Speaker
Description
The calcium isotopes are the ideal system to investigate the evolution of shell structure and magic numbers due to the closed proton shell with Z=20. The first experimental evidence of the N=34 sub-shell closure was found in 54Ca at RIKEN [1]. To study the magicity of N=34 and nuclear structure towards N=60, several in-beam gamma-ray spectroscopy measurements with proton-induced nucleon knockout reactions at around 200 MeV/u were performed at RIBF RIKEN, with the use of MINOS liquid hydrogen target device coupled with DALI2 gamma spectrometer. Exclusive cross sections and parallel momentum distributions were obtained. In 54Ca(p,pn)53Ca, a significantly larger cross section to the p3/2 state compared to the f5/2 state was observed in the excitation of 53Ca, providing direct evidence for the nature of the N=34 shell closure [2]. Gamma decays were observed in 56Ca and 58Ca following the quasi-free one-proton knockout reactions from 57,59Sc beams. The first 2+ excitation energies of 56Ca and 58Ca, combined with shell model calculations, preclude the possibility for a doubly magic 60Ca and potentially drive the dripline of Ca isotopes to 70Ca or even beyond [3]. Moreover, the structures of deeply bound nucleons in 53Ca and 55Ca populated from 54,56Ca(𝑝,𝑝𝑛) reactions were also probed for the first time. The observed excitation energies and cross-sections in the resonance states of 53Ca and 55Ca point towards extremely localized and well separated strength distributions in 53Ca and 55Ca. The extracted shell gap of approximately 3 MeV between the f7∕2 and p3∕2 orbitals indicates the robustness of the N=28 shell closure, even with the deeply bound f7∕2 orbital [4]. In this talk, the experiment results and the state-of-the-art theoretical calculations will be presented.
References:
[1] D. Steppenbeck et al., Nature, 502, 207 (2013).
[2] S. Chen, J. Lee, P. Doornenbal et al., Phys. Rev. Lett 123, 142501 (2019).
[3] S. Chen, F. Browne, P. Doornenbal, J. Lee et al., Phys. Lett B 843, 138025 (2023).
[4] P.J. Li, J. Lee, P. Doornenbal, S. Chen et al., Phys. Lett. B 855, 138828 (2024).
