High-resolution $(p,t)$ study of low-spin states in ${}^{240}$Pu: Octupole excitations, $\alpha$ clustering, and other structure features

Abstract

A high-resolution $(p,t)$ experiment at $E_p$= 24 MeV was performed to populate low-spin states in the actinide nucleus ${}^{240}$Pu. The Q3D magnetic spectrograph of the Maier-Leibnitz Laboratory (MLL) in Munich (Germany) was used to identify the ejected tritons via $dE/E$ particle identification with its focal-plane detection system. Angular distributions were measured at nine different Q3D angles to assign spin and parity to the excited states based on a comparison with coupled-channels DWBA calculations. In total, 209 states have been excited in ${}^{240}$Pu up to an excitation energy of 3 MeV. Many previously known states have also been observed and their spin-parity assignments were confirmed. However, many of the populated states have been seen for the first time. The low-spin one-octupole phonon excitations, i.e. $K^{\pi }=0^{-},1^{-},2^{-},3^{-}$, could be observed and a new candidate for the $K=3$ projection is proposed. Furthermore, the double-octupole or $\alpha$-cluster structure of the $0_2^{+}$ state in ${}^{240}$Pu has been studied in more detail. It is shown that the $0_2^{+}$ state in ${}^{230}$Th has a distinctly different structure. In addition, strongly excited $1^{-}$ states have been observed at 1.5\,MeV and 1.8\,MeV in ${}^{240}$Pu. The present study suggests that similar states might be observed in ${}^{230}$Th. At least two different and distinct structures for $J^{\pi }=0^{+}$ states are present in the actinides. These are pairing states and states with enhanced octupole correlations. We have shown that it is crucial to consider negative-parity single-particle states being admixed to some $K^{\pi }=0_2^{+}$ rotational bands to understand the $\alpha$-decay hindrance factors and enhanced $E1$-decay rates. Based on our analysis, we have identified the double-octupole or $\alpha$-cluster $K^{\pi }=0^{+}$ candidates from ${}^{224}$Ra to ${}^{240}$Pu.

Comments: This manuscript has been published in Phys. Rev. C 97, 064319 (2018). This is the preprint version