Radioactive decay and nuclear radiation — AQA GCSE Combined Science

Activity describes how quickly unstable nuclei in a radioactive source decay. Each decay is a random event for one nucleus, but a large sample gives a steady average rate. Activity is therefore the number of nuclear decays per unit time, not the number of nuclei present and not the count rate measured by a detector. For example, a source with an activity of 200 Bq has, on average, 200 nuclei decaying each second. Activity decreases over time because fewer unstable nuclei remain, following the half-life pattern. Students should distinguish activity from count rate: count rate is what an instrument records, and it is lower than activity because not every emitted particle or wave reaches the detector.

The becquerel (Bq) is the unit of activity. One becquerel is one nuclear decay per second, so a source of 3000 Bq has 3000 unstable nuclei decaying each second on average. Because activity is a rate, values are written with the unit Bq, and larger activities may be quoted in kBq (10³ Bq) or MBq (10⁶ Bq). Students should use Bq correctly in calculations and when interpreting data. For example, if a detector records 40 counts per second from a source whose activity is 400 Bq, the count rate is much lower because most emissions miss the detector or are absorbed before reaching it. Activity is not measured in counts per second; counts per second is a count rate.

Radioactive decay is random, so a detector cannot predict when the next decay occurs. Instead, we measure the count-rate: the number of decays recorded each second by a detector such as a Geiger-Muller tube. The tube produces a pulse each time ionising radiation enters it, and a counter or data logger totals these pulses over a chosen time. Dividing the total count by the time in seconds gives the count-rate in counts per second (cps). For example, 600 counts in 20 s gives 600 ÷ 20 = 30 cps. Because decay is random, repeat readings vary slightly, so a mean of several measurements is more reliable. Count-rate is not the same as activity: activity is the number of decays per second in the source (measured in becquerels, Bq), while count-rate is what the detector records, which is lower because some radiation is absorbed or misses the tube.

When an unstable nucleus decays, it emits nuclear radiation. The three main types are alpha, beta and gamma, and they differ in nature, charge, penetrating power and ionising ability. An alpha particle is a helium nucleus, 2 protons and 2 neutrons, with charge 2⁺; it is highly ionising but stopped by a few centimetres of air or a sheet of paper. A beta particle is a fast-moving electron emitted when a neutron changes into a proton, with charge 1⁻; it is moderately ionising and stopped by a few millimetres of aluminium. Gamma radiation is a high-energy electromagnetic wave with no charge and no mass; it is weakly ionising and absorbed by thick lead or concrete. A neutron is also emitted in some decays. Knowing these properties lets students identify the radiation from absorption data and balance nuclear equations.

An alpha particle is a nuclear emission made of two protons and two neutrons, so it is identical to a helium-4 nucleus, ⁴₂He. It has a relative mass of 4 and a charge of 2⁺, because the two protons each carry 1⁺ and the neutrons are neutral. When a nucleus emits an alpha particle, its proton number decreases by 2 and its nucleon number decreases by 4, so the element changes. For example, radium-226 decays to radon-222: ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He. Alpha particles are highly ionising but have a short range in air, typically a few centimetres, and are stopped by paper or skin. Students should be able to describe the composition, write or complete nuclear equations, and explain why the emitting nucleus changes identity.

A beta particle is a high-speed electron emitted from the nucleus when a neutron changes into a proton. The nucleus gains a proton but loses a neutron, so the proton number increases by 1 while the nucleon number stays the same. The emitted beta particle is written as ⁰₋₁e or β, with a relative mass of 0 and a charge of 1⁻. For example, carbon-14 decays to nitrogen-14: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e. Beta particles are less ionising than alpha particles but have a longer range, typically a few metres in air, and are stopped by a few millimetres of aluminium. Students should describe the origin of the beta particle, balance nuclear equations, and explain why the element changes without the nucleon number changing.

A gamma ray is a high-energy electromagnetic wave emitted from an unstable nucleus during radioactive decay. Unlike alpha or beta particles, it has no mass and no charge, so its emission does not change the proton number or the nucleon number of the nucleus. It travels at the speed of light and is highly penetrating: it passes through paper and several centimetres of aluminium, and is only significantly reduced by thick lead or concrete. For example, after a nucleus emits an alpha or beta particle, it may still hold excess energy; it then releases that energy as one or more gamma rays. In equations, gamma emission is shown by adding γ to the products, with the mass and atomic numbers unchanged. Because gamma is electromagnetic radiation, it is part of the electromagnetic spectrum and can ionise atoms indirectly.

A neutron is a neutral subatomic particle found in the nucleus of an atom. It has a relative mass of 1 and no electric charge, so it is written as n in nuclear equations. As well as alpha, beta, and gamma radiation, a neutron can be emitted from an unstable nucleus as a form of radioactive decay. Because a neutron has no charge, it is not deflected by electric or magnetic fields and can penetrate deeply into materials. In a nuclear equation, a neutron is shown with a mass number of 1 and an atomic number of 0, for example ¹₀n. The number of neutrons in a nucleus is found by subtracting the atomic number from the mass number. For example, carbon-12 has 6 protons and 6 neutrons because 12 − 6 = 6.

Choosing a radioactive source for a job means matching its penetration, range and ionising power to the task while keeping exposure as low as reasonably practicable. For thickness monitoring of thin metal foil, a beta source such as strontium-90 is suitable because some beta passes through the foil and the count rate changes as thickness varies; alpha would be stopped by the foil and gamma would pass through almost unchanged. For smoke detectors, an alpha source such as americium-241 ionises air between electrodes; the smoke absorbs the ions and the current falls, triggering the alarm. For sterilising medical equipment or treating tumours, gamma from cobalt-60 penetrates packaging or tissue. Evaluation means weighing benefits against the hazard, choosing the least penetrating radiation that still does the job, and using shielding, distance and short exposure time.