Speaker
Description
The Island of Inversion in the neutron-rich $N=20$ region arises in part due to a significant reduction in the energy gap between the sd and fp shells. Recent theoretical calculations [1] and experimental results in $^{30}$Mg [2] favor a much smoother transition towards the Island of Inversion than previously thought, with considerable fp admixtures in the ground state of $^{30}$Mg and small fp admixtures down to $^{28}$Mg. If such admixtures are present already in $^{28}$Mg, they are expected to influence the g factor of its $2^+_1$ state as the magnetic dipole moments are especially sensitive to the mixing of single-particle configurations.
To test this hypothesis, the first application of the Time Differential Recoil In Vacuum (TDRIV) method [3,4] on a radioactive ion beam aimed to measure the g factor of the $2^+_1$ state in $^{28}$Mg. The experiment was carried out at HIE-ISOLDE in 2017 using the MINIBALL HPGe detector array, a CD DSSSD for particle detection and the MINIBALL plunger device, and the state of interest was populated via Coulomb excitation of the post-accelerated $^{28}$Mg beam. The TDRIV method is based on observing the Larmor frequency, proportional to the g factor, at which the nuclear and atomic spins precess around the total spin of the projectile as it recoils between the target and a secondary foil within a plunger device. In the same experiment a calibration TDRIV measurement of the supposedly well-known g factor of the $2^+_1$ state in $^{22}$Ne was also performed as a test of the plunger system and in order to determine the plunger zero-offset distance, needed to constrain the $^{28}$Mg TDRIV analysis. A striking disagreement was observed between the newly-obtained results from the $^{22}$Ne measurement and the adopted g-factor value from the 1970s [5], which introduced significant systematic uncertainties for the $^{28}$Mg g-factor result.
In order to reduce the systematic uncertainties of the $^{28}$Mg measurement and to resolve the discovered discrepancy in $^{22}$Ne an experiment to re-measure the g factor of the $2^+_1$ state in $^{22}$Ne was performed in September 2024 at GANIL. The experimental setup consisted of the EXOGAM $\gamma$-ray spectrometer coupled to the Orsay Universal Plunger System and the newly-developed Orsay Particle Scintillator Array (OPSA). With the provided $10^9$ pps $^{22}$Ne beam intensity we were able to collect high-statistics particle-$\gamma$ coincidence data that will allow us to obtain a high-precision and high-accuracy value for the g factor of the $2^+_1$ state in $^{22}$Ne. The results from the preliminary analysis of this data set will be presented and compared to theoretical predictions. In addition, the implications of these results on the $^{28}$Mg g-factor measurement from ISOLDE, and on the extent of the $N=20$ Island of Inversion will be discussed.
