Radioactive decay and nuclear radiation — AQA GCSE Physics
Test yourself on Radioactive decay and nuclear radiation with AQA GCSE practice questions.
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Radioactive decay and nuclear radiation explained
A nucleus with an unsuitable balance of protons and neutrons is unstable, and at some point it emits radiation and becomes more stable.
Read the full explanation
It is the nucleus that decays, not the electrons and not the atom as a chemical object, which is why nothing you do to the outside of the material changes the rate: heating it, cooling it, dissolving it or squeezing it all leave the decay untouched. Decay is random in a precise sense. You cannot predict which nucleus in a sample will decay next, and you cannot predict when any particular nucleus will decay. Only across very large numbers of nuclei does a reliable average emerge, and that average is what half-life describes.
Activity is the rate at which a source of unstable nuclei decays.
The measurement of radioactive decay focuses on how frequently unstable nuclei emit radiation, quantified in becquerels (Bq), where 1 Bq equals one decay per second. This frequency depends on the total number of unstable nuclei present and the specific isotope's half-life. Given identical starting quantities of two isotopes, the one with a shorter half-life exhibits a higher initial frequency of decay events. For example, a sample of polonium-210 (half-life 138 days) registers significantly more counts per second on a detector than an equal sample of polonium-209 (half-life 125 years). As unstable nuclei deplete over time, the frequency of decay events naturally decreases, meaning the sample becomes less radioactive.
Activity is measured in becquerel (Bq) Count-rate is the number of decays recorded each second by a detector (eg Geiger-Muller tube).
The unit of activity is the becquerel: 1 Bq is one decay per second, so a source of 500 Bq has 500 nuclei decaying every second. Count-rate is what a detector such as a Geiger-Muller tube actually records each second, and it is always smaller than the activity. Radiation is emitted in every direction, so the tube intercepts only a fraction of it, some radiation is absorbed by the air or by the tube window, and the detector itself is not perfectly efficient. The reading also includes background radiation, which must be measured with the source removed and subtracted before the count-rate from the source is used.
The nuclear radiation emitted may be: an alpha particle (α) – this consists of two neutrons and two protons, it is the same as a helium nucleus a beta particle (β) – a high speed electron ejected from the nucleus as a neutron turns into a proton a gamma ray (γ) – electromagnetic radiation from the nucleus a neutron (n).
Four kinds of nuclear radiation appear in this specification. An alpha particle is two protons and two neutrons bound together, identical to a helium nucleus, so it has a relative mass of 4 and a charge of +2. A beta particle is a high-speed electron thrown out of the nucleus at the moment a neutron changes into a proton; it has a charge of -1 and negligible mass, and it does not come from the electron shells. A gamma ray is electromagnetic radiation emitted from the nucleus, with no mass and no charge. A neutron may also be emitted, carrying a relative mass of 1 and no charge. All four leave the nucleus, never the electron arrangement.
Required knowledge of the properties of alpha particles, beta particles and gamma rays is limited to their penetration through materials, their range in air and ionising power.
Three properties, and they run in opposite directions. Alpha is stopped by a sheet of paper or by skin, travels only a few centimetres in air, and is the most strongly ionising because its charge of +2 and slow speed let it interact with many atoms. Beta passes through paper but is stopped by a few millimetres of aluminium, travels about a metre in air, and is moderately ionising. Gamma is only reduced, never fully stopped, by thick lead or metres of concrete, travels very large distances in air, and is the least ionising. Range experiments use this: if the count-rate drops to background after about 3 cm of air, the source is emitting alpha only.
Students should be able to apply their knowledge to the uses of radiation and evaluate the best sources of radiation to use in a given situation.
Choosing a source means matching penetration, ionising power and half-life to the job. A smoke alarm uses alpha, because it ionises the air in the chamber strongly yet cannot reach anyone outside the detector. A thickness gauge for aluminium sheet uses beta, because alpha would be absorbed completely and gamma would pass straight through, so only beta gives a reading that changes with thickness. A medical tracer uses gamma, because it must pass out of the body to reach a detector while ionising as little tissue as possible on the way. Half-life matters too: long enough for the source to be useful, short enough that the activity does not persist and cause harm.
Your focus
- Explain what is meant by saying radioactive decay is a random process, giving both features of randomness.
- Explain why heating a radioactive sample does not change its rate of decay.
- Suggest two reasons why rolling a large number of dice and removing the sixes is a good model of the random nature of radioactive decay.
Show all 18 objectives
- State the unit of activity and define it in terms of decays per second.
- Explain why the activity of a radioactive source decreases over time.
- Compare the activities of two different isotopes present in equal numbers, based on their half-lives.
- State the unit of activity and what one becquerel means.
- Explain why the count-rate recorded by a detector is lower than the activity of the source.
- Correct a measured count-rate for background radiation and explain why the correction is needed.
- State the composition, charge and relative mass of an alpha particle, a beta particle and a gamma ray.
- Explain where a beta particle comes from, referring to the change of a neutron into a proton.
- Identify which of the four nuclear radiations carry charge and which do not.
- Compare alpha, beta and gamma radiation by naming the material that stops each one, its range in air and its rank for ionising power.
- Explain how an aluminium absorber can be used to decide whether a source emits beta or gamma radiation.
- Interpret a graph of count-rate against distance in air to identify the type of radiation a source emits.
- Select a suitable radiation type and half-life for a stated application and justify both choices.
- Explain why a beta source rather than an alpha or gamma source is used to monitor the thickness of aluminium sheet.
- Evaluate two sources given in a table by comparing their radiation types and half-lives against the requirements of the job.
Radioactive decay and nuclear radiation exam tips
Marking Points
- one mark for stating that some nuclei are unstable and emit radiation to become more stable
- one mark for stating that it cannot be predicted which nucleus will decay next
- one mark for stating that it cannot be predicted when a given nucleus will decay
- one mark for stating that the decay rate is unaffected by external conditions such as temperature
- Identifies that the activity of a radioactive source is measured in becquerels (Bq)
- States that one becquerel is equivalent to one nuclear decay per second
- Explains that activity decreases over time as the number of unstable nuclei in the sample reduces
- Compares two sources to conclude that for an equal number of nuclei, a shorter half-life results in a higher activity
- one mark for naming the becquerel, Bq, as the unit of activity
- one mark for stating that 1 Bq is one decay per second
- one mark for count-rate being the number of decays recorded each second by a detector such as a Geiger-Muller tube
- one mark for stating that the count-rate is lower than the activity, and that the background count must be subtracted
- one mark for an alpha particle being two protons and two neutrons, or a helium nucleus
- one mark for a beta particle being a high-speed electron emitted from the nucleus
- one mark for stating that a neutron in the nucleus changes into a proton when a beta particle is emitted
- one mark for a gamma ray being electromagnetic radiation emitted from the nucleus
- one mark for alpha being stopped by paper or skin, with a range of only a few centimetres in air
- one mark for beta passing through paper but being stopped by a few millimetres of aluminium
- one mark for gamma requiring thick lead or concrete and having a very long range in air
- one mark for the order of ionising power, alpha the most ionising and gamma the least
- one mark for using the fall of count-rate to background at a stated distance as evidence of the type emitted
- one mark for naming the type of radiation suited to the situation described
- one mark for justifying the choice by penetration or range, for example gamma passing out of the body for a tracer
- one mark for justifying the choice by ionising power or by the risk to people nearby
- one mark for commenting on a suitable half-life, long enough to be useful but short enough to decay away
Examiner Tips
- 💡Give both halves of randomness, which nucleus and when, because they are usually separate marks.
- 💡Say nucleus, not atom, throughout any answer about decay.
- 💡A dice analogy is accepted: you cannot say which dice will show a six, or on which throw.
- 💡When defining activity, always include the unit becquerel (Bq) and state that 1 Bq equals 1 decay per second.
- 💡If asked to compare the activity of two samples, check both the number of unstable nuclei and their respective half-lives before concluding which is more active.
- 💡If a question says give the unit, write becquerel or Bq; a description of the unit may not be credited.
- 💡Always check whether a count-rate in a table has already had background subtracted.
- 💡Explain a low count-rate by the detector catching only part of the radiation emitted in all directions.
- 💡Learn a table of the four radiations with mass, charge and origin; one-mark recall questions are common.
- 💡Say helium nucleus, not helium atom, because an alpha particle has no electrons.
- 💡In beta decay, name the change as neutron to proton; that phrase is worth a mark on its own.
- 💡Learn the absorber, the range and the ionising power for all three radiations as a three by three table.
- 💡When choosing an absorber to tell beta from gamma, say a sheet of aluminium of a stated thickness.
- 💡Argue from the count-rate falling to background, not simply falling, when identifying a source.
- 💡Structure an evaluation as choice, then penetration, then ionising power, then half-life.
- 💡Say why the rejected options fail as well as why your choice works; evaluate means weigh the alternatives.
- 💡Use the data in the table the question supplies rather than remembered values.
Common Mistakes
- saying the atom decays rather than the nucleus
- claiming decay can be speeded up by heating or by a chemical reaction
- reading random as meaning there is no pattern at all, even across a large sample
- saying the nucleus becomes unstable as a result of emitting radiation, rather than before it
- Confusing activity with the total number of undecayed nuclei remaining in a sample; activity is the rate of decay (decays per second), not the absolute number of unstable nuclei present.
- Assuming that a longer half-life means a higher activity; a longer half-life means the nuclei are more stable and decay less frequently, resulting in a lower activity for a given number of nuclei.
- Believing that the activity of a specific radioactive source remains constant over time; as unstable nuclei decay, there are fewer left to decay, so the activity decreases over time.
- writing decays per second when the question asks for the name of the unit
- treating count-rate and activity as identical quantities
- forgetting to subtract the background count-rate before using a reading
- quoting a count rather than a count-rate, so leaving out the per second
- saying the beta particle comes from the electron shells outside the nucleus
- describing an alpha particle as a helium atom rather than a helium nucleus
- giving a gamma ray a small mass or a small charge
- stating that a proton turns into a neutron when a beta particle is emitted, which is the wrong way round
- calling gamma the most dangerous in every situation, without saying whether the source is inside or outside the body
- describing alpha radiation as harmless because it is stopped by skin
- naming aluminium foil as the absorber without stating a thickness, which is ignored by the mark scheme
- forgetting to subtract background radiation before deciding that a source has been completely absorbed
- choosing a source only because it is safe, without checking that the radiation could reach the detector at all
- suggesting an alpha source as a medical tracer, when the radiation cannot leave the body
- ignoring half-life completely in an evaluation question
- calling a source safe because it is sealed, without saying which radiation it emits