Matching radiation to the job

1. Every use of a radioactive source is chosen by matching two properties of the radiation to the job: how penetrating it is and how strongly ionising it is.
2. Alpha radiation ($\alpha$) is a helium nucleus, ${}_2^{4}He$, so it is the most strongly ionising type and the least penetrating; it travels a few centimetres in air and is absorbed by a sheet of paper.
3. Beta radiation (${\beta }^{-}$) is a fast-moving electron, ${}_{-1}^{0}e$, so it is moderately ionising and moderately penetrating; it passes through paper but is absorbed by about $3mm$ of aluminium.
4. Gamma radiation ($\gamma$) is a high-energy electromagnetic wave, so it is the least ionising and by far the most penetrating; it is only reduced, never fully stopped, by thick lead or concrete.
5. Penetration decides whether the radiation can reach the thing it must affect or the detector that must measure it.
6. Ionising power decides how much change the radiation makes when it arrives, whether that change is ionising air, damaging the DNA of a microorganism or damaging a tumour cell.
7. The activity of the source, measured in becquerel ($Bq$), is chosen alongside the type: a smoke alarm needs a tiny activity, whereas a sterilisation plant needs a very large one.

Smoke alarms

1. An ionisation smoke alarm contains a very small alpha source, usually americium-241, sealed inside an ionisation chamber.
2. Two metal electrodes in the chamber are connected across a potential difference, with a gap of air between them.
3. Alpha particles ionise the air in the gap, knocking electrons off air molecules to create positive ions and free electrons.
4. These charged particles are attracted to the electrodes, so a small steady current flows across the gap even though air is normally an insulator.
5. When smoke enters the chamber, smoke particles absorb alpha particles before they can ionise much of the air.
6. Fewer ions are produced each second, so fewer charge carriers cross the gap and the current falls.
7. An electronic circuit monitors the current and sounds the alarm as soon as it drops below a set value.
8. Alpha is the right choice because it is strongly ionising, so a tiny activity still produces enough ions to give a measurable current, and because its range in air is only a few centimetres, so the radiation cannot escape the casing into the room.
9. Gamma would fail on both counts: it is weakly ionising, it would pass through the smoke without being absorbed, and it would escape from the alarm.

Irradiating food

1. Food travels on a conveyor into a heavily shielded chamber containing a gamma source, typically cobalt-60.
2. Gamma rays pass through the packaging and through the full depth of the food, ionising molecules inside any bacteria, fungi, moulds and insect eggs present.
3. Ionisation damages the DNA of those organisms, so they are killed or can no longer reproduce.
4. With far fewer microorganisms alive, the food spoils more slowly and its shelf life increases, which cuts waste and lowers the risk of food poisoning.
5. Gamma is essential because alpha and beta would be absorbed by the packaging and would never reach the food inside.
6. The food is exposed to radiation but no radioactive material is put into it, so it does not become radioactive and is safe to eat.
7. Irradiation can slightly reduce the vitamin content of some foods, and irradiated food must be labelled so shoppers can still choose whether to buy it.

Sterilising equipment

1. Syringes, scalpels, dressings and surgical instruments are sealed inside their packaging first and then passed through a gamma irradiation plant.
2. Gamma rays penetrate the packaging and the instrument, ionising molecules inside any microorganisms on the surfaces and killing them.
3. Because the item is sealed before it is irradiated, nothing can land on it afterwards, so it stays sterile until the pack is opened.
4. Gamma sterilises without heating the object, so plastic syringes and delicate instruments are not melted or damaged, which is the main advantage over steam sterilisation in an autoclave.
5. The source is kept in a thick concrete cell and loading is done remotely, because the activity needed to sterilise in a reasonable time is very large.

Industrial tracers

1. To find a leak in an underground pipe, a gamma-emitting tracer is added to the liquid flowing through it.
2. A Geiger-Muller tube connected to a counter is moved along the ground above the pipe and the count rate is recorded at each position.
3. Along an intact section the count rate stays roughly steady, because the tracer inside the pipe is the same distance from the detector at every point.
4. At a leak, tracer escapes into the soil and collects there, so the detector records a sharp rise in count rate at that position.
5. The engineers then dig at that one position instead of excavating the whole length of the pipe.
6. Gamma is the only sensible choice because the radiation must pass through the pipe wall and the soil to reach a detector at the surface, and alpha and beta would both be absorbed long before that.
7. The tracer is chosen with a half-life long enough to complete the survey but short enough that its activity soon falls to a safe level.
8. The same method tracks the movement of sewage and silt at sea and finds blockages inside industrial pipework.

Thickness gauging

1. In a paper or aluminium foil mill a beta source such as strontium-90 sits above the moving sheet and a detector sits directly below it.
2. Some beta particles are absorbed by the sheet and the rest reach the detector, so the count rate depends on the thickness of material between them.
3. If the sheet becomes too thick, more beta particles are absorbed and the detector records a lower count rate.
4. If the sheet becomes too thin, fewer beta particles are absorbed and the detector records a higher count rate.
5. A control system compares the count rate with the target value and moves the rollers closer together or further apart until the reading returns to that value.
6. Beta is used because it is partly absorbed, which is exactly what makes the count rate sensitive to small changes in thickness.
7. Alpha would be absorbed completely, so the detector would read only background whatever the thickness, and gamma would pass almost entirely through, so the reading would barely change.
8. A source with a long half-life is chosen so that the activity, and therefore the target count rate, does not drift during the life of the machine.
9. Thicker or denser products such as rolled steel use a gamma source instead, because beta would be completely absorbed by steel.

Finding and treating cancer

1. A gamma-emitting tracer can be swallowed or injected, and it collects in the organ the doctor wants to examine.
2. Gamma is used because it is penetrating enough to leave the body and reach a detector outside it, whereas alpha and beta would be absorbed by the surrounding tissue.
3. A tumour is treated by aiming high-energy gamma radiation at it, so that ionisation damages the DNA of the tumour cells and stops them dividing.
4. Cancer cells divide more often than most healthy cells, which is why they are more strongly affected by the same dose.
5. Healthy cells in the path of the beam are damaged as well, which causes side effects, so the beam must be aimed accurately at the tumour and no wider.