Geometrical effect on the measurement of stopping power: Angle-dependent energy loss of 7-MeV protons in Cu foils and computer simulation

The emergence-angle dependence of energy loss of 7-MeV protons in Cu foils, the thicknesses of which are 3.743 and 7.576 mg/${\mathrm{cm}}^2$, has been measured with very high angular resolution. It was found that in both cases the energy loss increases as the emergence angle increases. The amount of the increase of the energy loss depends strongly on the thickness of the sample foils. Computer simulation was performed in order to investigate the origin of this phenomenon. Individual collision which protons undergo within the target was found to be restricted by the determination of detection angles. Considering properly the dependence of energy loss on impact parameter, we have obtained a good reproduction of the experimental results.