PHYSICAL PROCESSES IN LIVING MATTER 



Incident proton 

 M 



5 = 2e^lmv^ 

 e = 2/mv^ • e^b 



Fig. 2. Approximate mechanics of simple knock-on collision. The distance the 

 struck electron is displaced during the encounter, perpendicular to the path of the 

 incident proton, is called 5; the angle of deflection of the incident proton (considered 

 small) is 6; the energy transfer to the initially stationary electron which moves finally 

 with velocity Vf is AT, and E'mc = Mv'^/2 is the initial energy of the incoming proton. 

 The distance b, often called the impact parameter, is the distance at which the inci- 

 dent particle would pass if there were no force of attraction. Small b means good 

 aim. Note especially how large energy transfers are associated with large angles of 

 deflection, small impact parameters, and big displacements, 5. The faster the inci- 

 dent particle, the greater the energy transfer for a definite deflection, but the smaller 

 must be the impact parameter, and therefore the better the "aim." High energy 

 transfers are evidently less likely. 



200 



o 100 



a. 







Electron 

 s; Proton 



8 mev 



Fig. 3. Space rate of energy transfer, measured in kev per micron of tissue, for 

 various charged particles as a function of energy. 



