This paper presents the details of g-ray measurements of fusion-born a particles, which were carried out in JET 'trace tritium' discharges, i.e. in majority deuterium plasma after seeding with a small population of tritium Neutral Beam Injection (NBI) fast ions. The g-ray emission from the nuclear reaction 9Be(a,ng)12C is used to measure changes in the density of the fast a particles with energy Ea>1.7 MeV in the post-NBI period. This important diagnostic nuclear reaction has already been applied to detect the presence of the fast a-particles in JET experiments, where ion-cyclotron-resonance heating of 4He-beam ions was used to accelerate 4He to the MeV range. In this paper we demonstrate how a nuclear diagnostic based on the g-ray spectrometry of the interaction between a's and Be impurity in plasmas is used in JET and how it could be used in future magnetic fusion machines to obtain essential information on the slowing down and confinement of the fast a particles.