300 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



A similar counter system described by Chang and Rosenblum 1 

 and illustrated in Fig. (5-2)21 consists of a wire stretched tautly 

 parallel to a metal plane. The plane has rounded edges to reduce 

 the, field strength and to prevent flashover at these ends. The 

 counter has a plateau region between 4,500 and 6,500 volts. The rate 

 of spurious discharges is less than 1 count in 4 hr, and the dead 

 time is about 10~ 4 sec. 2 



5-23. Solid-state Transducers for Ionizing Radiation 



a. Crystal Counters. The crystal counter, schematically illustrated 

 in Fig. (5-2)22, consists of a slab of a crystal such as silver chloride 



Fig. (5-2)21. Spark counter with wire 

 electrode W held in a position parallel to 

 a plate P by means of insulators /. 



R E 



Fig. (5-2)22. Crystal counter, 

 schematic diagram. 



(AgCl) or diamond (C) on which thin metal electrodes are deposited. 

 A voltage source E causes a field strength of the order of several 

 kilovolts j)er centimeter in the crystal. If ionizing radiation, for 

 instance an energetic beta particle, enters the crystal, it will cause 

 free electrons in the vicinity of its path, analogous to ionization in 

 a gas. Accelerated by the applied field, the electrons will move 

 through the crystal until they are trapped or reach the positive 

 electrode, thus forming a current and a voltage pulse. The magni- 

 tude of the pulse is of the order of 1 mV. If an ionizing particle 

 enters the crystal and causes the release of w electrons in the 

 vicinity of the negative electrode, the migration of the released 

 electrons gives rise to a voltage pulse of the magnitude 



E ° = -cd {l 



-rf/A 



) 



(1) 



where e is the electron charge, C the capacitance of the crystal plus 

 that of the associated circuit, and d the crystal thickness. The 



1 W. Y. Chang and S. Rosenblum, Phys. Rev., 67, 222 (1945). 



2 R. M. Payne, J. Sci. Instr., 26, 321 (1949). 



