The neutral particle analyzer ISEP (Ion SEParator) NPA was applied to measure the tritium neutral flux during Tritium Trace Experiment (TTE) on JET. The energy dependence (in the 5­28keV energy range) of the tritium neutral flux rise time after a short ~100ms tritium gas puff into deuterium plasmas has been observed for the first time. The dependence has been interpreted as being due to the penetration of the tritium ions from the plasma boundary into the core and has been used for the calculation of the tritium diffusion coefficient and convective velocity values.