionisation electrons produced by the radiation. Subsequently, these highly energetic electrons collide with the neutral gas atoms resulting in secondary ionisations. Hence, a cascade of electrons is eventually collected at the cathode after some energy deposition by the incident radiation. Typically, inert gases such as xenon are used for detecting photons. The cathode normally consists of a single thin wire, but a fine grid of wires can be utilized to measure energy deposition as a function of position within the detector. Such position-sensitive Multi-wire Proportional Chambers (MWPC) have been used in high-energy physics for a long time, and PET scanners have been developed based upon such a detector [5, 6]. However, the disadvantage of these detectors for use in PET is the low density of the gas, leading to a reduced stopping efficiency for 511 keV photons, as well as poor energy resolution.

Another class of radiation detectors is the semiconductor or solid-state detectors. In these detectors, incident radiation causes excitation of tightly bound (valence band) electrons such that they are free to migrate within the crystal (conduction band). An applied electric field will then result in a flow of charge through the detector after the initial energy deposition by the photons. Semiconductor detectors have excellent energy resolution but because of their production process, the stopping efficiency for 511 keV photons is low.

The third category of radiation detectors, which are of most interest to us, are the scintillation detectors. These detectors consist of an inorganic crystal (scintillator) which emits visible (scintillation) light photons after the interaction of photons within the detector. A photo-detector is used to detect and measure the number of scintillation photons emitted by an interaction. The number of scintillation photons (or intensity of light) is generally proportional to the energy deposited within the crystal. Due to their high atomic numbers and therefore density, scintillation detectors provide the highest stopping efficiency for 511 keV photons. The energy resolution, though much better than the proportional chambers, is not as good as that attained with the semiconductor detectors. This is due to the inefficient process of converting deposited energy into scintillation photons, as well as the subsequent detection by the photo-detectors. However, for PET, where both high stopping efficiency as well as good energy resolution are desired, scintillation detectors are most commonly used. For a more thorough treatment of radiation detection and measurement the reader is referred to Knoll (1988) [7].

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