A new instrument has been developed at JET to measure losses of fast particles in the MeV energy range. Neutron fluxes are recorded as well. The measurement technique is based on activation of in vessel samples. These samples are used as flux monitors. They are installed on a specifically designed holder, into six slots at different angular positions with respect to the magnetic field. This important design feature allows 1) a measurement of the local pitch angle distribution of fast particles and 2) a strong discrimination between neutron-induced and charge particle induced activation because neutron-induced activation is independent on the magnetic field direction. A special technique for low level activation, 'Ultra low-level gamma ray spectroscopy', was used to investigate the JET samples with enhanced sensitivity. First application of the instrument in helium plasmas experiments on JET is reported. Some measured activation products are a result of neutroninduced activation. A neutron spectrum was inferred from these measurements in agreement with the results of another neutron spectrometer. 7Be, an activation product involving mostly high energy protons and deuterons was measured in signi cant amount. Two nuclear reactions possibly due to alpha particles were registered, though at a level close to the detection limit.