In this video Myles Power shows how to make a cloud chamber. A cloud chamber, also known as the Wilson chamber, is a device which makes visible the paths of charged particles emitted as a result of radioactive decay. A very simplified version of the cloud chamber called a “diffusion cloud chamber” can be constructed at schools from available scientific supply companies. This experiment is perfect for teteaching students about radioactive decay and ionising radiation.
The Science Behind The Cloud Chamber
Cloud chambers have played an Important role in experimental particle physics between 1920s to the 1950s. The discovery of the positron and the Muon by Nobel Prize lorriet Carl Anderson and the Kaon were made using cloud chambers as detectors. Cloud chambers (AKA Wilson chamber) were invented by Scottish physicist Charles Thomson Rees Wilson in 1911. They consist of a sealed environment containing a supersaturated vapour of water or alcohol. When a charged particle travels within the chamber it ionises some of the molecules. The resulting ions act as condensation nuclei, around which a mist will form leaving a trail in the path of the charted particle. These trails have distinctive shapes for example alpha particles, which are relatively heavy, will produce straight dense trails. Beta particles leave wispy irregular trails.
Materials and Equipement
- Petri dish
- Black tape
- Black felt
- Radiation source – available through scientific supply companies
- Iso-propanol (rubbing alcohol)
- Torch (flash light)
- Dry ice
- Safety gloves
Dry ice is very cold: -78C. Gloves must be warn when handling as contact with skin must be avoided. Dry ice also sublimes (turns from a solid to gas) and carbon dioxide is an asphyxiant gas which is heavier then air. It is important to remember that a small chip of dry ice will sublime to a large volume of gas.
Radioactive sample should be handled carefully, and prolonged exposure to even low levels of radiation can be dangerous.
- Cover the bottom of the large part of the petri dish with black tape.
- Cut a long thin strip of felt the same hight as the large part of the petri dish and wrap around the inside.
- Douse the felt with iso-propanol.
- Secure the radiation in the middle of the petri dish.
- Place the lid on the petri dish and place on a slab of dry ice.
- To prevent damage to table surface from the dry ice place onto of a towel.
- Leave the experiment to cool down for between 5-10 minutes.
- Warm the top of the perry dish with your hand. Being very careful not to touch the dry ice.
- Turn off the light and angle the torch to see the misty trails.