The paper investigates the evolution of g-ray emission induced by fusion alphas colliding with beryllium impurity ions in tritium NBI blip experiments on JET. We assess the possible evaluation of slowing down and alpha loss rates from measurements of the delay time between gamma emission and alpha production in DT fusions. Such measurements are then compared with 3D Fokker-Planck modelling. Losses additional to first orbit loss of fusion alphas are validated for low I / low B and Current Hole (CH) plasmas. Finally, we identify the requirements for g-ray diagnostics as well as for the plasma and the tritium NBI conditions that are optimised to provide information on the confinement of fusion born a-particles.