Radiation Detectors

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 which 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 which 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 which combine ionization chambers with optical detection.