36 RADIATION BIOLOGY 



excitation energy for most heavy nuclei with a mass number above 210. The 

 immediate disintegration is prevented by the barrier effect. Tlie spontaneous 

 ejection of a particles observed in many radioactive elements is due to occasional 

 penetration of particles through the barrier. 



The ejection of a neutron or a proton from a nucleus requires an excitation 

 energy which lies, with few exceptions, between 5 and 10 Mev. The ejection of a 

 proton is hindered by the barrier effect. Therefore the ejection of a neutron 

 constitutes the most likely result of moderate excitation for nuclei of medium or 

 high atomic weight. 



Excitation with energies from 15 to 20 Mev upward may lead to the ejection of 

 two or more particles. Owing to the subdivision of energy among the particles 

 of a nucleus, particles are unlikely to take away much energy in excess of that 

 required to leave the nucleus. Neutrons usually escape with a kinetic energy of 

 the order of 1 Mev; protons, with a similar energy in addition to the energy 

 required to overcome the barrier effect. The successive escape of a number of 

 particles with moderate kinetic energies is called "evaporation" by analogy to 

 the escape of molecules from a liquid. 



The analysis of multiple evaporations resulting from excitation energies of the 

 order of 100 Mev is in progress at the time of this writing (1950). Neutrons and 

 protons, as well as larger aggregates of nuclear particles, are ejected in these 

 processes. Very-high-energy particles (100 Mev or more) appear to escape from 

 nuclei without sharing much, or even any, of their energy with other particles. 

 In general, the coupling of nuclear particles becomes less effective in the range of 

 very high energies. 



"Fission" is a form of nuclear disintegration in which a nucleus breaks up into 

 two or more fragments of comparable size. Fission occurs when the electric 

 repulsion among the positive charges of the protons within a nucleus overcomes 

 the cohesion due to the nuclear forces. Therefore fission is a characteristic 

 mode of disintegration of the nuclei containing the highest numbers of protons. 

 A substantial level of excitation is required to "activate" fission in all the nuclei 

 present in nature. The necessary excitation energy equals 5 to 6 Mev for ura- 

 nium {Z = 92) but is already much higher, possibly 50 Mev, for bismuth (Z = 83). 

 Fission is the most likely mode of disintegration when the activation is adequate. 



2-2. ELEMENTARY PROCESSES INVOLVING RADIATION 

 AND FREE PARTICLES 



Radiation may transfer to a single electron or nucleus within matter, in 

 a single elementary process, an amount of energy far in excess of the 

 amount required to remove the particle from the atom or molecule to 

 which it belongs. When this happens, the forces between the particle 

 and the rest of the atom do not have much influence on the effect of 

 radiation. The process can then be treated with considerable accuracy 

 under the simplifying assumption that the particle is totally free at the 

 start. In the following discussion atomic nuclei are regarded as single, 

 heavy, charged particles, under the assumption that the radiation does 

 not disturb their internal structure. 



