103 Scintillation Counters

The name of this detector comes from the fact that the interaction of a particle with some materials gives rise to a scintillation or flash of light. The basic phenomenon is familiar-many substances can be stimulated to glow visibly on exposure to ultraviolet light, and the images on a color television screen are the result of electron bombardment. Molecules of materials classed as phosphors are excited by radiation such as charged particles and subsequently emit pulses of light. The substances used in the scintillation detector are inorganic, e.g., sodium iodide or lithium iodide, or organic, in one of various forms -crystalline, plastic, liquid, or gas.

The amount of light released when a particle strikes a phosphor is often proportional to the energy deposited, and thus makes the detector especially useful for the determination of particle energies. Since charged particles have a short range, most of their energy appears in the substance. Gamma rays also give rise to an energy deposition through electron recoil in both the photoelectric effect and Compton scattering, and through the pair production-annihilation process. A schematic diagram of a detector system is shown in Fig. 10.3. Some of the light released in the phosphor is collected in the photomultiplier tube, which consists of a set of electrodes with photosensitive surfaces. When a photon strikes the surface, an electron is emitted by the photoelectric effect, it is accelerated to the next surface where it dislodges more electrons, and so on, and a multiplication of current is achieved. An amplifier then increases the electrical signal to a level convenient for counting or recording.

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Radiation workers are required to wear personal detectors called dosimeters in order to determine the amount of exposure to X- or gamma rays or neutrons. Among the most reliable and accurate types is the thermoluminescent dosimeter (TLD), which measures the energy of radiation absorbed. It contains crystalline materials such as CaF2 or LiF which store energy in excited states of the lattice called traps. When the substance is heated, it releases light in a typical glow curve as shown in Fig. 10.4. The dosimeter consists of a small vacuum tube with a coated cylinder that can be heated by a built-in filament when the tube is plugged into a voltage supply. A photomultiplier reads the peak of the glow curve and gives values of the accumulated energy absorbed, i.e., the dose. The device is linear in its response over a very wide range of exposures; it can be used over and over with little change in behavior.

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