Measurement of radiation is more complex than the familiar ways of determining properties such as mass, distance, or temperature. This is partly because there are several types of radiation, and partly because measuring how much radiation is coming from a source is different from measuring the effect of that radiation on an object or person.
“Radioactivity” describes how radiologically active a substance is. Radioactivity is something that can be measured and compared.
Each element (such as carbon or uranium) actually has several types of atoms that have different masses: these are called isotopes. For example, uranium-238 is an isotope that has three more particles in its nucleus than uranium-235 does. Some isotopes are less stable than others. When an atom breaks down (a process called “radioactive decay”), it ejects a subatomic particle and energy – and becomes a different type of atom. The part of the atom that is ejected is radiation; what is left is a new type of atom.
Each isotope has a unique rate of decay, and that rate is called a “half-life”. This is the amount of time it takes until half of that substance has undergone a radioactive decay and become something else – and therefore only half is left.
Uranium-238, for example, is very stable, with a half-life of 4.5 billion years – and because its rate of decay is so low, it is hardly radioactive at all. Fluorine-18, on the other hand, has a half-life of just under two hours, which makes it very radioactive, and useful in nuclear medicine – but it does not last very long.
The “radioactivity” of an object is the amount of radiation it is emitting.
The standard international unit for radioactivity is the becquerel (Bq). One becquerel equals the radioactive decay of one atom per second, and the resulting emission of a particle and/or photons).
This is a tiny amount. For context, bananas contain potassium, and a certain amount of that potassium will be potassium-40, a radioactive isotope; an average banana has about 15 Bq of radioactivity. The human body is much more radioactive, averaging about 20,000 Bq.
Some substances are much more radioactive than others. For example, one milligram of uranium-238 has 10 Bq of activity, whereas the same amount of Americium-241 has 100 MBq (or 100,000,000 Bq).
Another, more traditional unit of radioactivity is the curie (Ci), which is greater than a becquerel by a factor of 3.7 billion.
While the becquerel measures the radioactivity of an object, the radiation dose received by a person depends on the distance of the source and the time of exposure. So, another unit is needed to measure it.
The most important radiation unit is the sievert (Sv). More commonly discussed at the millisievert (mSv) or microsievert (μSv) level, the sievert is a measure of the effective dose of radiation to the body.
The sievert is not actually a physical measurement – it is an estimate, derived through calculation of many factors, such as the amount of energy physically imparted by the ionizations that radiation causes, the type of radiation (as alphas are more powerful than gammas, for example), and the sensitivity of the parts of the body exposed.
Radiation is part of the natural environment: all people are constantly receiving an annual average dose of approximately 2.4 mSv. Various medical, occupational, and recreational activities can increase this dose, as shown in the chart below.
Studies have shown that there are no observable health risks to humans below annual doses of 100 mSv. Nevertheless, Canada’s nuclear industries work hard to maintain doses “as low as reasonably achievable” (or the “ALARA principle”). For example, while the maximum regulated dose for members of the public is 1 mSv, the average dose within a few kilometers of Canadian nuclear power plants is kept, on average, 1000 times lower than that.