3.5.5.3. Scintillation Detector

Scintillation detectors (Figure 3.5.5.3) consist of a phosphor-coated screen and a photomultiplier tube (PM tube). X-rays (or electrons) incident upon the phosphor produce photons in the ultraviolet part of the spectrum, which are amplified by the PM tube. Since solids are denser than gas, the active volume of phosphor is relatively small and the production of photons from the phosphor is only about 20% efficient. The PM tube is only about 5% efficient and the combination results in total losses of about 100%, which means that although only about 3 eV are need to produce a photon from the phosphor, it really needs to be about 300 eV for detection.

Figure 3.5.5.3. Schematic diagram of a NaI(Tl) scintillation detector and photomultiplier assembly (after Potts 1987).

The deadtime for a scintillation detector is about 40% that of a gas-flow detector, but the associated counting circuitry raises this to about 75%, so there is no real difference. Like gas-flow detectors, scintillation detectors produce proportional counts and are best used with monochromatic radiation because of their poor energy resolution. Resolution of these detectors is about three times poorer than for gas-flow detectors.