The ratio R varied with the target thickness. The red solid point and blue solid point represent the experiment values for 0.8 MeV/u and 0.9 MeV/u H 2 + incident foils, and the black solid line is only used to guide the eye. The ratio R first increased then decreased with the target thickness, except for the case of 0.9 MeV/u H 2 + in 3–5 layers graphene foil which was abnormal.