Particle Leaking, Cross-Section Ratio 10B(n,α)/238U(n,fission), and Excitation Function of the Reaction 10B(n,α)7Li at MeV Energies

The 10B(n,α)7Li reaction was studied in the energy range between 1.5 MeV and 5.6 MeV at the 7-MV Van de Graaff accelerator of IRMM by using a gridded ionisation chamber, signal digitisation, and an intrinsic 238U neutron monitor. The aim was to obtain accurate data for the IAEA Coordinated Research Project (CRP) on the improvement of standard cross sections for light elements. The effect of particle leaking was discovered and its implications investigated. The determination of the cross section σ(α0+α1) strongly benefits from it but measurements of angular distributions, individual cross sections σ(α0) and σ(α1), and the branching ratio α0/α1 are negatively affected. The correct number of reaction events was obtained by identification of unknown particle signatures in the energy spectra as 10B(n,α)7Li events in the form of quasi 7Li+α particles created by particle leaking. The cross-section ratio 10B(n,α)7Li/238U(n,fission) was measured and the excitation function of 10B(n,α)7Li determined by simultaneously detecting the charged particles from the boron disintegration in the forward hemisphere and the 238U fission fragments in the backward hemisphere. The IRMM cross sections are compared to experimental data of other groups and to predictions of the ENDF/B-VI.8, JENDL-3.3, and JEF-2.2 evaluations