Lesson 2: Alpha, Beta and Gamma
In this lesson we'll introduce the three most important kinds of nuclear radiation: alpha, beta and gamma. We’ll see how their different properties affect what they can be used for and in what ways they may be harmful.
What is alpha, beta and gamma radiation?
We're not going to go into details about the exact origins of alpha, beta and gamma radiation in this lesson.
For the moment we'll say that alpha and beta radiation consist of tiny particles, much smaller than an atom. They move incredibly fast, perhaps thousands of kilometres per second.
Gamma radiation is a sort of invisible, very high-energy light.
Alpha, beta and gamma are the first three letters of the Greek alphabet. The types of radiation are named in the order that they were discovered.
Americium-241, an alpha source
Americium (pronounced a-meh-rees-ee-um) is a metal. It's radioactive and gives off alpha radiation so we call it an alpha source.
In school experiments we only use a tiny radioactive source the size of a grain of sand. It's normally enclosed in a steel tube with a wire mesh covering one end. This means you can't touch the source directly and the radiation only escapes in one direction. There's often a prong so you can pick up the source with pliers.
Alpha radiation doesn't go far but is very damaging
Alpha radiation gets stopped by a few centimetres of air or a thin sheet of paper. You may think that this means alpha radiation is quite weak but in fact the opposite is the case.
Alpha radiation is like a big heavy ball rolled across a lawn. It doesn't go very far because it loses a lot of energy flattening out the bumps in the ground. In other words the heavy ball interacts strongly with the ground.
This is what alpha radiation does to air. Each alpha particle loses its energy by ripping the air atoms to pieces as it flies past. Eventually it loses all its energy and just stops harmlessly.
The difference between irradiation and contamination
The best way to stay safe is to keep away from an alpha emitter, like americium-241. You don't have to be very far, half a metre is fine.
This is easy if the americium is a solid block but if it’s dissolved in a liquid or crushed into dust then you need to be much more careful. Dust can be blown long distances by the wind. If you inhaled some americium dust then your lung linings could be damaged as the alpha radiation tore up the molecules in your cells.
When radioactive dust lands on something we say that thing has been ‘contaminated’. Contamination is about the stuff that’s emitting the radiation, like dust or water. Contamination can be a risk over long distances.
Irradiation is about the radiation itself like alpha, beta or gamma. Radiation can’t travel far so is not a risk over long distances.
Alpha particles cause lots of ionization in a short distance
Alpha radiation is up to twenty times more damaging than other kinds because it tears up atoms so much. This tearing up process is called ‘ionization’ and we’ll see what it means in more detail later. The torn-up air particles (or “ions”) have an electric charge so they can be part of an electric circuit.
Using americium-241 in a smoke detector
One use of americium-241 is in smoke detectors. The alpha particles tear up the neutral air molecules.
The resulting positive and negative ions can be part of an electric circuit. In normal use this circuit is complete.
When there's a fire smoke particles enter the detector. The ions stick to the much bigger smoke particles because charged things are attracted to uncharged things. This breaks the circuit. A different circuit senses the break and sets off the alarm.
Strontium-90, a beta source
Strontium-90 is a soft, highly reactive metal. It gives off beta radiation.
Beta particles can go through a few metres of air. They can pass through paper and thin aluminium easily but they get stopped by even a thin piece of lead.
Beta goes further than alpha but is less damaging
A beta particle can get through a few metres of air. It's like a golf ball rolled fast along our grassy lawn.
The ball jumps and bounces over the ground. You can see it’s path through the grass but it doesn’t flatten everything like alpha. The ‘beta radiation ball’ breaks some of the blades of grass but most of them spring back unharmed.
We say that beta radiation is not so strongly ‘ionizing’ as alpha because it doesn’t rip atoms to bits as much as it passes.
Does this mean beta radiation is safer? Yes, it does. Beta is safer than alpha.
Beta radiation will do less harm to a cell as it passes through. But it can reach more cells that it can harm a bit. Radiation is most harmful if a cell is badly damaged but not killed.
Beta’s longer range in air means you have to be a few metres away from the radioactive source in order to be safe. So you can protect yourself from exposure to beta radiation by keeping your distance. You could also use a thin lead shield but sometimes this can produce X-rays, which carry their own risk.
It's is much harder to keep your distance if the beta emitter is a dust or carried in water so it can spread throughout the environment. Again, beta radiation is most dangerous if you breath in or swallow a substance that emits beta radiation. Remember it’s the radioactive substance that gets breathed in. You can’t breath in ‘radiation’.
Using beta particles to measure thickness
Imagine we want to manufacture some thin sheet aluminium. A good way of controlling the thickness is to shine beta radiation through the aluminium and measure how much gets through.
The less beta radiation that gets through, the thicker the sheet is.
Beta gauges are very sensitive but the real advantage is that you can measure the aluminium without having to touch it as it races past. Often the system is computer controlled so the rollers are moved automatically to keep the thickness the same.
Tiny beta capsules can be used to treat cancer
The capsules are injected around the cancer and the beta radiation kills the cancer cells.
Radiation is particularly damaging to cells that are in the process of dividing. Cancer cells divide much more often than healthy cells. This means cancer cells tend to be killed while most of the healthy cells are unharmed.
Gamma radiation is often emitted with alpha and beta
Gamma radiation is a type of invisible, very high energy light. It's the same type of stuff as light, infrared, radiowaves and X-rays. These are all types of electromagnetic radiation.
Gamma radiation isn't emitted by itself, only after another event like alpha or beta decay. Normally it's emitted at almost exactly the same time.
Gamma rays can pass through lead but aren't very damaging
Gamma rays can pass through a thin sheet of lead with very little effect. You need about 10 cm of lead to stop most gamma rays completely.
Gamma rays are like a wind blowing over our lawn. It occasionally blows down a blade of grass but mostly it just passes through undisturbed.
Gamma rays are weakly ‘ionizing’. They can rip an atom to pieces but they don’t do it very often.
Gamma radiation spreads out
All radiation spreads out as it gets further from the source. But alpha and beta radiation can be absorbed by the surroundings quite easily so this spreading is more difficult to see.
Gamma radiation on the other hand passes through most things without really noticing so the spreading out is much easier to detect.
So gamma radiation tends to pass through your body without interfering with the molecules that make up your cells too much. But you need to be quite a long way away from a gamma source in order not to receive much radiation.
One safety precaution is to keep gamma sources in lead containers. Wearing a lead apron also helps protect people whose job means they have to handle gamma sources regularly. Since lead can itself be toxic another metal like titanium can be used instead for clothing.
Gamma rays are commonly used for tracers
Gamma can easily pass through solid materials unlike alpha and beta. If we use a source that doesn’t give off very much gamma radiation then we can use it as a ‘tracer’ in the environment.
For example, maybe we know an oil pipeline is probably leaking but we don’t know where. First of all a gamma emitter is mixed with the oil. The oil carries the emitter to the leak.
The leaking oil mixes in with the earth around the pipe and ends up closer to the ground. The gamma radiation can pass through the ground and be picked up by the gamma detector so you can tell where the leak is.
Gamma radiation can sterilize medical equipment
Intense gamma radiation can kill bacteria and other microbes. This makes it useful for sterilizing medical equipment.
The gamma radiation can reach inside even the most complex shapes and it doesn't expose delicate equipment to high temperatures. There's also no need to rinse off chemicals afterwards.
Gamma radiation can be used to see inside a patient
In low concentrations gamma radiation is not very damaging to living cells because it tends to pass straight through them.
Doctors can use gamma radiation to ‘see’ inside a patient. But instead of shining gamma radiation at a patient from the outside they inject a gamma emitter into the patient and look at the radiation that comes out.
Say the doctor wants to get an image of a tumour in the patient’s brain. X-rays are no good because X-rays only show bones and denser tissues. She selects a harmless chemical that will tend to accumulate in the brain tumour. Then she reacts the chemical with another chemical that emits gamma radiation.
This is called radioactive ‘tagging’. A common tag is called technetium-99m.
The tagged chemical is injected into the patient’s vein. It spreads around the body but tends to build up in the brain tumour. So the brain tumour gives off gamma radiation.
The doctor then uses a special camera called a ‘gamma camera', which is sensitive to the gamma radiation given off. The image is built up quite slowly because the gamma source used is not very ‘bright’ for improved safety. So it takes a while for the camera to capture enough packets of gamma radiation.
The doctor may takes lots of images of the tumour from different angles. Sophisticated computer software can then build a 3-D image of the tumour. The computer can then display slices through the tumour. Displaying slices is called ‘tomography’.
Gamma radiation can be used as a 'knife'
Gamma radiation can also be used to treat some kinds of cancer. This is called ‘external radiotherapy’ because the radioactive source is not injected into the patient.
This machine is called a ‘gamma knife’. The gamma rays can pass through hundreds of holes in a lead ring. It’s only in the middle of the ring that the gamma radiation is very concentrated.
The patient is moved round so that the gamma radiation kills the cancer cells.