New investigation of the neutron-neutron and neutron-proton final-state interaction in the n-d breakup reaction

The neutron-neutron final-state interaction (FSI) has been investigated in the H-2(n,np)n reaction at 25.3 and 16.6 MeV, detecting neutrons and protons in coincidence in a geometry which should enable a practically model-independent determination of the S-1(o) neutron-neutron scattering length a(nn). The analysis was performed by means of detailed Monte Carlo simulations based on rigorous three-body calculations with realistic nucleon-nucleon potentials. At 25.3 MeV, the value of a,, deduced from the absolute cross section in the FSI peak is - -16.3+/-0.4 fm while the relative cross section, normalized in the region of neutron-proton quasifree scattering, gives - 16.1+/-0.4 fm. The relative data obtained at 16.6 MeV yielded a(nn) = - 16.2+/-0.3 fm. In addition, the S-1(o) neutron-proton scattering length was measured at 25.2 MeV in the same configuration for comparison. While our results for a(nn) are incompatible with those of a similar investigation performed recently at 13 MeV where the two neutrons were detected, both results for a(np) are in good agreement with the accurately known value from free n-p scattering.