PRINCIPLES OF RADIOLOGICAL PHYSICS 5 



counter. They rely on the ability of radiation particles to eject electrons 

 when they collide with gas molecules and thus to create pairs of separated 

 charges of opposite sign, i.e., pairs of "ions." 



The cloud chamber (Fig. 1-2) is a vessel filled with a gas which is 

 saturated with water vapor. The chamber is suddenly expanded, at 

 intervals, by the action of a piston. As a result, the gas is cooled and the 

 vapor becomes supersaturated. In the absence of impurity particles 

 which act as condensation centers, the vapor does not condense into 

 water droplets. However, since ions act as condensation centers, the 

 ionized path of a charged particle which has recently traversed the 

 chamber is marked by a line of fine water droplets. This line of cloud 

 can easily be seen or photographed (Fig. 1-3) (see Gentner et al., 1940). 



1 1 

 1 1 



1 1 



LIGHT 



PISTON 



SUDDENLY 



DROPS 



Fig. 1-2. Diagram of a cloud 

 chamber. 



Fig. 1-3. Tracks of a particles in a 

 cloud chamber. {Gentner et al., 

 1940.) 



A Geiger counter consists essentially of a wire with a tube surrounding 

 it. An electric potential difference is established between wire and tube 

 which is not quite sufficient to cause a discharge under normal conditions. 

 When a particle traverses the counter and creates a pair of ions in the 

 filler gas, the electric force accelerates the ions and thereby starts a 

 sudden discharge. After this, the wire and the tube recharge and no dis- 

 charge passes until a new ionizing particle comes by. Thus one counter 

 detects the passage of a single particle at a time, while a set of counters 

 may serve to trace the path of a particle. 



For information on counting devices the reader is referred to a report 

 by Wilson, Corson, and Baker (1950). 



Another method of observing radiations, similarly based on the ejection 

 of electrons from gas molecules, becomes possible at somewhat higher 

 levels of intensity when the total separation of positive and negative 

 electric charges ("ionization") is adequate for measurement by electric 

 techniques. Ionization is measured by attracting the charges of opposite 



