PEINCIPLES OF RADIOLOGICAL PHYSICS 59 



nuclear transformations. The complication of nuclear phenomena is 

 akin to the complication of chemical phenomena which still defies detailed 

 theoretical prediction in the majority of cases. 



A fairly large amount of experimental information on nuclear trans- 

 formations is available (see, for example, Way et at., 1950; Hornyak et al., 

 1950). 



The various possible types of nuclear transformations are usually indi- 

 cated by a symbol in parentheses which contains an indication of the 

 incident particle followed by a comma and by an indication of the particle 

 or particles ejected. For example: 



{n,a) means a reaction consisting of the capture of a neutron followed 

 by the ejection of an a particle. 



(n,pn) means capture of a neutron followed by ejection of one proton 

 and one neutron. 



{d,2n) means capture of a deuteron followed by the ejection of two 

 neutrons. 



{n,n) means capture and ejection of a neutron and implies the outward 

 appearance of an inelastic scattering since the nucleus generally remains 

 in an excited state. 



Similar symbols apply also to the absorption and emission of electro- 

 magnetic radiation. For example, (71,7) means the simple capture of a 

 neutron followed by release of the excitation energy by 7 radiation, and 

 (7,?)) means the ejection of a proton induced by the absorption of a 

 photon. 



The ordinary symbols of the initial and final nuclei, including their 

 atomic and mass number, serve to complete the identification of the trans- 

 formation. For example, 29Cu^^(n.,2n)29Cu'^- indicates the production 

 of copper of mass number 62 by the bombardment of Cu^^ with neutrons 

 leading to capture of one neutron followed by the ejection of two neutrons. 



The preceding discussion pertains to nuclear collisions in which the 

 energy of the incident particle does not exceed 50 or 100 Mev. Higher 

 energy particles seem to move more freely across nuclei, in a manner a 

 little more similar to the motion of particles across atoms. The produc- 

 tion of mesons results from nuclear collisions above approximately 

 150 Mev. The study of high-energy collisions has just been passing 

 through its initial stages in the years 1948 to 1950. 



3. DISSIPATION OF RADIATION ENERGY IN MATTER 



In this section we begin the discussion of the over-all process of action 

 of radiation on matter, which constitutes the main topic of our treatment. 



The biological effects of radiation which are accessible to observation 

 derive presumably from some disturbance of the atomic structure of 



