Diagnosing alpha particles in reactor-scale fusion plasmas remain difficult and progress are needed in view of ITER. A new method for in-situ cross calibration of time-resolved alpha particle loss diagnostics at JET ­ scintillator and faraday cups - using the D-3He fusion reactions and using an activation technique. is presented in this paper. The method is in analogy with neutrons, where cross-calibration of neutron yield monitors is obtained using a neutron activation foil system. A calibration factor is determined below for the scintillator plate which allows to compare quantitatively with model estimates the time-resolved scintillator measurements of MHD events. The faraday cups are not considered in this calibration work. Considerable effort was dedicated to fusion product loss diagnostics on TFTR. However, no complete in situ calibration of the lost alpha detector could be performed. Typical 60% uncertainty for P46 scintillator material (Y3Al5O12:Ce3+ ) was reported. In ASDEX Upgrade experiment, a faraday cup was embedded recently behind a scintillator plate of the TG green type (SrGa2S4:Eu2+) in an attempt to measure the total ion rate to the scintillator plate.