30 



RADIATION BIOLOGY 



tionary states" of each kind of atom (Fig. 1-21). The stationary state in 

 which the total energy of all electrons is lowest is called the "funda- 

 mental" or "normal" state of the atom. 



The other stationary states of motion of the atomic electrons are 

 called the "excited" states. The "excitation" consists of an excess 

 kinetic energy which is usually concentrated on a single electron. The 

 excited electron keeps farther away from the nucleus than it would if 

 it were not excited, on account of its more active motion. The binding 

 energy of an excited electron is accordingly reduced. 



2'Sy,[5'?]^]m'*ior-i 



2^ [3'D3/^ m =+ ^ or4 3^/2 [4^%] m=* fori 



Fig. 1-21. Diagrams of the density distribution of the electron of a hydrogen atom 

 in various states. {H. E. White, 1934.) 



The excitation of an external electron requires a certain minimum 

 amount of energy which varies from a few to about twenty electron volts 

 depending on the number of electrons in the external layers of the atom. 

 This energy is, normally, 2 to 4 ev lower than the binding energy of the 

 electron. The excitation of internal electrons requires a much larger 

 amount of energy, which is very nearly equal to their binding energy. 



Although each atomic electron is in constant motion according to a 

 fixed pattern, while an atom remains in a stationary state, the statistical 

 average of its motion yields no net oscillating current. However, an 

 oscillating current arises while the motion of the electrons changes over 

 from one to another stationary state. The frequency v of this current 

 is proportional to the energy difference AE gained or lost by the atom in 

 connection with the change of motion, according to the basic relation 



A^ = hu (12a) 



where h is the Planck constant [see Eq. (9)]. No change of motion from 

 one to another stationary state would occur but for disturbances caused 

 by other atoms or by radiation. 



