Sec. 5-2] RADIATION TRANSDUCERS 299 



the order of several hundred volts. The counting rate versus voltage 

 characteristic is similar to that shown in Fig. (5-2)17. For stable 

 counter operation a series resistor greater than H) 5 ohms must be 

 used, as well as a quenching agent. 1 



The discharge in the parallel-plate counter does not spread 

 throughout the entire counter but remains localized in that region 

 where the incident particle causes the liberation of a primary elec- 

 tron. The discharge is terminated when the voltage across the 

 counter has fallen below the minimum sustaining value. The capaci- 

 tance of the parallel-plate capacitor is larger than that of the cylin- 

 drical or the point counter; the dead time and the recovery time are 



Fig. (5-2)20. Spark counter, 

 schematic diagram. 

 L. 



large, therefore, and incident ionizing particles that arrive during 

 these times at any point of the counter are not recorded. Lion and 

 Vanderschmidt 2 have overcome this disadvantage by placing a 

 resistive layer L (ferrite) in the counter, as shown in Fig. (5-2)19. 

 d. Spark Counters. The discharge in a non-self-quenching counter 

 can be terminated by a series resistance which causes a reduction of 

 the voltage across the counter whenever a discharge occurs (see 

 Region D, Geiger-counter Region, 5-22a). If a capacitor C is con- 

 nected in parallel to the discharge gap, as shown in Fig. (5-2)20, 

 the charge stored in it causes the gas discharge to assume the 

 character of a spark that is strong enough to be seen or photographed 

 or heard without further amplification (spark counter). 3 Of course, 

 the large time constant of the counter, imposed by the resistance R 

 and the capacitance C, reduces the time resolution of the counter; 

 yet the system is simple and useful for a number of applications, 

 e.g., for X-ray dosimetry. The discharge in the Greinacher counter 

 takes place in a point-to-plane gap or between two small spheres 

 operated in air at atmospheric pressure. The counter character- 

 istic does not exhibit a plateau. 



1 R. W. Pidd and L. Madansky, Phys. Rev., 75, 1175 (1949). 



2 Lion and Vanderschmidt, loc. cit. 



3 H. Greinacher, Helv. Physica Acta, 7, 360 and 514 (1934); other references 

 by P. Frey, Helv. Physica Acta, 19, 41 (1946). 



