36 BIOPHYSICALLY ACTIVE X-RAYS 



known as chromosomes, which carry the hereditary factors in the form 

 of physicochemical units called genes. The remarkable effect following 

 the absorption of the radiant energy is an alteration of the genetic nature 

 of the cell (Goodspeed and Uber [1939]), which results from the ioniza- 

 tion accompanying the passage of high-speed electrons through the 

 nucleus and chromosomes. The effect of the ionizing radiations is to 

 increase markedly the frequency of mutations, so that geneticists need 

 not wait for the very rare probability of spontaneous genetic changes in 

 order to study the problems of heredity and variations. For purposes 

 of study those changes which cause differences in the physical appear- 

 ance of Drosophila have been found most convenient. 



For a further discussion of these very fundamental problems see 

 Chapter II, " The Biological Roentgen," and Chapter IV, " Effect of 

 Ultraviolet Radiation on Bacteria." 



X-Ray Protection 



The protective materials commonly used in radiology are: (1) sheet 

 lead; (2) lead-impregnated glass and rubber; (3) concrete. 



The absorptive values of sheet lead for various thicknesses and various 

 wavelengths of radiation are given by Mutscheller [1925] and reproduced 

 in Table 1-7. These results show that absolute protection is impossible 

 even with 8 mm of lead. 



Under practical conditions faced by an x-ray operator, a protection 

 comparable to 8 mm of lead is seldom attainable. Mutscheller has, 

 however, found that an operator can safely subject himself to a " toler- 

 ance dose " of 1/100 of an erythema dose in 30 days' exposure. 



On the basis of this definition we can assume that during radiographic 



o 



work with an effective wavelength equal to 0.17 A, with 50 exposures 

 per day, each of 5 seconds at 20 milliamperes current, the operator 

 standing 10 feet from the tube would receive about 1 erythema dose in 

 1 month. To reduce this to 1/100 of an erythema dose, lead protection 

 of 1.2 mm is needed. If fluoroscopic work is undertaken during 2 hours 

 per day, using 4 ma, about 6 erythema doses would be received in the 

 same time and distance. Under these working conditions 1.8 mm of 

 lead as a protection is necessary. 



During radiotherapy work, where 4 ma for 10 hours a day may be 

 used, 28.5 erythema doses per month under similar conditions are re- 

 ceived and from 5 to 6 mm of lead are needed for protection. 



Protective gloves or gauntlets and aprons are constructed of rubber 

 impregnated with 55 per # cent lead oxide. They are comparatively 

 heavy but ensure protection. Lead-rubber aprons can be obtained in 

 different weights. The usual 7-lb apron is made of -j^in. leaded rubber 



