Orateur
Description
The magnetic dipole moment of a nuclear state provides unique insight on the single-particle structure of the nucleus. Its experimental observable, the $g$ factor, can yield valuable information e.g. on the effective single-particle energies of proton and neutron orbitals, and shell evolution away from the valley of stability. One of the available methods for measuring $g$ factors of excited nuclear states with picosecond lifetimes, is the Time-Differential Recoil In Vacuum (TDRIV) method. This 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.
А modification of the TDRIV method was proposed [1] to overcome one of its limitations, namely applying it on radioactive ion beams. After a successful proof-of-principle experiment using a stable $^{24}$Mg beam [2], the first application of the modified TDRIV method on a radioactive $^{28}$Mg beam was performed at the HIE-ISOLDE facility to probe the nature of the transition towards the $N=20$ Island of Inversion around $^{32}$Mg along the Mg isotopic chain, where modern theoretical models give varying predictions [3,4]. An additional TDRIV measurement was performed using a stable $^{22}$Ne beam in order to constrain some experimental parameters, making use of the precise literature value for the $g$ factor of the $2^+_1$ state in this nucleus [5]. However, the results obtained from the $^{22}$Ne measurement put into question the accuracy and precision of the adopted $g$-factor value, and in turn hindered the achievable precision for the $2^+_1$ $g$ factor in $^{28}$Mg.
These discrepancies led to an initiative to develop better tools in order to reach new levels of precision for $g$-factor measurements in both stable and radioactive ion beam measurements, such as the Orsay Particle Scintillator Array (OPSA), designed specifically for TDRIV measurements. For its first use in an experiment, the OPSA array was coupled to the EXOGAM $\gamma$-ray spectrometer and the OUPS plunger device in a dedicated TDRIV measurement on $^{22}$Ne performed at GANIL in 2024. The main goals of the experiment were to resolve the observed discrepancy in $^{22}$Ne, to minimize systematic uncertainties for the $^{28}$Mg results from HIE-ISOLDE, and to establish the application of the TDRIV method at GANIL for future studies on stable and radioactive ion beams.
Details on the above-mentioned experiments will be reported with a main focus on the successful experiment at GANIL and the current status of the data analysis. The implications of the obtained preliminary results on the transition towards the $N=20$ Island of Inversion in Mg isotopes will be highlighted. The directions for further advances with the TDRIV method as well as plans for future measurements on radioactive ion beams will be mentioned.
[1] A. E. Stuchbery, P. F. Mantica and A. N. Wilson, Phys. Rev. C 71 047302 (2005).
[2] A. Kusoglu, A. E. Stuchbery, G. Georgiev et al. Phys. Rev. Lett. 114 062501 (2015).
[3] N. Tsunoda et al., Phys. Rev. C 95 021304 (2017).
[4] T. Miyagi et al., Phys. Rev. C 102, 034320 (2020).
[5] R. E. Horstman et al., Nucl. Phys. A, 275(1) 1977.