Indirect Radiation Effects

Water is the main constituent of all living systems. In somatic and vegetative cells its fraction lies between 40 and 70%, even in bacterial spores it amounts still around 20%). Therefore, in irradiated cells, most of the energy is absorbed by water molecules, which are either excited or ionized (Equation 1). Excitation of a water molecule is often followed by splitting of the molecule (Equation 2).

Hence, the primary products are Hv »OH, H20+ and electrons. All these species possess unpaired electrons, thus being highly reactive free radicals. The electrons are particularly reactive and capture another water molecule thus forming a negatively charged ion (Equation 3).

The ions H20+ and H20 are not stable and almost immediately (10"16 seconds) dissociate into H+ ions and 'OH radicals as well as into OH ions and H' radicals (Equation 4).

There will be a number of reactions among the free radicals themselves, thereby either reconstituting water (Equation 5) or forming molecular hydrogen and hydrogen peroxide (Equation 6). The interactions of free radicals both among themselves and with their own reaction products are dependent primarily on how closely they have been formed. After they are formed, they must diffuse through the medium until they encounter something with which they may interact. The probabilities of these reactions are favored within spurs, blobs and tracks. Interactions with other solute molecules are only possible, if the primary species are able to escape these zones.

(Recombination) [5]

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Figure 7-06. Tracks in photo-emulsions o f electrons produced by y-ravs and tracks of different nuclei of the primary cosmic radiation moving at relativistic velocities. For biological radiation effects, the efficiency of a radiation type increases as the ion density along the tracks increases.

Figure 7-06. Tracks in photo-emulsions o f electrons produced by y-ravs and tracks of different nuclei of the primary cosmic radiation moving at relativistic velocities. For biological radiation effects, the efficiency of a radiation type increases as the ion density along the tracks increases.

With increasing density of ionization, i.e., with increasing Linear Energy: Transfer (LET), the number of changed molecules increases leading to an increase of radiation effects in cells. Densely ionizing radiations, such as the heavy ions and a particles of radiations in space, produce clusters of ions and radicals that are very close together (Figure 7-06). Consequently, there will be a high probability of interactions between free radicals as well as with the key molecules of the cell (e.g., proteins and nucleic acids) leading to a broad spectrum of DNA lesions including damage to nucleotide bases, cross-linking, and DNA single- and double-strand breaks (Figure 7-07). In summary, for the indirect radiation effects the number of inactivated molecules depends on the dose and on the concentration of the water molecules.

Figure 7-07. Different types of DNA damage induced by ionizing radiation and other genotoxic agents.

Different types of DNA damages h | • {I I H ) \ : I I ! 11 T I ! S j n I I H I

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Figure 7-07. Different types of DNA damage induced by ionizing radiation and other genotoxic agents.

TTT I I 1 1 I 1 . I i ■ I m ÔTYr inili'jh I ! [j 1 j I t 1 :

DMA nucleotides GC AT

base pairing CGTA

TTT I I 1 1 I 1 . I i ■ I m ÔTYr inili'jh I ! [j 1 j I t 1 :

double-strand break 1 adduet

DNA proletn ctosstink s rn g le-strand break

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