Structure of superheavy nuclei along decay chains of element 115

A recent high-resolution $\alpha$, X-ray, and $\gamma$-ray coincidence-spectroscopy experiment offered the first glimpse of excitation schemes of isotopes along $\alpha$-decay chains of $Z=115$. To understand these observations and to make predictions about shell structure of superheavy nuclei below ${}^{288}115$, we employ two complementary mean-field models: the self-consistent Skyrme energy density functional approach and the macroscopic-microscopic Nilsson model. We discuss the spectroscopic information carried by the new data. In particular, candidates for the experimentally observed $E1$ transitions in ${}^{276}\mathrm{Mt}$ are proposed. We find that the presence and nature of low-energy $E1$ transitions in well-deformed nuclei around $Z=110,N=168$ strongly depends on the strength of the spin-orbit coupling; hence, it provides an excellent constraint on theoretical models of superheavy nuclei. To clarify competing theoretical scenarios, an experimental search for $E1$ transitions in odd-$A$ systems ${}^{277}\mathrm{Mt}\mathrm{,}{}^{275}\mathrm{Hs}$, and ${}^{277}\mathrm{Ds}$ is strongly recommended.

Figure 1

Quadrupole moments $Q_2$ of even-even nuclei forming the $\alpha$-decay chain ${}^{296}120\to \cdots {\to }^{264}$Rf, calculated with unedf1${}^{\mathrm{SO}}$ and unedf1 SEDF models and the NS approach.

Figure 2

Nilsson diagram for neutrons (top) and protons (bottom) for nuclei along the $\alpha$-decay chain of ${}^{296}$120 using the MO potential of Ref. [29]. The orbits are labeled by the standard asymptotic Nilsson numbers [28]. The positive (negative) parity levels are marked by solid (dashed) lines. The Fermi levels of nuclei in Fig. 1 are indicated by dots. The quadrupole moment was determined from shape deformations ${\epsilon }_2$ and ${\epsilon }_4$ [34]: $Q_{20}=0.8A{R}_0^2({\epsilon }_2+0.5{\epsilon }_2^2+0.758{\epsilon }_4^2-{\epsilon }_2{\epsilon }_4)$ with $A=280$, $R_0=r_0{A}^{1/3}$, and $r_0=1.217$ fm.

Figure 3

Single-neutron (top) and single-proton (bottom) canonical energies of unedf1${}^{\mathrm{SO}}$ for nuclei along the $\alpha$-decay chain of ${}^{296}$120 as in Fig. 1. The orbits are labeled by the standard asymptotic Nilsson numbers corresponding to the dominant components of the SHFB canonical wave functions. The positive (negative) parity levels are marked by solid (dashed) lines. The Fermi levels are indicated by thick dotted lines.

Figure 4

Similar as in Fig. 3 but for unedf1.

Figure 5

One-q.p. spectra for nuclei forming the $\alpha$-decay chain ${}^{287}\mathrm{Lv}$ $\to \cdots \to$ ${}^{275}\mathrm{Ds}$ predicted with unedf1${}^{\mathrm{SO}}$ (upper sequence) and unedf1 (lower sequence). $Q_{\alpha }$ values for g.s.$\to$g.s. transitions are marked. The binding energy differences between different nuclei are shifted arbitrarily, whereas the excitation energies within a given nucleus are shown to scale.

Figure 6

Similar as in Fig. 5 but for the $\alpha$-decay chain ${}^{293}117$ $\to \cdots \to$ ${}^{277}\mathrm{Mt}$.

Figure 7

Results of 2-q.p.-plus-rotor NS calculations for ${}^{276}\mathrm{Mt}$. States connected with lines have the same dominating single-particle configurations; they can be interpreted as rotational bands built on 2-q.p. bandheads indicated. Solid (dashed) lines mark bands with positive (negative) parity. Positive (negative) parity neutron configurations are indicated by asterisks (triangles). The candidates for stretched $\Delta \Omega =0,\pm 1$ $E1$ transitions are marked by thick arrows.

Figure 8

Similar as in Fig. 7 but for ${}^{272}\mathrm{Bh}$.