Reactions in water radiolysis - producing hydroxyl radicals There’s an excellent question on Stack Exchange regarding the radiolysis of water here but I have one simple outstanding question that perhaps someone might now; in radiobiology, a common DNA damaging free radial is caused by the radiolysis of water. This reaction is something like 

$$\ce{H2O + $h\nu$ -&gt; H2O+ + e- }$$

$$\ce{H2O+ + H2O -&gt;  H3O+ + OH. }$$

which produces a hydroxyl radical which can prove fatal to cells. I follow the logic here, but given the massive energy of the ionizing radiation, how come a cleaving reaction like the following isn’t common? 

$$\ce{H2O + $h\nu$ -&gt; H. + OH. }$$

I’ve seen the energy quoted to break water this way at around $\pu{493.4 kJ/mol}$, which is way below the energy ($h\nu$) of an x-ray photon, even after compton scattering. Is there a reason why this reaction isn’t valid or common?
H (atomic hydrogen radical) is in fact formed.  It is 9% of the radial species formed by radiolysis.  

See Reactivity of the Hydroxyl Radical in Aqueous Solution and references cited therein for further information.