IGCSE Physics / Atoms and radioactivity
Alpha, beta and gamma radiation revision notes
Edexcel International GCSE Physics (4PH1), chapter 22: Atoms and radioactivity, specification point 7.4, 7.5. Free lesson.
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Unstable nuclei emit alpha, beta or gamma radiation at random. The three types differ in what they are, how strongly they ionise atoms and how far they penetrate.
What ionising radiation does
Ionising radiation knocks electrons off atoms. The atoms then become charged particles called ions. Alpha, beta and gamma radiation are all ionising. Describe ionisation as removing electrons from atoms.
Why radioactive decay is random
You cannot predict when one particular unstable nucleus will decay, or which nucleus will decay next. Decay is random. With a huge number of nuclei, you can only say how many will decay on average.
How alpha, beta and gamma differ
The three types of radiation come from unstable nuclei, but they have different forms and charges. Their ability to ionise is linked to how far they travel: the more strongly radiation ionises, the less far it gets. Alpha (α) is a helium nucleus containing 2 protons and 2 neutrons. It has a charge of +2 and is strongly ionising. Paper or a few centimetres of air stops it. Beta (β⁻) is a fast electron emitted from the nucleus. It has a charge of −1 and is moderately ionising. A few millimetres of aluminium stops it. Gamma (γ) is an electromagnetic wave. It has no charge and is…
Why alpha has a short range
An alpha particle is relatively large and has a charge of +2. It knocks electrons off many atoms as it passes, so it ionises strongly. Each ionisation takes some of its energy, which means it loses energy quickly and travels only a short distance. Gamma is the opposite: it ionises weakly, so it can travel a long way.
Identify radiation using what stops it
Use the material that stops the radiation to identify it. In this example, the source's count is unchanged by paper, but 5 mm of aluminium stops all the radiation. It is beta radiation. It is not alpha, because paper would stop alpha. It is not gamma, because aluminium would not stop gamma.