First direct mass-measurement of the two-neutron halo nucleus 6He and improved mass for the four-neutron halo 8He

TL;DR

This paper reports the first direct mass measurement of the two-neutron halo nucleus 6He and an improved measurement for 8He, providing new data for nuclear structure analysis and theoretical comparison.

Contribution

It presents the first direct mass measurement of 6He using a Penning trap and an improved mass value for 8He, with implications for nuclear theory and three-nucleon forces.

Findings

Mass of 6He: 6.018885883(57) u, deviates from literature by 4σ

Mass of 8He: 8.03393444(11) u, with improved precision

Charge radii: 2.060(8) fm for 6He and 1.959(16) fm for 8He

Abstract

The first direct mass-measurement of $^{6}$He has been performed with the TITAN Penning trap mass spectrometer at the ISAC facility. In addition, the mass of $^{8}$He was determined with improved precision over our previous measurement. The obtained masses are $m$($^{6}$He) = 6.018 885 883(57) u and $m$($^{8}$He) = 8.033 934 44(11) u. The $^{6}$He value shows a deviation from the literature of 4$\sigma$. 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 $^{6}$He and $^{8}$He respectively. We present a detailed comparison to nuclear theory for $^6$He, 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.