Branching ratios for secondary processes of water ions induced by proton beams in radiation therapy of cancer

New results are reported that can improve proton beam instrumentation in radiation medicine and cancer therapy. The destructive effects of proton beams are primarily based on direct collision ionization and radiolysis of water. Resulting fragments subsequently damage DNA and alter the cell cycle with different efficiency. We therefore computed the water ion and hydronium potential energy surfaces and obtained fragment branching ratios for processes that further involve low energy electrons, OH radicals and metastable O+ ions. In addition, all related data available in the literatures are shortly summarized and reviewed. State-selective excitation and electron capture cross sections reported in this work allow for solving the rate equations of the ion production cascade.