4.4.3.1Background radiation
Physics only — not assessed in Combined Science: Trilogy.
Background radiation is around us all of the time. It comes from:
• natural sources such as rocks and cosmic rays from space • man-made sources such as the fallout from nuclear weapons testing and nuclear accidents.
The level of background radiation and radiation dose may be affected by occupation and/or location.
Radiation dose is measured in sieverts (Sv), and 1000 millisieverts (mSv) = 1 sievert (Sv). You will not need to recall the unit of radiation dose, so it will be given if a question needs it.
Background radiation matters practically as well: when measuring the count-rate from a source, the background count must be measured first and subtracted, or the source will appear more active than it is.
Common mistake
Most background radiation is natural, not man-made. Rocks and cosmic rays contribute far more than weapons testing and accidents for almost everyone.
- Background radiation
- radiation around us all the time, from natural sources such as rocks and cosmic rays and from man-made sources such as fallout from weapons testing and nuclear accidents
- Radiation dose
- a measure of the risk of harm from the body being exposed to radiation, measured in sieverts
4.4.3.2Different half-lives of radioactive isotopes
Physics only — not assessed in Combined Science: Trilogy.
Radioactive isotopes have a very wide range of half-life values — from fractions of a second to billions of years. You should be able to use data presented in standard form, which is how such a range has to be written.
You should be able to explain why the hazards associated with radioactive material differ according to the half-life involved.
The answer to "which is more hazardous" is therefore "it depends what you mean", and a good answer says so. A short half-life means intense radiation now but a source that soon becomes safe. A long half-life means weaker radiation but a hazard that persists for generations — which is why radioactive waste is so difficult to deal with.
This explains the choice of isotopes in medicine: a tracer put into a patient should have a short half-life, so that it stops emitting soon after the scan is finished and the patient's total dose stays low.
4.4.3.3Uses of nuclear radiation
Physics only — not assessed in Combined Science: Trilogy.
Nuclear radiations are used in medicine for the:
• exploration of internal organs • control or destruction of unwanted tissue.
You should be able to describe and evaluate the uses of nuclear radiations for both, and to evaluate the perceived risks of using nuclear radiations in relation to given data and consequences.
A tracer should also have a short half-life, so that its activity falls quickly once the scan is over, and it should be chosen so that the body eliminates it rather than storing it.
Evaluating the risk means weighing the benefit against the harm using the data given. Radiation is ionising and can damage cells and cause cancer, so every dose carries some risk. But an undetected tumour or an untreated cancer carries a much larger and more immediate risk. The judgement is whether the information gained, or the tissue destroyed, is worth the dose received — and that is a comparison, not an absolute.
Worked example
A patient is offered a gamma scan carrying a dose of 5 mSv. The average annual background dose is about 2.7 mSv. Evaluate whether the scan is justified.
- Compare the dose with something familiar: 5 mSv is a little under two years of background radiation.
- State the risk: the radiation is ionising and could damage cells, slightly raising the lifetime risk of cancer.
- State the benefit: the scan may detect a condition that is treatable if found early and dangerous if missed.
- Weigh them, and note what would change the answer.
Answer: The dose is comparable to about two years of natural background, so the added risk is small, while the benefit of detecting a serious condition early is large — so the scan is justified. It would be harder to justify for a patient at very low risk of the condition, or if the scan were to be repeated many times.
Common mistake
"Perceived risks" means the risks as people judge them, which may not match the data. An evaluation should use the figures given rather than a general statement that radiation is dangerous.
Before you practise
- This sub-topic is physics only — it is not assessed in Combined Science: Trilogy.
- Background radiation comes from natural sources such as rocks and cosmic rays, and man-made sources such as fallout from weapons testing and nuclear accidents.
- Background level and radiation dose are affected by occupation and location.
- Radiation dose is measured in sieverts; 1000 mSv = 1 Sv, and you do not need to recall the unit.
- Short half-life means high activity but a hazard that ends quickly; long half-life means lower activity but a hazard lasting a very long time.
- Nuclear radiation is used in medicine to explore internal organs and to control or destroy unwanted tissue.
- A tracer should be gamma-emitting with a short half-life, so it reaches the detector and stops emitting soon afterwards.
- Evaluate risk by comparing the dose and its consequences against the benefit, using the data you are given.