An experimental method for the determination of the normalization integral (denominator of perturbation expression) has been developed and used in three fast assemblies. The procedure consists of measuring the apparent reactivity associated with a calibrated 252Cf neutron source at a known power level. The method is independent of the geometrical configuration of the nuclear reactor. Additionally the spatial distributions of the importance of fission neutrons were measured by observing the rate of the linear increase of the neutron population due to the 252Cf source as a function of position in each fast assembly. The measured values of the normalization integrals (N.I.) as well as the measured distribution of were higher than those obtained with multigroup calculations.