118 RADIATION BIOLOGY 



farther than the heavy charged particles which are the main by-product of 

 neutron colhsions. The jS rays follow the direct action of neutrons with very 

 considerable delay; delays of days or years are not uncommon. 



The distribution in space of the energy dissipation by /3 rays does not quite 

 parallel the distribution of the points of direct action of neutrons because of the 

 appreciable range of the electrons. The situation resembles that of energy dis- 

 sipation by high-energy electrons arising from the action of X rays. However, 

 the direction of the rays does not bear any relation to the direction of arrival of 

 the neutrons. The distribution of the |8-ray energies should be studied as a 

 problem of electron penetration starting from a knowledge of the distribution of 

 the jS-ray-emitting nuclei. 



4-5. LIGHT 



Light penetration in homogeneous media is limited by outright absorp- 

 tion. Therefore the light intensity decreases in the course of penetration, 

 like the penetration of X rays, when only the photoelectric effect is 

 important, i.e., according to the simple exponential law of Eq. (40), 



I(x) = 7(0)6^"- 



The absorption coefficient for light varies greatly as a function of the 

 frequency and from one substance to another. A general discussion of 

 the absorption of light was given in Sect. 2-3. The absorption is exceed- 

 ingly high in metals owing to energy dissipation by free electrons. 



Catalogues of data on the absorption coefficients of homogeneous 

 chemical substances are given in the International Critical Tables (1929). 



The mass absorption coefficient of a homogeneous mixture of substances is 

 obtained by combining the coefficients of the component substances Nos. 1, 2, 

 . . . according to the composition by weight of the substance, as in Eq. (43) : 



H = pifjLi + P-tJi-2 + • • • 



In calculating the light penetration in a material we should take into account, of 

 course, the surface reflectivity of the material. 



The penetration in nonhomogeneous materials presents a more complicated 

 prol)lem. The effect of macroscopic inhomogeneities, such as a 1-mm air 

 bubble suspended in water, constitutes a standard problem of optics. Micro- 

 scopic inhomogeneities, in which the properties of the medium vary considerably 

 over distances of the general order of magnitude of the fight wave length (0.1 to 

 1 ju) ])roduce a scattering of fight. 



To understand how scattering arises, we may consider, in the first place, the 

 simple propagation of light through a homogeneous medium. As mentioned in 

 Sect. 2-3, electromagnetic radiation exerts an oscillating disturbance on atomic 

 electrons and thus induces oscillating currents within atoms and molecules. 

 These currents constitute the source of new disturbances which tend to propagate 

 away in all directions in the form of scattered radiation. 



When atoms are distributed uniformly over a large region of space, the dis- 

 tm-bances scattered by different atoms in any one direction neutralize each other 



