88 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



curie is a measure of how much radiation is emitted by a source and may 

 therefore reach the absorbing material, whereas the roentgen indicates the 

 amount of energy dissipated in the absorbing material. It must also be 

 noted that the roentgen is a unit of quantity of dosage. The number of 

 roentgens per unit time is necessary to express the dosage rate. For 

 example, if 100 r/hr is delivered for 8 hr, the total dose delivered would be 

 800 r. 



Another concept that is confusing but must be understood is that the 

 roentgen refers to energy absorbed per unit volume of air. Thus, if a man 

 receives the equivalent of 1000 r just on his hand, there would be no 

 serious effects. However, if the same dosage, 1000 r, were delivered to 

 the entire body, there would be considerably more energy absorbed, 

 undoubtedly enough to produce death. To evaluate the dosage received, 

 one must therefore take into account the dosage rate, the time, and the 

 volume irradiated. 



From the biological standpoint the interest is primarily in the energy 

 absorbed by tissue rather than by air. Also there is an interest in radia- 

 tion other than gamma rays or X rays (e.g., beta rays, alpha rays, and 

 neutrons). An unambiguous unit for tissue dose that has been officially 

 adopted (4) is the erg per gram. However, the roentgen has been so widely 

 used that there is some merit in having a unit for dose which is related to 

 the roentgen. This is feasible because with photon energies below 3 Mev 

 there is an essentially constant factor of proportionality between the 

 exposure of biological tissues as expressed in roentgens and the energy 

 absorption. The International Commission on Radiological Units in 

 1953 proposed a new unit for absorbed dose called the rad, which is equal 

 to 100 ergs/g (25). A similar unit that appears in the literature had pre- 

 viously been proposed by Parker (26) ; this was called the roentgen- 

 equivalent-physical (rep), which corresponded to the amount of ionization 

 that would bring about the same absorption of energy per unit of air or 

 tissue as would be caused by 1 r of gamma or X rays. It should be noted 

 that the roentgen actually corresponds to the absorption of 83 ergs/g of 

 air and about 93 ergs/g of tissue. The use of the rad avoids confusion as 

 to the actual erg-per-gram value that has been used for any particular 

 reported value. Another unit proposed by Parker which appears in the 

 literature is the roentgen-equivalent-man (or -mammal) (rem). This is 

 the amount of energy absorbed in tissue which is biologically equivalent 

 in man to 1 r of gamma or X rays. 



In regard to relative biological effectiveness (RBE), it is considered 

 that gamma and beta rays are equivalent and that alpha rays are twenty 

 times more effective. References (27 to 29) are of interest in connection 

 with units and dosimetry. 



To give an idea of the magnitude of the roentgen unit in terms of radia- 



