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UBC Theses and Dissertations

First direct mass measurement of the two and four neutron halos ⁶He and ⁸He using the TITAN Penning trap mass spectrometer Brodeur, Maxime


Neutron halo nuclei are distinguished by an extended matter radius and a small neutron separation energy compared to other nuclei. They are comprised of a core surrounded by at least one loosely bound neutron forming a halo structure. The size of the core is associated with the charge radius while the extent of the diffuse halo region, owing to quantum mechanical leakage of the valence neutron(s) wavefunction, depends exponentially on their separation energy. Predicting accurately the extreme behaviour of these nuclei is challenging for nuclear theory. These nuclei provide an ideal testing grounds of nuclear theory, leading to a deeper understanding of the strong force and nuclear interactions. Halo nuclei can be found amongst light nuclei and hence have few (A~10) nucleons. This makes them treatable using ab-initio methods. To test these theoretical approaches and consequently refine our knowledge of the nucleus, one requires both precise and accurate experimental data, such as ground state properties: masses (or separation energies) and sizes (charge and matter radii). In this thesis we present the mass measurement of the two- and four-neutrons halo ⁶,⁸He (t₁/₂ = 808 ms, 119 ms respectively) using the TITAN Penning trap as well as systematic studies of this system. The obtained mass are m(⁶He) = 6 018 885.883(57) u and m(⁸He) = 8 033 934.435(114) u. These values show deviations with literature of 4.0 and 1.7σ. Using our new masses, we re-evaluated the charge radius and obtained (rc²)¹/²(⁶He) = 2.056(10) fm and (rc²)c¹/²(⁸He) = 1.955(18) fm, which correspond to an improvement in the precision of 9% and 36% respectively. Using the charge radii and the binding energies of ⁶,⁸He, obtained from our more precise and accurate masses, we show that one can test the predictions of advanced ab-initio nuclear theories for these extreme systems. Using such comparison, we point to the needs of three-body interactions in order to explain the experimental observables.

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