Speaker
Description
Magnetic moments of excited nuclear states provide sensitive probes to the contributions of single-particle configurations to the nuclear wave functions. This is especially true close to shell closures where the wave functions are expected to be quite pure, and the magnetic moments should be close to the Schmidt values of the single-particle approximation. However, similarly to the effective charges for protons and neutrons, effective spin $g$ factors are often used in calculations to improve agreement between experiment and theory.
The nickel isotopic chain, with three doubly-magic isotopes, makes for a fruitful testing ground for various theoretical models. A peculiar case is that of $^{68}$Ni which lies at the $N=40$ sub-shell closure between the fp shell and the $g_{9/2}$ orbital. It notably exhibits some properties associated with doubly-magic nuclei, such as a $0^+$ first excited state [1] and a low $B(E2)$ value of its $2^+_1$ state [2,3]. However, no sign of magicity at $N=40$ was observed in the $S_{2n}$ from mass measurements [4]. Further insight into the nuclear structure of $^{68}$Ni could be obtained through the magnetic moments (or $g$ factors) of excited states. Targeting directly the excited states in $^{68}$Ni experimentally is still quite challenging, so instead, as a first step, the $g$ factors of $g_{9/2}$ isomeric states in neighboring isotopes were used to shed light on the nuclear structure in the region, with measurements on the $9/2^+$ isomers in $^{65}$Ni [5] and $^{67}$Ni [6].
A follow-up experiment aiming to measure the magnetic moments of $g_{9/2}$ isomeric states in the vicinity of $^{68}$Ni was performed at the NSCL facility at MSU, USA. The main goals of the experiment were to determine the $g$ factors of the $8^+$ isomer in $^{70}$Ni ($T_{1/2}=232(1)$ ns) and the $(19/2^-)$ isomer in $^{69}$Ni ($T_{1/2}=439(3)$ ns), together with a re-measurement of the $g$ factor of the $9/2^+$ isomer in $^{67}$Ni ($T_{1/2}=13.3(2)~\mu$s). The nuclei of interest were produced following a projectile-fragmentation reaction of a $^{76}$Ge primary beam (130 MeV/A) on a Be target. The fragments passed through the A1900 separator and were implanted at the center of a dedicated detector setup.
The half-lives of these isomers, as well as those of isomers in $^{71,72}$Cu present in the data, were measured, leading to improved precision in some cases. The Time-Dependent Perturbed Angular Distribution (TDPAD) method was employed to extract the $g$ factors of the isomeric states of interest by observing the Larmor precession of the oriented nuclear spins in an external magnetic field. The results obtained in this experiment [7] will be presented and compared to state-of-the-art shell-model calculations with the LNPS interaction, showing evidence that calculations in large valence spaces can mostly negate the need for effective spin $g$ factors. Ideas for future measurements of magnetic moments following projectile-fragmentation reactions for both long- and short-lived isomeric states will also be discussed.
References:
[1] M. Bernas et al. Phys. Lett. B, 113(4):279-282, 1982.
[2] O. Sorlin et al. Phys. Rev. Lett., 88:092501, Feb 2002.
[3] N. Bree et al. Phys. Rev. C, 78:047301, Oct 2008.
[4] C. Guenaut et al. Phys. Rev. C, 75:044303, Apr 2007.
[5] G. Georgiev et al. Journal of Physics G, 31(10):1439, 2005.
[6] G. Georgiev et al. Journal of Physics G, 28(12):2993, 2002.
[7] K. Stoychev, G. Georgiev et al., submitted to Phys. Rev. C, 2025.
| Contribution category | Experiment |
|---|---|
| Presenter status | Postdoc |