IONIZATION YIELD 17 



lar not clear that different biological effects could not originate from ion 

 clusters of different size, even with fixed mean spacing. In such a case, 

 the whole correlation analysis would be washed out by the ion-cluster- 

 size distribution. Morrison] 



Ionization Yield 



Ionization constitutes a particularly drastic form of molecular acti- 

 vation. When an electron is ejected from an atom, the resulting separa- 

 tion of electric charges lasts for a much longer time than the minor dis- 

 locations of atomic electrons which accompany simple excitations. It 

 is uncertain whether the somewhat larger energy involved in ionization 

 processes than in excitations and the greater permanency of charge 

 separation have a particularly great significance in relation to biological 

 effectiveness. 



The separation of charges which results from ionization processes in 

 gases affords a convenient and sensitive method for the physical measure- 

 ment of radiation effects. It is frequently assumed, on somewhat 

 uncertain grounds, that essentially equal amounts of ionization are pro- 

 duced in a given amount of matter whether the matter is in gaseous, 

 liquid, or solid state. Our information on the subject of ionization con- 

 cerns mostly the occurrence of this phenomenon in gases. 



The amount of ionization produced in a gas generally serves as an 

 index of the total energy dissipation. The main reason for this stems 

 from the following considerations. I shall be speaking primarily about 

 the effect of glancing collisions, but the smaller number of knock-on 

 collisions does not modify the qualitative conclusions. 



Some of the glancing collisions merely raise the external electrons of 

 an atom or molecule to an excited state ; others transfer more energy and 

 lead to an ionization. The relative frequency of occurrence of transitions 

 to different levels of excitation and ionization can be inferred from 

 theoretical or experimental data on the absorption spectrum of the 

 particular atom or molecule, since the glancing collision has the same 

 effect as an electromagnetic radiation with a continuous spectrum of 

 uniform intensity. 



Loosely attached external electrons, that is electrons with a low 

 ionization potential, are generally apt to oscillate with comparatively 

 low frequency and with great intensity while being raised to low excited 

 states, whereas the opposite is true for electrons that are stiffly held. 

 Therefore, excitations are relatively more probable than ionizations 



