Gamma-ray spectra from nuclear reactions between fusion-born alpha (a) particles and Be impurities were measured for the first time in deuterium plasmas in the Joint European Torus. The time dependence of the measured spectra allowed the determination of the density evolution of fast α particles. Correlation between the decay time of the γ-ray emission and the plasma parameters in different plasma scenarios was established. Results of the measurements are consistent with classical slowing down of the a particles in discharges with high plasma currents (Ip ≥ 2.0 MeV) and monotonic q-profiles. In low plasma current discharges and in the discharges with large on-axis current holes (and, hence, extreme central magnetic shear reversal) the γ-ray emission decay times are shorter than the classical slowing down times, indicating an α-particle confinement degradation in such discharges in line with theoretical predictions. The obtained data provides essential information for studying α-particle confinement and for assessing the potential of the diagnostic technique in the perspective of ITER.