6.28P, 6.33P–6.35PUses of nuclear radiation
Physics only — not on the Combined Science specification
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.
6.29–6.32Radioactive contamination
Radioactive contamination is the unwanted presence of materials containing radioactive atoms on other materials. The hazard from contamination is due to the decay of the contaminating atoms. The type of radiation emitted affects the level of hazard.
Irradiation is the process of exposing an object to nuclear radiation. The irradiated object does not become radioactive.
That last sentence is the whole distinction, and it is examined constantly. Contamination means the radioactive material itself is on or in the object, so the hazard continues until it is removed. Irradiation means only the radiation reached the object; move the object away, or remove the source, and the exposure stops immediately.
You should be able to compare the hazards associated with contamination and irradiation. Alpha is the greatest hazard inside the body because it is strongly ionising and all its energy is deposited in a very short distance in nearby tissue. From outside the body it is the least hazardous, because it cannot even penetrate skin. Gamma is the reverse on both counts.
Suitable precautions must be taken to protect against any hazard that the radioactive source used in the process of irradiation may present — for example handling sources with tongs, storing them in lead-lined containers, keeping exposure times short, and standing well back.
You should also understand that it is important for the findings of studies into the effects of radiation on humans to be published and shared with other scientists, so that the findings can be checked by peer review. Peer review lets other scientists examine the method and the data before conclusions are accepted, which catches errors and bias — and on a subject as consequential as radiation exposure, that matters.
- Radioactive contamination
- the unwanted presence of materials containing radioactive atoms on other materials; the hazard comes from the decay of the contaminating atoms
- Irradiation
- the process of exposing an object to nuclear radiation; the irradiated object does not become radioactive
Before you practise
- Match the isotope to the job through its radiation type and half-life — that pairing is what gets the marks.
- Smoke alarms use alpha; medical tracers use gamma with a short half-life; sterilising and treating tumours use gamma.
- Irradiation is being exposed to radiation from a source outside you; contamination is getting the radioactive material onto or into you.
- Contamination is the more dangerous of the two, because the source stays with you and keeps emitting.
- Ionising radiation damages tissue and can cause mutations that lead to cancer.
- Precautions: shielding, limiting the time of exposure, keeping your distance, and handling sources with tongs.
- A tracer for a PET scan has to be made nearby because its half-life is too short to survive transport.