Fast ions measurements become increasingly important as we move closer to ITER or burning plasma experiments. Further R&D is especially required in the field of fast ion losses measurements.In this perspective, a boron target mounted on a manipulator was exposed to deuterium plasmas during reversed field experiments at JET.Due to the 10B (p,a )7 Be reaction,7Be is produced in the boron target by MeV protons escaping the plasma.The proton losses are deduced from measurements of the intensity of decay emission of 7Be. Excellent agreement is found between measurements and prediction from fast ion classical drift losses. The Mcorbit Monte Carlo simulation code was used to predict unconfined orbits fraction. Further developments are presented, including a new dedicated instrument to be used in experiments already planned at JET with ICRF accelerated alpha particles. The obtained results will be essential for the design of lost alpha diagnostics on ITER based on activation technique.