Pionic fusion study of the 6He halo

The halo nucleus 6He has been studied in a pionic fusion experiment at the CELSIUS ring in Uppsala. The aim of the experiment was to investigate, in particular, the high-momentum part of the halo wave function by measuring the differential cross section for the 4He(d, 6He)π+ reaction 0.6, 1.2 and 5.0 MeV above threshold in the centre-of-mass frame. The 6He ions were detected in a ΔE−E solid-state detector telescope inserted into the ring vacuum. The result for the total cross section is, respectively, 22(1), 38(1) and 57(9) nb with a common systematic uncertainty of ±35%. The differential cross section is clearly anisotropic: [dσdΩ(0°)−dσdΩ(180°)]/[dσdΩ(0°)+dσdΩ(180°)]=−0.25(5), −0.29(5) and −0.35(15) at the same three energies. The preferred direction of emission for the pion is parallel to the momentum of the 4He ion in the initial state. In terms of a simple model for the reaction the results depend sensitively on the state of motion of the centre of mass of the halo relative to that of the core.