Speaker
Description
The structural evolution of neutron-rich Ca isotopes (Z = 20) has drawn significant experimental and theoretical interest, particularly concerning the emergence of sub-shell closures at N = 32 [1] and N = 34 [2]. While these closures are supported by excited-state energies [2], mass measurements [1, 3], and direct reaction cross sections [4, 5], the large charge radii observed in 50,52Ca [6] indicate potential (core breaking) proton excitations, challenging the magicity of 52Ca [7]. To elucidate the driving mechanism of the shell evolution in this exotic region, and to benchmark theoretical calculations towards the potentially doubly magic 60Ca, measurements of transition strengths are critical.
In this contribution, we report on two complementary experiments performed at the RIBF facility of the RIKEN Nishina Center, investigating electromagnetic transition strengths in 50-54Ca.
In the first experiment, we performed high-resolution gamma-ray spectroscopy to measure the lifetimes of excited states in 53Ca. The excited states were populated via multi-nucleon removal reactions of a Sc secondary beam on C and CH2 targets. Utilizing the HiCARI
(High-resolution Cluster Array at RIBF) array, excited state lifetimes are extracted through line-shape analysis, providing direct access to transition strengths.
Complementing this, a systematic study of E2 transition strengths in
50-54Ca was conducted using intermediate-energy Coulomb excitation. Neutron-rich Ca beams, produced via the fragmentation of a 345 MeV/nucleon 70Zn primary beam, were directed onto a 1-mm Au target. The BigRIPS-ZeroDegree beamline and DALI2+/HYPATIA array were utilized to measure the excitation cross sections, with nuclear excitation contributions disentangled using a 6-mm Be target.
The presentation will detail the experimental setups, the current status of both data analyses, and preliminary results. The measured transition strengths will be compared with state-of-the-art theoretical calculations to further discuss the complex shell structure along
the Ca isotopic chain beyond N = 28.
References:
[1] Wienholtz, F. et al. Nature 498, 346–349 (2013).
[2] Steppenbeck, D. et al. Nature 502, 207–210 (2013).
[3] Michimasa, S. et al. Phys. Rev. Lett. 121, 022506 (2018).
[4] Chen, S. et al. Phys. Rev. Lett. 123, 142501 (2019).
[5] Enciu, M. et al. Phys. Rev. Lett. 129, 262501 (2022).
[6] Garcia Ruiz, R. F. et al. Nature Phys. 12, 549–598 (2016).
[7] Gade, A. et al. Phys. Rev. C 74, 021302(R) (2006).