404 1SOTOP1C TRACERS AND NUCLEAR RADIATIONS [Chap. 16 



absorbed and of ionization may be denned so that they are more or less inde- 

 pendent of the type of radiation and absorbing medium, dose expressed in 

 such physical units may be accurately reproduced in different tissue and can 

 be correlated with any biological or clinical effects that occur. 



Units of energy absorbed per unit mass or volume of medium have, at the 

 present time, found universal although informal acceptance for expressing 

 dose delivered by corpuscular radiations. The important features of such 

 units are their evaluation in terms of fundamental physical units, the accuracy 

 and convenience with which they are measured, at least in principle, and the 

 fact that they are amenable to calculation when the source of radiation is 

 given. On the other hand, the ionization produced per unit mass of tissue 

 possesses essentially the same qualifications as energy for a unit of dose in 

 addition to the important physical distinction of being the immediate and 

 only directly observable product of radiation interaction with tissue. Ioniza- 

 tion as such is in effect the only measurable precursor of the biological, 

 clinical, and chemical effects that occur. From the standpoint of dose 

 measurement, some advantage is to be found in units based on the number of 

 ion pairs formed per unit mass since the dose is then given in terms of the 

 quantity directly measured with the ionization chamber. 



The units of energy absorbed which have found the most widespread use 

 include the roentgen-equivalent-physical (U. S.), the gram-roentgen (British), 

 and the energy-unit (British). The only unit of ionization is the J, proposed 

 by the British Committee for Radiological Units. The roentgen, which 

 applies only to x- and gamma radiation, is actually a partial description of 

 the radiation at any specified point (not necessarily in an absorbing medium) , 

 but it is regarded by some as essentially a unit of energy absorbed in air, 

 i.e., 83 ergs per gm of air — a meaning that is not strictly correct. 



Whichever unit of dose is employed, the physical quantity actually 

 measured is, in all instances, the ionization produced per unit volume per 

 unit time. In the case of dose expressed in roentgens, the ionization meas- 

 ured must, by the definition of the roentgen, be that produced in air at the 

 point under consideration. In order to make use of conveniently small 

 ionization chambers and yet measure true air ionization, it is necessary to 

 use "air-wall" chambers, i.e., chambers with solid wall material consisting 

 of airlike atomic composition, in determining dose in roentgens. Dose 

 expressed in the various energy units requires conversion of the detected 

 ionization to ergs or mev of energy absorbed. This can be done only when 

 the average energy W required to form one ion pair in the medium is known. 

 For ions produced in air by electrons, W = 32.5 ev. In the absence of con- 

 clusive experimental data on tissue, this value is sometimes adapted to 

 tissue dose assuming air and tissue to be nearly the same in atomic stopping 

 power for electrons. 



