412 1S0T0P1C TRACERS AND NUCLEAR RADIATIONS [Chap. 16 



The roentgen is in some respects a measure of virtual energy absorbed, 

 i.e., the time integral of the energy absorbed if the medium were replaced 

 with air. A dose of 1 r is delivered only when 1 esu of charge is formed per 

 cubic centimeter of air, but this amount of ionization may be produced in 

 any arbitrary interval of time. The time is introduced only in reference to 

 dosage rate expressed in roentgens per unit time. For any given gamma-ray 

 energy the dosage rate is directly proportional to the intensity of radiation. 



b. Roentgen Equivalent Physical {proposed by H. M. Parker [19]). A dose 

 of one roentgen-equivalent-physical means 



83 ergs absorbed per cc tissue 



5.2 X 10 7 ev absorbed per cc of tissue 



Definition: "That dose of any ionizing radiation which produces energy 

 absorption of 83 ergs per cubic centimeter of tissue is 1 rep" [19]. 



The rep is a dose unit applicable to all corpuscular radiations such as beta 

 and alpha particles, protons, and deuterons and is a measure of energy 

 absorbed in tissue exposed to these radiations. Its value is established on the 

 basis of the energy absorbed in air exposed to 1 r, but the rep is not, in general, 

 equal to the energy absorbed per gram of tissue exposed to 1 r. The energy 

 absorbed in tissue exposed to gamma-radiation depends on the atomic com- 

 position and density of the tissue as well as on the energy of the radiation, 

 whereas, the rep is always 83 ergs per gram of tissue, independent of tissue 

 composition, type of corpuscular radiation, and energy. In soft tissue, for 

 example, a dose of 1 r corresponds to the absorption of about 93 ergs per 

 gram, whereas this amount of energy absorbed from corpuscular radiation 

 corresponds to a dose of 1.1 rep. If it is known either from computation or 

 experiment that Ei ergs or E 2 mev of energy are absorbed per gram of tissue, 

 the dose delivered expressed in rep is Ei/83 or E 2 /5.2 X 10 7 , respectively. 



The rep is essentially the same unit as the equivalent roentgen (er), used 

 by Marinelli, et al. [13], and such units as tissue roentgen and roentgen 

 equivalent proposed by others. 



c. Grain Roentgen (proposed by W. V. Mayneord [12]). A dose of one gram 

 roentgen means 



83 ergs energy absorbed 



5.2 X 10 7 ev energy absorbed 



Definition: the gram-roentgen is that amount of gamma-ray energy con- 

 verted into kinetic energy of secondary electrons which is equal to the energy 

 absorbed by 1 gm of air exposed to 1 roentgen. 



The gram roentgen is a unit of energy conversion but without regard to the 

 quantity of tissue in which this amount of energy is absorbed. It is, there- 

 fore, a unit of integral dose and should be clearly distinguished from other 



