Measurements of the decay of gamma (g) rays (from interactions of alpha particles with beryllium) in JET Trace Tritium Experiments have demonstrated a significant sensitivity ofg-ray emission to the magnetic shear in the plasma core. In discharges with relatively high currents (I>2MA) the g-decay rate observed after a short tritium NBI blip corresponded with the classical slowing down rate of fusion alphas in the case of monotonic q-profile. However, in discharges with a current hole of the size of about 1/3 of the plasma radius and in low current (1MA) discharges the measured decay time was much shorter than the classical slowing down time. Thus a strong reversed shear indicates a degradation of fast alpha confinement similar to that at low current (Ip = 1MA). Here we present first results of the numerical simulation of the relaxation of fast alpha distribution based on a kinetic model for NBI ions and fusion alphas in post blip plasmas. Modelling results are in a qualitative agreement with measurements in reversed shear and monotonic current plasmas.