108 



RADIATION BIOLOGY 



a layer of material vs. the thickness of the layer no longer follows a 

 straight line, as it does for monochromatic X rays under narrow beam 

 conditions. The total intensity follows the downward concave trend 

 indicated in Fig. 1-67. 



o 



o 



v" 



in 



>■ 

 < 



I 



X 



0.01 



0.1 - 



BUILD-UP FACTOR 



■ BUILD-UP 

 FACTOR 



4 8 12 16 20 



CONCRETE ABSORBER THICKNESS, In. 



Fig. 1-67. Narrow-beam and broad-beam absorption curves in concrete for X rays 

 from a tube operated at 800 kv with heavy filtration. Notice the build-up of sec- 

 ondary X rays under broad-beam conditions. {Courtesy R. J. Kennedy and H. 0. 

 Wyckoff.) 



Notice that the curve which includes the scattered radiation does not 

 coincide with the ''narrow-beam" curve even at "zero depth" of pene- 

 tration. The excess radiation at the surface consists of backscattered 

 X rays which are flowing out of the entrance surface. 



The downward concavity may be compensated for or even over- 

 compensated for by the upward concavity due to nonmonochromaticity 

 of the incident beam (see Fig. 1-66). Presumably, many experimental 

 attenuation plots which appear to follow a straight line indicate approxi- 

 mate cancellation of the effects of non-monochromaticity and of scattered 

 X rays rather than good monochromaticity and adequate narrow beam 

 conditions. 



The ratio of the intensity actually observed to the intensity expected 

 from the incident radiation alone is called the "build-up factor." The 

 scattered radiation builds up to a very high level particularly for high 

 primary energies and in "light" materials, in which a photon frequently 

 undergoes 10 or more Compton scatterings before eventual absorption 

 by photoelectric effect. A high build-up of X rays of lower energy than 

 the primaries also takes place at the tail end of cascade showers, as men- 

 tioned in Sect. 4-2c. 



