Sign up to access the complete lesson and track your progress!
Unlock This CourseFor every job, engineers ask the same two questions you met in Uses of Radioactivity in Medicine. Which radiation? (alpha, beta or gamma, from Alpha, Beta and Gamma Radiation) and which half-life? (from Activity and Half-Life). This lesson covers the industrial uses. Medical uses are in the previous lesson.
Remember the penetrating powers. Alpha is stopped by paper or a few centimetres of air. Beta is stopped by a thin sheet of aluminium. Gamma passes through most things and is only reduced by thick lead or concrete.
Key terms:
Paper, plastic film and metal foil are made as one long moving sheet. The sheet must be the same thickness all along. Here is how a beta source helps.
Alpha would be stopped by even the thinnest paper, so nothing would reach the detector. Gamma would pass through almost any sheet, so a change in thickness would hardly change the count. Beta is absorbed a little, and by an amount that depends on the thickness. So the count rate changes when the thickness changes.
The source runs all day, every day. If its half-life were short, the activity would fall and the count rate would drop for a reason that has nothing to do with the sheet. A long half-life (many years) keeps the activity almost steady, so a change in count really does mean a change in thickness.
A pipe buried under a road may leak, or become blocked, and digging up the whole length would be slow and expensive. Instead, a small amount of a radioactive tracer is added to the liquid in the pipe.
The detector is above ground and the tracer is under the soil, so the radiation has to travel through earth and the pipe wall. Alpha and beta would be absorbed. Gamma rays are penetrating enough to reach the detector.
The tracer ends up in the soil and perhaps in water. A short half-life (hours or a few days) means its activity falls to a very low level soon after the job, so it does not stay in the ground as a lasting hazard.
Remember from Half-Life Calculations and Graphs that after several half-lives the activity is tiny. After a day or two, a tracer with a half-life of a few hours has almost gone, which is exactly why a short half-life is chosen.
Many smoke alarms contain a very small alpha source. Inside the alarm, the alpha particles ionise the air in a small gap between two metal plates. The ions are charged, so a small electric current flows across the gap, and the alarm circuit measures it.
When smoke enters the gap, the smoke particles absorb the alpha particles. Fewer ions are made, so the current falls. The circuit detects the drop and sounds the alarm.
Alpha is the most strongly ionising radiation, so it makes plenty of ions in the air. It is also stopped by a few centimetres of air and by the plastic case, so the people in the room are not exposed to it.
Nobody wants to replace the source every few weeks. A half-life of hundreds of years means the activity stays the same for the whole life of the alarm, so the current in clean air is steady.
Food can be treated with gamma rays to kill bacteria and moulds, so it stays fresh for longer. You met this idea in Uses of Electromagnetic Waves. Here is why gamma is the radiation chosen.
The food is only irradiated. The gamma rays pass through it and do not make the food radioactive.
| Use | Radiation | Half-life | Reason |
|---|---|---|---|
| Thickness control | Beta | Long | Partly absorbed by the sheet, so the count shows thickness. Steady activity. |
| Pipe tracer | Gamma | Short (hours or days) | Reaches a detector above ground. Fades soon, so little is left in the soil. |
| Smoke alarm | Alpha | Long | Ionises air strongly, stopped by the case. Lasts for years. |
| Sterilising food | Gamma | Long | Goes through packaging. Steady source. |
Here are five sources. They are made-up examples, to practise choosing.
| Source | Radiation | Half-life |
|---|---|---|
| P | Alpha | 400 years |
| Q | Beta | 30 years |
| R | Gamma | 6 hours |
| S | Gamma | 5 years |
| T | Beta | 2 minutes |
Pick the best source for each job: (1) checking the thickness of aluminium foil, (2) finding a leak in a buried water pipe, (3) a smoke alarm, (4) sterilising packed bandages.
(1) Q: beta, with a long half-life. T is beta but would fade in minutes.
(2) R: gamma to reach the detector, with a short half-life so it fades.
(3) P: the only alpha source, and its long half-life suits a smoke alarm.
(4) S: gamma goes through the packs, and a long half-life keeps it steady. R would fade within a day.
A factory makes plastic film on a moving roll. A radioactive source is on one side of the film and a detector is on the other side.
(a) State which type of radiation the source should give out. [1 mark]
(b) Explain why the other two types of radiation are not suitable. [2 marks]
(c) The film becomes thinner. State what happens to the count rate on the detector. [1 mark]
(d) Explain why the source needs a long half-life. [2 marks]
(a) Beta (1)
(b) Alpha would be stopped by the film, so nothing reaches the detector (1). Gamma would pass straight through, so a change in thickness would make almost no difference to the count rate (1)
(c) The count rate increases (1)
(d) A long half-life means the activity stays almost constant (1), so a change in count rate is due to the thickness changing and not to the source fading (1)
In "explain why not" questions, give a reason for each rejected radiation. One says it is stopped, the other says it passes through.