How does radiation cause cell death?
Radiation Chemistry 
Radiation acts on living systems through direct and indirect effects. When the energy of a photon or secondary electron ionizes biologic macromolecules, the effect is termed direct. Alternatively, a photon may be absorbed by water in an organism, ionizing some of its water molecules. The resulting ions form free radicals (radiolysis of water) that in turn interact with and produce changes in biologic molecules. Because intermediate changes involving water molecules are required to alter the biologic molecules, this series of events is termed indirect.
Charring of nucleoproteins
Destroying their mitochondria
Ionization
Disruption of cytosol