248 RADIATION BIOLOGY 



enon insofar as the shape of the survival curve, and effect of variation of 

 dose rate, are concerned. However, the insensitivity to variation of iy^pe 

 of high-energy radiation (e.g., to variation of wave length of X rays) must 

 be expected to be a qualitative result which might not be substantiated 

 quantitatively by refined experiment. And above all, the experimentally 

 determined "target area" would not be correctable meaningfully with 

 the geometrical area of the molecule or of any chemically defined portion 

 of it; rather, it would be a complicated average of the cross sections for 

 various types of excitation multipUed by their respective probabilities 

 for causing the observed effect. 



If the biological unit experiences several "simultaneous" excitation or 

 ionization events (arising either from the random fluctuations of primary 

 acts, or from a greater average density of ionization; cf. Sect. 4-6), the 

 result will usually be a greater tendency for all possible structural changes. 

 Obviously, if a single event is sufficient to cause an observable effect, 

 several events can do no more, and this may lead to an apparent decrease 

 in the number of hits per primary event. On the other hand, if a single 

 event is not enough to effect the observable change, the use of radiation 

 for which multiple events are frequent may cause changes otherwise un- 

 detectable, or less frequent. In the extreme case of very densely ionizing 

 radiation, the disruption is so great that it is difficult to treat theoretically. 

 In this case, however, the idea of a high-temperature column advanced 

 by Dessauer may be a vaUd, or at least a suggestive, approximation. A 

 very simple example of additivity of primary events is provided by the 

 mechanism of protein inactivation suggested below. 



Observed radiobiological effects of the "direct" type may be classified 

 according to whether the primary process brings about a localized action, 

 affecting the chemical structure of a more or less recognizable molecular 

 group, or a nonlocalized action, influencing the organization of a relatively 

 great region. Localized action may include, for example, the direct chem- 

 ical transformation of a prosthetic group of a protein following its elec- 

 tronic excitation^s q^. ionization. The primary act may lead to a dis- 

 sociation which cleaves off an atom or radical and results in oxidation of 

 the group ; it may lead to an internal conversion which has a certain prob- 

 ability for chemical effect; or it may even consist of the transfer of an 



23 In some cases the prosthetic group need not be excited directly by the incident 

 radiation: the excitation energy may be transferred to it after (say) initial absorption 

 in an amino group of the protein. For this to occur, it is not necessary that the amino 

 group be fluorescent with observable yield for if the spectrum of this fluorescence, 

 however weak, includes wave lengths which the prosthetic group can absorb, the 

 excitation energy can be communicated directly from the amino group to the prosthe- 

 tic group by a process akin to that responsible for sensitized fluorescence. Indeed, 

 many amino groups can contribute to the probability for indirect excitation of the 

 prosthetic group, the actual energy migration being less probable, the greater the 

 distance through which it must occur. 



