Novel 4π Detection System for the Measurement of the 6Li(n,α)3H Reaction Cross Section

A dedicated one-dimensional Time Projection Chamber (1D-TPC) was designed and produced at IRMM to determine the Li-6(n,alpha)H-3 cross section in the 0.4-2.8 MeV energy range, aiming at 5% accuracy. The basic TPC components were a twin gridded ionisation chamber (GIC) with interwired electrodes and fast digitisation of the anode and cathode signals. The energy of both reaction products emitted from a thin (LiF)-Li-6 sample at the common TPC cathode was measured. A Kr(97%)CO2(3%) mixture was used as the detector gas at a pressure up to 3.5 bar. A U-238 sample mounted on the cathode of an ionisation chamber without grid was used as the neutron flux monitor. Special care was taken to reduce the experimental sources of uncertainty. The beam-monitor U-238 sample was characterised at IRMM by low-geometry alpha-counting with an accuracy of 0.1%. A Li-6 sample was produced at IRMM by vacuum evaporation of (LiF)-Li-6 onto transparent aluminium backing. The number of Li-6 atoms will be measured via Thermal Neutron Depth Profiling with an expected accuracy of 2% with respect to an IRMM Standard Reference Material. First test measurements were performed using a monoenergetic neutron beam produced by the T(p,n)He-3 reaction at the IRMM 7 MV Van de Graaff accelerator. The experimental method and preliminary results are presented.