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ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 12 



k 



M 



measured is a small potential difference produced by the collected charge. 

 Although the millivolt is the absolute quantity measured directly, such 

 instruments may be calibrated in terms of roentgens, curies of source mate- 

 rial, or in arbitrary units of radiation intensity. 



The instruments most frequently adapted to ionization-chamber measure- 

 ments are summarized below. 



a. Quartz-fiber Electroscope. Of the numerous types of electroscopes that 

 have been used for radiation detection, the form developed by Lauritsen is 

 the most frequently used in this country. It consists of a metal-coated 



quartz fiber, 6 mm long and 0.005 

 mm in diameter, mounted parallel to 

 a rigid wire and connected mechan- 

 ically to it, as shown in Fig. 97. 

 An aluminum shield covers the fiber 

 assembly to serve as the cathode and 

 to protect the fiber from damage, air 

 currents, and dust. The displace- 

 ment of the end of the fiber is 

 observed with a compound micro- 

 scope in which an accurately ruled 

 scale is mounted in the focal plane of 

 the eyepiece. Although this type of 

 electroscope is designed to be used 

 independently, the microscope and 

 fiber assembly can be mounted on an ionization chamber with the fiber con- 

 nected directly to the collecting electrode. 



The fiber is initially displaced by charging it with a potential of approxi- 

 mately + 100 volts before a measurement is to be made. Then, as ions of 

 the opposite charge are collected, the fiber drifts back at a rate proportional 

 to the rate at which ions are produced in the sensitive region surrounding the 

 fiber or in an ionization chamber connected to it. Small displacements of 

 the fiber are very nearly proportional to the number of ions collected, and a 

 linear relationship can usually be assumed if the displacement is not large. 



Normally, the background activity from cosmic rays and alpha particles 

 gives a drift rate of five to ten scale divisions per hour. 



b. Moving-vane Electrometer [1]. The most suitable electrometers of this 

 type for ion detection are those with a metal-coated quartz fiber mounted 

 on a torsion fiber in the gap between constant potential electrodes. Both 

 the Hoffman [2] and Lindemann [3] forms have been widely used with 

 ionization chambers, but at the present time the Lindemann electrometer 

 generally is preferred (see Fig. 98). It is highly sensitive, its calibration is 

 independent of orientation, and it can be made portable. 



Fig. 97. Schematic diagram of Lauritsen 

 electroscope. A, amber insulator for sup- 

 porting fiber assembly; C, switch for charg- 

 ing fiber; F, metal-coated quartz fiber with 

 "T" fiber mounted at free end; M, com- 

 pound microscope; S, scale for reading 

 displacement of fiber; W, stiff wire support 

 for fiber. 



