Figure
1
). Free radicals are the fragments of atoms that remain after being ionized. Free radicals have an
unpaired or odd number of orbital electrons, resulting in a high degree of chemical instability.
Such free radicals can easily break chemical bonds, and are a main cause of damage from
radiation exposure. Free radicals react with cellular molecules within fractions of a second after
their formation, and thus have a short life. The OH• free radical, formed by the ionization of
cellular water, is among the most common because of the abundance of water in all biological
tissues (about 80% of the mass of a living cell is water). To appreciate the quantity of free
radicals produced, consider their concentration (expressed in terms of a G-value, defined as the
number of radicals produced per 100 eV of energy absorbed in the medium). The G-value for the
OH• radical is 2.6 (IAEA, 2010). Thus, if a 5 MeV alpha particle were to dissipate all of its
energy within cellular water, some 50 000 free radicals of OH• could theoretically be produced.
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