50 



RADIATION BIOLOGY 



t-t — MM ABSENCE 

 ' OF ATOMIC 

 1 ELECTRONS 



"~^ ACTUAL 



2° 4° 6° 

 DEFLECTION 

 Fig. 1-35. Diagram of 

 the cross section for 

 the elastic scattering 

 of charged particles in 

 different directions. 



large mass of the nuclei. An elastic collision reduces to a simple Ruther- 

 ford scattering (see Sect. 2-2c) when the incident particle comes close to 

 an atomic nucleus. Otherwise, the negative electronic charge which 

 surrounds the nucleus effectively cancels the electric force due to the 



nuclear charge and reduces the probability of 

 scattering (see Fig. 1-35). 



Radiative collisions involve the emission of elec- 

 tromagnetic radiation by the incident particle in 

 the course of an otherwise elastic collision. They 

 have already been discussed with regard to the 

 production of X rays (see Sect. l-4b). 



Inelastic collisions involve the excitation or ioni- 

 zation of an atom or molecule and result primarily 

 from the interaction of the incident particle with the 

 atomic electrons. Since electrons are light and 

 yield easily to the incident particle, these collisions 

 are particularly frequent and take up, on the whole, 

 most of the energy of the incident particle even 

 though each collision usually leads only to a small 

 energy transfer. 



As we shall see further on, the inelastic collisions 

 of charged particles constitute the most important mechanism for the 

 action of all high-energy radiations on matter. 



A given collision may appear to be elastic or inelastic depending on how we 

 look at it. Therefore some care is required to gauge the meaning of these terms 

 in a particular context. A collision between two specified bodies is called "elas- 

 tic" if no part of the initial kinetic energy is transferred to the internal energy of 

 the bodies. Thus if an incident proton knocks an electron off an atom, the 

 collision between the proton and the electron may be regarded as elastic. On 

 the other hand, the same collision appears to be inelastic if the whole atom is 

 regarded as undergoing a collision with the proton, because the energy taken up 

 by the outgoing electron is then classed as internal energy of the atom which has 

 been raised to a highly excited state. 



2-4a. Mechanism of Inelastic Collisions. The electric force exerted by 

 an incident charged particle upon each atomic electron increases in 

 strength as the particle approaches and then decreases as the particle 

 moves away. This disturbance acts as a sudden blow if the incident 

 particle moves sufficiently fast and passes sufficiently close to the atomic 

 electron. We then speak of a ''fast" collision. 



On the other hand, we speak of a "slow" collision when the electric 

 force increases and decreases rather slowly and the motion of the atomic 

 electron has time to readjust itself reversibly to the proximity of the 

 incident particle. The reversibility of this disturbance implies that a 

 slow colhsion is inherently an elastic one. A colUsion may generally be 



