Description
Studying the evolution of shell closures approaching the drip-lines is a powerful way to probe nuclear forces and refine the theoretical description of nuclear structure. The particular case of the spin-orbit π=28 magic number has been intensively investigated in various types of experiments. It has been suggested that the loss of magicity below 48Ca arises from a subtle interplay between proton- and neutron- induced collectivity. Indeed, at π=28, the proton d3/2 and s1/2 orbitals become degenerate, enabling quadrupole excitations for nuclei with π<20. Moreover, when removing protons from the d3/2 orbital, the attractive residual interaction ππ3/2βπ7/2ππbeing stronger than ππ3/2βπ3/2ππleads to a weakening of the π=28 shell gap, again favouring quadrupole excitations. However, other effects such as three-body forces or the coupling to the continuum have also been suggested to play a role. Nuclear structure changes in this region remain not fully understood.
Mass spectrometry offers an alternative way to probe shell closures, by examining trends in one- and two-neutron separation energies (ππ and π2π) along isotopic chains. In particular, the strength of a closure is directly related to a sudden drop in ππ and π2π values, when crossing a magic number, making the masses at π=29 and π=30 especially critical. Along these isotonic chains, only the masses of K (π=19) and Ar (π=18) have been measured. Neutron-rich Cl have recently been measured at the LEBIT Penning Trap at MSU, but only up to π=28. The mass of 46Cl (π=29) has been measured, but the uncertainty remains large, whereas the 47Cl mass remains unmeasured.
Therefore, we propose to measure the masses of 45-47Cl, and thus probe the π=28 shell gap for π<18 through binding energy trends, providing critical insights into the evolution of shell structure far from stability.