The Measurement of Radiation 185 



of the above, so that there was considerable fear that the ex- 

 pression of gamma-ray quantity in rontgens was without 

 meaning. 



This disharmony was resolved by Kaye and Binks in 1937/^ 

 who showed conclusively that on accotmt of the large range in 

 air of the secondary electrons produced by the gamma radiation, 

 the dimensions of the ''free-air" chamber need to be very much 

 greater than in the case of X-radiation, for the equilibrium in- 

 tensity of the secondary electrons to be reached, and for their 

 energy to be fully utilized in producing ionization. The current 

 obtained from the "free-air" chamber with gamma radiation no 

 longer originates in the simple "ionized volume" as in the case 

 of X-rays but, provided the dimensions are large enough, a 

 geometrical argument shows that full compensation exists and 

 the same simple calculation is valid. Kaye and Binks ^^ found 

 a value of approximately 8.0 r per- hour for the specific gamma- 

 ray dose rate of radium. 



Friedrich provided further confirmation in 1938 '^ by measur- 

 ing the ionization in a small thin-walled chamber suspended in 

 air in the center of a large hall, so that it was influenced solely 

 by the secondary electrons (in ec[uilibrium) produced in the air. 

 In this way, a value of 7.8 r per hour was found for the constant. 

 Lastly, Taylor and Singer in 1940 "^ made very precise measure- 

 ments with a "free-air" chamber operated at ten atmospheres' 

 pressure, in order to reduce the size, and obtained the figure 

 8.16 r per hour. All doubts as to the legitimacy of measuring 

 gamma rays in rontgens have thus been finally dispelled. 



True Energy Absorption and the Theory of the 

 ''Thimble'' Chamber 



Of greater fundamental physical importance, however, was 

 the work on the "thimble" chamber method of measurement, 

 referred to several times above. The essence of this idea was 

 provided in 1911 by Bragg,^ rediscovered by Fricke and Glasser 

 in 1925,^ and again independently by Gray in 1929. Innumer- 

 able other workers have made contributions of various kinds, but 



