Possible experimental signature of octupole correlations in the 0${}_2^{+}$ states of the actinides

$J^{\pi }=0^{+}$ states have been investigated in the actinide nucleus ${}^{240}$Pu up to an excitation energy of 3 MeV with a high-resolution ($p,t$) experiment at $E_p$ = 24 MeV. To test the recently proposed $J^{\pi }=0_2^{+}$ double-octupole structure, the phenomenological approach of the spdf-interacting boson model has been chosen. In addition, the total 0${}^{+}$ strength distribution and the $0^{+}$ strength fragmentation have been compared to the model predictions as well as to the previously studied ($p,t$) reactions in the actinides. The results suggest that the structure of the 0${}_2^{+}$ states in the actinides might be more complex than the usually discussed pairing isomers. Instead, the octupole degree of freedom might contribute significantly. The signature of two close-lying 0${}^{+}$ states below the 2-quasiparticle energy is presented as a possible manifestation of strong octupole correlations in the structure of the 0${}_2^{+}$ states in the actinides.

Figure 1

The ${}^{242}$Pu$(p,t){}^{240}$Pu spectra at 10${}^{\circ }$ for both magnetic settings which were used for the energy calibration of the focal plane detection system. The overlap regions are marked by dashed lines. Prominent peaks are highlighted with their excitation energy in keV. In addition, the proposed double-octupole phonon $J^{\pi }=0_2^{+}$ state [10, 11] is marked with a grey shaded area.

Figure 2

Angular distributions of the (a) $J^{\pi }=0_1^{+}$ ground state, (b) $J^{\pi }=0_2^{+}$ state at 861.2(1) keV, and (c) $J^{\pi }=0_3^{+}$ state at 1090.3(1) keV in ${}^{240}$Pu. The angular distributions for the corresponding $L$ = 0 transfer calculated with the chuck3 code [28] are also shown. For the latter, two-neutron transfer configurations of orbitals close to the Fermi surface were chosen. For further details of the chuck3 calculations see, e.g., Ref. [23].

Figure 3

Running relative transfer strength as a function of energy up to the 2QP energy (solid arrows) in ${}^{228}$Th. The neutron-pairing energy ${\Delta }_n$ was calculated from the odd-even mass differences [32]. Except for ${}^{240}$Pu, the data are from Refs. [18, 23, 24].

Figure 4

Evolution of the excitation energies of the (a) $3_1^{-}$ and (b) 0${}_2^{+}$ states in actinides. (c) Energy difference $\Delta E$ of 0${}_2^{+}$ and 0${}_3^{+}$ states where known [18, 23, 24, 33].

Figure 5

(a) Experimental and (b) spdf-IBM distribution of the relative transfer strength for (firmly assigned) 0${}^{+}$ states in ${}^{240}$Pu in logarithmic scale. The boson structure predicted by the IBM is added to Fig. 5b. While $sd$ highlights a quadrupole structure of the excited 0${}^{+}$ state, 2$pf$ indicates the presence of two negative-parity bosons in the wave function. Most of the double-dipole/octupole states have a predominant double-octupole structure. In (c) the running sum of the transfer strength as a function of energy is shown.