296 BELL SYSTEM TECHNICAL JOURNAL 



somewhere in the path of the X-rays to absorb the A'/3 radiation from the 

 copper target, it may be used here to seal the entrance of the chamber. 



The Geiger-Miiller counter is similar to the ionization chamber, but is 

 operated at such high voltage that the gas is always near breakdown, 

 the passing of X-rays supplying the impetus to complete the breakdown. 

 Because of the high voltage the ionization continues to be strong as the 

 intensity of the radiation declines. Such a lag is likely to cause erroneous 

 maximum readings in crystal measurement. 



3.3 Physiological Effects of X-Rays 



Even a few minutes direct exposure to X-rays from such a source as the 

 G. E. CA6 tube will result in a burn that will become apparent in a day or 

 so. In most cases such a burn, if not repeated, will heal without ill effects, 

 but because the physiological effect of X-rays is cumulative, repeated 

 exposure to direct radiation could result in a burn that would not heal 

 and might become cancerous. 



Pinhole leaks which permit direct radiation to escape are thus exceedingly 

 dangerous. They may be detected by use of a fluorescent screen in a 

 darkened room. A sheet of lead-glass j to | inch thick should be held 

 between the fluorescent screen and the observer. 



X-rays are scattered from all substances which they strike and repeated 

 exposure to this scattered radiation may result in a harmful decrease in 

 the white blood corpuscles, in sterility, and perhaps in serious burns. 



To test for scattered radiation dental X-ray films should be arranged 

 as close to the instrument on all sides as any part of the operator's body 

 can get and left for a period of two weeks of normal operation. If there 

 is no position in which the film becomes fogged during the two weeks, the 

 operator is safe. A narrow lead strip across the film will provide an un- 

 exposed portion for comparison with the exposed portion. It should be 

 emphasized that a dental film carried in the pocket is an inadequate safety 

 test, since the hands are frequently the most dangerously exposed part 

 of the body. 



The absorptive power of shielding materials is proportional to the density 

 of the material. Minimum adequate shielding is provided by 1.5 mm. 

 or about pg- inch of lead or its equivalent for protection against X-rays 

 generated at 70 kv.^ Equivalent thicknesses of shielding materials are 

 as follows: 



Lead y6 inch 



Lead rubber \-\ inch 



Lead glass |-| inch 



Steel \ inch 



Bricks and concrete 6 inches 



Woods 60 inches 



1 Davey, W. P., "Study of Crystal Structure and its Applications," McGraw-Hill, 1934. 



