Radiation Detectors

One type of optical detector is the film detector. This is the oldest, most simple type and one that closely resembles the analogy of tracks in snow. The film detector works much like everyday photographic film, which is sensitive to visible light. A film detector changes its appearance in spots where it encounters radiation. For instance, a film detector may be white in its pure form and subsequently turn black when hit by beta particles. Each beta particle which passes through the film will leave a black spot. Later, a person can count the spots (using a microscope), and the total number reveals the level of beta radiation for that environment.

Since film detectors are good at determining radiation levels, they are commonly used for radiation safety. People who work near radioactive materials can wear pieces of film appropriate for the type of radiation. By regularly examining the film, they can monitor their exposure to radiation and stay within safety guidelines. The science of determining how much radiation a person has absorbed is called dosimetry. Film detectors do have limitations. Someone studying the film cannot tell exactly when the radiation passed or how energetic it was.

An optical radiation detector more useful for experiments is a scintillation detector. These devices are all based on materials called scintillators, which give off bursts of light when bombarded by radiation. In principle, an observer can sit and watch a scintillator until it flashes. In practice, however, light bursts come in little packages called photons, and the human eye has a hard time detecting them individually. Most scintillator detectors make use of a photo multiplier, which turns visible light (i.e., optical photons) into measurable electrical signals. The signals can then be recorded by a computer. If the incoming radiation has a lot of energy, then the scintillator releases more light, and a larger signal is recorded. Hence, scintillation detectors can record both the energy of the radiation and the time it arrived.

Materials used in scintillation detectors include certain liquids, plastics, organic crystals, (such as anthracene), and inorganic crystals. Most scintillating materials show a preference for which type of radiation they will find. Sodium iodide is a commonly used inorganic crystal which is especially good at finding x rays and gamma rays. In recent years, sodium iodide has received increasing competition from barium fluoride, which is much better at determining the exact time of an event.