ASSAY OF RADIOACTIVITY 



relative to the metal case; when negatively charged electrons are collected 

 the fibre moves back by an amount which is, for small displacements, linearly 

 related to the total charge, and hence to the total ionization that has occurred 

 within the sensitive volume. The movements of the fibre can be observed 

 directly with a lens system, against a scale calibrated in standard units of 

 radiation (the roentgen). 



Since ionization chambers do not take advantage of the magnification 

 that can be obtained by gas multiplication, their sensitivity is low. If, for 

 example, the total capacitance of the collecting electrode system is 10 ///^F, 

 the potential change on collection of a single electron is only about 0-016 /zV, 

 which is not measurable. In contrast, therefore, to proportional and Geiger- 

 Miiller counters, they are inconvenient for the detection of individual 

 ionizing particles, but will measure the integrated ionization resulting from 

 larger amounts of radiation. This limits their application in biological tracer 

 experiments, where maximum sensitivity is usually desirable, and often 

 essential. Used in conjunction with a vibrating reed electrometer, ionization 

 chambers have been applied successfully by some workers to the determination 

 of ^^C, introduced directly into the chamber as COg, but their principal value 

 is for monitoring large sources of radiation. A number of radiation monitors 

 using ionization chambers are available commercially, as are pocket quartz- 

 fibre dosimeters, which are especially valuable for measuring the total doses 

 of radiation received by an individual in the course of handling radioactive 

 materials. 



FORMATION OF THE AVALANCHE IN 

 GEIGER-MULLER COUNTERS 



The basic construction of two commonly used types of Geiger tube is 

 illustrated in Figure 31.4. In both of them the anode is a thin wire coaxial 

 with a cylindrical cathode; normally they are used with the cathode at 

 earth potential, and the anode at a suitable positive potential. They differ 

 in the thickness of wall through which the incident radiation has to penetrate 

 before it reaches the sensitive volume. The 'end-window' tube has a relatively 

 thick metal cathode, but its end window, usually of mica, can weigh as little 

 as l-Omg/cm^, thus permitting the entry of even the weakest ^-radiation 

 (excepting that of ^H) without excessive absorption by the window. The 

 'thin-walled' tube is designed for the measurement of rather stronger /S- 

 and y-radiation, having a thin glass wall weighing about 30 mg/cm^; the 

 cathode, immediately inside the glass wall, may take the form of a layer of 

 carbon, an open spiral of stainless steel wire, or (particularly for soft y- 

 radiation) of a thin sheet of lead. 



It has already been mentioned that in the Geiger-MUller region {£) of 

 Figure 31.1 the formation of a single ion pair within the sensitive volume of 

 the chamber leads to the discharge of a very large number of electrons, the 

 so-called 'Townsend avalanche', at the anode. The mechanism of this process 

 must now be considered in slightly greater detail. Immediately after an 

 ionizing particle (either emitted directly by a /5-emitter or produced indirectly 

 by the passage of a y-ray) has formed a few ion pairs in the gas (usually argon 

 or neon) filling the chamber, the electrons begin to move towards the anode ; 



426 



