Spectroscopy of deeply bound orbitals in neutron-rich Ca isotopes

• Main Authors: Li, P. J, Lee, J, Doornenbal, P, Chen, S, Wang, S, Obertelli, A, Chazono, Y, Holt, J. D, Hu, B. S, Ogata, K, Utsuno, Y, Yoshida, K, Achouri, N. L, Baba, H, Browne, F, Calvet, D, Château, F, Chiga, N, Corsi, A, Cortés, M. L, Delbart, A, Gheller, J-M, Giganon, A, Gillibert, A, Hilaire, C, Isobe, T, Kobayashi, T, Kubota, Y, Lapoux, V, Liu, H. N, Motobayashi, T, Murray, I, Otsu, H, Panin, V, Paul, N, Rodriguez, W, Sakurai, H, Sasano, M, Steppenbeck, D, Stuhl, L, Sun, Y. L, Togano, Y, Uesaka, T, Wimmer, K, Yoneda, K, Aktas, O, Aumann, T, Boretzky, K, Caesar, C, Chung, L. X, Flavigny, F, Franchoo, S, Gasparic, I, Gerst, R. -B, Gibelin, J, Hahn, K. I, Kahlbow, J, Kim, D, Koiwai, T, Kondo, Y, Körper, D, Koseoglou, P, Lehr, C, Linh, B. D, Lokotko, T, MacCormick, M, Miki, K, Moschner, K, Nakamura, T, Park, S. Y, Rossi, D, Sahin, E, Schindler, F, Simon, H, Söderström, P-A, Sohler, D, Takeuchi, S, Toernqvist, H, Tscheuschner, J, Vaquero, V, Wagner, V, Werner, V, Xu, X, Yamada, H, Yan, D, Yang, Z, Yasuda, M, Zanetti, L
• Format: Journal Article
• Language: English
• Published: 05-07-2024
• Subjects: Physics - Nuclear Experiment; Physics - Nuclear Theory
• DOI: 10.48550/arxiv.2407.04529

Phys. Lett. B, 855 (2024),138828 The calcium isotopes are an ideal system to investigate the evolution of
shell structure and magic numbers. Although the properties of surface nucleons
in calcium have been well studied, probing the structure of deeply bound
nucleons remains a challenge. Here, we report on the first measurement of
unbound states in $^{53}$Ca and $^{55}$Ca, populated from \ts{54,56}Ca($p,pn$)
reactions at a beam energy of around 216 MeV/nucleon at the RIKEN Radioactive
Isotopes Beam Factory. The resonance properties, partial cross sections, and
momentum distributions of these unbound states were analyzed. Orbital angular
momentum $l$ assignments were extracted from momentum distributions based on
calculations using the distorted wave impulse approximation (DWIA) reaction
model. The resonances at excitation energies of 5516(41)\,keV in $^{53}$Ca and
6000(250)\,keV in $^{55}$Ca indicate a significant $l$\, =\,3 component,
providing the first experimental evidence for the $\nu 0f_{7/2}$
single-particle strength of unbound hole states in the neutron-rich Ca
isotopes. The observed excitation energies and cross-sections point towards
extremely localized and well separated strength distributions, with some
fragmentation for the $\nu 0f_{7/2}$ orbital in $^{55}$Ca. These results are in
good agreement with predictions from shell-model calculations using the
effective GXPF1Bs interaction and \textit{ab initio} calculations and diverge
markedly from the experimental distributions in the nickel isotones at $Z=28$.