New short-lived isotope 223Np was discovered at the SHANS gas-filled separator in IMP. The experimental results indicate the absence of the Z = 92 sub-shell closure around N = 126.
The existence of super heavy elements is based on the nuclear shell effects. Isotopes with special numbers of neutrons and protons (called magic number , Z or N = 2, 8, 20, 28, 50, 82 and N = 126) are particularly stable. The major shells are characterized by huge level gaps ~5 MeV at the shell closure defined by the magic number. The study of the evolution of shell structure above Z = 82, the largest magic number of proton, is important for the exploration of the stability of super heavy nuclei. Sub-shells also have significant effect on the nuclear stability but weaker than the major shells. Theoretically, the Z = 92 sub-shell has been controversial for a long time. The relativistic mean-field and macroscopic-microscopic models predicted the presence of Z = 92 subshell, while the large-scale shell model denied it. An energy of 9477(44) keV and a half-life were measured. The proton separation energy was calculated, the smooth behavior of which along N =130 isotones indicates the absence of the Z = 92 subshell.
223Np was produced in the 40Ar (187Re, 4n) reaction and identified through the 223Np→219Pa→215Ac→211Fr a decay chain by the spatial-temporal correlation. The daughter nucleus 219Pa(T1/2 ~ 60 ns) is the shortest-lived a emitter directly measured in the chart of nuclide. Digital pulse processing technique and fast rise-time preamplifiers were used in the experiment to resolve the pile-up signals. As examples, three preamplifier traces are shown in Figure 1 where 223Np implant and subsequent α-decays of 223Np and 219Pa were registered.
This work was jointly supported by the National Natural Science Foundation of China, the ‘Hundred Talented Project’ of the Chinese Academy of Sciences and 973 project of Ministry of Science and Technology. The results were published in Physics Letters B. The article link:http://www.sciencedirect.com/science/article/pii/S0370269317302654
Figure 1: Examples of traces where 223Np implants and subsequent α-decays of 223Np and 219Pa were registered