S4.11.5 Chapter 5

Radiation and safety

A worker stands near a strong gamma source for an hour and then walks away. Is the worker now radioactive?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Yes, the radiation has made them radioactive.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Yes, but only until they wash.
You picked B. Read on. At the end, the chapter comes back to your guess.
C No. They were irradiated, and they stay as they were.
You picked C. Read on. At the end, the chapter comes back to your guess.
D No, because gamma rays cannot reach the body.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Compare contamination and irradiation
Explain how distance, shielding and time reduce a dose

The radiation in this book is called ionising radiation, because it ionises atoms. In living cells that can damage the molecules, including DNA. A cell with a little damage usually repairs itself. Too much damage, and the cell may die or divide wrongly.

A very large dose in a short time kills many cells and causes radiation sickness. Smaller doses cause no sickness. A moderate dose adds a small extra chance of cancer, years later. For very small doses, like those from background, the extra chance is too small to measure. Safety rules assume that it is there and that it rises with the dose, so they keep every dose low.

So the aim is clear: keep every dose as low as is reasonable. Some risk is accepted when the gain is greater, as with a medical scan. This chapter shows how people with radioactive materials do it, and it begins with two ways of being exposed.

Irradiation is exposure to radiation from a source outside the body. The source may be a sealed container. While you are near it, your body receives a dose. When you move away, or the source is taken away, it stops. You do not become radioactive. The alpha, beta and gamma rays from a source cannot make your body radioactive. Neutrons can make things radioactive, and they are found inside a working reactor, not near ordinary sources.

Irradiation: the source stays outside. Take the source away and the exposure ends.

Contamination is having radioactive material on or in the body, or on an object. It may be dust or liquid. The source is now on you, and it goes where you go. It keeps on irradiating you until it is washed off, passes out of the body, or decays.

Contamination: the radioactive dust is on the person, so the source goes with them.

Contamination also spreads. Dust can pass from hands to door handles, to food, to other people. That is why workers wear gloves and protective clothing, wash, and leave their outer clothes at the door. Irradiation does not spread in this way.

The kind of radiation matters, and it matters in different ways. Outside the body, alpha is stopped by dead skin and a few centimetres of air, so it does little harm. Gamma and fast beta go in, so a source of those outside the body can be a risk.

Inside the body, it is the other way. A source of alpha that is swallowed or breathed in is next to living cells. The alpha particles ionise strongly and all their energy goes into a very small space. So alpha is the most harmful kind when it is inside.

A wrong idea many people have

Alpha is the most dangerous kind of radiation, in every case.

Why it feels right: Alpha is the most ionising, and ionising is what harms cells.

The test that breaks it: Think about where the particles go. Outside the body, a few centimetres of air and dead skin stop them. Take the same source into the lungs as dust, and nothing stops them.

What is true: Alpha is the most harmful inside the body and the least harmful outside it. Gamma is the opposite: it is a greater risk from outside. Where the source is matters as much as what it is.

Now to lowering a dose. There are three ways, used together. Keep your distance. Put a shield in the way. Spend as little time as you can near the source.

Distance. The rays from a small source spread out in all directions. The further away you are, the thinner they are. The picture counts 16 rays that fall on squares of the same size. At twice the distance they cover four times the area.

Double the distance and each square gets a quarter of the rays.

So for gamma rays from a small source, doubling the distance cuts the dose rate to one quarter. Trebling it cuts it to one ninth. This holds when the air between takes in very little, which is true for gamma. It does not hold for alpha, which stops in a few centimetres of air. Nor does it hold for beta, which the air also takes in.

Shielding. A shield takes in radiation before it reaches you. Paper stops alpha. A few millimetres of aluminium, or about a centimetre of plastic, stop beta. Thick lead or concrete reduce gamma. Each layer takes away a share of the gamma, and none takes it all.

Thick lead takes most of the gamma rays, but some pass through.

Time. While the rate stays the same, the dose is the dose rate multiplied by the time. dose = dose rate × time. Halve the time and you halve the dose.

At a steady 20 µSv an hour, the dose grows in a straight line.

Worked example

A worker stands where the dose rate is 20 µSv an hour. The worker stays for 3 hours. What dose does the worker receive?

dose = dose rate × time.
dose = 20 × 3.
dose = 60 µSv.

Answer: 60 µSv

Try it

Plan a job to lower the dose

You need: paper, a pencil.

Imagine a job of 30 min near a small gamma source. The dose rate at 1 m is 96 µSv an hour.
Work out the dose for 30 min at 1 m. Remember that 30 min is half an hour.
Now stand at 2 m. The dose rate falls to a quarter. Work out the new dose rate and the dose for 30 min.
Now do the job in 15 min at 2 m. Work out the dose.
Write the three doses in a table.

What you will see: Each change lowers the dose. Distance cut it to a quarter, and the shorter time halved it again. A lead shield would lower it further. Workers use all three together.

If you cannot try it, this is what happens.

People who work with radiation also wear a badge that records their dose. There are limits on the dose. Many countries follow the advice of the International Commission on Radiological Protection. A member of the public should get no more than 1 mSv a year from all the ways people use radiation. Natural background and medical care are not counted in this. A radiation worker may get up to 20 mSv a year, as an average over five years. It must never be more than 50 mSv in one year. Workers are allowed more. They are adults who chose the work. They are trained to keep their dose low, and their dose is measured all the time.

Yearly limits advised by the International Commission on Radiological Protection. For workers, 20 mSv is an average over five years. Natural background and medical care are left out.

A wrong idea many people have

A person who has been irradiated is radioactive and a danger to others.

Why it feels right: In films, people who have been hit by radiation glow, and everyone avoids them.

The test that breaks it: Take a person who has had a medical X-ray. A counter held near them reads the same as before. The X-rays passed through and left nothing behind.

What is true: Irradiation does not make a person radioactive. Contamination is different: if radioactive dust is on the person, they can pass it on until it is removed.

Key words

irradiation: exposure to radiation from a source outside the body.
contamination: having radioactive material on or inside the body or an object.
shielding: material that takes in radiation before it reaches you.
dose rate: the dose received each hour or each second.

Back to your guess

The answer is C: No. They were irradiated, and they stay as they were.

Gamma rays do pass into the body and can do harm, but they do not make it radioactive. The worker received a dose and is not a source. It would be different if radioactive dust were on the worker.

Keep it

Copy this into your exercise book.

Irradiation: a source outside, it stops when you leave. Contamination: radioactive material on or in you, it goes with you. Alpha is worst inside, gamma worst outside. Lower a dose by distance, shielding and time. dose = rate × time.

Draw: A person with a source outside and rays arriving, beside a person with dots on their body.