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Octupole correlations, resulting from the long-range, octupole-octupole interaction between nucleons occupying pairs of orbitals with Δj=Δl=3 have been proved to be one of the major cornerstone in nuclear structure physics. However, the evidence for static octupole deformation leading to a heart-shaped nucleus has been rare [1]. The band structure involving alternate parity states and associated with static octupole deformation has recently been explained by invoking condensation of rotational- aligned-octupole-phonons [1] having their angular momentum aligned with the rotational axis. In an ideal scenario, the synchronization of the rotations of the condensate and quadrupole shape of the nuclei results in a collective motion that corresponds to the rotation of a static octupole-deformed nucleus.
Recently, the behaviors of yrast positive and negative parity states in $^{220}$Ra have been found to be consistent with the interpretation of condensation of octupole phonons [1]. Subsequently, strong octupole correlations observed in $^{240}$Pu has been successfully interpreted employing this concept [2]. Apart from these two cases, static octupole deformation has remained rather elusive. Excited levels in the candidate $^{150}$Sm nucleus were populated employing the $^{150}$Nd(α,4n) reaction at a beam energy of 52 MeV from the K-130 cyclotron at VECC, Kolkata. Deexciting γ-rays were detected using the INGA spectrometer.
The data have revealed significant new information on the high-spin states of this nucleus. The positive parity ground state band (gsb) has been extended up to spin, $I = 22^+$. The negative parity band (npb) has been confirmed up to its previously reported spin ($I^π = 21^-$). Considering all the evidence, it was suggested earlier that above spin I=8, $^{150}$Sm shows very strong octupole correlations probably developing an octupole shape [3]. The most significant finding of the present work is the reemergence of strong interband E1 transitions linking the states of the gsb and npb above spin 17ħ where the two bands appear to merge as a single-sequence. This observation is similar to the $^{240}$Pu where the octupole phonons condensate at higher spin. The measured branching ratios (B(E1)/B(E2)) further corroborate the return of strong octupole correlation at high-spin in $^{150}$Sm. The probable 2-phonon band has been extended, and all three of them (0-, 1- and 2-phonon) have been found to approach each other with the 2-phonon band merging with the 1-phonon at the highest observed rotational frequency. All these experimental evidences suggest that 150Sm is only the 3rd nuclei (and the first lanthanide) in the periodic table to be consistent with the picture of octupole phonon condensation after $^{220}$Ra and $^{240}$Pu.
[1] PHYSICAL REVIEW C77,021304(R)(2008).
[2] PHYSICAL REVIEW LETTERS102,122501(2009).
[3] PHYSICS LETTERS B185,331(1987).