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Edexcel GCSE Physics · 1PH0
Edexcel 1PH0 · 6.30P Check the specification (PDF) (opens in a new tab)
Ionising radiation can damage living tissue, so a radioactive material presents a hazard while its unstable nuclei are decaying. tells us how quickly the number of undecayed nuclei—and therefore the —falls.
Activity is the rate of decay. An activity of 1 becquerel (Bq) means one nuclear decay each second. For two samples containing the same number of radioactive nuclei, the isotope with the shorter half-life undergoes more decays each second and therefore has the greater initial activity. It can deliver a large amount of radiation in a short time.
This comparison needs care: half-life is not the only factor controlling danger. The amount of radioactive material, the type of radiation, distance, shielding and whether material enters the body also matter. A tiny short-lived sample need not be more hazardous than a large long-lived one.
A short half-life means nuclei decay rapidly. The source may initially emit radiation at a high rate, increasing the risk of a large exposure over a short period. However, its activity also falls rapidly: after each half-life, only half of the previous activity remains.
Precautions must therefore control the relatively intense initial radiation. Use only the minimum necessary quantity, minimise the time spent near the source, maximise distance where possible, and use suitable shielding or remote-handling equipment. After several half-lives, the source becomes much less active.
For the same initial number of radioactive nuclei, a long-lived isotope decays more slowly and has a lower initial activity. Its danger does not disappear quickly, however. It remains radioactive for many years—or potentially much longer—and can continue to expose people or contaminate the environment.
Long-lived material therefore requires durable containment, secure labelled storage, suitable shielding and long-term monitoring or disposal arrangements. Containment is especially important because it prevents radioactive material from leaking or spreading. If long-lived material contaminates soil, water or a person, the hazard can persist for a long time.
For equal initial numbers of radioactive nuclei, a short half-life gives a greater initial activity but a rapidly declining hazard; a long half-life gives a more persistent hazard requiring long-term control.
The central trade-off is rate against persistence:
A complete explanation should connect the half-life to the decay rate, then to the particular precaution. For example, minimising handling time reduces exposure from a highly active short-lived source, whereas secure long-term containment prevents a long-lived source from spreading while it remains radioactive.
Short half-life: high activity over a short time; control the immediate exposure using a small quantity, short handling time, distance, shielding and remote handling.
Long half-life: slower decay but a persistent hazard; use durable containment, secure storage, shielding and long-term monitoring or disposal.
Half-life → rate and duration of emission → nature of the danger → matched precaution.
Do not judge danger from half-life alone: amount, radiation type, distance, shielding and contamination also affect the risk.
Do not write simply that a short half-life is always more dangerous. For equal numbers of radioactive nuclei, it gives a higher activity, but the danger falls more quickly.
Link each timescale to a suitable precaution: short half-life → control the intense initial exposure; long half-life → prevent release and maintain secure storage for a long time.
State what is being held constant when comparing activity. The relationship between shorter half-life and greater activity assumes equal numbers of radioactive nuclei.
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Ionising radiation
Radiation energetic enough to remove electrons from atoms, which can damage cells and cause mutations.
Activity
The number of nuclear decays occurring each second in a radioactive source, measured in becquerels (Bq).
Half-life
The time taken for half the undecayed nuclei in a sample to decay, or for its activity to fall to half its value.
Radioactive contamination
The unwanted presence of radioactive material on or inside an object or organism.
Put your knowledge into practice — try past paper questions for Physics
Ionising radiation
Radiation energetic enough to remove electrons from atoms, which can damage cells and cause mutations.
Activity
The number of nuclear decays occurring each second in a radioactive source, measured in becquerels (Bq).
Half-life
The time taken for half the undecayed nuclei in a sample to decay, or for its activity to fall to half its value.
Radioactive contamination
The unwanted presence of radioactive material on or inside an object or organism.