High-resolution γ-ray measurements were carried out on the Joint European Torus (JET) in an experiment aimed at accelerating 4 He ions in the MeV range by coupling third harmonic radio frequency heating to an injected 4 He beam. For the first time, Doppler broadening of γ-ray peaks from the 12 C (d, pγ) 13 C and 9 Be (α, nγ) 12 C reactions was observed and interpreted with dedicated Monte Carlo codes based on the detailed nuclear physics of the processes. Information on the confined 4 He and deuteron energy distribution was inferred, and confined 4 He ions with energies as high as 6 MeV were assessed. A signature of MHD activity in γ-ray traces was also detected. The reported results have a bearing on diagnostics for fast ions in the MeV range in next step fusion devices.