Plastic scintillators are often used to reject charged particles, while thicker CsI, bismuth germanate ("BGO"), or other active shielding materials are used to detect and veto gamma-ray events of non-cosmic origin. A typical configuration might have a NaI scintillator almost completely surrounded by a thick CsI anticoincidence shield, with a hole or holes to allow the desired gamma rays to enter from the cosmic source under study. A plastic scintillator may be used across the front which is reasonably transparent to gamma rays, but efficiently rejects the high fluxes of cosmic-ray protons present in space.