Recently the researchers from IMP(Institute of Modern Physics, Chinese Academy of Sciences) reported the discovery of a new isotope ²¹⁹Np, which was produced via the fusion reaction ¹⁸⁷Re⁽³⁶Ar,4n)²¹⁹Np at the gas-filled recoil separator SHANS (Spectrometer for Heavy Atoms and Nuclear Structure) at the Heavy Ion Research Facility in Lanzhou (HIRFL). The a-decay properties of ²¹⁹Np were measured for the first time. 

 Actinide nuclei with N=126 have been reported up to ²¹⁸U. It very important to extend the study to unknown nuclei such as ²¹⁹Np and ²²⁰Pu with N=126 to explore the evolution of the N=126 shell closure beyond Z=92 and the possible ways to enter so far unknown regions in the upper part of the chart of nuclei. To study ²¹⁹Np, the experiment of ¹⁸⁷Re(³⁶Ar,4n)²¹⁹Np was carried out at the gas-filled recoil separator SHANS and a beam of ³⁶Ar was delivered by the sector focusing cyclotron of HIRFL in Lanzhou, China. The incident beam energy was 191.5 MeV and the beam intensities were 300-400 pnA. The targets consisted of 380 mg/cm² thick layers of ¹⁸⁷Re, which were evaporated on 45 mg/cm² thick carbon foils. The targets with the backing upstream were mounted on a stationary frame. The reaction products were identified using spatial and time correlations between the implants and subsequent a decays. According to the observed a-decay chain (shown in Fig. 1), an alpha-particle energy of E_a=9039(40) keV and a half-life of T_1/2=0.15(+0.72/-0.07) ms were determined for ²¹⁹Np. The deduced proton binding energy of -301(83) keV for ²¹⁹Np fits well into the systematics (shown in Fig. 2), which gives another evidence of that there is no sub-shell closure at Z=92. Negative value of proton binding energy indicates that ²¹⁹Np is a semi-magic nucleus beyond the proton drip-line. In addition, the influence of the N=126 shell closure on the stability of Np isotopes was also discussed within the framework of a-decay reduced widths. 

The research was supported by the National Natural Science Foundation of China, the National Basic Research Program of China and Chinese Academy of Sciences. The co-researchers came from the Institute of Theoretical Physics, Chinese Academy of Sciences and Nanjing University. The article has been published in Physics Letters B. 

The article link  http://www.sciencedirect.com/science/article/pii/S0370269317309942 

Fig.1. The observed a-decay chain assigned to the new isotope ²¹⁹Np 

Fig.2. (a) Proton binding energies S_p of odd-Z Tl-Np isotopes and (b) reduced a-decay widthsδ²of odd-Z At-Np isotopes as a function of proton number. The red solid circles refer to the values of ²¹⁹Np deduced in present work