Radiation: Key Terms and Definitions

Activity

The activity of a piece of radioactive material (called a ‘source’) is a measure of how many radioactive decays taking place per second. If you compare two uranium sources for instance, the one with the higher activity would be emitting more energy. There are three kinds of radioactive decays: alpha, beta and gamma.

Alpha Radiation

An alpha decay is a nuclear process by which a helium nucleus is emitted. The nucleus of an atom is simply the atom without its usual complement of electrons. The nucleus of a helium atom consists of two protons and two neutrons. Therefore the alpha particle is relatively heavy compared to an electron (about 2000 times as heavy), and highly electrically charged (+2 charge because of two protons). Since they are relatively heavy, they tend to move relatively slowly. Due to their properties, they do not penetrate far into materials that they hit, but they do deliver a relatively large amount of energy to the target material per decay compared to the other forms of radiation.

Beta Radiation

A beta decay is a nuclear process by which an electron is emitted. Electrons are relatively light and highly charged. Due to these properties, they can travel very quickly, but interact strongly upon collision with matter. They can penetrate a relatively small distance into most matter. However they do penetrate further than alpha particles. On a per-particle basis, they carry less energy than alpha particles.

Gamma Radiation

A gamma ray is an electromagnetic wave. Visible light is another type of electromagnetic wave. These waves can penetrate deeply into materials. They are never fully absorbed, only ‘attenuated’. Attenuation is the process by which the energy of the light is reduced as it travels through materials. After a sufficient distance, the gamma rays are attenuated down to a point where they are not detectable.

Half-Life

Half-life is the amount of time that it takes half of an amount radioactive substance to decay. For example, pretend there is 1 gram of substance A, which has a half-life of one minute. After one minute has passed, half of the substance has decayed, leaving only half of the original amount behind. After another minute, there is only one-quarter the original amount.

In general, substances that have a half-life anywhere up to several hours or days can be extremely dangerous even in small quantities. Medium half-life substances of several years to dozens of years can be dangerous if there is a large quantity of material, or if a human is exposed to them long-term or if ingested. Substances with a very high half-life, up to many millions of years, are generally not radioactively dangerous except in extremely large quantities. Quantities being equal, the longer the half-life of a material, the less of an immediate danger it is in terms of its radiation.

The world we live in is slightly radioactive. It does not make sense to compare the radioactivity emitted by nuclear reactions to an imaginary ‘zero point’, because no such point exists. We receive radiation doses constantly from our environment, including the life-giving sun and almost all of the materials around us. The only way to measure radioactive doses that makes sense is to compare them to what we normally experience everyday. We call this everyday radioactivity the radioactive ‘background’.  Radioactive background is usually estimated to be around 3.5 millisieverts per year. The ‘sievert’ is a standard unit for measuring the radioactive dose. Regulations in Canada stipulate that citizens cannot receive more than 1 millisievert per year from any industrial or research radioactive sources.

Decay Chain

When a nucleus emits radiation, it changes into a similar nucleus that may have very different properties. This new nucleus may or may not be radioactive as well. Sometimes it takes several steps to reach a stable (non-radioactive) isotope. This series of steps is called a decay chain. It should be noted that the same nucleus can decay in different ways. This means that there may be more than one possibility for path that a nucleus will take towards a stable state. You can think of this as a branching decay tree. Depending on the path the nucleus takes, it may be a different number of steps to a stable isotope. For a more detailed explanation as well as examples, see Wikipedia’s article about decay chains.

Exposure

Exposure, or absorbed radiation dose, is a measure of the amount of ionizing energy that happens to an target. It is measured in units of energy per unit mass, so in the metric system, this is Joules/Kilogram, which is called a gray. The plural of gray is ‘gray’. Gray are symbolized as “Gy”. A general heuristic is that exposure around one gray can be fairly dangerous. Exposure of several grays can be fatal. The effect of exposure on humans depends on many factors. The type of radiation, the duration of the exposure, and the areas of the body exposed are all very important factors. There are areas of the body that can handle relatively intense radiation without serious consequences, while other areas are much more sensitive. These differences are taken into account using a term called the radioactive ‘dose’.

Radioactive Dose

Dose, or ‘dose equivalent’, is a numerical measure of the harm done to the human body through exposure to radiation. The unit of dose is the sievert. Just as with the gray, sieverts are Joules / Kilogram. Despite this similarity, the units are used for different things. Gray are a measure of exposure, meaning the literal number of joules of ionizing energy incident on a given target object. Conversely, sieverts are a unit that signifies actual radioactive dose delivered to a human being. Exposure to sensitive areas of the human body causes the dose to increase dramatically, while exposure to other areas will cause less of an increase in dose. An example of a sensitive area would be the chest, while an example of an insensitive area would be the hands. The number of sieverts is essentially a measure of how much damage is being done to the body. Sieverts are symbolized as “Sv”. An older unit, called the rem, is related to the sievert by the unit 100 rem  = 1 Sv.

Background radiation on the surface of the earth is usually estimated to be around 3.5 millisieverts per year. Regulations in Canada stipulate that citizens cannot receive more than 1 millisievert per year from any industrial or research radioactive sources.