Molecular Alpha-Particle Clustering inRa218; Dipole Collectivity in the Vicinity of Nuclear Shell Closures

Abstract

Evidence for the presence of alpha-particle clustering in 218 Ra comes from a number of observables: binding energy, S 2n9 Q a , ^(J^), 6 a 2 , and F a . That this clustering is a signature for the new dipole collectivity suggested by Iachello and Jackson follows from observation of simultaneous enhancement of selected El 9 E2, and E3 deexcitation transition matrix elements; of these the El enhancement is most pronounced as would be expected for dipole collectivity.PACS numbers: 21.10. Re, 21.60.Gx, 27.80,+w The existence of alpha clustering in the lowlying states of light nuclei, particularly in the 16 O-20 Ne region, is now well established. 1 Recently published 2 systematics for ground-state alpha-particle widths for heavy nuclei also indicate large reduced alpha-particle widths in the vicinity of the Z= 50 and 82 shell closures. It bears noting that the largest ground-state reduced alpha-particle width for any heavy nucleus yet reported is that 2 for 218 Ra; it exhausts 75% of the Wigner sum-rule limit. These large groundstate reduced widths (0 a 2 ) suggest that configurations such as a +A 2 may indeed exist in the vicinity of shell closures. If the overlap between the alpha cluster and the core nucleus (A 2 ) is small (as suggested by the large 9 a 2 ), it may be appropriate to view these states as physically real molecular alpha-particle cluster states. Such states could then be viewed as the nuclear equivalents of the HBr or HI diatomic molecules which also invoice consitituents of very different sizes.Recently, the possible importance of such alphaparticle clustering in heavy nuclei was emphasized and a phenomenological description was proposed 3 in which the cluster states are associated with a new molecular dipole degree of freedom. 4 The model is developed within the context of a spectrum-generating algebra to emphasize the intrinsic symmetries involved; in this case the cluster is characterized by the length and orientation of the vector separating the a. and A 2 centers of mass and thus is associated with a dipole degree of freedom and with 5 and P (-1 ^ l± ^1) bosons-the generators of U(4).In addition to such molecular states it would be expected, of course, to find the normal quadrupole collective states in the low-energy excitation spectra of these heavy nuclei-and indeed dipole and quadrupole states having the same J 7r would be expected to mix. The result of such mixing of the normal quadrupole ground-state band having a sequence 0 + , 2 + , 4 + , 6 + ,..., and the molecular dipole band 3 having sequence 0 + , 1 ~, 2 + , 3", 4 + ,... then leads to low-lying 1" states, as observed, 5 a sequence 0 + , 2 + , 1", 4 + , 3", again as observed, as well as large ground-state alphaparticle reduced widths and small relative alphaparticle hindrance factors for excited states. In addition, higher-lying states-0 2 + , 2 2 ,-with small hindrance factors are expected. For highspin states it remains possible that the mixing of the ground-state band and the cluster band may not be as important, leaving the i...

Identification of theJπ=1−state inRa218populated viaαParr1, Smith2, Greenlees3 et al. 2016Phys. Rev. C60101View full textAdd to dashboardCite

“…This proposed assignment is not in agreement with the previous tentatively assigned J π = 1 − state at 713 keV [10]. Also, population of the 1 − state by α decay would be expected from consideration of the intensity with which the J π = 3 − state is populated.…”

“…Energies of the 1 − and 3 − states produced by this α-particle clustering mechanism in Ra and Th were calculated by Daley and Iachello [15] and were shown to agree with the available experimental data for even 218−228 Ra and 222−230 Th. The comparison included the tentative assignment for the J π = 1 − state in 218 Ra at 713 keV, 81 keV below the J π = 3 − state [10]; a result contradicted by the present work. The inverted ordering of the J π = 1 − and 3 − levels suggests that neither α-particle clustering or rotation of an asymmetric ground state are responsible for the low-lying negative parity states in 218 Ra.…”

“…1(a), the state populated by the α decay is likely to subsequently de-excite to the 2 + state via a 404-keV γ ray. The transition energies from 3 − → 2 + and 2 + → 0 + in 218 Ra have previously been established as 404 keV and 389 keV, respectively, from in-beam studies [10][11][12][13]. The 7205-keV α-particle peak is therefore assigned to the decay which directly populates the J π = 3 − state in 218 Ra.…”

“…The ground state of 218 Ra is expected to be spherical [8], but it has been shown that at high spins, the yrast states form an alternating-parity sequence, with enhanced E1 transitions between the positive-and negative-parity states [9][10][11][12][13]. This high-spin structure is characteristic of strong octupole correlations, and it has been suggested that the octupole shape in this nucleus is stabilised by rotation [12,14].…”

“…In the 1980s, Gai et al [10] tentatively assigned the J π = 1 − state with an excitation energy of 713 keV, approximately 80 keV below the established J π = 3 − level. This observation was consistent with the interpretation that the states result from α-particle clustering, fitting well with the theoretical predictions of α-cluster models [15].…”