Alpha particle tracks
Teaching Guidance for 14-16
bullets from radioactive atoms make the tracks in a cloud chamber. They hurtle through the air,
wet with alcohol vapour, detaching an electron from atom after atom, leaving a trail of ions in their path. Tiny drops of alcohol can easily form on these ions to mark the trail.
The trail of ions is made up of some ‘air molecules’ that have lost an electron (leaving them with a positive charge) and some that have picked up the freed electrons, giving them a negative charge.
There is no sighting of the particle which caused the ionisation, because it has left the ‘scene’ before the condensation happens. If you count the number of droplets an alpha particle might produce 100,000 pairs of ions by pulling an electron from 100,000 atoms.
When the alpha particle has lost all its energy in collisions with the ‘air molecules’ it stops moving and is absorbed.