XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 543 



be adopted for most purposes as representing the energy expended 

 per ion pair formed by electrons. Consistent with the rise in We 

 with decreasing electron speed is the fact that the value of W for the 

 more slowly moving protons and a particles is greater than that for 

 electrons. The best experimental values appear to be (TFp) ^ir = 

 36 e.v. for protons of energy between 2 and 8 m.e.v. and (IFa)air = 

 35.1 e.v. as the mean over the whole range of RaC a particles, and 

 (TFa)air = 35.6 e.v. for polonium a particles. 



4. Design of Ionization Chambers and Choice of Conditions 

 of Irradiation for Experiments with Particular Sources 



External Beta-Ray Sources. Thought given to the choice of the 

 conditions of irradiation will often greatly facilitate the accurate 

 measurement of the dose received by the specimens. For example, 

 when using j8 radiation it is necessary to bear in mind not only the 

 ease with which slow /3 particles are scattered through large angles but 

 also the fact that the extent of the scattering varies with the atomic 

 number of the scattering medium. If specimens such as bacteria are 

 irradiated in thin films of gelatin, the dose is probably most accu- 

 rately estimated by means of a very shallow ionization chamber having 

 top and bottom walls made out of the thinnest obtainable paper or 

 nylon, rendered conducting by graphite, and an open grid collecting 

 electrode. The chamber is situated beneath a stop, as in Figure 16a. 

 The stop is made of low atomic number elements and defines the /3- 

 ray beam. By using stops of different diameter, reflection from the 

 edges can be allowed for. Only the depth of the chamber needs to 

 be accurately defined. 



If I is the total measured ionization in electrostatic units per 

 minute, R the radius of the upper surface of the stop, d the perpen- 

 dicular distance of this surface from the source, t the depth of the 

 chamber, the dose rate to the specimen placed on the level of the 

 upper surface of the stop is given by : 



H 



1 ^ 



(10) 



[1 + (R' 

 which, when R/d <C 1, reduces to: 



kl/nRH (11) 



