42 



RADIATION BIOLOGY 



and exerts an opposite pull upon the other particle (Fig. 1-31). Colli- 

 sions of this kind have the effect of spreading out, or "scattering," the 

 particles of an incident beam. 



We speak of "elastic" collisions when the total kinetic energy of the 

 two particles has the same value after as before the collision. (An 

 inelastic "radiative" collision occurs when an X-ray photon is emitted as 

 described in Sect. l-4b.) 



Rutherford was the first to analyze the elastic collisions of charged 

 particles experimentally and theoretically. He exposed a thin metal foil 



.^^ 



^ 



Fig. 1-31. Diagram showing the deflecting forces as a positively charged particle 

 passes near a nucleus. Notice reaction forces on nucleus. 



to a beam of a particles and observed whether particles emerge in various 

 directions and how many (Fig. 1-32). The overwhelming majority of 

 a particles experience hardly any deflection, a few are deflected by a small 

 angle, and an occasional particle is even deflected backwards. Ruther- 

 ford realized that the chance of a large deflection depends on whether a 

 particle happens to approach some atomic nucleus within the foil so 

 closely as to experience a particularly strong repulsion. (In fact, the 

 observations on the a-particle scattering supplied the first and main 



evidence of the concentration of posi- 

 tive atomic charges within small 

 nuclei.) 



Rutherford and his co-workers 



showed experimentally and theoreti- 

 ^ DETECTOR OF cally (see, for example, Rutherford 

 PARTICLES 6^ ol-i 1930) that: 



RADIOACTIVE 

 SOURCE 



COLLIMATOR 



\ SCATTERING 



/FOIL 



y 



MAIN BEAM 



Fig. 1-32. Diagram of the Rutherford 

 scattering experiment. 



(a) The probability of deflection by any 



given angle is proportional to the square 



of the electric charges carried by the incident particle and by the scatterer particle. 



{h) The probability of deflection by any given angle is inversely proportional 



to the square of the kinetic energy of the incident particle. 



(c) The relative probability of large and small deflections does not depend on 

 the energy of the incident particle. 



(d) The probability of any given deflection is inversely proportional to the 

 fourth power of the momentum change experienced by the particle (Fig. 1-33). 

 Accordingly, small deflections are far more likely than large ones. 



