MEASUREMENT OP IONIZING RADIATIONS 149 



specific proposal of employing air ionization as an index of X-ray dose in 

 biology. 



In general, the ionization produced in a gas by charged particles is pro- 

 portional, within wide hmits of particle energy and particle mass, to the 

 energy absorbed by the gas; namely, the average energy W expended by 

 an ionizing particle per ion pair formed in a gas is, to a first approxima- 

 tion, a constant for the gas. It is obvious, therefore, that an ionization 

 measurement can express the energy Dy absorbed per unit mass of a gas if 

 the average energy W expended per ion pair formed is known, since in 

 this case 



D, = WJ (2) 



where J is the number of ion pairs per unit mass of the gas. It is cogent 

 to ask at this point what relation D^, as measured, bears to D of Eq. (1), 

 namely, to the energy absorbed by the biological object. By reasoning 

 similar to that followed there, 



D,=F-er el (2a) 



where F, e^, and €„ retain the previous meaning and ^ is a superscript 

 indicating their pertinence to the gas. It must be concluded from Eqs. 

 (1), (2), and (2a) that D can be calculated as 



D = J ■ W ■ ee • eje", ■ el (3) 



and that, in principle, D is determinable directly from the ionization 

 ,/ if W and the ratios e^/e^ and e^/e^ are known. Expression (3) is of 

 general application, whatever the type of radiation encountered, and it 

 is an implicit statement of the principle underlying dosage calculations 

 based on ionization measurements. 



Relation between Dose and Exposure in Roentgens. In view of the wide- 

 spread use of the official radiological unit — the roentgen — it is desirable 

 to estabUsh the relation between this unit and the dose in ergs of energy 

 absorbed per gram; this can be done readily by the use of Eq. (2a). It 

 will be recalled that the roentgen is defined as "the quantity of X or 7 

 radiation such that the associated corpuscular emission per 0.001293 g of 

 air produces, in air, ions carrying one electrostatic unit of quantity of 

 electricity of either sign"; and that, in the terminology adopted here, the 

 coefficient e^ of Eq. (2a) is obviously equal to unity. It follows, therefore, 

 that 



„ TI^ X ions per esu 5. 18 X 10-^^ X 2.08 X 10^ _ ^o . ,,,^/^ 



^^ = 0001293 = 1.293 X 10"^ ^^^'^ ^'^'/^ 



• 

 on the assumption that Tfair = 32.5 ev = 5.18 X lO'i^ erg for all electron 

 energies and that the charge of the electron e = 4.81 X 10"^° esu. 



