Observation of excited states in Mg20 sheds light on nuclear forces and shell evolution

Le résumé fourni par la source

The exotic Borromean nucleus $^{20}\mathrm{Mg}$ with $N=8$, located at the proton drip line provides a unique testing ground for nuclear forces and the evolution of shell structure in the neutron-deficient region. We report on the first observation of proton unbound resonances together with bound states in $^{20}\mathrm{Mg}$ from the $^{20}\mathrm{Mg}(d,{d}^{\ensuremath{'}})$ reaction performed at TRIUMF. Phenomenological shell-model calculations offer a reasonable description. However, our experimental results present a challenge for current first-principles nuclear structure approaches and point to the need for improved chiral forces and ab initio calculations. Furthermore, the differential cross section of the first excited state is compared with distorted-wave Born approximation calculations to deduce a neutron quadrupole deformation parameter of ${\ensuremath{\beta}}_{n}=0.46\ifmmode\pm\else\textpm\fi{}0.21$. This provides the first indication of a possible weakening of the $N=8$ shell closure at the proton drip line.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.