Short-Lived α-Emitting Isotope ^{222}Np and the Stability of the N=126 Magic Shell.

A new, very short-lived neutron-deficient isotope Np-222 was produced in the complete-fusion reaction Re-187(Ar-40, 5n) Np-222, and observed at the gas-filled recoil separator SHANS. The new isotope Np-222 was identified by employing a recoil-alpha correlation measurement, and six alpha-decay chains were established for it. The decay properties of Np-222 with E-alpha = 10016(33) keV and T-1/2 = 380(-110)(+260) ns were determined experimentally. The alpha-decay systematics of Np isotopes is improved by adding the new data for Np-222, which validates the N = 126 shell effect in Np isotopes. The evolution of the N = 126 shell closure is discussed in the neutron-deficient nuclei up to Np within the framework of alpha-decay reduced width.