Ion Cyclotron Radio Frequency (ICRF) heating is one of the main techniques proposed for auxiliary plasma heating at ITER. ICRF heating can produce fast ion populations with energies reaching up to several MeV. In this paper, we study fast deuterons accelerated by 3rd harmonic ICRF at JET. We present theoretical predictions of the fast deuterium distribution function (fd) and benchmark these with measurements using neutron emission spectroscopy.