Joint momentum distribution of p + n in 6Li

The joint momentum distribution for p + n in 6Li was measured with the 6Li(α, 2α)pn reaction at 118 MeV for 0 ⩽ Q ⩽ 1.2 fm− and 0 ⩽ k ⩽ 1.0 fm−; Q is the wavenumber of pn c.m. motion in the lab, and k is that for the motion of either nucleon in their c.m. system. The four-body absolute cross sections near Q = 0 were reasonably-well fitted by an impulse approximation model when the np pair was described by a harmonic oscillator wavefunction in the initial state and by Yamaguchi, Paris, or Reid wavefunctions in the final state, and when the primary αα interaction was evaluated by the final-state energy prescription. Poorer fits were obtained using final plane waves and/or the initial-state prescription; in particular, measured yields at large Q exceeded the predictions by large factors. For 0 ⩽ k ⩽ 0.6 fm−1, the impulse approximation model predicted the three- to four-body cross section ratio within about a factor of two, but required an oscillator parameter which gave too small a charge radius for 6Li.