Atomic mass measurements of neutron-rich nuclides on the path to 78 Ni with a β-TOF-equipped MRTOF

W. Xian^1,2,3*M. Rosenbusch⁴V. H. Phong⁴M. Wada³P. Schury³D. Hou⁵A. Takamine^4,6S. Chen^2,3T. Niwase^3,4,6Y. Hirayama³H. Ishiyama⁴S. Iimura⁴Y. Ito⁷T. M. Kojima⁴S. Kimura⁴J. Liu²J. Lee²S. Michimasa⁸H. Miyatake³J. Y. Moon⁹M. Mukai^3,4S. Nishimura⁴S. Naimi^10,4T. Sonoda⁴Y. X. Watanabe³H. Wollnik¹¹S. Yan¹²

• ¹Sun Yat-sen University, Sino-French Institute of Nuclear Engineering and Technology, Zhuhai, China
• ²Department of Physics, The University of Hong Kong, Hong Kong, Hong Kong, SAR China
• ³KEK Wako Nuclear Science Center, Wako, Japan
• ⁴RIKEN Nishina Center, Wako, Japan
• ⁵Institute of Modern Physics Chinese Academy of Sciences, Lanzhou, China
• ⁶Kyushu University, Fukuoka, Japan
• ⁷Advanced Science Research Center, Japan Atomic Energy Agency, Naka-gun, Japan
• ⁸Center of Nuclear Study (CNS), The University of Tokyo, Tokyo, Japan
• ⁹Institute for Basic Science, Yuseong-gu, Republic of Korea
• ¹⁰Laboratoire de Physique des 2 Infinis Irene Joliot-Curie, Orsay, France
• ¹¹New Mexico State University, Las Cruces, United States
• ¹²Institute of Mass Spectrometry and Atmospheric Environment, Jinan University, Guangzhou, China

We report using a multi-reflection time-of-flight to perform atomic mass measurements of the unstable nuclides $^{73-75}$Ni, $^{73-78}$Cu, and $^{74-78}$Zn, which have been accomplished using new technical developments to resolve challenges for exotic-isotope identification and selection. These isotopes were produced in-flight at the RIKEN Radioactive Ion Beam Facility and delivered to the combined gas cell and multi-reflection system installed downstream of the ZeroDegree spectrometer. The incoming high-energy beam was energy-degraded and subsequently stopped in a helium gas cell. The energy degrader thickness was optimized using a new method that employs signals from plastic scintillators located upstream and downstream of the helium-filled gas cell. Extracted isotopes of interest were mass-selected by the in-MRTOF deflector method, which will be described in this work. Notably, the simultaneous selection of multiple and non-consecutive isobar chains is reported. The ions of interest were identified unambiguously using $\beta$-decay-correlated mass measurements for the first time, which we demonstrate for $^{78}$Zn. The new mass values are compared with the literature, including recent measurements performed at JYFLTRAP and ISOLTRAP, with which an agreement is observed.