High-resolution (p,t) study of low-spin states in 240Pu: Octupole excitations, α clustering, and other structure features
Abstract
A high-resolution (p,t) experiment at Ep= 24 MeV was performed to populate low-spin states in the actinide nucleus 240Pu. 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 240Pu 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. Kπ = 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 240Pu has been studied in more detail. It is shown that the 0+2 state in 230Th has a distinctly different structure. In addition, strongly excited 1- states have been observed at 1.5\,MeV and 1.8\,MeV in 240Pu. The present study suggests that similar states might be observed in 230Th. At least two different and distinct structures for Jπ = 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 Kπ = 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 Kπ = 0+ candidates from 224Ra to 240Pu.
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