188 



Cellular Events Produced by Ionizing Radiations /I0 : 2 



1 nvt = thermal neutron flux per cm 2 times time in seconds. 



1 Mdwjct = 3 x 10 17 nvt. 



1 pile unit = 10 17 nvt plus associated gammas and fast neutrons. 



These units are all used in the literature. The r was originally 

 defined for X rays and is too firmly imbedded in medical terminology to 

 be completely discarded. It would appear far more desirable to express 

 dosage either in terms of energy loss per gram without extra symbols or 

 in terms of the number of particles and their energy. The list of 

 different dosage units is included here because they are all used to 

 describe experiments in biophysics. 



The ratio of the rem to the r is sometimes called the relative bio- 

 logical effectiveness, abbreviated RBE. The accompanying table gives 

 some RBE factors. It should be repeated that these values will vary 

 widely depending on the criterion used. 



TABLE I 

 Some RBE Factors 



Radiation RBE 



X 1 



gamma 1 



1.0 Mev beta particle 1 



0.1 Mev beta particle 1.08 



Thermal neutron 2-5 



1.0 Mev proton 8.5 



0.1 Mev proton 10 



Fast neutron 10 



5 Mev alpha 15 



1 Mev alpha 20 



Various committees have set up maximum permissible doses for 

 persons working near radiation. The maximum permissible dose is 

 defined as the highest level at which the probability of producing harm- 

 ful somatic effects is so low that it cannot be measured. Over the course 

 of years, various maximum permissible doses have been chosen. As 

 more knowledge has been obtained, these have decreased steadily. 

 Thus, from 1935 to 1947, an accumulated dose of 0.1 rem per day was 

 considered permissible. This was then lowered to a maximum of 0.3 

 rem per week. In 1957, the maximum permissible dose was further 

 lowered to 5 rem per year for each year over eighteen years of age. 

 Even these levels might produce appreciable genetic damage and might 

 give rise to malignant tumors. It seems likely that the maximum 

 permissible doses will be further lowered in the future. These values are 

 for whole-body irradiation of radiation workers. Levels for the entire 



