Diffusion cloud chamber

Class practical

The Taylor diffusion cloud chamber is a simple piece of equipment which will clearly show alpha particle tracks. It is cheap enough to allow students, in groups, the opportunity to do their own experiment. Students are fascinated by the tracks and watch them for a long time. This is something to be enjoyed and not hurried.

You can do this as a demonstration. However, students will prefer waiting for their own apparatus to produce results rather than yours. Also, if you have eight to ten groups of students, each with their own cloud chambers, you are more likely to get some results sooner or later.

Apparatus and Materials

For each student or student group:

• Taylor diffusion cloud chamber
• Lamp, 12 V, 24 W and power supply (shining through 1 cm wide slit)

Available to the class/teacher:

• Flexicam or webcam linked to a projector (optional)

Dry ice

or

Carbon dioxide cylinder

...and dry ice attachment (see guidance note).

Health & Safety and Technical Notes

This demonstration uses a weak radioactive source. If any radioactive paint has flaked off the source inside the chamber, do NOT use it.

Since ethanol is in use, there must be no naked flames in the room.

Wear eye protection and gauntlet-style leather gloves when making or handling solid carbon dioxide.

Read our standard health & safety guidance

• The cloud chamber works by allowing a super-saturated vapour to build up close to the base of the chamber. The air at the top of the chamber should become saturated with ethanol vapour. Any air that sinks to the bottom of the chamber is cooled by the dry ice underneath. This makes the air super-saturated and the vapour will condense if given the opportunity – i.e. one or more condensation nuclei. These are provided by alpha particles from the thoron source.
• When putting the alchol into the chamber it is essential that none of it falls on the source, otherwise alpha particles may not penetrate it.
• Surprisingly little dry ice is needed in these chambers. Practice will show you how much is required, usually about 2 or 3 cm³ .
• The radioactive source is normally a spot of radioactive paint containing thorium or radium.
• Insert the wire source holder in the cork and place the cork in the hole in the side of the chamber, with the source near the floor. Position the source in the gap between the metal foils by rotating the wire.

• Place the chamber on the three levelling wedges; clean the underside of the Perspex lid before replacing.
• Direct a flat beam of light across the chamber towards the radioactive source. (The foils should be bent back slightly so that they do not reflect light onto the chamber floor.)
• NB Suppliers of diffusion-type cloud chambers:
• Ideas for Education in Co. Fermanagh, N. Ireland, telephone number 028 6863 1209\. (Also supplied by:

• Timstar

  ...and...

  Scientific & Chemical Supplies

  ...Uses dry ice.
• PASCO SE-7943\. Uses ice water.
• An alternative radioactive source is fully described here...

  Alternative radioactive source for the diffusion cloud chamber

Procedure

1. It is very important that the class should have plenty of time for this experiment. Allocate the cloud chambers so that there is one for every three or four students.
2. The laboratory will need to blacked out, but the light from the 12 V lamps is enough for everyone to see what they are doing (see guidance note

   Classroom management in semi-darkness

3. To set up the chambers, put alchol on the padding inside the top of the chamber using a dropper. A drop or two may also be put on the black base of the chamber and allowed to spread over it. Make sure none gets onto the thoron source.
4. Unscrew the base of the whole apparatus and put a little

   dry ice

   in contact with the base plate. Put the foam back to keep the dry ice in contact with the plate. Screw the base cap on again, and turn the chamber the right way up.
5. It is important that the cloud chamber is level. Place it on the three wedges provided. These can be adjusted to get it level. If it is not level, you will see convection currents moving in the chamber and these can be used as guides in levelling.
6. The top must be put back on the chamber. Rubbing it with a clean duster will charge it sufficiently to provide an adequate electric field inside the chamber to sweep away old ions.
7. Illumination is important. Adjust the 12 V lamps so that there is a layer of illumination a few millimetres above the base plate.
8. Usually within 30 seconds of setting it up, you should see alpha tracks coming from the weak radioactive source which is inserted in the side of the chamber.
9. If the tracks are not sharp, try rubbing the top again to improve the electric field. This cleans out any stray ions in the air.

Teaching Notes

• Tell the class that what they can see is the effect of alpha radiation. They are not seeing the radiation itself, but the condensation which has formed on ions left behind by the radiation. By the time the condensation forms, the alpha particle has long gone. There is a nice analogy in the guidance note on

  Alpha particle tracks

• Draw attention to the amount of ionisation that each alpha particle produces and to the length of its track.
• You could also draw attention to the fact that the tracks are straight, showing that nearly all the collisions are with something much lighter (usually removing an electron from an atom). Forked tracks may be seen when the alpha particle strikes a more massive particle such as one of the constituents of air.
• If students watch the cloud chamber for long enough, and the chambers are well balanced, they may well see the tracks of high energy electrons from cosmic rays.
• Short, thin spiralling tracks may be seen which are electrons or β particles in the Earth's magnetic field.
• A fast group could swing the source behind the thin foil. This will absorb the α particles but let the β particles through. The wavering tracks of the β particles may be seen if conditions are optimum.
• If you start to get some good results, you could use a flexicam to project the live tracks onto a screen or whiteboard. You could even record a short movie for posterity and to refer back to in later lessons. Similarly, if you have access to a digital camera, you could take some still photographs and use them in a wall display or PowerPoint presentation in a follow-up lesson. You could offer a prize for a forked track!

This experiment was safety-tested in August 2007