346 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 12 



— 2 



\\\\V\\\\\\\^ 



To Detecting 

 Device 



WWWWWNN^ 



Fig. 96. Thimble ionization chamber. 



construction materials are determined by the particular circumstances of the 

 measurements. Despite great variations in design, the chambers are, in most 

 instances, built around two basic forms of collecting electrode geometry. 

 The first is a parallel-plate condenser shown in Fig. 95. The central portion 

 of one plate of the condenser is highly insulated and connected directly to 

 the detecting instrument as well as to some means for charging it to the 

 desired potential, often ground potential, either through a charging (or 

 grounding) switch or a high grid resistance if a vacuum-tube amplifier is 

 used. The remainder of the plate bounding the collecting electrode serves 

 as a guard ring and is maintained at the same potential to define accurately 

 the sensitive volume of the chamber and to prevent leakage of current across 



the insulator supporting the collect- 

 ing electrode. The opposite plate of 

 the condenser is maintained at the 

 potential necessary to provide the 

 requisite field strength between the 

 electrodes. The second form of 

 chamber frequently used consists of a 

 cylindrical anode (or cathode) within 

 which a highly insulated rod-shaped collecting electrode is coaxially mounted, 

 as shown in Fig. 96. 



The functions of the guard ring, which is maintained at the same potential 

 as the collecting electrode, are especially important in chambers intended for 

 measurement of weak ionization. The least important of these is to define 

 the sensitive volume. Its more important functions are (1) prevention of 

 leakage currents across the insulator supporting the collecting electrode by 

 maintaining zero potential gradient, (2) prevention of insulator stresses due 

 to potential gradients, since polarization and relaxation of the insulator may 

 induce voltages comparable to those produced by the ions, and (3) elimina- 

 tion of steep potential gradients near any part of the collecting electrode 

 system. 



All ionizing radiations can be detected with appropriate forms of chambers. 

 Heavily ionizing particles such as protons, alpha particles, and fission frag- 

 ments produce ionization of sufficient intensity to be detected and counted 

 as separate events using the same techniques and external counting circuits 

 as for Geiger-Miiller and proportional counters. Less strongly ionizing 

 radiations such as gamma rays and beta particles cannot be detected and 

 counted as separate events but must be measured by the average charge 

 collected per unit time at the electrodes. 



Chambers designed for radiation detection by the time rate of accumulation 

 of charge normally will operate satisfactorily when filled with any gas of 

 low molecular weight, including those gases, such as oxygen, water, carbon 



