Lesson 11: Ionization and detection
In this lesson we’ll look at ionization in more detail. We’ll find out how gamma rays are absorbed in lead and we’ll see how ionization can be used to measure radiation.
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Alpha particles in a cloud chamber
We know that alpha and beta particles ionize air atoms by knocking electrons off them. One way to see the ion tracks left by alpha or beta particles is to use a cloud chamber.
Alpha particles have a big mass and a charge of +2. The positive charge means they cause lots of ionization in a short distance.
Each time the alpha particle rips an electron off a molecule it loses some energy and slows down a bit. An alpha particle might make 100 000 ionizations before it loses all its energy. It grabs two electrons and becomes a helium atom.
An alpha particle will ionize most air molecules it passes close to, which is why it loses energy so quickly and has a short range in air. This is why the cloud trails are short and fat.
The alpha particle has quite a big mass. So it’s difficult to change its direction. This makes the path straight.
Ionizations get closer together as the particle slows down
You may also notice that the paths start thin and then get fatter. This is because the alpha is more ionizing when it’s going slower.
Low energy alphas make thicker tracks than high energy ones because a low energy alpha spends more time near the atom so has more chance to ionize it.
The paths are all the same length. This is because alphas from the same source have the same energy.
Beta particles in a cloud chamber
With high energy beta the ionizations are much further apart. The beta’s single negative charge means it ionizes say every 100 or so air molecules it comes across. Again each ionization costs the beta particle some energy and it gradually slows down.
The tracks made by the low energy beta are thicker than for the high energy beta. Again, this is because the beta particle is more ionizing the slower it goes, since it spends more time near each air molecule.
Low energy beta tracks also tend to wiggle. This is because a beta particle has a tiny mass compared with an air molecule and so can be fairly easily deflected every time it causes an ionization.
With low energy beta the paths are different lengths. This is because beta particles from the same source come with a range of energies. The ones that happen to have higher energy go further. Higer energy betas often end up hitting the walls of the cloud chamber so you can't tell how long their paths would be.
It's how close to together ionizations are, not how many
A beta particle and an alpha particle of the same energy will make about the same total number of ionizations before stopping.
But the alpha particle causes those ionizations in a shorter distance than the beta. This is why alpha particles have a shorter range in air and why we say that alpha is more ionizing than beta.
Each gamma ray photon only causes a single ionization
Alpha and beta particles cause many ionizations. But a gamma ray causes only one ionization and then disappears.
Gamma is only emitted as a result of alpha or beta radiation but we’ve used a lead plug as a filter so only gamma gets through. In a cloud chamber the single gamma ionization tends to happen in the wall or the floor of the chamber rather than in the air.
The ionized electron gains most of the gamma ray’s energy and has a very high velocity. The gamma hits the wall and knocks out an electron in a random direction. The electron leaves a track like a beta particle.
Gamma doesn't have a particular range
An alpha or beta particle of a given energy will have a particular range in, say, air. A few centimeters for alpha, a few metres for beta. It won't ever make it much more or less than this before it loses all its energy ionizing the particles it passes close to.
Gamma rays don't behave like this. With gamma rays we use the idea of half-thickness rather than range.
Geiger-Mueller tubes: using ionization to count radiation particles
Alpha, beta and gamma radiation all cause ionization. This ionization can be detected to show that radiation is present.
This is how a Geiger-Mueller (GM) tube works. The GM tube is the part that detects the radiation in a Geiger counter.
The GM tube has a thin ‘window’ of a mineral called mica which allows alpha particles to pass through. In the middle of the tube there is a thin wire that is at a high voltage and it’s filled with a low pressure gas.
When an alpha or beta particle enters the tube it causes some ionization and some electrons are freed. The high voltage wire attracts these electrons. The electrons accelerate and knock off even more electrons from other atoms.
This cascade of electrons means a small current flows in the wire. The current is detected and one ‘count’ is registered.
GM tubes have their advantages and limitations
GM tubes are good at detecting alpha and beta radiation because they cause lots of ionization in the gas. But gamma rays normally pass straight through the gas without causing any ionization. This makes them difficult to detect.
Gamma rays can cause ionization in the side of the GM tube, as we saw in the cloud chamber, so often it’s better to turn it on its side.
The advantage of a GM tube is that it's robust and easy to use. You can detect single particles so it’s also very good at detecting low levels of radiation.
Sometimes the counter is set up to tell you how many counts per second are being detected. Other times it will just count and you need to time the counts in order to calculate the counts per second.
A GM tube only detects a small fraction of the radiation around because most of it doesn't actually hit the tube. If you want to monitor the total amount of radiation given off by a source then you need to enclose it in an ionization chamber.