S4.11.6 Chapter 6

Uses of radiation

A doctor gives a patient a small amount of radioactive material to scan the body. Which kind of radiation should it give out?

Your guess

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

Your guess

A Alpha, because it ionises the most.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Beta, because the body stops it.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Alpha or beta, because they are the strongest.
You picked C. Read on. At the end, the chapter comes back to your guess.
D Gamma, because it passes out of the body to a detector.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain medical uses, such as tracers and cancer treatment
Explain how carbon dating shows how old remains are

Radiation that can harm can also help, when the dose and place are controlled. People choose the kind of radiation and the half-life to fit the job. This chapter looks at jobs in medicine and industry, and then at dating old remains.

A tracer is a small amount of a radioactive isotope used to follow where something goes. In medicine, a patient swallows or is injected with a tracer. It travels round the body. A gamma camera outside the body picks up the rays and makes a picture.

A tracer shows how an organ works, because the picture shows where the tracer goes and how fast.

The tracer must give out gamma, so the rays leave the body. It must also have a short half-life, of a few hours. A long half-life would leave the patient with a dose for days or years, for no gain. One tracer is technetium 99m, with a half-life of about 6 hours.

Worked example

A tracer with a half-life of 6 hours gives 640 counts/min when it is given. What does it give after 24 hours?

Half-lives = time ÷ half-life.
Half-lives = 24 ÷ 6 = 4.
Halve the count rate 4 times: 640 → 320 → 160 → 80 → 40.
The count rate after 24 hours is 40 counts/min.

Answer: 40 counts/min

Another use is to treat cancer. This is radiotherapy. High doses of radiation kill cells, and cancer cells are killed more easily than most healthy ones. A machine sends a beam of gamma rays or X-rays into the body. The dose is given in many small amounts, so healthy cells can recover.

A beam passes through healthy tissue on its way in and on its way out. One beam gives the tissue in its path about the same dose as the tumour, or more. So the beam is sent from different directions.

One beam. All the tissue in its path gets a dose.

Three beams cross at the tumour. Each path of healthy tissue gets one beam, and the tumour gets all three.

With three beams crossing at the tumour, it gets the dose from all three. Any path of healthy tissue gets only one. Another way is to place a small radioactive source in or beside the tumour. Its radiation travels only a short way.

Gamma rays also kill bacteria. Syringes, dressings and some food are sealed in packets and passed through a beam of gamma rays. This sterilises them. The things are not made radioactive. Its source has a long half-life, such as cobalt 60 at about 5.3 years, so it works for years.

Industry also uses radiation. A paper mill checks its paper with a source of beta particles on one side and a detector on the other. A thicker sheet takes in more of the beta, so the count rate falls. A reading that is too low means the paper is too thick, and the rollers are adjusted.

A thin sheet. Most of the beta particles get through.

A thick sheet takes in more of the beta. The reading falls, so the machine knows the sheet is thick.

Beta is chosen for paper because it goes through thin sheets but not through thick ones. Alpha would be stopped by even thin paper, so the detector would always read about zero. Gamma goes through almost unchanged, so the reading would hardly change with thickness.

A smoke alarm holds a tiny source of americium 241, which gives out alpha. The alpha particles ionise the air between two plates, and a small current flows. Smoke takes in the alpha particles, so the current falls, and the alarm sounds. A sealed source is safe in a home. It must not be taken apart.

The alarm works because alpha is stopped so easily.

A wrong idea many people have

A tracer should have a long half-life, so that it can be followed for longer.

Why it feels right: A source that lasts seems more useful, because it would still be there for a late picture.

The test that breaks it: Think about what a long half-life means. Most of the tracer is still in the body, and still irradiating it, days or years later. The picture was done in hours.

What is true: A medical tracer has a short half-life, of hours, and gives out gamma. The patient gets the picture and a small dose, and the tracer is soon gone. A source for sterilising has a long half-life instead, to work for years.

Now for dating old remains. Carbon dioxide in the air has a tiny, nearly steady share of carbon 14, about one atom in a million million. Cosmic rays make it in the upper air. Plants take it in as they grow. Animals eat the plants, so all living things hold the same share.

When a living thing dies, it takes in no more carbon. The carbon 14 that it holds decays, with a half-life of about 5 700 years. So the share falls. Measure how much carbon 14 is left in a bone or in wood and you can say how long ago it died. This is carbon dating.

Carbon 14 in a dead sample. Living wood gives about 14 decays each minute for each gram of carbon. The graph uses 16 so that the numbers stay whole.

Worked example

Living wood gives 16 counts/min for each gram of carbon. A piece of old wood gives 4. The half-life of carbon 14 is 5 700 years. How old is the wood?

Halve the living rate until you reach the old rate: 16 → 8 → 4.
That is 2 half-lives.
age = 2 × 5 700 years.
age = 11 400 years.

Answer: 11 400 years

Carbon dating has limits. It works only on things that were once alive, such as wood, bone, cloth and charcoal. It cannot date rock or metal. After about 50 000 years, too little carbon 14 is left to measure. Scientists also correct dates using tree rings, because the share in the air has changed a little over time.

Try it

Share out the dose on paper

You need: paper, a pencil.

Draw a large circle for the body. Draw a small circle at its centre for the tumour.
Draw one beam as a straight line right across the body, through the tumour. Shade its path.
Say that each beam gives 1 unit of dose along its path. How many units does the tumour get? How many does a point on the path get?
Now draw 3 beams from different sides. Each must run straight through the tumour.
Count the units again for the tumour, and for a point on the path of one beam.

What you will see: With one beam, the tumour gets 1 unit and so does the healthy tissue in the path. With three beams that cross at the tumour, the tumour gets 3 units and a point on one path gets 1. The tumour now gets three times as much as that tissue.

If you cannot try it, this is what happens.

A wrong idea many people have

Carbon dating can give the age of a rock or a metal tool.

Why it feels right: It is the best known way to date the past, so it seems to work on anything old.

The test that breaks it: Look at what carbon dating measures. It measures carbon 14 taken in while the thing was alive. A rock was never alive and never took any in.

What is true: Carbon dating works only on remains of living things, such as wood, bone and cloth, up to about 50 000 years old. Other methods date rocks.

Key words

tracer: a small amount of a radioactive isotope used to follow where something goes.
radiotherapy: treating cancer with beams of radiation that kill cells.
sterilise: to kill bacteria, for example with gamma rays.
carbon dating: finding the age of remains from the carbon 14 left in them.

Back to your guess

The answer is D: Gamma, because it passes out of the body to a detector.

To make a picture, the radiation must get out of the body and reach a camera. Only gamma passes through the body in large amounts. Alpha and beta would be stopped inside, and give a dose with no picture.

Keep it

Copy this into your exercise book.

Tracers: gamma, short half-life. Radiotherapy: beams cross at the tumour. Beta gauges thickness, alpha works in smoke alarms. Carbon dating: carbon 14 halves every 5 700 years. Only for once-living things, to about 50 000 years.

Draw: A circle for the body with three beams crossing at a small circle for the tumour.