The fission cross section of 235U from 0,01 eV to 0,1 eV and its absolute value at 0,0253 eV

The absolute fission cross section of ²³⁵U at the reference neutron velocity of 2200 m/sec was measured. The counting rate of a ¹⁰B fission chamber was compared with that of a ionization chamber by alternate measurements in a neutron beam emergent from the thermal column of BRI. The energy selection was done with a slow chopper previously calibrated with respect to the sharp breaks of σ, for α Fe, Be, Bi, Al, C (graphite). The efficiency of the ¹⁰B chamber was compared to the well-known absorption cross section of ¹⁹⁷Au by activation measurements with a (β-γ) coincidence technique. The thickness of the layer was determined by comparison with α-standards of the same isotopic composition, and controlled by a massapectrometric method. Our determination yields:
σ_f⁰= (587 ± 6) barns. Some information is also obtained about the nearby negative energy levels.