Pairing Forces Govern Population of Doubly Magic <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>Ca</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>54</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math> from Direct Reactions
F. Browne, S. Chen, P. Doornenbal, A. Obertelli, Kazuyuki Ogata, Y. Utsuno, K. Yoshida, N. L. Achouri, H. Baba, D. Calvet, F. Château, N. Chiga, A. Corsi, M. L. Cortés, A. Delbart, J.-M. Gheller, A. Giganon, A. Gillibert, C. Hilaire, T. Isobe, T. Kobayashi, Y. Kubota, V. Lapoux, Hongna Liu, T. Motobayashi, I. Murray, H. Otsu, V. Panin, N. Paul, W. Rodríguez, H. Sakuraï, M. Sasano, D. Steppenbeck, L. Stuhl, Y. Sun, Y. Togano, Т. Уесака, K. Wimmer, K. Yoneda, Ö. Aktaş, T. Aumann, K. Boretzky, C. Caesar, L. X. Chung, F. Flavigny, S. Franchoo, I. Gašparić, R.-B. Gerst, J. Gibelin, K. I. Hahn, M. Holl, J. Kahlbow, D. Kim, D. Körper, T. Koiwai, Y. Kondo, P. Koseoglou, J. Lee, C. Lehr, B. D. Linh, T. Lokotko, M. MacCormick, K. Miki, K. Moschner, T. Nakamura, S. Y. Park, D. Rossi, E. Şahin, F. Schindler, H. Simon, P.-A. Söderström, D. Sohler, Satoshi Takeuchi, H. Törnqvist, J. Tscheuschner, V. Vaquero, V. Wagner, S. Wang, V. Werner, X. Xu, Hiroki Yamada, D. Yan, Z. Yang, Masahiro Yasuda, L. Zanetti
Abstract
Direct proton-knockout reactions of ^{55}Sc at ∼220 MeV/nucleon were studied at the RIKEN Radioactive Isotope Beam Factory. Populated states of ^{54}Ca were investigated through γ-ray and invariant-mass spectroscopy. Level energies were calculated from the nuclear shell model employing a phenomenological internucleon interaction. Theoretical cross sections to states were calculated from distorted-wave impulse approximation estimates multiplied by the shell model spectroscopic factors, which describe the wave function overlap of the ^{55}Sc ground state with states in ^{54}Ca. Despite the calculations showing a significant amplitude of excited neutron configurations in the ground-state of ^{55}Sc, valence proton removals populated predominantly the ground state of ^{54}Ca. This counterintuitive result is attributed to pairing effects leading to a dominance of the ground-state spectroscopic factor. Owing to the ubiquity of the pairing interaction, this argument should be generally applicable to direct knockout reactions from odd-even to even-even nuclei.