Examining the evolution of alpha particle distributions produced during blips of tritium NBI into deuterium plasmas in JET is a new and promising approach in the study of fast ion behaviour in tokamak plasmas. Thus the decay of g-rays emitted in nuclear interactions of fusion-born alphas with beryllium impurity ions was measured after a tritium NBI blip into a D-plasma [1,2]. In current hole and in low current JET discharges higher decay rates of g-emission were detected, while some high current discharges without reversed magnetic shear demonstrated an anomalously long decay time tg of g-intensity. The wide range of g-decay times (tg ~ 50% to 150% of the slowing down time in the plasma centre) at high currents with peaked profiles (resulting in good confinement of a-s) indicates the sensitivity of alpha relaxation to the shape of the fusion source, to auxiliary heating and bulk plasma profiles, which were varied in these discharges. An important factor is the energy spectrum difference between fusion a-s produced in plasmas with small beam deuteron population and those born if the beam ion fraction is substantial. Here we extend the Fokker-Plank model used for simulation of a-induced g-emission [3] to accommodate the effects of anisotropy and energy broadening in the a-source.