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(T_1/2 ~ 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. 

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