When they pass through water, the energy causes the formation of ions and radicals. These can interact with proteins and DNA and destroy them. Called ionizing radiation. Formation of ion pairs and free radicals. Formation of reactive species in water that react with cellular molecules to kill the cell.
Most killing is due to indirect effects.
Many of the species formed are very strong oxidizing agents that can damage the cell. Can be used to inactivate enzymes, kill bacterial cells, kill bacterial spores, kill insects. The amount of energy to kill insects is much lower that that required to kill bacterial spores. Need to limit indirect effects on food components. The same ions that kill cells can also cause breakdown of food molecules.