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arXiv · 1807.07139

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

M. Spieker·S. Pascu·D. Bucurescu·T. M. Shneidman·T. Faestermann·R. Hertenberger·H. -F. Wirth·N. -V. Zamfir·A. Zilges
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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. $K^π = 0^-,1^-,2^-,3^-$, could be observed and a new candidate for the $K = 3$ projection is proposed. Furthermore, the double-octupole or $α$-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 $J^π = 0^+$ 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 $K^π = 0^+_2$ rotational bands to understand the $α$-decay hindrance factors and enhanced $E1$-decay rates. Based on our analysis, we have identified the double-octupole or $α$-cluster $K^π = 0^+$ candidates from $^{224}$Ra to $^{240}$Pu.

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M. Spieker, S. Pascu, D. Bucurescu, T. M. Shneidman, T. Faestermann, R. Hertenberger, H. -F. Wirth, N. -V. Zamfir, A. Zilges. 2018-07-18. High-resolution $(p,t)$ study of low-spin states in $^{240}$Pu: Octupole excitations, $α$ clustering, and other structure features. https://doi.org/10.1103/physrevc.97.064319

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