Language issues for teaching radioactivity
The word decay can be used quite legitimately in two separate contexts
- A nucleus decays when it changes from an unstable to a stable state and emits a particle
- The radioactivity of a sample decays (i.e. decreases) over time
The most important thing to bear in mind is that decay never means disappear. When a nucleus decays its structure changes but it doesn't just vanish.
Radiation, nuclear radiation, ionizing radiation
Because these lessons are all about radioactivity we've tended to use the word radiation instead of the more complete nuclear radiation or ionizing radiation simply to avoid repetition but it's worthwhile being able to distinguish between them.
Radiation is commonly used to mean electromagnetic radiation, such as microwaves, X-rays and ultraviolet. This is really a matter of convention and if you wanted to insist that this is what you mean by radiation then really you should always qualify it with the epithet electromagnetic.
Nuclear radiation has its origins in the nucleus. We deal with three main types: alpha, beta and gamma though there are also less commonly known ones such as beta-plus radiation, neutrons and protons. The defining feature of nuclear radiation is that you can't do anything to change it unless you use the sort of extreme conditions you find in stars.
Ionizing radiation is any radiation that can cause ionization of matter that it strikes, like air. This is normally taken to mean all nuclear radiation plus X-rays and perhaps high-energy ultraviolet. The risks, principally cancer, and types of safety precautions you'd take (shielding, limiting exposure, distance, monitoring) are similar for all ionizing radiation, which is why they tend to be lumped together. In other words it's a way of concentrating on their effects rather than how they are produced.
Substance, source, isotope, nuclide
Technically the correct way of referring to a particular nuclear species is to use the term nuclide. So carbon-14 is strictly a nuclide rather than an isotope but this distinction is rather blurred.
In the earlier lessons we've tried to avoid using the term isotope and have opted for the more general ideas of radioactive sources and radioactive substances. Sometimes this avoids potentially confusing technical language and at other times would annoy the pickier physicist.
Atoms, molecules, particles
These words can be a bit of a minefield but our approach is not to be too hysterical about them.
The terms atom and molecule have very precise meanings in chemistry. But in physics our feeling is that you can afford to be a little more free and easy to try and avoid rather long and clumsy sentences. For example you might refer to a cosmic ray hitting the nucleus of an air atom in the atmosphere rather than a cosmic ray hitting the nucleus of an atom of a molecule in the atmosphere.
The problem with the catch-all term particle is that it can be used legitmately at the atomic and macroscopic scales, for example when referring to a smoke particle.
It becomes even more difficult when you're trying to find a way of making a general statement about nuclear radiation but you want to emphasize it's discrete nature. Though you could argue that gamma photons are particles, they're not particles in the way that alpha and beta are.
We occasionally have used the term radioactive particles though it's true that the particles themselves aren't radioactive. Radiation particles seems worse somehow. We don't pretend to have an elegant answer to these questions.
Open and closed radioactive sources
An open source is any source that is not confined in a container to stop it spreading into the environment. All tracers are open sources, whether injected into a patient or dropped into a river. Open sources are chosen with short half-lives.
Closed sources are contained somehow. For example the beta source in a beta thickness gauge is a closed source. The half-life of closed sources can be allowed to be longer though they have to be disposed of carefully when they are no longer needed.