Radiation in Action

Radiation in Action

Radioactivity sounds like something locked away in a reactor, but low-level radiation is passing through you right now, and doctors deliberately use it to save lives. This module (the sequel to The Radiation Lab) is about the radiation around us, and the radiation we put to work.

The radiation around us

Background radiation is the low-level radiation present all the time, everywhere. It is not the same everywhere, though: it depends on where you are (high altitude means more cosmic rays, granite country means more radon) and on what you do (pilots and cabin crew meet more cosmic rays, radiographers and nuclear workers more of it at work). The sources fall into two groups:

Mark the claims

Five statements about background radiation. Tap the TWO that are WRONG.

• Background radiation is present all the time, everywhere, at a low level.
• Radon gas seeping out of rocks is a natural source of background radiation.
• Background radiation is the same wherever you stand in the UK.
• An airline pilot receives more background radiation than an office worker.
• Almost all background radiation comes from the nuclear power industry.

Two towns, two doses

Two towns are at the same altitude and neither has any nuclear industry, yet one records a much higher background radiation dose than the other. What is the most likely reason?

• It is built on granite, so more radon gas seeps out of the ground
• It receives more cosmic rays
• People there eat more food, and food contains radioactive isotopes
• More weapons-testing fallout landed there

Fast or slow to fade

Remember half-life: the time for the activity to halve. It shapes how hazardous a source is, in two opposite ways: - A short half-life source is very active, so it is more hazardous now, but it decays away quickly. - A long half-life source is less intense, but it stays radioactive and hazardous for a very long time. The tracer injected for a medical scan has a half-life of a few hours: intensely active while the camera is watching, and gone by the next day. Some of the waste from a reactor has a half-life of thousands of years: far weaker second by second, and still a problem long after everyone alive today has gone.

Why store it for so long?

Waste from a reactor contains an isotope with a half-life of 24,000 years. Its activity is far lower than a fresh medical tracer's. Why does it still have to be sealed away for millennia?

• Because a long half-life means the activity falls very slowly, so it stays hazardous for an enormous time
• Because a long half-life makes a source more intensely active
• Because its activity will grow over the centuries
• Because long half-life sources always emit gamma radiation

Radiation put to work

Used carefully, radiation is hugely useful. In every case the job decides the emission and the half-life, not the other way round:

Pick the source for the job

To sterilise surgical instruments inside their sealed packaging you need _____ radiation, because it is penetrating enough to get through the wrapping. A thickness gauge for aluminium foil uses a _____ source instead, because that radiation is only partly absorbed, so the reading changes when the foil does. A medical tracer needs a _____ half-life, so it decays away soon after the scan. The source inside a thickness gauge needs the opposite, a _____ half-life, so the factory is not replacing it every week.

Choose a medical tracer

A hospital needs a radioactive tracer to image a patient's kidneys. Choose its properties, one at a time.

• The tracer will be inside the body, but a camera outside must detect it. Which radiation should it emit?
• You do not want the tracer irradiating the patient for weeks. What half-life should it have?
• Any radiation carries some risk. Is using the tracer justified?

The thickness gauge

A factory monitors the thickness of aluminium foil with a radioactive source on one side and a detector on the other. Which source works best, and why?

• A beta source: beta is partly absorbed, so the amount getting through changes if the foil thickness changes
• An alpha source: alpha passes straight through metal unchanged
• A gamma source: gamma passes through almost unchanged whatever the thickness
• No source works: radiation cannot measure thickness at all

Justify the scan

A patient is offered a kidney scan using an injected radioactive tracer, and is worried about having radiation put inside her. Explain how a hospital decides that the scan is justified.

• Name the emission the tracer uses and say why that type is chosen
• Say what half-life it has and why that limits the patient's exposure
• State honestly what the risk is, rather than claiming there is none
• Say what the benefit is, and how the two are weighed against each other

Action summary

Most background radiation is _____, coming from sources like radon gas in rocks and cosmic rays, and the dose depends on where you live and what you do for a living. A source with a _____ half-life is very active but decays away quickly. Medical tracers use _____ radiation with a short half-life so it can be detected outside the body and soon decays. Every use must weigh the _____ against the benefit.