Sec. 12.3] IONIZATION CHAMBERS 351 



fi = fraction of atoms of the kind i 



ji = average energy of recoil particles from atom ol kind i in units of 

 primary radiation energy 



a = collision cross section for atom of kind i 

 The bar over the product ye indicates its average value taken over the energy 

 spectrum of the primary radiation. When applied to gamma rays in the 

 energy range where only Compton scattering is important, the formula can 

 be used in a somewhat simpler form. 



E = N e $>y e <r e ev/cc/sec 



where N e — number of electrons per cc of wall material 

 y e = average energy of recoil electrons 

 a e = electronic scattering absorption coefficient (see Sec. 2.3) 



A. special form of cavity chamber has been extensively used by Failla [11] 

 for the investigation of the dose delivered by x- and gamma radiation at small 

 depths in tissuelike substances. Its importance derives from the fact that 

 it provides a method for measuring the primary radiation energy absorbed 

 per unit volume of solid at depths from the surface corresponding to the 

 transition region before radiative equilibrium is reached between the primary 

 and secondary radiation. The usual form of the chamber consists of a 

 small shallow chamber, less than 1 mm deep, machined in a block of material 

 of the desired atomic composition. Extremely thin coats of graphite serve as 

 electrodes since, though sufficient to conduct charge, they do not influence 

 the ionization. The dose curve throughout the transition region can now be 

 plotted by successively decreasing the wall thickness through which the 

 radiation enters normal to the chamber axis. When sufficient points are 

 determined, the curve can be readily extrapolated back to zero wall thickness, 

 thus giving the dosage rate or the energy absorbed at the surface and at any 

 depth in the substance. The surface dose indicated by this method is the 

 true dose since the contributions from scattered primary and secondary 

 radiation appear implicitly in the measurements. 



12.3. Charge -measuring Instruments. The charge collected at the elec- 

 trodes of an ionization chamber requires for its measurement instruments of 

 the highest possible sensitivity. In most instances the rate of charge forma- 

 tion is smaller than 1CT 10 amp and may be as little as 10~ 16 amp. It is 

 evident, therefore, that both the chamber and the charge-measuring device 

 must be not only sensitive but must be made relatively free from extraneous 

 influences such as excessive insulator leakage, stray fields, thermal coeffi- 

 cients, contact potentials, voltages due to thermal agitation, and fields 

 induced by insulator stresses which can produce spurious responses of the 

 same order of magnitude as the charge to be measured. In all detecting 

 instruments employed with ionization chambers, the physical quant ity 



