Experimental evidence is presented of a discrepancy between the predicted and measured D-D fusion products rates on MAST. Both the neutron and proton production rates, measured independently with a neutron camera and charged fusion product detector array, are approximately 40 % lower than those predicted by TRANSP/NUBEAM codes. This deficit is scenario independent and can not be explained by uncertainties in the typical plasma parameters suspected for such discrepancies such as the electron temperature, the plasma effective charge and the injected neutral beam power. Instead, a possible explanation is an overestimate of the neutron emissivity due to the guiding center approximation used in NUBEAM to model the fast ion orbits.
