Describe radioactive decay and compare the nature, charge and penetrating power of alpha, beta and gamma radiation

What this dot point is asking

SEAB wants you to describe radioactive decay as the random giving-out of radiation from unstable nuclei, and to compare the three types of radiation (alpha, beta and gamma) by what they are made of, their charge, and how far they can travel through materials. The big idea is that some nuclei are unstable, so they throw out radiation to become more stable, and the three types behave very differently.

The answer

What radioactive decay is

Some atoms have a nucleus that is unstable. To become more stable, the nucleus gives out radiation. This is called radioactive decay. The atom does this on its own; nothing we do, such as heating it or putting it under pressure, changes whether or how fast it happens.

Radioactive decay is a random process. This means we cannot say exactly when a particular nucleus will decay. Each unstable nucleus simply has a chance of decaying in any given second, but which one goes, and when, is unpredictable.

Background radiation

There is always a low level of radiation around us called background radiation. It comes from natural sources such as radon gas from rocks and soil, radioactive rocks, food, and cosmic rays from space. There are also some man-made sources, such as medical X-rays. When measuring a source, we first measure the background count so we can subtract it.

The three types of radiation

Unstable nuclei can give out three types of radiation:

• Alpha ($\alpha$): a helium nucleus, made of $2$ protons and $2$ neutrons. It has a charge of $+2$. It is heavy and slow.
• Beta ($\beta$): a fast-moving electron thrown out of the nucleus. It has a charge of $-1$. It is light and fast.
• Gamma ($\gamma$): a high-energy electromagnetic wave (like a very energetic light wave). It has no charge and no mass.

Penetrating power

Penetrating power means how far the radiation can travel through a material before it is stopped. The three types are very different:

• Alpha is the least penetrating. It is stopped by a sheet of paper or a few centimetres of air.
• Beta is more penetrating. It passes through paper but is stopped by a few millimetres of aluminium.
• Gamma is the most penetrating. It is only greatly reduced by thick lead or concrete, and is never fully stopped.

A useful order to remember is: alpha is stopped by paper, beta by aluminium, gamma by lead.

Ionising ability

When radiation passes through matter it can knock electrons off atoms, turning them into ions. This is called ionising. The order is the opposite of penetrating power: alpha is the most strongly ionising, beta is in the middle, and gamma is the least ionising. Alpha gives up its energy quickly to nearby atoms, which is why it does not travel far but ionises strongly.

Examples in context

Example 1. Smoke detectors. Many smoke alarms use a weak alpha source. Alpha radiation ionises the air in a small gap, letting a tiny current flow. When smoke enters, it absorbs the alpha and the current drops, setting off the alarm. Alpha is chosen because it is stopped by a short distance of air and so stays safely inside the detector.

Example 2. Choosing shielding. Workers near a gamma source stand behind thick lead or concrete because gamma is the most penetrating type and cannot be stopped by thin materials. For a beta source, a few millimetres of aluminium or plastic is enough, which links to the penetrating-power order above.

Try this

• Cue. State what alpha, beta and gamma radiation each are made of. [3 marks] Alpha is a helium nucleus ($2$ protons and $2$ neutrons); beta is a fast-moving electron; gamma is a high-energy electromagnetic wave.

• Cue. Put alpha, beta and gamma in order of penetrating power, from least to most. [2 marks] Alpha (least, stopped by paper), then beta (stopped by aluminium), then gamma (most penetrating, needs thick lead).

• Cue. Explain why we measure the background count rate before measuring a source. [2 marks] Background radiation is always present, so it is subtracted from the readings to find the count rate that is actually due to the source.