XIV. X U A V S AND X I H H A D I A T ION 461 



For sevenil different flies the ///o value for silver X radiation was 

 0.894; for copper, 0.530; and for chromium, 0.246. The square 

 roots of these values give 94, 72, and 49%, respectivel}^, of the incident 

 surface radiation reaching the center of the fly. The respective X 

 radiations from these three metal targets Avere filtered through palla- 

 dium, nickel, and vanadium windows. The dose incident to the fly 

 must be corrected by these percentages to get the estimated dose to 

 the sperm. 



3. Monochromatic X Radiation 



The difficulties attached to studies of wavelength effects bring uul 

 other points for consideration. For these studies one would like 

 monochromatic X radiation. Unlike visible light, an X-ray beam 

 cannot be readil.y separated into its constituent wavelengths. Mono- 

 chromatic X radiation thus far has had to be obtained by diffraction 

 from crystals or gratings. This results not only in a great reduction 

 in intensity at each wavelength but also in a very small area ovei' 

 which the X rays are monochromatic. These facts make this method 

 impractical for most biological work. As the next best alternative 

 the experimenter attempts to restrict the radiation to particular 

 bands. This is accomplished largely through filtering out the X rays 

 of long wavelength and through the use of particular metals as win- 

 dows. The filters for the long wavelength components are generally 

 made of aluminum or copper. The general radiation reaching the 

 tissues is therefore restricted in some degree to the short waves of the 

 spectrum. 



Besides this general absorption effect, there is a characteristic 

 absorption effect. Every element has a characteristic absorption 

 curve; these curves are marked by discrete peaks or discontinuities 

 at specific wavelengths. Use is made of this fact in the choice of 

 filter materials for X-ray windows. In using a copper target tube, 

 for example, the continuous background of general X radiation has 

 imposed upon it ladiation characteristic of copper. The "/v''- 

 characteristic radiation of copper occurs at a wavelength of approxi- 

 mately 1.5 A. The presence of this characteristic radiation has the 

 effect of making a large peak in the radiation curve when intensity is 

 plotted against wavelength (c/. Fig. 4). A thin sheet of nickel em- 

 ployed as a window in the X-ray tube thus possesses the imiciue prop- 

 erty of transmitting nearly all of this copper peak radiation at 1.5 A. 



