How Wilson Cloud Chamber Works

Radiation Detectors 

Such devices which are used to indicate the presence of fast moving charged atomic or nuclear particles by observation of the electrical disturbance created by a particle as it passes through the device are known as radiation detectors.

Wilson Cloud Chamber

This cloud chamber was invented by C.T.R Wilson in 1911. In it, the tracks of ionization particles can be made visible observable. Then the tracks can be seen or photographed.

Principle

Its principle is based on Wilson's discovery in 1897 that in an air saturated with water vapors, the ions or dust particles serve as centers of condensation. It makes use of the fact that supersaturated vapors condense preferentially on ions. If an ionizing particle passes through a region in which the cloud droplets are about to form, the droplets will form first along the particle's path, showing the path as a trial of droplet's.

Construction

 

The apparatus consist of a cylindrical glass chamber closed at the upper end by a glass window and at the lower end of a movable piston. A black felt pad soaked in alcohol is placed on a metal plate inside the chamber.

Working

Due to alcohol vapors, the air inside the chamber becomes saturated. When the pump is given a sharp pull, the pressure in the chamber is decreased, consequently, the air and vapors expand. The occurring of sudden cooling due to adiabatic expansion helps to form saturated vapors. As radiation passes through the chamber, ions are produced along the path. The tiny droplets of moisture condense about these ions and form vapor tracks showing the path of the radiation.

Tracks of  Radiation

The fog tracks are illuminated with a lamp and may be seen through a glass window. The cloud chamber tracks followed by alpha, beta and gamma are such as discussed below:

• Tracks of alpha particles

The alpha particles leave thick, straight and continuous tracks due to intense ionization produced by them.

• Tracks of beta particles

The beta particles form thin discontinuous tracks extending in the erratic manner showing frequent deflections.

• Tracks of gamma particles

The gamma rays leave no definite tracks along their way, they eject electrons. These ejected electrons have sufficient energy to ionize atoms along their path. All such paths originate on the path of X-rays. The tracks of the ejected electrons are similar to those produced by beta particles. The length of a track has been found proportional to the energy of the incident particle.