4 PHYSICAL PROCESSES IN LIVING MATTER 



falls off with energy as 1/E^. The secondaries can ionize in their turn, 

 and the total energy removed from particle motion converted by such 

 collisions can be computed. For such encounters, one electron is like 

 another, and the energy removed is independent of the kind of atom 

 traversed, except in so far as the composition determines the total 

 electron density of the medium. The space rate of energy loss depends, 

 for knock-ons, only on the velocity — not the mass — of the incoming 

 particle and increases as the square of its charge. If the incident particle 

 is an alpha, or an even heavier ion, like the recoil C^* nucleus of slow 

 neutron capture in nitrogen, the charge will not remain constant. 

 Passage of the heavy ion through the atoms of the material can be 

 regarded in a different frame of reference as a kind of bombardment of 

 a stationary ion by the electrons of the matter; sometimes the not 

 completely stripped ion will lose an electron by ionization; sometimes 

 the ion will pick up an electron moving just in its direction, and the 

 charge will be reduced. The process reaches a kind of slowly shifting 

 equilibrium as the particle slows down. It is most important for slow 

 and heavy ions, for which the simple considerations will no longer be 

 adequate, and ionizations may proceed strongly even if the ion is on 

 the average nearly neutral. 



Further to complicate the problem there are the very important 

 collisions, often called glancing collisions, in which the ionizing particle 

 does not approach any electron very closely. It may pass through the 

 atom, or even many angstroms away. The distinguishing feature of 

 these collisions is a small energy transfer, not overriding the atomic 

 binding forces, and corresponding to a very slight deflection of the 

 incident particle. The effect of such an undeflected and more or less 

 remote charge sweeping by can to a good approximation be replaced by 

 a strong, rapid, electric pulse, very like a burst of light uniformly dis- 

 tributed in frequency. The equivalent electromagnetic radiation may 

 excite or even ionize the atom as a whole, just as a beam of real photons 

 would do. For this type of collision the simple mechanics of electro- 

 static forces is entirely inadequate; the effect of light of all colors on the 

 atom must be known. Evidently the problem is more complex and 

 demands a full knowledge of the quantum mechanics of atomic struc- 

 ture (2). In particular, we must recall the following: 



1. The atomic electrons now move during these relatively slow and 

 weak encounters. The accurate treatment of the whole process requires 

 a knowledge of the electron orbits so complete that the effect has been 

 worked out in detail only for hydrogen atoms. We have to depend upon 

 this calculation for a guide and supplement it with observed semi- 

 empirical regularities (3). 



