32 RADIATION BIOLOGY 



of adjacent molecules. These distortions give rise to a net attraction 

 which is still weaker than polar forces and is called a "van der Waals 

 force." Notice that both kinds of cohesive forces are of electric nature 

 even though they act between molecules that do not carry any net charge. 

 The various kinds of aggregates of atoms are treated by Pauling (1940); 

 Rice and Teller (1949); Slater (1939), Chaps. 22 ff. 



The motion of electrons within a molecule or crystal can proceed 

 steadily in a variety of stationary states, as in atoms. Moreover, the 

 atoms within a molecule or crystal move bodily with respect to one 

 another in oscillatory or rotary fashion in a variety of stationary states. 

 Therefore the combined set of stationary states is far more numerous 

 and varied for an aggregate of atoms than for a single atom. 



The energy required to excite the bodily motion of atoms in a 

 molecule with respect to one another equals only a small fraction of the 

 energy released by the formation of chemical bonds. This excitation 

 energy is smaller than the excitation energy of electrons, in general by 

 one or more factors of 10. It is always smaller, often much smaller, than 

 1 ev. In fact, many forms of excitation of interatomic motion require an 

 energy comparable with the kinetic energy of gas molecules at room 

 temperature, 0.1 ev or less. The heat, i.e., the thermal energy, which 

 is contained in any material, enables these forms of motion to spend most 

 of the time in one or another excited state. 



2-lc. Ejfects of Excitation. Radiations act upon atoms and molecules, 

 in essence, by delivering energy to single atomic particles, i.e., by exciting 

 various forms of internal motion. The consequences of the various 

 kinds of excitation depend on the internal mechanics of the atom or 

 molecule and on the amount of excitation but do not depend on the 

 source of the excitation. 



An electron which has been excited to a sufficiently high level of energy 

 cjuickly leaves the atom or molecule to which it belongs. This process is 

 called "ionization" because it results in an outright separation of electric 

 charges. The electron has little chance of sharing its excitation energy 

 with any other particle because its kinetic energy is generally large as 

 compared with the electric force of interaction with any one of the other 

 electrons. 



Nuclear particles are much more apt than electrons to subdivide their 

 energies because each particle interacts strongly with its immediate 

 neighbors. Therefore a neutron or proton which receives sufficient 

 energy to break out of the nucleus has little chance of doing so at once 

 (unless the nucleus consists of a very few particles only and each particle 

 lies practically on the outer surface). If and when a particle even- 

 tually breaks loose from the nucleus, as a result of an excitation, the 

 remaining nucleus is likely to keep some energy of excitation. The 

 ejection of a particle from a nucleus is called a "disintegration." The 



