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

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 Ca55-57 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 Ca-54, comparable to the gap between the neutron 2p(3/2) and 2p(1/2) orbitals in Ca-52. For the Ca-56 nucleus, an open-shell property in neutrons is suggested.