Pionic fusion study of the He-6 halo

摘要

The halo nucleus He-6 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 He-4(d, He-6)pi(+) reaction 0.6, 1.2 and 5.0 MeV above threshold in the centre-of-mass frame. The He-6 ions were detected in a Delta E - E solid-state detector telescope inserted into the ring vacuum. The result for the total cross section is, respectively, 22(l), 38(l) and 57(9) nb with a common systematic uncertainty of +/- 35%. The differential cross section is clearly anisotropic: [(d sigma)/(d Omega)(0 degrees) - (d sigma)/(d Omega)(180 degrees)]/[(d sigma)/(d Omega)(0 degrees) + (d sigma)/(d Omega)(180 degrees)]=-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. (c) 2006 Elsevier B.V. All rights reserved.