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Unlock This CourseIn the last lesson you met the nucleus, the tiny centre of the atom. Most nuclei just sit there for ever. Some do not. In some atoms the nucleus has the wrong mix of protons and neutrons, so it cannot stay as it is. Sooner or later it throws out a particle or a burst of energy to become more settled. What it throws out is called radiation.
Key terms:
Think of popcorn in a hot pan. You cannot say which kernel will pop next, or when. Radioactive decay is the same. Each unstable nucleus has its own chance of going off at any moment, and nothing you do can make it happen sooner or later. With a huge number of nuclei you can say roughly how many will go off in a minute, but never which ones.
An atom is normally neutral. When fast radiation hits an atom it can knock an electron clean off. The atom is left with a positive charge, and the electron goes off on its own. Charged atoms are called ions, so the radiation is called ionising. This is why these radiations can harm living cells, which you will study in a later lesson.
All three radiations in this lesson, alpha, beta and gamma, come from unstable nuclei, are given out in a random process and are ionising.
An alpha particle is the same as a helium nucleus: 2 protons and 2 neutrons stuck together. It has a charge of +2. It is the largest and heaviest of the three, and it moves fairly slowly.
A beta particle is a fast electron that comes from the nucleus, not from the outer shells of the atom. It has a charge of −1. It is far smaller than an alpha particle.
A gamma ray is not a particle. It is a high-frequency electromagnetic wave, like X-rays but even more energetic. It has no charge and no mass.
Alpha particles are big and heavy, so they bump into atoms very quickly and lose their energy. Gamma rays have no charge and slip past most atoms, so they go a long way.
Radiation loses energy each time it ionises an atom. Alpha particles are big and have a charge of +2, so they strongly pull electrons off the atoms they meet. They ionise the most, but they use up their energy in a very short distance. Gamma rays ionise the least, which is why they go so far. Beta sits in the middle.
So the more a radiation ionises, the less it penetrates. The two go in opposite directions.
| Radiation | What it is | Charge | Stopped by | Ionising ability |
|---|---|---|---|---|
| Alpha (α) | 2 protons and 2 neutrons (a helium nucleus) | +2 | Paper or a few cm of air | Most |
| Beta (β−) | A fast electron from the nucleus | −1 | A few mm of aluminium | Medium |
| Gamma (γ) | A high-frequency electromagnetic wave | 0 | Only reduced by thick lead or concrete | Least |
A radioactive source gives out alpha particles, beta particles and gamma rays.
(a) State what is meant by ionising radiation. [1 mark]
(b) Describe what an alpha particle is. [2 marks]
(c) A student holds a sheet of paper between the source and a detector. Explain which of the three radiations will be mostly stopped. [2 marks]
(a) Radiation that can knock electrons off atoms to make ions. (1)
(b) A helium nucleus (1), made of 2 protons and 2 neutrons, with a charge of +2. (1)
(c) Alpha particles (1) because they ionise the most, so they lose their energy quickly and are stopped by paper. Beta and gamma pass through. (1)
For a question on stopping radiation, give the material for each type: paper for alpha, aluminium for beta, thick lead for gamma. Then link penetrating power to ionising ability.