Collectivity of neutron-rich Cr and Fe toward N=50

¹ Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany
² CEA, Centre de Saclay, IRFU, F-91191 Gif-sur-Yvette, France
³ RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
⁴ IPHC, CNRS/IN2P3, Université de Strasbourg, F-67037 Strasbourg, France
⁵ Center for Nuclear Study, University of Tokyo, RIKEN campus, Wako, Saitama 351-0198, Japan
⁶ Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
⁷ Department of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima, Tokyo 172-8501, Japan
⁸ School of Computing Engineering and Mathematics, University of Brighton, Brighton BN2 4GJ, United Kingdom
⁹ Institute for Nuclear Science & Technology, VAEI, 179 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam
¹⁰ MTA Atomki, P.O. Box 51, Debrecen H-4001, Hungary
¹¹ Institut de Physique Nucléaire Orsay, IN2P3-CNRS, 91406 Orsay Cedex, France
¹² Department of Physics, University of Oslo, N-0316 Oslo, Norway
¹³ Department of Physics, The University of Hong Kong, Pokfulam, Hong Kong
¹⁴ Department of Physics, University of Surrey, Guildford GU2 7XH, United Kingdom
¹⁵ Department of Physics, Tohoku University, Sendai 980-8578, Japan
¹⁶ State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, P.R. China

Published online: 19 January 2016

Abstract

Within the SEASTAR project at RIKEN-RIBF, ⁶⁶Cr and ^70,72Fe have been produced via protonknockout reactions, and their first excited 2⁺ and 4⁺ states have been discovered. The combination of the liquid-hydrogen target and TPC system MINOS has been used in combination with the DALI2 detector array for the first time. A 345 MeV/u ²³⁸U beam with a mean intensity of about 12 pnA impinged on a Be target. Fission fragments were separated and identified using the BigRIPS spectrograph, and reaction products were analyzed using the ZeroDegree spectrograph. A plateau of excitation energies, with a small change in the systematic trends past N = 44, reveals an extension of the N = 40 region of collectivity toward N = 50. Hence, the isotopes of interest are located within the N = 40 island of inversion. An interpretation of the observed trends is offered through large scale shell model calculations.