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 the unknown regions in the upper part of the chart of nuclei.

Recently, the researchers from Institute of Modern Physics of 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 Spectrometer for Heavy Atoms and Nuclear Structure (SHANS) at the Heavy Ion Research Facility in Lanzhou (HIRFL). The a-decay properties of ²¹⁹Np were measured for the first time. This study was published in Physics Letters B.

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, 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, which gave another evidence of that there is no sub-shell closure at Z=92. 

Negative value of proton binding energy indicated 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 of Chinese Academy of Sciences and Nanjing University.