However, today it is estimated that about two-thirds of the damage caused by x rays is due to indirect action . This occurs when the liberated electron does not directly strike the DNA, but instead strikes an ordinary water molecule. This ionizes the water molecule, eventually producing what is known as a free radical . A free radical reacts very strongly with other molecules as it seeks to restore a stable configuration of electrons. This also increases the possibility that other substances could be introduced that would neutralize free radicals before they do damage.
Neutrons act quite differently.
A fast neutron will bypass orbiting electrons and occasionally crash directly into an atomic nucleus, knocking out large particles such as alpha particles, protons, or larger fragments of the nucleus. The most common collisions are with carbon or oxygen nuclei. The particles created will themselves then set about ionizing nearby electrons. A slow neutron will not have the energy to knock out large particles when it strikes a nucleus. Instead, the neutron and the nucleus will bounce off each other, like billiard balls.