544 



L. H. G R A Y 



where k is the ratio of the mass stopping powers of tissue and air for 

 the electrons in question if the dose is expressed in r.e.p. per minute 

 and the ratio of the electronic stopping powers if doses are expressed 

 in energy units per minute. 



{a) 



T Source 

 \ 

 \ 



d \ 



Material of low atomic number 



^\\\\N\\y 



To electrometer 



V"\ ■ ; -Xxx^ Thickness not les: 

 V ' .N\N\Ni ttian electron ran 



ess 

 ge 



Thin paper 



Metal guard ring 



(A) 



(C) 



To electrometer -G 



Thin paper 



Metal guard ring 



To electrometer 



-- Source 



icf\ 



Metal guard ring 



-^ 



About 500 V. 



0.01 mm. nylon 



Fig. 16. Arrangement of ionization chambers for measming (a) the dose of 

 i8 rays when the biological material is irradiated in a thin film, (b) the dose of 

 /3 rays to the superficial cells of a portion of tissue, and (c) the dose of a rays 

 delivered to unicellular organisms. 



If the specimens are more conveniently irradiated on the surface 

 of agar, or if we are interested in the superficial layer of cells of a mass 

 of tissue, a small c.ylindrical chamber may be arranged somewhat as 

 shown in Figure 166 to measure the dose. Lateral scattering, which 

 will occur in the irradiation of the specimen, is provided by wood. If 



