176 RADIATION BIOLOGY 



virus or bacteria) placed immediately below D can be calculated as 



^ I -W • pm tan2 d , 



Dose = ^ p,., ^rr. ergs/g 



2xi2"(l — COS e)tb 



where R and ^ are in centimeters, 5 is the density of the air in grams per 

 cubic centimeter, / is the total number of ion pairs, d is the angle sub- 

 tended by R at the source, and W (in ergs per ion pair formed) and p„. 

 retain their previous meanings. In the interest of accuracy, the distance 

 between chamber diaphragm as well as the electrode separation should 

 be small compared to the range of the particles in air, and scattering 

 from the edges of the diaphragm should be investigated by varying its 

 diameter. 



A more versatile chamber for the measurement of particle radiation is 

 the extrapolation chamber of Failla (1937), diagramatically represented 

 in Fig. 2-11. This chamber has been recently described in the literature 



h X SOURCE 



s- TO BATTERY 



A 



TO ELECTROMETER 

 Fig. 2-11. Failla's extrapolation chamber. 



in some detail (Failla, Rossi, et al, 1947; Sheppard and Abele, 1949; Bass 

 et al., 1949; Bortner, 1951; Quimby and Focht, 1943). Schematically, it 

 consists of an effective volume of air, usually cylindrical in shape, limited 

 in its height h by the inner surfaces of two plane electrodes, and of cross 

 section equal to that of the collecting electrode C, which is coplanar with 

 the annular guard ring B. By extrapolating to zero both the thickness 

 of the upper electrode A and the spacing h, it is possible to measure the 

 number of ions /„ per unit mass of air at the surface of the collecting 

 electrode. The energy Dc absorbed per unit mass in the material of the 

 collecting electrode will be given by 



D, = p,n-J- W X 1.44 X 10-12 erg/g 



where the symbols retain their previous meaning. 



It becomes obvious, therefore, that if the electrodes of the chamber are 

 made of materials containing elements prevalent in biological material 

 (such as bakeUte, polystyrene, and resins), the dose D in thin biological 

 specimens (virus, bacteria, a few cell layers, for example can be calculated 

 as 



D = piD, 



