XVII. RADIOACTIVE TRACERS 615 



placed at various heights thus allowing a change in the solid angle and 

 providing a much wider range of usefulness. These supports can be 

 fastened to the chamber with screws that fit into drilled and tapped 

 holes in the chamber. A wooden box of proper height to bring the 

 optical system to eye level can also contain the charging power pack, 

 and a voltage control and switches can be located conveniently on this 

 box. 



A number of other types of quartz fiber electroscopes are now on 

 the market. In the main these are designed as monitoring instru- 

 ments and are discussed in Section G of this chapter. 



3. Counters 



As the voltage on a gas-filled chamber is increased, the chamber 

 undergoes several distinct types of behavior toward radiation. A 

 similar effect is observed if the gas pressure in the chamber is gradu- 

 ally reduced. These changes bring one to the region of counters, 

 in which individual ionizing events due to single particles can be 

 counted {26). In obtaining the desired characteristics in the counter, 

 the separation and form of the electrodes and the composition of the 

 gas as well as the pressure and voltage are of importance. At volt- 

 ages around 1 kv. wath air or nitrogen at one atmosphere and with 

 electrodes that are parallel plates separated by 0.5 to 1 cm., an indi- 

 vidual a particle will produce enough ionization so that it can be 

 counted. When the particle passes through the air gap it produces 

 electrons and positive ions, which travel to the electrodes rapidly and 

 rapidly reduce the voltage across the gap. This reduction of voltage 

 constitutes a pulse, which is terminated after the ions are collected. 

 The system then recharges itself and is ready for the next a particle. 

 Because of the much lower ionization that they produce within this 

 short path length, /3 particles do not give enough of a pulse and only 

 a particles can be counted. Even so, the chamber requires high am- 

 plification, which can be given by a linear amplifier circuit. 



Increasing the voltage on the system brings a region in which the 

 electrons, the negative half of the ion pairs produced by the incident 

 radiation, are caused by the electrical field to move so rapidly that 

 they produce additional ion pairs. This process of gas amplification 

 (referred to as an avalanche) is aided if the positive electrode is a 

 straight wire and the negative electrode a cyhnder surrounding it. 

 With the aid of gas amplification the further electronic amplification 



