Stopping powers and energy loss straggling for (0.9–3.4) MeV protons in a kapton polyimide thin film

Abstract

The energy loss and energy loss straggling widths have been measured in transmission for Ep≈(0.9–3.4) MeV protons traversing a thin kapton polyimide foil. In a prior step, the thickness and non-uniformity of the target foil were carefully investigated. The overall relative uncertainties in the stopping power and energy loss straggling variance data amount, respectively, to less than 2% and 8%. The S(E) experimental data show to be in excellent agreement with available previous ones and with those compiled in the ICRU-49 report. They are fully consistent with the predictions of Sigmund-Schinner’s binary collision theory of electronic stopping over the whole proton energy range explored. An average deviation of ∼2.5% relative to values calculated by the SRIM-2008 code, likely due to effects of valence electrons involving the CH, CC and CO bonds, is however observed at low proton velocities. The measured energy loss straggling data, which are unique to our knowledge, are found to be in good agreement with values derived by the classical Bohr formula for Ep≳1300keV but they significantly exceed Bohr’s collisional energy loss straggling at lower proton velocities where target electrons can no longer be considered as free. They also show to be consistent with the predictions of the Bethe-Livingston and Sigmund-Schinner theories over the low proton velocity region (Ep<1300keV). However, they are significantly overestimated by these theories over the intermediate and high proton velocity regions, which may be due to bunching effect by inner shell electrons of the polymer target. Besides, our energy loss straggling data are in better overall consistency with the Yang, O’Connor and Wang empirical formula for Ep>1300keV, while deviations above the latter amounting up to ∼18% are observed at lower proton velocities.