First direct mass-measurement of the two-neutron halo nucleus 6He and improved mass for the four-neutron halo 8He
Résumé
The first direct mass-measurement of 6He has been performed with the TITAN Penning trap mass spectrometer at the ISAC facility. In addition, the mass of 8He was determined with improved precision over our previous measurement. The obtained masses are m(6He) = 6.018 885 883(57) u and m(8He) = 8.033 934 44(11) u. The 6He value shows a deviation from the literature of 4σ. With these new mass values and the previously measured atomic isotope shifts we obtain charge radii of 2.060(8) fm and 1.959(16) fm for 6He and 8He respectively. We present a detailed comparison to nuclear theory for 6He, including new hyperspherical harmonics results. A correlation plot of the point-proton radius with the two-neutron separation energy demonstrates clearly the importance of three-nucleon forces.