Radiation Detectors

Radiation detectors are devices which sense and relay information about incoming radiation. Though the name brings to mind images of nuclear power plants and science fiction films, radiation detectors have found homes in such fields as medicine, geology, physics, and biology. The term radiation refers to energies or particles given off by radioactive matter. Mostly, radiation takes the form of alpha particles, beta particles, gamma rays, and x rays. Some of these are more easily detected than others, but all are incredibly tiny and invisible to the human eye. This is why scientists originally started building radiation detectors. Since people cannot sense radiation, they need assistance to observe and understand it.

It is important to note that people are always subjected to a certain amount of radiation because the earth contains radioactive minerals and cosmic rays bombard the earth from space. These omnipresent sources are called background radiation, and all radiation detectors have to cope with it. Some detector applications subtract off the background signals, leaving only the signals of local radioactive sources.

In general, radiation detectors do not capture radiated particles. In fact, they usually do not even witness the radiation itself. The detectors look for footprints that it leaves behind. Each type of radiation leaves specific clues; physicists often refer to these clues as a signature. The goal in detector design is to create an environment in which the signature may be clearly written.

For example, if someone wants to study nocturnal animals, it might be wise to consider the ground covering. Looking at a layer of pine needles by day, one finds few, if any, tracks or markings. However, one can choose to study a region of soft soil and find many more animal prints. The best choice yet is fresh snow. In this case, one can clearly see the tracks of every animal that moved during the night. Moreover, the behavior of an animal can be documented. Where the little prints of a fox are deep and far apart, it was probably running, and where A handheld Geiger counter. Photograph by Hank Morgan. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. its prints are more shallow and more closely spaced, it was probably walking. Designing a radiation detector presents a similar situation. Radiation can leave its mark clearly, but only in special circumstances.

Clues are created when radiation passes too close to, (or even collides with), another object—commonly, an atom. What detectors eventually find is the atom's reaction to such an encounter. Scientists often refer to a single encounter between radiation and the detector as an event. Given a material which is sensitive to radiation, there are two main ways to tell that radiation has passed through it: optical signals, in which the material reacts in a visible way; and electrical signals, in which it reacts with a small, but measurable voltage.