410 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 16 



a. Roentgen. A dose of one roentgen, received at any point, means 



1 esu of ion pairs produced per cc air 

 2.083 X 10 9 ion pairs produced per cc air 

 1.61 X 10 12 ion pairs produced per gm air 

 6.77 X 10 4 mev absorbed per cc air 

 5.2 X 10 7 mev absorbed per gm air 

 83 ergs absorbed per gm air 



Air refers to dry air at 0°C, 760 mm Hg. Values of energy absorbed are 

 based on 32.5 ev absorbed to form one ion pair by an electron in air [9,12]. 



Definition: The roentgen is "that quantity of x- or gamma radiation 

 such that the associated corpuscular emission per 0.001293 gm of air, dry, 

 0°C, 760 mm Hg produces, in air, ions carrying 1 electrostatic unit of quantity 

 of electricity of either sign" [1]. 



The roentgen, by its definition, is a partial description of the electro- 

 magnetic radiation at a point measured in terms of the ionization produced in 

 air. It is a unit not of intensity, energy, or flux but rather of the time 

 integral of the flux density evaluated according to its ability to ionize air. 

 The relationship that does exist between the energy absorbed in air exposed to 

 one roentgen and energy flux (number of photons per square centimeter X 

 energy, hv, per photon) shows a complicated dependence on gamma-ray 

 energy. As shown in Fig. 114, x- or gamma rays of very low energy, corre- 

 sponding to the energy range in which the photoelectric effect is the principal 

 absorption process, produce the greatest ionization or dose for a given energy 

 flux. Only in the range from 0.08 to about 1.2 mev, where Compton scatter- 

 ing is the principal interaction process, is the roentgen nearly independent of 

 gamma-ray energy for constant flux. In this region 1 r corresponds to 

 approximately 2,800 ergs per cm 2 . In general, however, for a given energy 

 flux the dose in roentgens decreases with increasing gamma-ray energy, at 

 least for energies less than about 30 mev. 



An important feature of the definition of the roentgen is its reference to 

 air. One roentgen of gamma rays is the quantity of radiation such that about 

 83 ergs are absorbed per gram of air, but in substances of different atomic 

 number and density the amount of energy absorbed for this same quantity 

 of radiation will be different. Thus, in soft tissue the energy absorbed per 

 gram per roentgen is about 93 ergs, and in bone it may be several hundred 

 ergs. Despite the great variation in the relative amounts of energy absorbed 

 in different substances, the dose is still 1 r if the same quantity of radiation 

 produces 1 esu of charge of either sign per cubic centimeter of air at the 

 point under consideration. The dose expressed in roentgens is totally inde- 

 pendent of the absorbing medium that is exposed and of the amount of 

 energy that the particular medium absorbs. 



