58 RADIATION BIOLOGY 



the absorption of electromagnetic radiations. This probability turns out to be 

 small, mainly because of the heavy mass of nuclear particles. The excitation of 

 nuclei by the impact of electrons has been detected experimentally. {Excitation 

 due to glancing collisions of heavy charged particles has not been distinguished 

 clearly from the excitation due to the effect of nuclear forces. 



2-5. NUCLEAR COLLISIONS 



There remains to be considered the direct effect of nuclear particle 

 radiations (protons, neutrons, deuterons, etc.) on atomic nuclei. 



Since the nuclear forces are very strong, an incident particle tends to 

 lose its identity and to become mixed with the other particles of the 

 nucleus immediately upon impact unless its kinetic energy is excessively 

 high. Therefore it is generally convenient to analyze the processes of 

 nuclear collision into two steps. First, the incident particle becomes 

 incorporated into the nucleus. Thus a new "compound nucleus" 

 arises from the old one. 



This nucleus finds itself in an excited stationary state because much 

 energy normally results from the attraction which the nucleus exerts upon 

 the incoming particle. The excitation energy normally amounts to 

 about 6 to 10 Mev — roughly 8 Mev on the average — for every neutron or 

 proton added to the nucleus. The compound nucleus disposes of its 

 excitation energy, usually by disintegration, according to the principles 

 discussed in Sect. 2-1 d. 



Once a particle hits the nucleus, the formation of a compound nucleus 

 in an excited stationary state is always possible, no matter what the 

 kinetic energy of the incident particle, provided this energy is sufficiently 

 high. At lower energies the excited energy levels of the compound 

 nucleus are still clearly distinct from each other and do not yet blend 

 into a continuous band (see end of Sect. 2-3). No compound nucleus can 

 then be formed unless the kinetic energy of the incident particle added to 

 the energy released by the capture happens to coincide with one of the 

 excited energy levels of the compound nucleus. 



This difficulty generally hinders the capture of neutrons of energy 

 below 1 Mev by nuclei of medium or high atomic weight. The cor- 

 responding capture of comparatively slow charged particles is hindered 

 anyhow by the electric repulsion. When the energy of the incident 

 particle happens to be just right for the formation of an excited state, we 

 speak of a "resonance effect," by analogy with the resonance of character- 

 istic oscillations induced by electromagnetic radiations. The whole 

 concept of formation of a compound nucleus applies less characteristically 

 to the capture of particles by light nuclei. 



No detailed systematics of the results of nuclear collisions has been 

 worked out. The qualitative understanding of the general course of the 

 phenomena does not yield quantitative predictions on the probability of 



