The light output of ZnS scintillators used to detect escaping fusion products in the TFTR Tokamak is studied with 3.5 MeV alpha and 3 MeV proton beams. The emitted light first increases linearly with beam current and then saturates. In all cases investigated, the onset of the saturation corresponds to a power of about 1 mW absorbed within the ZnS powder. The scintillators have adequate time response up to 50--100 kHz
