74 , RADIATION BIOLOGY 



leads to the formation of a metastable nucleus, e.g., a /3-radioactive 

 nucleus. 



Elastic collisions of neutrons against hydrogen nuclei (protons) absorb 

 most of the neutron energy whenever a hydrogen-rich material, such as 

 tissue, is exposed to neutrons. In the first place a neutron-proton impact 

 has a much greater chance of yielding an elastic collision than capture. 

 In the second place protons, being about as heavy as neutrons, are 

 capable of recoiling violently under the impact. The recoiling proton 

 may take up any fraction of the neutron energy. Each elastic collision 

 leads, on the average, to an even sharing of energy between neutron and 

 proton. As a result, a material containing hydrogen, which is exposed 

 to neutrons, is traversed by protons of all energies from zero up to the 

 initial kinetic energy of the neutrons. Elastic collisions also result with 

 highest frequency from the impact of neutrons on the nuclei of other 

 light elements, notably helium, carbon, and oxygen. 



The dissipation of energy by all possible mechanisms must be evaluated 

 in detail, for each specific application, whenever the dissipation by proton 

 recoil does not constitute the main mechanism or whenever an accurate 

 knowledge is desired. For example. Table 1-5 summarizes the result of 

 an evaluation of the energy dissipated by neutrons, mostly of low energy, 

 in a biological material. The introduction in a material of an abnormally 

 large amount of some highly neutron-absorbing element, like boron, 

 serves to increase locally the energy dissipated by slow neutrons. 



3-4. ACTIVATION BY INFRARED, VISIBLE, AND ULTRAVIOLET LIGHT 



As mentioned before, the potency of light lies in the lower portion of 

 the chemical range. Light energy is delivered directly to individual 

 atoms and molecules by absorption of individual photons here and there 

 throughout a material exposed to light. Photon absorption leads then 

 generally to a simple excitation, whose possible consequences have been 

 considered briefly in Sects. 2-lc and 2-3. 



One further point must be considered with regard to the action of light 

 on materials such as living matter, or even on less complex organic sub- 

 stances. Most pure non-electrically-conducting substances are trans- 

 parent to near-infrared, visible, and near-ultraviolet light. Therefore 

 the absorption of these radiations in most organic materials takes place 

 exclusively in special molecular groups which are called " chromophoric " 

 and are present in greater or smaller concentration. The specific absorp- 

 tion of visible light of wave length between 6000 and 7000 A by chloro- 

 phyll in plants offers a familiar example of this mechanism. The absorp- 

 tion of ultraviolet light near 2600 A by the purine and pyrimidine groups 

 of nucleic acids offers another well-known example. 



Thus we see that special groups of atoms constitute the point of attack 

 of the primary action of light on a large number of materials, particularly 



