Edexcel IGCSE Physics · Spec 7.4
Half-Life, Uses & Dangers
What half-life means, and the uses and dangers of radioactivity.
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Half-Life, Uses & Dangers
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Explained
Half-life in practice, and handling radiation safely
Radioactive decay is random. No one can say when a particular nucleus will decay, only that each has the same chance in any given moment. With a large enough number of nuclei that randomness averages into a completely predictable pattern, and half-life is the measure of it.
Half-life is the time for half the radioactive nuclei in a sample to decay, or equivalently for the activity to halve.
Reading a decay curve
Start at the initial activity on the vertical axis, halve it, read across to the curve and down to the time axis. That time is the half-life.
Do it a second time from a different starting point as a check. Genuine exponential decay gives the same half-life wherever you start, and that is the property being tested.
Remember to subtract the background count before using any reading, since the detector registers background whether the source is there or not.
Choosing an isotope for a job
The half-life and the type of radiation together decide what an isotope is good for, and questions usually ask you to justify a choice.
A medical tracer needs a short half-life, so it stops being active soon after the scan, and gamma radiation, so it can escape the body to be detected. Technetium is used for this.
Sterilising equipment or food needs gamma, because it penetrates the sealed packaging, and a long half-life so the source does not need replacing.
Smoke detectors use alpha, because alpha ionises the air and is stopped by the smoke, and a long half-life so the detector lasts for years.
Checking the thickness of sheet metal or paper uses beta, because alpha would all be absorbed and gamma would all pass through, so neither would respond to a change in thickness.
Carbon dating uses carbon 14, whose half-life of about 5700 years is comparable with the ages being measured. An isotope with a very short or very long half-life would be useless for the same job.
What the examiners say about this topic
A principal examiner report for Edexcel International GCSE Physics describes a three mark question on shielding, which required candidates to explain why it is necessary, how it works, and what it prevents. Some responses were too general, referring to danger, which the report says is less specific than the harm caused by radiation escaping.
It then contrasts two answers. One scored two marks for saying that the shielding prevents radiation escaping and that radiation is harmful, but did not explain how it does so. Another scored all three by stating that the shielding absorbs the radiation, so it cannot escape, and that the radiation causes cancer.
Absorbs is the word that carries the mechanism. Prevents describes the outcome without explaining it, and harmful is vague where cancer is specific.
The same report notes two vocabulary errors on nearby parts. Some candidates mixed up spontaneous with unpredictable, and many simply reworded the stem of the question. And on ionisation, some wrote that electrons could be gained, which was not credited, and some referred to objects or substances being ionised rather than atoms or molecules.
The dangers, and reducing them
Ionising radiation removes electrons from atoms in living cells. That can damage or mutate DNA, which may cause cancer, and a large dose kills cells outright, causing radiation sickness.
Outside the body, gamma is the most dangerous because it penetrates furthest. Inside the body the order reverses: alpha becomes the most dangerous, because it is the most strongly ionising and all of its energy is deposited in a small volume of tissue.
Protection works in three ways: shielding with lead or concrete, which absorbs the radiation; distance, since intensity falls sharply as you move away; and time, by limiting how long anyone is exposed. Handling sources with tongs and storing them in lead lined boxes are applications of the first two.
Spec 7.4
What you need to know
- Define half-life
- Read a decay curve
- Give uses and dangers of radioactivity
Active recall
Quick check
Answer each question before opening the answer.
Define half-life.
The time taken for half the radioactive nuclei in a sample to decay (or for the activity to halve).
Give one use and one danger of radioactivity.
Use — e.g. medical tracers, treating cancer, smoke detectors; danger — it can damage or kill cells and cause cancer.
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