302 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



HW " 



B 



-C— ! 



■¥- 



a 



v. 



A number of materials have been found suitable for counting, 

 the most frequently used crystals are silver chloride, silver bromide, 

 and thallium bromide-thallium iodide at the temperature of liquid 

 air, and diamond, sulfur, and cadmium sulfide at room temperature. 

 Two groups of cadmium sulfide crystals exist, a nonluminescent 

 group and an impurity-activated luminescent group. The former 

 have normal counting properties, the pulse size being determined 

 by the number of electrons released by the incident particle. The 

 luminescent cadmium sulfide crystal exhibits a multiplication effect; 

 the incident ionizing particle causes the release of electrons which 

 give rise to a semipermanent conduction phenomenon in the counter 

 and, in further sequence, to a voltage pulse that is enormously 

 larger than in the nonluminescent crystal. A single alpha particle 

 may cause a pulse of 10 volts; the duration of this pulse is of the 



order of 0.1 sec, several orders of 

 magnitude larger than that of the 

 ordinary (silver chloride) crystal 

 counter. 



Many materials require careful 

 preparation before they count, 

 and not all specimens of the same 

 material show any counting action. 

 Annealing and careful heat treat- 

 ment are required for silver and 

 thallium halides. Suitable meas- 

 ures have to be taken to avoid 

 strain; gas ionization in the vicin- 

 ity of the crystal can be avoided 

 by evacuation. Adequate insulation and low-capacitance mounting 

 must be provided. The amplifier following the counter must have a 

 fast rise time and a low noise level. The crystal should be kept in 

 darkness to avoid photoconduction. 



Extensive lists of references may be found in the review papers by R. 

 Hofstadter, Nucleonics, 4, 2 (April, 1949), and 29 (May, 1949); Proc. IRE, 

 38, 721 (1950). 



b. Radiation-induced Photoconductivity. Alkali halide crystals 

 that are exposed to high-energy radiation (e.g., gamma or X rays) 

 become colored and in this state are photoconductive. If such crys- 

 tals are afterwards exposed to light, they will be bleached again. 

 If a voltage is applied to the crystal, as shown in Fig. (5-2)23, a 

 current will flow during the bleaching process. The magnitude of 



X E 



Fig. (5-2)23. Arrangement for the 

 measurement of radiation-induced 

 photoconductivity: X, crystal; E, 

 voltage source (300 volts); G, guard 

 ring to eliminate the effect of surface 

 current; B. compensation arrange- 

 ment to eliminate the effect of dark 

 current. 



