Stopping powers of LiF thin filrns deposited onto self-supporting Al foils for swift protons

摘要

The energy losses of ～(0.273-3.334) MeV protons in LiF thin films deposited by vacuum evaporation onto self-supporting Al foils have been measured using the transmission method. The thicknesses of selected and used LiF/Al target samples were accurately determined via systematic energy loss measurements for alpha particles from a very thin mixed ~(241)Am/~(239)Pu/~(233)U radioactive source. The samples were investigated in detail for their stoichiometry and their impurity contents by backscattering Rutherford spec-trometry and nuclear reaction analysis. Then, LiF stopping powers have been determined with overall relative uncertainty of less than 2.7% arising mainly from errors in the determination of target sample thicknesses. These S(E) data are reported and discussed in comparison to previous experimental data sets from the literature and to values calculated by the Sigmund-Schinner binary collision stopping theory both for molecular LiF, and for the LiF compound assuming Bragg-Kleeman's additivity rule. Our S(£) data show to be in excellent agreement with the latter theory for molecular LiF over the whole proton energy range explored, which supports the use of modified electronic hydrogen wave functions for evaluating atomic shell corrections in the case of low-Z_2 target materials. In contrast, they exhibit a slightly increasing deviation from theoretical values derived for the LiF compound with assuming stopping force additivity as the proton energy decreases from E ≈ 400 keV towards lower proton velocities. This deviation in excess relative to experimental data, amounting only up to (at most) ～2.5%, can be ascribed to strong effects of 2s-state valence electrons of Li atoms within the LiF compound. Besides, the comparison to values calculated by the SRIM-2008 computer code indicates that this program satisfactorily accounts for our S(E) data above E≈1.30 MeV but underestimates them with substantially increasing deviations (up to ～11%) towards lower proton velocities where the Bragg-Kleeman additivity rule therefore appears to be inapplicable.