The paper investigates the temporal evolution of γ-ray emission induced by nuclear interactions between energetic fusion-born alpha particles with beryllium impurity ions in JET plasmas. The alpha slowing down time and loss rates are assessed from the delay time between γ-emission and alpha production in deuteron-triton fusion reactions during tritium NBI blips into a JET deuterium plasma. The use of γ-diagnostics is also demonstrated for examining the energy distributions of ICRH accelerated 4He-ions in the MeV energy range.