The paper presents the results of investigations of nuclear-physical characteristics of neutron fields generated in a massive uranium target irradiated by deuterons with an energy of 0.66 GeV. 23 natural uranium samples spatially arranged in a sub-critical assembly Quinta (at the Joint Institute for Nuclear Research, Dubna , Russia), were irradiated with spallation neutrons. We have processed the experimental data based on gamma-ray spectrometry in order to reach (obtain) the number of neutron induced fissions and neutron captures in the detector foils. Applying the try and error method we have found that the neutron energy for which the ratio of the fission cross section to the capture cross section of the natural uranium from the nuclear data base is equal to the measured ratio of the spectral indexes. The retrieved distinct fission and capture cross sections for the distinct neutron energy from the nuclear data base describe the average values which enabled us to evaluate the average neutron flux and neutron fluency distribution in the assembly.