What is half-life, how do we calculate it, and how is radioactivity used and made safe?
The definition of half-life, calculating the remaining activity after a number of half-lives, the uses of radioactive sources (medical tracers, treatment and dating), the difference between irradiation and contamination, and how to reduce the risks of radiation.
A focused answer to the OCR Gateway GCSE Combined Science A topic P4 on half-life and applications, covering the definition of half-life and how to calculate remaining activity, the uses of radioactive sources, the difference between irradiation and contamination, and reducing the risks of radiation.
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What this topic is asking
OCR wants you to define half-life, calculate the activity remaining after a number of half-lives, describe the uses of radioactive sources, distinguish irradiation and contamination, and describe how to reduce the risks of radiation.
Half-life
Because radioactive decay is random, you cannot say when a particular nucleus will decay, but for a large sample the activity falls in a predictable way: it halves every half-life. So after one half-life half the original nuclei remain, after two half-lives a quarter remain, after three an eighth, and so on. Half-lives vary enormously between isotopes, from fractions of a second to billions of years, and a longer half-life means the source stays radioactive for longer but has a lower activity for a given amount.
Calculating with half-life
This calculation is a frequent Paper 6 question, sometimes given as a table or a decay graph from which you read off the time for the activity to halve. The pattern is always the same: count the half-lives, then halve repeatedly.
Uses, irradiation, contamination and safety
Radioactive sources have many uses, chosen by matching the radiation type and half-life to the job:
- Medical tracers: a source (often a gamma or beta emitter with a short half-life) is put into the body and tracked with a detector to study how an organ is working.
- Treating cancer: beams of gamma rays, or sources placed near a tumour, kill cancer cells (radiotherapy).
- Industry: sources are used to monitor and control the thickness of materials such as paper or metal sheets, and to check for leaks.
- Dating: the known half-life of carbon-14 is used to date once-living material, and other isotopes to date rocks.
It is important to distinguish two hazards. Irradiation is being exposed to radiation from a source outside the body; the object is not made radioactive, and the exposure stops when the source is removed or shielded. Contamination is when radioactive material gets onto or into an object or the body; it keeps exposing the person until it is removed, and it can be breathed in or swallowed. The risks are reduced by shielding (lead or concrete), keeping your distance from the source (using tongs or handling remotely), and limiting the time of exposure, as well as storing sources in lead-lined containers.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksA radioactive source has an activity of 800 Bq and a half-life of 5 hours. Calculate its activity after 15 hours, and explain what half-life means.Show worked answer →
A Physics Paper 6 half-life calculation. Method: hours is half-lives. The activity halves each half-life: after one half-life Bq, after two Bq, after three Bq. So the activity after hours is Bq. Half-life means the time taken for the number of undecayed (radioactive) nuclei in a sample to halve, or equivalently the time for the activity (count rate) to fall to half its original value. Markers credit working out the number of half-lives (3), halving the activity each time to reach Bq, and a correct definition of half-life.
OCR 20214 marksExplain the difference between irradiation and contamination, and give one way of reducing the risk to a worker handling a radioactive source.Show worked answer →
A P4 question on radiation safety. Reward: irradiation is being exposed to radiation from a source that is outside the body; the object or person is not made radioactive, and the exposure stops when the source is removed or shielded. Contamination is when radioactive material (atoms) gets onto or into an object or the body; it continues to expose the person to radiation until it is removed, and can be breathed in or swallowed. One way to reduce risk: keep your distance from the source (use tongs or handle remotely), limit the time of exposure, wear shielding such as a lead apron, or store sources in lead-lined containers. Markers credit the clear distinction (external exposure versus radioactive material on or in you) and one valid protective measure.
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