New $\alpha$-Emitting Isotope ${}^{214}\mathrm{U}$ and Abnormal Enhancement of $\alpha$-Particle Clustering in Lightest Uranium Isotopes

A new $\alpha$-emitting isotope ${}^{214}\mathrm{U}$, produced by the fusion-evaporation reaction ${}^{182}\mathrm{W}({}^{36}\mathrm{Ar},4\mathrm{n}){}^{214}\mathrm{U}$, was identified by employing the gas-filled recoil separator SHANS and the recoil-$\alpha$ correlation technique. More precise $\alpha$-decay properties of even-even nuclei ${}^{216,218}\mathrm{U}$ were also measured in the reactions of ${}^{40}\mathrm{Ar}$, ${}^{40}\mathrm{Ca}$ beams with ${}^{180,182,184}\mathrm{W}$ targets. By combining the experimental data, improved $\alpha$-decay reduced widths ${\delta }^2$ for the even-even Po-Pu nuclei in the vicinity of the magic neutron number $N=126$ are deduced. Their systematic trends are discussed in terms of the $N_p{N}_n$ scheme in order to study the influence of proton-neutron interaction on $\alpha$ decay in this region of nuclei. It is strikingly found that the reduced widths of ${}^{214,216}\mathrm{U}$ are significantly enhanced by a factor of two as compared with the $N_p{N}_n$ systematics for the $84\le Z\le 90$ and $N<126$ even-even nuclei. The abnormal enhancement is interpreted by the strong monopole interaction between the valence protons and neutrons occupying the $\pi 1f_{7/2}$ and $\nu 1f_{5/2}$ spin-orbit partner orbits, which is supported by the large-scale shell model calculation.

Figure 1

(a) Energy spectrum for $\alpha$-decay events following recoil implantations within a time window of 10 ms. (b) Two-dimensional plot of mother and daughter $\alpha$-particle energies for $\mathrm{ER}-{\alpha }_m-{\alpha }_d$ correlations in the ${}^{36}\mathrm{Ar}+{}^{182}\mathrm{W}$ reaction. Maximum search times for the $\mathrm{ER}-{\alpha }_m$ and ${\alpha }_m-{\alpha }_d$ pairs are 10 ms and 50 s, respectively. The decay events from the new isotope ${}^{214}\mathrm{U}$ labeled as “chain 1” and “chain 2” are indicated by red arrows. The ${}^{210}\mathrm{Th}$ event is from the decay of daughter nucleus of ${}^{214}\mathrm{U}$ in chain 1. The details of these decay chains can be found in Fig. 2.

Figure 2

Observed $\alpha$-decay chains for ${}^{214}\mathrm{U}$. For each chain, the implantation energy of ERs ($E_{\mathrm{imp}}$), the $\alpha$-particle energy ($E_{\alpha }$), the decay time ($T$), and the position ($P$) in the strip detector are shown. The reconstructed energies for escaping $\alpha$ decays are given as the sum of the PSSD and SSD energies.

Figure 3

(a) Systematics of reduced widths for g.s.-to-g.s. $\alpha$ decays of even-even $84\le Z\le 94$ isotopes as a function of neutron number. The decay properties are taken from Refs. [11, 12, 42, 50, 51, 52, 53]. The values for ${}^{214,216,218}\mathrm{U}$ from this work are shown by filled circles. The errors of reduced widths are only determined by half-life uncertainties. (b) Same as (a) but against $N_p{N}_n$ for even-even Po to U isotopes. The $N_p$ and $N_n$ values are calculated relative to $Z=82$ and $N=126$ closed shells, respectively, with an exception of ${{}_{84}{}^{186}\mathrm{Po}}_{102}$, for which $N_n=-20$, relative to the closest $N=82$ neutron shell.

Figure 4

(a) Calculated proton occupation numbers for the $\pi 0h_{9/2}$ (square), $\pi 1f_{7/2}$ (circle), and $\pi 0i_{13/2}$ (triangle) orbits in $N=122$, 124, and 126 even-$Z$ isotones of Po-Pu. (b) Monopole matrix elements of $p\text{−}n$ interaction calculated for the $Z>82$ and $N\le 126$ nuclei. The inset in panel (b) shows the single-particle orbits near the ${}^{208}\mathrm{Pb}$ doubly closed shells, and the strong interaction between the $1f_{7/2}$ protons and $1f_{5/2}$ neutrons is marked.