106 RADIATION BIOLOGY 



deflection or change of energy. This fraction may be determined experi- 

 mentally provided that the reception of unwanted radiation is minimized 

 by the device which measures the transmitted intensity I(x). 



The arrangement shown in Fig. 1-65 attains this goal to a degree which 

 depends on the narrowness of the beam after passing the collimator and 



SOURCE ^^^ ^ DETECTOR 



ABSORBER 



COLLIMATOR 

 Fig. 1-65. Schematic layout for measurements of narrow-beam absorption coefficients. 



on the distance of the source and of the detector from the absorber. 

 These geometrical characteristics of the arrangement cannot be improved 

 indefinitely without an excessive reduction of the X-ray intensity. The 

 geometrical recjuirements are not too strict at lower X-ray energies, since 

 most of the attenuation is due to photoelectric effect. On the contrary, 

 very small angle scattering becomes quite important at high energies, 

 and an adequate measurement requires a high-precision geometry. 



Since a direct measurement of /x is performed with a narrow beam of 

 X rays, this quantity is frequently referred to, more specifically, as the 

 '^narrow beam absorption coefficient." 



4-3b. N arrow-beam Penetration of Non-monochromatic X Rays. When 

 a non-monochromatic beam of X rays traverses a material, each mono- 

 chromatic component is absorbed or scattered independently of the 

 others, at a rate determined by the value of the absorption coefficient 

 which corresponds to its energy. 



Generally, the lower energy, or "softer," components are absorbed at a 

 higher rate than the higher energy ones. (The downward trend of the 

 absorption coefficient with increasing energy is reversed only at very high 

 energies.) Therefore the spectral composition of the beam changes 

 progressively. The softer components disappear and the higher energy, 

 or "harder," components become comparatively more prominent. The 

 average rate of attenuation of the whole beam decreases in the course of 

 penetration. 



Therefore, the slope of a semilogarithmic plot of intensity vs. depth 

 keeps decreasing, as in Fig. 1-66, instead of remaining constant as in 

 Fig. 1-46. The slope approaches progressively the value corresponding 

 to the most penetrating component of the incident beam. 



These considerations apply to the penetration determined under the 

 "narrow beam" conditions. 



4-3c. Scattered and Secondary X Rays. The "narrow beam" condi- 



