What Is a Scintillation Counter?

A scintillation counter is a device used to detect and measure emissions from radioactive elements. Radioactivity is a release of particles or energy from certain elements that contain too many neutrons, and can be hazardous to humans, animals and plants. The scintillation counter combines a chemical that creates light when struck by radioactive emissions, and a detector to sense and count the light pulses.

Many elements have isotopes, molecules that contain different numbers of neutrons with the same number of protons and electrons. Most isotopes are stable, and nothing will happen to change their chemical makeup over time. A number of radioactive isotopes, however, will not hold the neutrons in place and begin to radioactively decay.

There are three main types of radioactive decay, and each has different characteristics. Alpha radiation is a particle combining protons and neutrons and has a relatively low energy, allowing it to be stopped by water or thin metal plates. Beta radiation is high-energy electrons released from the element, and can penetrate body tissues and layers of protective shielding. Gamma radiation is not a particle but rather an electromagnetic wave, similar to light, which has a very high energy and can only be shielded by layers of dense lead plate.

All three types cause cellular damage to plants and animals because they cause molecules to change when struck by radiation. As a radioactive particle or gamma radiation strikes a molecule, it will release electrons into surrounding tissues or the air. If the radiation strikes a chemical that gives a flash of light when struck, and the light can be detected, a scintillation counter has been created.

There are three types of solid scintillation chemicals, called phosphors, used in counters, and include inorganic, organic and plastics. Inorganic chemicals that can release light, called photons, when struck by radiation include metal iodides and zinc sulfide. Organic phosphors can include naphthalene, anthracene, and other benzene-related compounds. Plastics by themselves are not typically phosphors, but chemicals can be combined with a plastic to form a photon generator.

Inorganic chemicals are the best detectors for gamma radiation, organics are optimum for beta particles, and plastic-embedded phosphors work well for neutron detection. Radioactive isotopes may decay using a variety of methods, so detectors can contain more than one type of detection element. The counting software used in detectors is critical for determining the amount of radiation, because higher counts indicate more radioactive element is present or the counter is near the radioactivity.

Once photons of light are created, the other important part is the detector, which both sees the photons and counts them. Many counters use a photomultiplier, which is a series of electrodes mounted in a vacuum tube. As a photon of light enters the tube, it is normally too faint to be detected by the electronic circuits in the scintillation counter. The photon strikes the first electrode, which has an electrical voltage applied to it.

When struck by the light, the electrode releases more electrons, which travel to the second electrode. Each time this occurs, more electrons are released, and the signal gets stronger. After several steps, which occur very rapidly with electrons traveling at the speed of light, the signal is strong enough for the counter to detect it, and it registers the presence of a photon of light and counts it. A photomultiplier is extremely sensitive, and can accurately detect very small flashes of light from decay.

Another type of scintillation counter is a liquid-phase unit. These counters may be useful in laboratory analysis, because a sample is placed directly in a liquid composed of a phosphor and a solvent. Any radioactive emission is detected immediately by the phosphors surrounding the sample, which are then counted.

This technology can be useful when decontaminating a radioactive spill, because wipe tests can be used to check for radioactivity. Small cloth samples are wiped over surfaces, and then placed in a liquid scintillation counter. This process can be repeated as necessary until the counter shows radioactivity to be at low levels, called background radiation.