Influence of the pH on molecular hydrogen primary yields in He2+ ion tracks in liquid water. A Monte Carlo study

Monte Carlo calculations are performed to investigate how the acidity of aqueous solutions at room temperature affects the molecular hydrogen (H2) yield as a function of time in 20 MeV-He2+ ion track segments. For pH values varying from 1 to 13, the time dependence of the calculated yields is nearly independent of pH in the time range 10−12-10−8 s and only weakly dependent in the time range 10−8-10−6 s. To understand this behaviour, the kinetic mechanisms governing H2 formation are examined as a function of time. It is found that the main reactions responsible for the H2 yield as a function of time are strongly pH-dependent at low and high pH values. The pH-dependences of the reaction yields are however such that the variations in the yields compensate each other. This is why the time dependence of the H2 yield is only weakly pH-dependent.