PRINCIPLES OF RADIOLOGICAL PHYSICS 45 



Nuclear forces exert only a minimal influence on electrons. 



A nucleus may behave as a rigid body even when nuclear forces are 

 effective in the course of a close range collision. The collision is then 

 "elastic," meaning that no energy is transferred to the internal motion of 

 the nucleus. 



When neutrons of moderate energy (up to a few million electron volts) 

 experience elastic collisions with nuclei, they rebound evenly in all 

 directions. Elastic collisions tend to predominate, in general, for neu- 

 trons with a kinetic energy below 1 Mev (see also Sect. 2-5). The fre- 

 quency of collisions along the path of neutrons frequently becomes quite 

 large for very low neutron energies. 



2-2f. Summary Table. In Table 1-2 are summarized the main ele- 

 mentary processes which result from the interaction of incident radia- 

 tions with "free" atomic particles. 



2-3. ACTION OF LIGHT AND X RAYS ON ATOMS AND MOLECULES 



Electromagnetic radiation acts upon atomic systems by inducing 

 internal oscillating currents within them. The induction of currents 

 may subtract from the energy of the incident radiation either by dis- 

 sipating energy within the atomic system ("absorption" proper) or by 

 reradiating energy in a new direction ("scattering"). 



The process of absorption of one photon of light or X rays is generally 

 localized within the particular atom or molecule which takes up the 

 energy. (The absorption in larger crystalline aggregates of atoms may 

 be localized less closely.) Therefore the absorption of light or X rays by 

 a material may be fairly resolved into separate "elementary processes" 

 which take place within particular atoms or groups of atoms. The 

 absorption of radiofrequency radiation constitutes a macroscopic process 

 and will not be considered here. 



The process of scattering of one photon by a portion of matter can be 

 localized within a particular atom or molecule if the atom or molecule 

 shows some aftereffect of the scattering. For example, the Compton 

 scattering of a high-energy X-ray photon results in the ejection of an 

 electron from an atom. 



However, the scattering of light or of low-energy X-ray photons gen- 

 erally leaves no aftereffect, for the following reason: Unless the recoil of 

 an electron due to the scattering of a photon is sufficiently strong to eject 

 the electron, the recoil is absorbed by the whole atom or molecule to 

 which the electron belongs. Owing to the large mass of atoms, their 

 recoil would entail a very small energy uptake, which would not even 

 overcome the cohesion of adjacent molecules. Therefore the scattering 

 of each photon does not necessarily leave a mark on a particular atom or 

 molecule (except in gases of sufficiently low density). 



