High resolution g-ray measurements were carried out on the Joint European Torus (JET) in an experiment aimed at accelerating 4He ions in the MeV range by coupling third harmonic radio frequency heating to an injected 4He beam. For the first time, Doppler broadening of g-ray peaks from the 12C(d, pg)13C and 9Be(a, ng)12C reactions was observed and interpreted with dedicated Monte Carlo codes based on the detailed nuclear physics of the processes. Information on the confined 4He and deuteron energy distribution was inferred and confined 4He ions with energies as high as 6MeV were assessed. A signature of MHD activity in g-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.