An experimental approach to cloud chambers

What the Activity is for

Understanding cloud chambers: this is a four-part activity that tells the story of the detection of ionising radiation using cloud chambers.

The cloud chamber is one of the very few activities that will enable students to see the effects of ionising radiation. You can set up a cloud chamber as a demonstration but it is better if students can work in groups with their own cloud chamber. Seeing the tracks being formed is hugely engaging and the subsequent discussion enables them to understand what happens during the process of ionisation.

The next demonstration shows how clouds are formed in a jar and demonstrates the need for condensation nuclei.

This is followed by a demonstration of ions forming a cloud using a high voltage to produce the ions.

Finally, the expansion cloud chamber provides a different method of producing tracks that includes a lot of the ideas already discussed.

What to Prepare

Cloud chambers per group:

• Taylor diffusion cloud chambers
• lamps (12 V, 24 W) and power supply

For the teacher:

• dry ice or CO₂ cylinder with attachment
• ethanol in a dropper bottle

Safety note: Cloud chambers contain weak radioactive sources and must not be used if there are any signs of flakes of radioactive paint inside the chamber. Most of the newer sources are enclosed. Make sure that there are no naked flames in the room when using ethanol. Wear eye protection and gauntlet-style leather gloves when making or handling solid carbon dioxide.

What Happens During this Activity

Demonstrate how to set up the cloud chamber. Start by removing the perspex lid and putting a few drops of ethanol on the black felt at the top of the chamber. Put a couple of drops on the black base of the chamber, making sure that no alcohol falls on the source. Put the perspex lid back on and turn the chamber upside-down. Remove the screw top and the foam. Wearing goggles and using appropriate gloves, put a handful of dry ice into the bottom of the chamber. You don't need very much dry ice – enough to cover the palm of your hand will work well (about a dessertspoonful). Replace the foam and screw the lid on. Turn it back the right way up. Make sure that the perspex lid is clean, particularly the underside. There are wedges that enable you to make the cloud chamber level and finally illuminate the chamber with the lamp, pointing it towards the source and slightly downwards. If you don't see tracks, try rubbing the top of the chamber with a cloth to charge it slightly. You should see faint straight tracks, like the vapour trails of aircraft.

A discussion, starting with the vapour trails

Teacher: Sometimes a plane is so high that you can't see it but you can see two vapour trails. Why is that?

Sam: It's too high up.

Teacher: Right! But the vapour trails look a bit like clouds. How do clouds form?

Sam: Is it like steam from a kettle? The water is heated by the Sun and when it cools down it makes clouds?

Teacher: That's partly right. You are right about the water evaporating because of heat from the Sun but it takes a bit more than just cooling down. Imagine the Sun evaporating water from the sea, and that water rising and cooling down. What would make those water molecules join together to make big enough droplets to reflect sunlight so we see them as white and fluffy?

Sam: Does gravity pull them together?

The conversation develops

Teacher: Good try, but no. First the air needs to have a lot of water molecules in it, so that we say that it is saturated. A bit like if you put loads and loads of sugar in your tea until eventually you can't get any more to dissolve. Then it needs to cool down, and then you need little tiny particles, like dust or ice. The water molecules condense on the dust or ice and make droplets, and lots of droplets make a cloud. So let's get back to planes. What might be coming out of the back of the aircraft that the water could condense on?

Sam: Exhaust fumes?

Teacher: Right idea. When the plane burns fossil fuels, one of the things produced is water vapour, so that helps to saturate the air. It's cold up there so all the conditions are right for making the water molecules condense to make droplets.

Teacher: So in our cloud chamber what did we saturate the air with? Was it water?

Sam: No, it was ethanol on the black felt.

Teacher: Good. So what was the ethanol condensing on?

Sam: The alpha particles?

Teacher: Think again. If it was condensing on each alpha particle as it was emitted, why would we see a long track? Would it not stop the alpha particle?

Sam: Something that the alpha particle is leaving behind, like the exhaust gases.

Teacher: Excellent. As the alpha particle moves through the air, the air is ionised. That means it removes electrons from air molecules leaving positively charged things called ions behind. The vapour that is cold and saturated condenses on the ions. We call them condensation nuclei – anything like dust, ice crystals or ions can do it. In fact, the ethanol works really well because an ethanol molecule is sort of oblong shaped with a positive charge at one end and a negative charge at the other. So it is attracted to anything charged. Ethanol molecules cluster around and make a droplet. Lots of droplets make a trail.

A second step: clouds in a jar demonstration

What the activity is for

• a 1 litre aspirator or large flask
• a bung to close lower outlet of aspirator
• a bung with glass tube
• a short length of rubber tubing
• a compact light source and power supply
• a large sheet of black card
• a box of matches

What happens during this activity

Start by putting a black screen behind the aspirator, and a bright light to the side. Then put a few millilitres of water into the aspirator, closing the bung and tube. Blow down the tube then pinch it with your fingers. Wait a minute for the air to cool. During this time you can talk about the similarities with the cloud chamber (saturating with water, cooling down). Turn the lights out, illuminate the aspirator and pull out the bung very quickly, allowing the air to expand. You should see a cloud forming.

Teacher: So what have I saturated the air with?

Sam: Water.

Teacher: Good. So why do we see a cloud forming when I take the bung out?

Sam: You have let some dirt in?

Teacher: Not quite. Think about the conditions for making vapour trails. The air is under pressure until I take the bung out. So what happens when I take the bung out?

Sam: The air cools?

Engaging students in a development

Teacher: Good. When gases expand quickly they cool down. Think about the opposite. When you pump up your bike tyre the pump gets hot because you are compressing the gas. So now we have cool air saturated with water. So what happens next?

Sam: The water molecules make little droplets and we see a cloud.

Teacher: Good. If you repeat this several times you should see the clouds becoming thinner, or even not forming at all. This is a very interesting discussion point.

Teacher: Why are the clouds not forming now?

Sam: Has it run out of water?

Teacher: Not quite. We can still see some at the bottom of the jar. So it's still saturated and I keep cooling it down. What's missing?

Sam: There can't be anything for the water to condense on.

Teacher: Why not?

Sam: You have let them all out?

Teacher: Some ions have been used up making the other clouds. Some ions have sunk to the bottom of the jar. Let's see what happens when we add some more.

If you throw a lighted match into the aspirator clouds will form again.

Teacher: So why do we get clouds again now?

Sam: The match is putting smoke into the air for the water to condense on.

Teacher: Good. Not just smoke particles. The flame makes ions, just like the alpha particles did. A water molecule is a bit like ethanol – one end is positively charged and the other end negatively charged. So if we want to get water molecules to get together, ions are a good thing.

A third step: ions start a cloud

What the activity is for

• a Van de Graaff generator
• a 500 ml flask with bung and glass tube
• a glass tube drawn to a jet
• a Bunsen burner
• a tripod
• a retort stand, boss, and clamp
• some connecting leads
• a flexicam or webcam linked to a projector (optional)
• a compact light source and power supply (optional)

Safety note: Take care with the steam from the nozzle. There is a risk of scalding.

What happens during this activity

To make the effect clearly visible to the class either project a shadow onto the wall or a screen, using the compact light source at a distance of 1 m from the nozzle, or use a flexicam or webcam connected through a computer to a projector or interactive whiteboard.

Set up the nozzle a long way from the Bunsen flame (about 1.5 metre) because the Bunsen makes so many ions that it will spoil the demonstration. Set up two wire electrodes to form a spark gap about 0.5 m wide and about 0.3 mm above the nozzle. This will ensure it is in the water vapour and not in the white cloud. Boil the water in the flask and observe the cloud formation as the vapour emerges from the jet. Highlight the gap between the cloud and the nozzle.

Developing an understanding through discussion

Teacher: Look at the gap between the cloud and the nozzle. That's a bit odd. Why do you think there is a gap?

Sam: The cloud is so thin you can't see it?

Teacher: Think again. Think of the clouds in the jar. There is lots of water vapour and lots to condense on. What's missing?

Sam: Cooling down. So it has to move away and cool down before you get a cloud?

Teacher: Excellent! So here there is nothing to cool the vapour but as it moves that distance from the nozzle it cools down enough to become supersaturated. Then a cloud forms when the steam condenses on the particles in the air, just like the elements that went into our cloud chamber. We needed ethanol for the vapour, dry ice to cool it and something to condense on.

Switch the Van de Graaff generator on so that a stream of small sparks passes through the vapour jet. The cloud will be seen to intensify due to the production of ions, which act as condensation nuclei.

Teacher: Why is the cloud thicker?

Sam: The water molecules are condensing on a spark?

Teacher: Well, the spark is definitely having an effect. Let's think back to what the alpha particle was doing in the cloud chamber.

Sam: Making ions?

Teacher: Excellent. So here the spark is a result of the air conducting electricity. The few ions in the air are attracted to the positive or negative terminals and accelerate. They hit other air molecules and ionise them, and then make a sort of avalanche effect. So why does that make the cloud thicker?

Sam: There must be more particles for the water to condense on – all the ions.

Teacher: Good. That's just like an alpha particle making a trail.

The expansion cloud chamber demonstration

What the activity is for

• an EHT power supply (not an HT power supply)
• a source of alpha radiation (if it is not part of the cloud chamber)
• a pair of large forceps or pliers, if required
• a bicycle pump or other device for producing expansion
• a lamp, lens and power supply

What happens during this activity

Expansion cloud chambers are all slightly different in their operation, so you will need to refer to the manufacturer's instructions. The position of the lamp is often critical. Some have an evacuation mechanism, such as a bicycle pump, which removes the air, while others change the pressure of a water column.

Working up an understanding of the apparatus

Teacher: Let's think about how this demonstration combines the cloud in the jar and the ions in the cloud demonstrations. Let's start with what is happening when I suddenly remove the air from the chamber. What else have I done in a different experiment that's a bit like this?

Sam: Is it like when you pulled the bung out?

Teacher: Exactly. In that case I had pressurised the gas and then let it out. Here I am removing some air with a pump. What effect does that have?

Sam: It cools the air down.

Teacher: Good. So then the air is saturated with ethanol and is cool. What happens next?

Sam: The alpha particles make ions and the ethanol condenses on the ions.

Teacher: Good. So that is just like our spark making our clouds thicker. After a while, though, we don't get tracks – they fade. In fact this cloud chamber is sometimes called a pulsed cloud chamber. It's not like the diffusion cloud chamber. So why don't the tracks last as long?

Sam: Have they run out of ions?

Teacher: Good try, but if that was the case that would happen in the diffusion cloud chambers as well. I'll give you a hint: this didn't happen in the diffusion cloud chamber because of the dry ice.

Sam: The air heats up?

Teacher: Exactly. If anything, in fact, there are too many ions. We have to keep a power supply connected to sweep away all the ions from the old tracks so we can see new ones when we expand the gas again.