66 RADIATION BIOLOGY 



Table 1-4. Typical Values of e, the Energy Dissipated per Ion Pair Produced 



Radiation Energy (e), Ev 



340-Mev protons in Ho 34.9 



340- Mev protons in air 32 . 2 



5.2-Mev <x particles in H2 36.0 



5.2-Mev a particles in air 31.1 



5.2 Mev a particles in He 42 . 7 



50-kev electrons in air 32 . 4 



For additional data and discussion see Rutherford et at. (1930), p. 80; Gray (1944); 



Bethe (1950); Bakker and Segre (1951); and Jesse et al. (1953). 



ratio €, which represents the average energy dissipated for every ioniza- 

 tion produced by various radiations in different gases. 



The energy e dissipated per ionization does not depend immediately 

 on the ionization potential (the energy actually required to rip off an 

 electron from an atom) but results from a grand average over all collisions, 

 among which some produce an ionization and some do not. In fact, the 

 values of e vary within narrow limits from one substance to another and 

 lie in the neighborhood of 30 to 35 ev per ionization for most gases. 



The value of e for each substance depends on how often external electrons are 

 raised to different levels of excitation and ionization as a result of collision. As 

 indicated in Sect. 2-4c, the probability of different outcomes of a collision depends 

 on the readiness of electrons to start oscillating with different frequencies after a 

 glancing disturbance. Electrons that are attached rather loosely, i.e., electrons 

 with a low ionization potential, are generally apt to oscillate wdth comparatively 

 low frequency and wath great intensity while being raised to low excited states, 

 but the opposite obtains for electrons that are held more rigidly. Therefore, 

 excitations are relatively more probable than ionizations just in those atoms and 

 molecules which require least energy to be ionized. In other words, just in those 

 substances in which an ionization can be produced cheaply, in terms of energy, a 

 larger amount of energy has to be spent in excitations. (In substances whose 

 ionization potential has a typical average value, around 10 ev, the relative fre- 

 quency of excitations and ionizations is of the order of 2 to 1.) This argument 

 accounts for the remarkable similarity of the values of e for different substances 

 (Fano, 1946). 



As a particle begins to slow down, there is no longer a great excess of glancing 

 collisions over collisions which involve a larger momentum transfer. Therefore a 

 somewhat paradoxical result is observed : The probability of excitation tends to 

 decrease, as compared to the probability of ionization (see Fig. 1-38) ; the over-all 

 yield of ionization increases and the value of e decreases a little. 



3-1 c. Energy Dissipation by Loxoer Energy Heavy Particles. Inelastic collisions 

 tend to become extremely unlikely when the incident particle slows down to and 

 below the velocity of atomic electrons (see Sect. 2-4a). A heavy particle may 

 move more slowly than any atomic electron within a material and still retain a 

 substantial kinetic energy. This obtains, for example, for protons whose energy 

 falls below approximately 20 kev or for nitrogen ions below 300 kev. The 

 energy dissipation by such "slow" particles does not appear important, on the 



