Short-Lived $\alpha$-Emitting Isotope ${}^{222}\mathrm{Np}$ and the Stability of the $N=126$ Magic Shell

A new, very short-lived neutron-deficient isotope ${}^{222}\mathrm{Np}$ was produced in the complete-fusion reaction ${}^{187}\mathrm{Re}({}^{40}\mathrm{Ar},5\mathrm{n}){}^{222}\mathrm{Np}$, and observed at the gas-filled recoil separator SHANS. The new isotope ${}^{222}\mathrm{Np}$ was identified by employing a recoil-$\alpha$ correlation measurement, and six $\alpha$-decay chains were established for it. The decay properties of ${}^{222}\mathrm{Np}$ with $E_{\alpha }=10016(33)\mathrm{keV}$ and $T_{1/2}=380_{-110}^{+260}\mathrm{ns}$ were determined experimentally. The $\alpha$-decay systematics of Np isotopes is improved by adding the new data for ${}^{222}\mathrm{Np}$, which validates the $N=126$ shell effect in Np isotopes. The evolution of the $N=126$ shell closure is discussed in the neutron-deficient nuclei up to Np within the framework of $\alpha$-decay reduced width.

Figure 1

Two-dimensional scatter plot of parent and daughter $\alpha$-particle energies for correlated ER-$\alpha 1\text{−}\alpha 2$ events measured in the PSSDs. Searching times for the ER-$\alpha 1$ and $\alpha 1\text{−}\alpha 2$ pairs are $10\mu \mathrm{s}$ and 1 ms, respectively. The decay events from the new isotope ${}^{222}\mathrm{Np}$ are labeled by red dots.

Figure 2

Energy spectra and decay curves of $\alpha$ particles emitted from ${}^{220}\mathrm{Pa}$ and ${}^{219}\mathrm{Th}$. The data were obtained by using a pulse shape fitting method given in Ref. [14].

Figure 3

Correlated $\alpha$-decay chains of ${}^{222}\mathrm{Np}$ in the present work. Each chain is labeled with an identification number, strip number of PSSDs, implantation energy of ER events, decay energies, time intervals, and vertical positions. All the events in each chain were observed within a position window of $\pm 2.0\mathrm{mm}$ for full energy deposited in the PSSDs. Case marked “missing $\alpha$” denotes an event with no-energy collected in the implantation PSSDs.

Figure 4

Examples of traces where ${}^{222}\mathrm{Np}$ was registered (dotted lines). The red solid lines are the fitted curves based on the method in Ref. [14]. Right panel: enlarged traces for the rising edge region.

Figure 5

(a) Systematics of $\alpha$-decay $Q_{\alpha }$ values and (b) $\alpha$-decay half-lives $T_{1/2}^{\alpha }$ of the g.s. to g.s. transitions for neutron-deficient $89\le Z\le 93$ isotopes as a function of proton number and neutron number. The solid spheres refer to literature values taken from Refs. [5, 6, 8, 9, 16, 20, 21]. The red bigger solid spheres represent the results measured in the present work. The solid lines refer to the theoretical predictions taken from Refs. [22, 23, 24].

Figure 6

Reduced $\alpha$-decay widths ${\delta }^2$ of even-$N$ (a) and odd-$N$ (b) Po-Np isotopes as a function of proton number. The reduced widths of ${}^{219}\mathrm{Pa}$, ${}^{219}\mathrm{U}$, and ${}^{215}\mathrm{At}$ were deduced from the latest data [6, 28, 29]. The red dots represent the values of Np isotopes deduced from Refs. [5, 6, 8] and this Letter.