Sec. 13.9] STANDARDIZATION OF RADIOACTIVE SAMPLES 369 



The ionization produced by a charged particle traversing a gaseous medium 

 is proportional to the energy loss of the particle to the medium. The factor 

 of proportionality appears to be constant over wide limits of particle energy. 

 Thus, ionization measurements become measurements of energy loss in the 

 medium when the factor of proportionality is known. The energy loss per 

 ion pair for beta particles in air is considered to be about 32.5 ev. In practice 

 absolute determination of energy loss by ionization becomes a more complex 

 problem because of the wall effects in the chamber. Both beta particles and 

 gamma rays are scattered back from the walls of the chamber, thus increasing 

 the ionization in the sensitive volume. Secondary electrons produced in the 

 walls by gamma rays also contribute to the ionization, often to an extent that 

 is difficult to estimate. Consequently, considerable caution is necessary in 

 the interpretation of ionization phenomena. In practice, two different 

 ionization-chamber constructions are possible in which these complex 

 conditions may be accurately analyzed. These are chambers in which the 

 wall effect is negligible and chambers in which the wall effect is made the 

 most important contribution to the ionization (see also Sec. 12.2). 



a. Gas-wall Chambers. These chambers are best suited to standardization 

 of low-energy gamma rays and low-energy beta emitters such as H 3 and C 14 . 

 Such instruments are usually constructed with a central electrode of fine wire 

 and a concentric cylindrical electrode consisting of a coarse mesh made of 

 fine wire. It is important that the solid walls of the chamber be placed 

 farther from the outer electrode than the greatest range of the beta particles 

 being measured or the range of the secondary electrons from the wall when 

 gamma rays are measured. Ionization chambers of this kind often operate 

 at higher than atmospheric pressure in order to reduce the linear dimensions 

 required to meet this condition. 



For an isotope in gaseous form, emitting beta particles of average energy 

 Ep, the disintegration rate R per unit gas volume may be expressed in terms 

 of the measured ion current / by the simple formula 



R = WI 



eVE 



where e = electronic charge 



W = average energy required to form one ion pair 

 V = volume of sensitive region 

 For x- and gamma rays the energy flux F may be determined from the 

 expression 



WI 



F = 



(impVe 



