Abstract
Background: Many nuclear-structure features have been observed in
actinides in recent decades. In particular, the octupole degree of
freedom has been discussed lately after the successful measurement of
the B(E3; 0(1)(+) -> 3(1)(-)) reduced transition strength in Ra-224.
Recent results stemming from gamma-spectroscopy experiments and
highresolution (p, t) experiments suggested that strong octupole
correlations might be observed for some positiveparity states of
actinide nuclei.
Purpose: This work completes a series of (p, t) experiments on actinide
nuclei by adding the data on Pu-240. The (p, t) experiments allow us to
study low-spin states up to J(pi) = 6(+). Besides two-nucleon transfer
cross sections, spin and parity can be assigned to excited states by
measuring angular distributions, and several rotational bands are
recognized based on these assignments.
Methods: A high-resolution (p, t) experiment at E-p = 24 MeV was
performed to populate low-spin states in the actinide nucleus Pu-240.
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-channel distorted-wave Born approximation calculations.
Results: In total, 209 states have been excited in Pu-240 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, e.g., 15 new
and firmly assigned J(pi) = 0(+) states. In addition, all 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 a-cluster structure of the
0+2 state in Pu-240 has been studied in more detail. It is shown that
the 0+2 state in Th-230 has a distinctly different structure. In
addition, strongly excited 1-states have been observed at 1.5 and 1.8
MeV in Pu-240. The present study suggests that similar states might be
observed in Th-230.
Conclusions: 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 Ra-224 to Pu-240.
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