Types of nuclear radiationEdexcel International A Level Physics: Revision notes

Background radiation is the ionising radiation that is always present around us. It comes from natural sources such as radon gas released from rocks like granite, cosmic rays from space, and rocks, soil, food and drink that contain radioactive isotopes. It also comes from artificial sources such as medical X-rays and fallout from nuclear weapons testing.

Background radiation is random, so it must be allowed for in any experiment:

1. Measure the background count rate with the source removed, over a long period, to get a reliable average.
2. Subtract it from the count rate measured with the source to get the corrected count rate.

Worked example: 120 counts in 10 minutes is 12 counts per minute. A source gives 1890 counts in 5.0 minutes, which is 378 counts per minute, so the corrected count rate is 378 − 12 = 366 counts per minute.

Three types of nuclear radiation are emitted by unstable nuclei:

• Alpha (α): a helium nucleus, two protons and two neutrons, with charge +2e.
• Beta-minus (β⁻): a fast-moving electron from the nucleus, with charge −e.
• Gamma (γ): a high-energy photon of electromagnetic radiation, with no charge.

Alpha and beta-minus are particles. Gamma radiation is a wave, so its behaviour in matter is different.

Ionising radiation knocks electrons from atoms, losing energy each time. The more strongly it ionises, the more quickly it loses energy, so the less far it penetrates.

• Alpha: strongly ionising, a range of a few centimetres in air, stopped by paper.
• Beta-minus: weakly ionising, a range of up to a few metres in air, stopped by a few millimetres of aluminium.
• Gamma: very weakly ionising, very long range in air, reduced (but never completely stopped) by thick lead.

The relationship is always the same: high ionising ability means short range and low penetration.

In a nuclear equation the total nucleon number $A$ and the total charge (proton number $Z$) are the same on both sides.

• Alpha decay: $A$ falls by 4 and $Z$ by 2. $^{226}_{88}\text{Ra} \rightarrow {}^{222}_{86}\text{Rn} + {}^{4}_{2}\text{He}$
• Beta-minus decay: $A$ is unchanged and $Z$ rises by 1, as a neutron becomes a proton. The electron $^{0}_{-1}\text{e}$ and an antineutrino are emitted. $^{14}_{6}\text{C} \rightarrow {}^{14}_{7}\text{N} + {}^{0}_{-1}\text{e} + \bar{\nu}_e$
• Gamma emission: $A$ and $Z$ are unchanged. The nucleus drops to a lower energy state.

Check by adding the top numbers and the bottom numbers on each side.

Aim: to investigate how the count rate of gamma radiation depends on the thickness of lead between the source and the detector.

• Measure the background count rate over a long time with the source away.
• Clamp the source and Geiger–Müller tube at a fixed distance apart.
• Place lead sheets of known thickness between them and record the count over a fixed time. Repeat and average. Subtract the background to get the corrected count rate.
• Safety: use tongs, point the source away from people, keep exposure short and return it to its lead-lined store.

Results: the corrected count rate falls exponentially with thickness. The same thickness of lead, the half-thickness, always halves the count rate, and it never falls to zero. For example, if 8.0 mm halves the rate, then 20.0 mm gives $640 \times (\tfrac{1}{2})^{2.5} \approx 113$ counts per minute from 640.