Magic Nature of Neutrons in $^{54}$Ca: First Mass Measurements of $^{55-57}$Ca

概要

We perform the first direct mass measurements of neutron-rich calcium isotopes beyond neutron number $34$ at the RIKEN Radioactive Isotope Beam Factory by using the time-of-flight magnetic-rigidity technique. The atomic mass excesses of $^{55–57}$Ca are determined for the first time to be $-18650$($160$),$-13510$($250$), and $−7370$($990$) keV, respectively. We examine the emergence of neutron magicity at $N=34$ based on the new atomic masses. The new masses provide experimental evidence for the appearance of a sizable energy gap between the neutron $2p_{1/2}$ and $1f_{5/2}$ orbitals in $^{54}$Ca, comparable to the gap between the neutron $2p_{3/2}$ and $2p_{1/2}$ orbitals in $^{52}$Ca. For the $^{56}$Ca nucleus, an open-shell property in neutrons is suggested.