Stopping of ∼0.2-3.4 MeV/amu 1H+ and 4He+ ions in polyvinyl formal

The stopping powers of polyvinyl formal resin for protons and alpha particles have been measured over the energy range ∼(0.2-3.4) MeV/amu with an overall relative uncertainty of less than 2.5% using the ion beam transmission method. The obtained results are discussed in comparison to scarce experimental data from the literature and to values calculated by the SRIM-2008 computer code assuming Bragg-Kleeman's additivity rule of stopping powers. Departures from additivity attaining ∼5.5% and ∼3.0% for the proton and alpha particle data, respectively, are observed at moderate projectile velocities, increasingly as one evolves towards the stopping power maximum dominated by charge exchange effects. They are presumed to be mainly due to valence structure effects involving the C-H bonds of the studied polymer compound. The less pronounced deviation for alpha particles is probably due to the projectile screening effect that occurs in distant collisions tending to reduce the contribution of valence electrons in the low projectile velocity regime.