Zakopane 2026 Conference on Nuclear Physics

Zhou Yan¹, Xiaofei Yang¹, on behalf of PLASEN collaboration
¹School of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China.

Nuclear charge radii and quadrupole moments are sensitive observables of nuclear deformation. Around neutron number N = 60, a region of deformation has been observed through measurements of nuclear ground-state properties of neutron-rich nuclei with Z = 37-41, including Rb, Sr, Y, Zr and Nb [1-3]. Among them, Rb is the lowest-Z element for which charge radii data beyond N = 60 are available, clearly exhibiting signatures of deformation. However, experimental data on nuclear radii and moments of neutron-rich Rb isotopes above N = 60 remain scarce, motivating further investigations [4,5].

Laser spectroscopy provides access to fundamental nuclear properties such as spins, magnetic dipole moments, electric quadrupole moments, and charge radii through measurements of hyperfine structure and isotope shifts in a nuclear-model-independent way [6]. Previous laser-spectroscopic studies have covered Rb isotopes from ⁷⁶Rb to ⁹⁸Rb, while the nuclear spin and other basic properties of ⁹⁹Rb have not yet been firmly established [4,5]. Recently, a high-resolution and high-sensitivity collinear resonance ionization laser spectroscopy system, PLASEN (Precision Laser Spectroscopy for Exotic Nuclei), has been developed for both off-line and on-line experiments, enabling systematic studies of exotic nuclei [7,8].

In this work, we report the first online collinear resonance ionization spectroscopy experiment performed with the PLASEN setup at the Beijing Radioactive Ion-beam Facility (BRIF). Hyperfine spectra of the $5s\ ^{2}\mathrm{S}_{1/2} → 5p\ ^{2}\mathrm{P}_{3/2}^\circ$ transition were measured for a series of Rb isotopes, including ⁹⁹Rb, with sufficient resolution to extract both magnetic dipole and electric quadrupole hyperfine constants. The extracted nuclear spin, moments, and charge radius of ⁹⁹Rb provide new information on nuclear deformation in this mass region.

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