Orateur
Description
Exotic nuclei offer a unique window into the fundamental properties of nuclear forces, particularly through the evolution of their shell structure. Far from the valley of stability, canonical magic numbers are no longer preserved: shell gaps can weaken or vanish entirely, driven by the proton-neutron interaction at the microscopic level. The monopole component of this interaction is understood to be a key driver, reshaping effective single-particle energies across the nuclear chart. However, isolating and quantifying the contributions of individual interaction terms remains an open challenge in nuclear structure.
The $N = 20$ isotonic chain, from $^{40}$Ca to $^{34}$Si, offers a favourable framework to isolate the spin-orbit interaction. In this region, proton occupation of the $2s_{1/2}$ orbital suppresses tensor-force contributions, making the neutron $d$-shell spin-orbit splitting a sensitive probe of the spin-orbit term and its dependence on nuclear density and isospin. Existing data for $^{39}$Ca and $^{35}$S already hint at deviations from semi-empirical trends, yet the chain remains incomplete toward $Z = 14$.
The nucleus $^{34}$Si stands out as a particularly interesting case. Several observables point to a doubly-magic character: the first excited state is a $0^+$, and the $2^+$ state lies above 3 MeV, indicative of a robust shell closure. At the same time, $^{34}$Si sits at the edge of the $N = 20$ island of inversion, where the weakening of the $N = 20$ shell gap favours intruder configurations involving particle-hole excitations across the $sd$-$pf$ shell gap, in contrast to the behaviour expected of a magic nucleus.
To address these open questions, we performed a one-neutron transfer experiment at GANIL using the reaction $^{34}$Si$(p,d)$$^{33}$Si in inverse kinematics at 50 MeV/u. A radioactive $^{34}$Si beam (produced by fragmentation and selected by the LISE spectrometer) impinged on a CH$_2$ target. The MUST2+EXOGAM+Zero-Degree Detection setup enabled the measurement of deuterons, $\gamma$ rays, and heavy recoils in coincidence. Via the missing-mass technique, this gave access to differential cross sections and spectroscopic factors for the ground state, isomeric, and
neutron-unbound states.
This experiment pursues two complementary physics goals: (i) measuring the neutron $1d_{3/2}$-$1d_{5/2}$ spin-orbit splitting in $^{33}$Si, thereby extending the isotonic chain to $Z = 14$; and (ii) probing the doubly-magic character of $^{34}$Si through the stiffness of its Fermi surface, complementing existing $^{34}$Si$(d,p)$$^{35}$Si data. Preliminary results and perspectives for comparison with theoretical calculations will be presented.