XIV. 



X RAYS AND X IRRADIATION 



459 



varies approximately as the fourth power of this mnnher. Since the 

 atomic mimber of lead is high, a comparatively thin lead screen may 

 give effective protection against X rays {55). In tissue, the absorp- 

 tion can be represented b}^ a weighted averaging of the atomic num- 

 bers of the atoms composing it. 



Another complicating factor in this absorption correction is that 

 of the wavelengths of the X ra3^s. It is not practical to obtain high 

 intensity monochromatic radiation with X rays for irradiating the 

 large areas encountered in most biological work. X-ray tubes give 

 out a continuous band of general radiation having wavelengths rang- 

 ing from 10 A. to some limiting value dependent upon the voltage put 



Characteristic 



A'- series rodiation 



Fig. 4. Variation of intensity 

 of general X radiation with 

 wavelength. Characteristic radi- 

 ation is superimposed. Dashed 

 Une shows intensity distribution 

 possible by using proper metal 

 filter. 



a 



z 

 o 

 o 



LlI 

 (/I 

 \ 



in 



z 



UJ 



UJ 



o 

 tr 



Absorption edge 

 of proper metal 

 filter 



Exciting voltage 

 limit to wave 

 engtti 



Ctiaracteristic 

 L- series 

 radiation 



General 

 radiation 



.1 10.0 



WAVELENGTH, angstrom units 



on the tube. A graph showing the absolute intensity as a function 

 of wavelength for general radiation is shown in Figin-e 4. General 

 radiation shows a continuous spectrum of wavelengths from a low 

 intensity value in the long wavelengths, rising to a modal value, and 

 then dropping more sharply to zero intensity at the short wavelength 

 cut off. This limiting wavelength in A. is given by 12,395/ F, where 

 V is the voltage. With 20 kv. on the tube, the wavelength limit 

 would be about 0.6 A. ; with 200 kv., 0.06 A. 



Besides general radiation there is superimposed radiation charac- 

 teristic of the metal used as target m the X-ray tube. Characteristic 



