542 L. H. G R A Y 



properties are less affected by ionizing radiations than those of most 

 other substances. The insulation between the earthed cylinder and 

 the chamber wall is less critical. The actual ionization chamber 

 should comprise the components shown in Figure 15&, namely, an 

 envelope to define the volume of air ionized (the inner surface of 

 which at least must be conducting), insulation, an earthed cyhnder, 

 high grade insulation, and the central wire supporting the collecting 

 electrode. The shape and material of the wall and electrode will be 

 varied according to the type of ionization to be measured. 



The apparatus described is suitable for the measurement of dose 

 rate. If it is desired to measure total dose similar apparatus may be 

 employed but with an integrating circuit following the electrometer 

 valve, or the "condenser dosimeter" method may be used {85, Ch. 

 l\;88). 



3. Energy Equivalent of One Electrostatic Unit of Ions 



Formed in Air 



Denoting the average energy lost by an ionizing particle in pro- 

 ducing a pair of ions in the gas within the chamber by W, the total 

 energy lost by the ionizing particles per unit mass of gas (-E'm)gas is 

 given by: 



(^TO)gas = 7] r— (9) 



(density) gas 



where J„ is the number of pairs of ions formed per unit volume. 



The quantity W has been measured experimentally under various 

 conditions for electrons, protons, and a particles. It is nearly, but 

 not quite independent of particle type and particle speed in the case 

 of air (c/. Table V). It is different for different gases, and is not 

 necessarily an additive property in the case of gas mixtures, so that 

 the value of W must be ascertained if gas mixtures not previously in- 

 vestigated are used in the chamber. 



Reviewing the literature. Gray {23) concluded that there are good 

 grounds for believing that the average energy expended by electrons 

 in producing a pair of ions in air is independent of electron energy 

 above about 300 kv. and is equal to 32.0 e.v. Below 300 kv. there 

 was a steady rise in W with decreasing electron energy, which is 

 slight at first and steeper at very low energies until at 1 kv. the value 

 is probably about 37 e.v. Neglecting finer details, it would seem that 

 the general mean value (TFe)air = 32.5 e.v. or 5.17 X 10~" erg may 



