Measurement of the 3He and 4He total 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 (3/2)+ resonance in the n+ 4He 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/sub n/approx.(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 3He, 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 3He and the sum of the partial n-3He cross sections. This discrepancy is removed by our new data