Radiation detectors are devices that sense information about ionizing radiation. Although light and radio waves are technically forms of electromagnetic radiation, devices that detect them are not considered “radiation” detectors.
Though the term “radiation detectors” may bring to mind images of nuclear power plants, these devices have found homes in such fields as medicine, geology, physics, and biology. The term radiation refers to high-energy electromagnetic waves or particles given off by radioactive matter or other sources. Most commonly, radiation takes the form of alpha particles (fast helium nuclei), beta particles (fast electrons), neutrons, gamma rays, and x rays. Some of these are more easily detected than others, but all are invisible to the human eye. Since people cannot sense radiation, they need mechanical devices to observe and understand it.
People are always subjected to a certain amount of radiation because earth contains radioactive minerals and cosmic rays bombard earth from space. These omnipresent sources of radiation are called background radiation, and all radiation detectors have to cope with it, which they often do by shielding it out. Some detector applications subtract off the background signals, leaving only the signals of local radioactive sources.
In general, radiation detectors do not 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 signatures. 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 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.
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 that 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.
Electrical detectors wait for radiation to ionize part of the detector. Ionization occurs when incoming radiation separates a molecule or atom into a negative piece (one or more electrons) and a positive piece (i.e., the ion, the remaining molecule, or atom with a “plus” electrical charge). When a material has some of its atoms ionized, its electrical characteristics change and, with a clever design, a detecting device can sense this change.
Many radiation detectors employ an ionization chamber. Fundamentally, such a chamber is simply a container of gas that is subjected to a voltage. This voltage can be created by placing an electrically positive plate and an electrically negative plate within the chamber. When radiation encounters a molecule of gas and ionizes it, the resulting electron moves toward the positive plate and the positive ion moves toward the negative plate. If enough voltage has been applied to the gas, the ionized parts move very quickly. In their haste, they bump into and ionize other gas molecules. The radiation has set off a chain reaction that results in a large electrical signal, called a pulse, on the plates. This pulse can be measured and recorded as data. The principles of the ionization chamber form the basis for both the Geiger-Müller detector and the proportional detector, two of the most common and useful radiation-sensing devices.
A Geiger-Müller counter in its basic form is a cylinder with a wire running through the inside from top to bottom. It is usually filled with a noble gas, like neon. The outside of the metal cylinder is given a negative charge, while the wire is given a positive charge. In this geometry, the wire and the cylinder function as the two plates of an ionization chamber. When electrons are knocked from the gas by radiation, they move to the wire, which can then relay the electrical pulse to counting equipment. The voltage applied to a Geiger-Müller detector is quite high and each ionization creates a large chain reaction. In this way, it gives the same-sized pulse regardless of the radiation's original speed or energy.
One version of the Geiger-Müller detector, the Geiger counter, channels the electrical pulses to a crude speaker which then makes a popping noise each time it detects an event. This is the most familiar of radiation detectors, particularly in films that depict radioactivity. When the detector nears a radioactive source, it finds more events and gives off a correspondingly greater number of popping sounds. Even in a more normal setting, such as the average street corner, it will pop once every few seconds because of background radiation.
A proportional detector is very similar to the Geiger-Müller detector, but a lower voltage is applied to the ionization chamber, and this allows the detector to find radiation energies. More energetic radiation ionizes more of the gas than less energetic radiation does; the proportional detector can sense the difference, and the sizes of its pulses are directly related to the radiation energies. A large pulse corresponds to highly energetic radiation, while a small pulse likewise corresponds to more lethargic events. Since it can record more information, the proportional counter is more commonly found in scientific experiments than the Geiger-Müller detector, which, like the film detector, is primarily used for radiation safety.
Physicists who search for rare subatomic particles have utilized the principles of ionization chambers. They have developed many types of exotic detectors that combine ionization chambers with optical detection.