Determination of the Absolute Neutron Fission Cross Section ofU235

The absolute neutron fission cross section of ${\mathrm{U}}^{235}$ at 0.0253 ev has been determined by measuring ($\frac{{\sigma }_t}{{\sigma }_f}$) at 5.3 A and combining this with scattering, absorption, and relative fission cross section measurements. Two multiplate ionization chambers, one containing ${\mathrm{B}}^{10}$ and the other ${\mathrm{U}}^{235}$, were constructed. The flux is determined by making the two chambers as alike as possible and comparing counting rates in the same neutron beam. The amounts of ${\mathrm{U}}^{235}$ and ${\mathrm{B}}^{10}$ in the beam are determined by transmission measurements. After corrections for chamber efficiencies, residual differences between chambers, and incoherent scattering, a value for $(1+\alpha )=1.171\mathrm{}\pm 0.009$ at 5.3 A was obtained. This leads to a value of 580±7 barns as the absolute fission cross section at 0.0253 ev. These results are independent of the absolute neutron cross section of ${\mathrm{B}}^{10}$.