New Isotope ${}^{220}\mathrm{Np}$: Probing the Robustness of the $N=126$ Shell Closure in Neptunium

A new short-lived neutron-deficient isotope ${}^{220}\mathrm{Np}$ was synthesized in the fusion-evaporation reaction ${}^{185}\mathrm{Re}({}^{40}\mathrm{Ar},5n){}^{220}\mathrm{Np}$ at the gas-filled recoil separator SHANS. Based on the measurement of the correlated $\alpha$-decay chains, the decay properties of ${}^{220}\mathrm{Np}$ with $E_{\alpha }=10040(18)\mathrm{keV}$ and $T_{1/2}=25_{-7}^{+14}\mu \mathrm{s}$ were determined, which are in good agreement with theoretical predictions. From the new experimental results coupled with the recently reported $\alpha$-decay data of ${}^{219,223}\mathrm{Np}$, the $\alpha$-decay systematics for Np isotopes around $N=126$ was established, which allows us for the first time to test the robustness of the $N=126$ shell closure in $Z=93$ Np isotopes. The results also indicate that, in the region of nuclei with $Z\ge 83$, the proton drip line has been reached for all odd-$Z$ isotopes up to Np.

Figure 1

(a) Energy spectrum for $\alpha$-decay events following recoil implants within a time window of 1 ms. (b) Two-dimensional plot of mother and daughter $\alpha$-particle energies for correlated $\mathrm{ER}-{\alpha }_m-{\alpha }_d$ events measured in the PSSDs. Maximum searching times for the $\mathrm{ER}-{\alpha }_m$ and ${\alpha }_m-{\alpha }_d$ pairs are 1 ms and 8 s, respectively. The decay events from the new isotope ${}^{220}\mathrm{Np}$ are labeled by red ellipses.

Figure 2

Left: Energy spectra of the correlated decay events from ${}^{220}\mathrm{Np}$. The events with the same color represent one correlated decay chain. The reconstructed $\alpha$-decay events (see Table 1) are labeled by red star markers. The measured $\alpha$-particle energies for ${}^{220}\mathrm{Np}$ and the literature values [26, 27] for its descendants are indicated. Right: Decay-time distributions for each chain member. The solid curves indicate the decay-time distribution functions, for which the mean lifetimes of ${}^{220}\mathrm{Np}$ and its descendants are taken from this work and Refs. [26, 28], respectively.

Figure 3

Systematics of (a) $\alpha$-decay $Q_{\alpha }$ values, (b) partial $\alpha$-decay half-lives $T_{1/2}^{\alpha }$ of g.s. to g.s. transitions for neutron-deficient $89\le Z\le 93$ isotopes, and (c) proton separation energies $S_p$ for odd-$Z$ Ac, Pa, and Np isotopes as a function of neutron number. Open symbols refer to the literature values taken from Refs. [12, 18, 19, 26, 32, 33, 34, 35, 36, 37, 38, 39]. The ${}^{220}\mathrm{Np}$ values from this work are marked by red solid squares. The dotted lines show the theoretical predictions taken from Refs. [40, 41, 42, 43]. For further discussions, see text.