Measurement of theHe3andHe4total neutron cross sections up to 40 MeV

The total neutron cross sections of the helium isotopes were measured in the energy range from 1.5 to 40 MeV. A continuous energy neutron source was used in connection with a 190 m long flight path and high resolution time-of-flight techniques. The energy resolution achieved enabled us to improve considerably the parameters of the narrow ${\frac{3}{2}}^{+}$ resonance in the n+${}_{}{}^4{}_{}{}^{}\mathrm{He}$ system. The position of the resonance was found to be 22 133±10 keV, with a total width of 76±12 keV and a neutron width of 37±5 keV. This agrees well with previous analyses, which yield ${\Gamma }_{\mathrm{n}}\sim \frac{1}{2}\Gamma$. Absolute values for the cross sections were determined with an overall error smaller than 3%. Comparison with older data shows agreement within the total errors. Our data are 4% to 5% higher than the most recent data set; however, in the case of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, the data are still below the predictions from an $R$-matrix analysis. It was pointed out recently that a slight disagreement exists between the total neutron cross section of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and the sum of the partial n-${}_{}{}^3{}_{}{}^{}\mathrm{He}$ cross sections. This discrepancy is removed by our new data.