  We study the effects of irradiating water with 3 MeV protons at high doses by
observing the motion of charged polystyrene beads outside the proton beam. By
single-particle tracking, we measure a radial velocity of the order of microns
per second. Combining electrokinetic theory with simulations of the
beam-generated reaction products and their outward diffusion, we find that the
bead motion is due to electrophoresis in the electric field induced by the
mobility contrast of cations and anions. This work sheds light on the
perturbation of biological systems by high-dose radiations and paves the way
for the manipulation of colloid or macromolecular dispersions by
radiation-induced diffusiophoresis.
