Octupole collectivity in the Sm isotopes

Microscopic models suggest the occurrence of strong octupole correlations in nuclei with $N\approx 88$. To examine the signatures of octupole correlations in this region, the spdf interacting boson approximation model is applied to Sm isotopes with $N=86\text{−}92$. The effects of including multiple negative-parity bosons in this basis are compared with more standard one negative-parity boson calculations and are analyzed in terms of signatures for strong octupole correlations. It is found that multiple negative-parity bosons are needed to describe properties at medium spin. Bands with strong octupole correlations (multiple negative-parity bosons) become yrast at medium spin in ${}^{148,150}\mathrm{Sm}$. This region shares some similarities with the light actinides, where strong octupole correlations were also found at medium spin.

Figure 1

The energies for the three types of bosons, $p,d$, and f, exhibit a smooth behavior across the Sm isotopes with masses ranging from A = 148 to A = 154.

Figure 2

Comparison of experimental values [14, 17, 15, 16] for energies of the first $K^{\pi }=0^{\pm }$ states (symbols) in ${}^{148\text{−}154}\mathrm{Sm}$ with the calculated energies in our Hamiltonian (curves).

Figure 3

Experimentally observed (left) and theoretically calculated (right) low-lying states in ${}^{148}\mathrm{Sm}$. Calculated bands with a higher content of negative-parity bosons are labeled with “pf.”

Figure 4

Same as Fig. 3, but for ${}^{150}\mathrm{Sm}$.

Figure 5

Same as Fig. 3, but for ${}^{152}\mathrm{Sm}$.

Figure 6

Same as Fig. 3, but for ${}^{154}\mathrm{Sm}$.

Figure 7

(Top) Experimental signature-splitting index $S(J)$ (symbols) for ${}^{148}\mathrm{Sm}$ (left) and ${}^{150}\mathrm{Sm}$ (right) compared with theory with $N_{\mathit{pf}}=1$ and 3 (lines). (Bottom) Ratio of electric dipole and quadrupole strengths for the same nuclei. The effective charges, $e_1=0.010\hspace{1em}e\hspace{1em}{b}^{1/2}$ and $e_2=0.13\hspace{1em}e\hspace{1em}b$, are the same for both isotopes.

Figure 8

Experimental signature-splitting index $S(J)$ (symbols) for ${}^{152}\mathrm{Sm}$ and ${}^{154}\mathrm{Sm}$ compared with theory with $N_{\mathit{pf}}=1$ (lines).

Figure 9

Comparison between experimental $B(E2)$ strengths from the 0${}_1^{+}$ bands in ${}^{148\text{−}154}\mathrm{Sm}$ and theory, with a constant effective charge $e_2=0.13$ e b [9].

Figure 10

Comparison of experimental (points) and predicted (lines) ratios of electric dipole strengths for ${}^{150,152,154}\mathrm{Sm}$.

Figure 11

Parameters of the $\stackrel{\wedge }{T}(E1)$ operator vary in only a small range from −0.25 to −0.35 for ${}^{148\text{−}154}\mathrm{Sm}$.

Figure 12

Evolution of absolute $B(E1)$ strengths in ${}^{148\text{−}154}\mathrm{Sm}$ for $1^{-}\to 0^{+}$ and $3^{-}\to 2^{+}$ transitions. The effective charge $e_1=0.010\hspace{1em}e\hspace{1em}{b}^{1/2}$ is kept constant for all Sm isotopes.

Figure 13

Ratio of expectation values for the number of p to the number of f bosons in the low-lying negative-parity states. The octupole nature in ${}^{148\text{−}150}\mathrm{Sm}$ is evident by the lack of p boson content. These are also the nuclei that exhibit alternating parity bands. The similarity of p and f boson content in ${}^{152\text{−}154}\mathrm{Sm}$ states is a reflection of the Hamiltonian nearing the (SU${}_{\mathit{spdf}}$) rotational limit in the full boson space. These near-rotational nuclei do not exhibit the alternating parity bands.