[en] Human lymphocytes were irradiated with monoenergetic neutrons at 0.8, 1.0, 1.2, and 2.35 MeV of incident neutron energy. The cross section for neutron elastic scattering against oxygen has a peak at 1.0 MeV of neutron energy and a marked trough at 2.35 MeV, and the relative contribution to the total dose from the oxygen recoils is different for the actual neutron energies. The heavy oxygen recoils have short ranges ( <1 μm) within the cell nucleus and LET values of the order of 300 keV μm^-1. The oxygen recoils were found to be about 2.5 times more efficient than proton recoils in producing chromosome breakage. The distribution of dicentrics between cells were rather close to a Poisson distribution for the proton recoils, whereas multiple aberrations were over-represented in the observed distribution for the oxygen recoils. This implies that chromosome threads located within the volume limited by the ranges of an oxygen recoil are broken with a high efficiency, and the number of chromosome threads within such a volume was estimated. The data indicate that the interphase chromosome threads are clustered within the cell nucleus. (author)