MEASUREMENT OF IONIZING RADIATIONS 157 



Under these conditions the experimental prerequisites necessary for 

 the measurement of radiation in roentgens with the thimble chamber 

 become difficult of attainment/ 



In addition to these obstacles, the roentgen itself loses much of its 

 sound basis as a unit of dose, mainly because it is concerned with the 

 energy converted at a certain point in the biological medium irrespective 

 of where this energy is utilized. This deficiency is not serious in the 

 low- or medium-energy X-ray range since the electrons produce ioniza- 

 tion very close to the point at which they are emitted, but it was shown 

 as early as 1937 by FaiUa, Twombley, and Marinelli, and later by others, 

 that the situation merited attention in the case of 7 rays from radium. 

 It has become understood (Fano and Taylor, 1950), therefore, that 

 although the roentgen as defined may become a useful index of radiation 

 output of an X-ray machine (measure of energy converted, i.e., F ■ e,'), 

 it serves no purpose as a general index of energy imparted to a given 

 region of the irradiated medium. 



MEASUREMENT OF DOSE BY MEANS OF CAVITY-IONIZATION CHAMBERS 



Gamma Rays. The evolution of the air-wall chamber as a reliable 

 instrument for the measurement of X and 7 radiation in roentgens can 

 be traced principally through the contributions of Bragg (1912), Fricke 

 and Glasser (1925), Laurence (1937), and Gray (1928, 1936). It is 

 mainly through the efforts of Gray, however, that its adaptability to 

 the measurement of energy absorbed in a medium is recognized today. 

 By considering first the rate of energy loss of ionizing particles generated 

 by 7 radiations in a medium and the ionization that results in a gas filhng 

 a small cavity in the medium (the experimental conditions being prac- 

 tically identical to those stated above for the measurement of the roent- 

 gen), he demonstrated (1936) that the energy expended per unit mass 

 ( /„ • W) in the gas is essentially related to the energy D absorbed per unit 

 mass in the solid by the following relation, 



D = Prn ■ J,n ' W (10) 



where J„, is the number of ions per unit mass of gas, W is the average 

 energy lost by an electron per ion pair formed, and p,„ is a proportionality 

 factor independent of the velocity of the ionizing particles, which can be 

 identified as the ratio of the mass stopping power {&Z.J o^ the medium 

 to that (^Lss) of the gas for the energy of the particles concerned. ^ 



4 At photon energies greater than the binding energy of the neutron in the nucleus — 

 about 11.1, 16.3, and 19 Mev for nitrogen, oxygen, and carbon, respectively— the 

 situation is complicated further by the appearance of nuclear reactions and by the 

 induction of radioactivity in the irradiated material. All the quantitative and qualita- 

 tive aspects of these phenomena are not known with great accuracy ; they have been 

 found, however, rather unimportant at the relatively low voltages investigated. 



8 pm and W are considered here strictly independent of the energy of the particle. 

 Although experimental data are, to a first approximation, consistent with this assump- 



