Dissociation of He-6

The dissociation reaction He-6 --> alpha + 2n was studied with He-6 projectiles at 23.9 MeV/nucleon in targets of C, Al, Cu, Sn, Pb, and U. Relative to Al, the 2n removal cross section sigma(-2n) with each of the other targets was determined. With U, the Coulomb part accounts for 2/3 of sigma(-2n). The widths of the alpha particle and neutron parallel momentum distributions increase with target Z. For the alpha particle, He-6 dissociation on C gave width sigma = 40.2+/-2.3 MeV/c, corresponding to an rms radius of the halo neutrons of (2.95+/-0.17) fm, which is (0.35 +/- 0.17) fm larger than the rms radius of all four neutrons. The relation between the widths of the alpha particle and neutron distributions indicates only a small correlation between the two neutrons. This conclusion is supported by the distribution function for the angle between the two neutrons, which was obtained in kinematically complete measurements. The latter measurements with the U target also yielded an E1 strength function, and it agreed with one determined in an experiment at ten times our beam energy.