Loss of MeV alpha particles from JET plasmas has been measured with a set of thin foil Faraday cup detectors during third harmonic heating of helium neutral beam ions. Tail temperatures of ~2MeV have been observed, with radial scrape off lengths of a few cm. Operational experience from this system indicates such detectors are potentially feasible for future large tokamaks, but careful attention to screening RF and MHD induced noise is essential.