Measurement of the 2H(d,2He)2n reaction at Ed=171 MeV and implications for the neutron-neutron scattering length -: art. no. 044003

A measurement of the H-2(d,He-2)(2)n charge-exchange reaction at E-d=171 MeV has been performed in order to study the interaction of the di-neutron system. Spectroscopy of the neutron-neutron final-state interaction with a resolution of about 115 keV (FWHM) was achieved by measuring the momentum of He-2 at forward scattering angles. In the experiment the two protons of the unbound He-2 are momentum analyzed with a magnetic spectrometer and detected with the same detector. A simple reaction theory is employed in order to obtain information about the neutron-neutron interaction from the measured cross section data. The validity of the impulse approximation for the d -> He-2 transition at intermediate energies is discussed and compared to the Watson-Migdal theory of the proton-proton final-state interaction. Predictions of the impulse approximation for the analog transition d ->(2)n are confronted with experimental data. A procedure to extract the neutron-neutron scattering length a(nn) from this comparison is presented. The result is compatible with the recommended value of a(nn)=-18.6(+/- 0.4) fm.