154 RADIATION BIOLOGY 



necessary to increase it to the equivalent of 3.5 meters, and Failla and 

 Marinelli (1937), by indirect means, find that compensation is hkely to 

 require close to the equivalent of 4.1 meters of air at NTP. As a con- 

 sequence of this state of affairs the air-wall chamber has been used almost 

 exclusively for the measurement in roentgens of 7 rays of energy above a 

 few tenths of a mega electron volt. 



Air-wall Thimble Chamber. This chamber derives its name from what 

 it tries to accomplish, namely, to surround a definite mass of air with solid 

 material which will interact with photonic as well as corpuscular radiation 

 as a solid layer of air would. Under these conditions the loss in ionization 

 due to the corpuscular radiation originating in the air, but not penetrat- 

 ing the chamber, is compensated by the ionization produced in the air 

 within the chamber by the corpuscular radiation originating in the walls 

 (White, 1950; Failla, 1950). The compensation is perfect irrespective of 

 the volume of the chamber if the wall is truly air equivalent (in atomic 

 composition), of adequate thickness, and subject to a uniform flux of 

 radiation.- Under these conditions e^" = e7^ 



Although these requirements cannot be met in practice, exhaustive 

 analysis (White, Marinelli, and Failla, 1940; Gray, 1928, 1936; Laurence, 

 1937) has proved that the degree of compensation can be calculated with 

 satisfactory accuracy if (1) the walls are of known chemical composition, 

 comprising elements of atomic number close to 7 ; (2) the linear dimensions 

 of the ionized gas volume are negligible in comparison to the range in free 

 air of the ionizing particles ; (3) the walls of the chamber do not attenuate 

 unduly the primary radiation but are thick enough so that further increase 

 in thickness does not change the number of ions collected in the air vol- 

 ume; (4) the distance between source and chamber is large in comparison 

 to the outer linear dimensions of the chamber; and (5) the corpuscular 

 radiation is due mainly to the Compton process. When these require- 

 ments are met it is possible to measure 7 radiation in roentgens by divid- 

 ing the experimental value of the volume density of ionization at NTP 

 (esu/cc) by a correction factor B of the form^ 



where Zu, and Za are, respectively, the atomic numbers of the wall and of 

 air, and J, K, and L are functions of the primary photon energy and of 

 the energy loss undergone by the corpuscular radiation m air. Graphs 



2 Recent investigations on stopping powers of condensed systems seem to indicate 

 that this is not strictly true under all circumstances (see p. 166). 



3 Condition (2) assumes that the ionization produced by the 7-ray flux in the gas 

 of the cavity is negligible; hence the total ionization in the chamber is due to the 

 energy conversion occurring in the wall and to the utilization of this energy in the air 

 volume. An explicit expression for B in terms of photon energy and stopping powers 

 of the gas and wall materials can be obtained by comparing Eqs. (10a) and (10b). 



