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All natural-parity states of 18 0 have been studied with high accuracy with the 14 C( 7 Li, ty) ls O coincidence and 14 Cfo,y) 18 0 radiative-capture reactions. The four-particle, twohole 0 2 + , 1", 2 3 + , and 3 3~ states deexcite with consecutive enhanced El and E2 crossover transitions having B (El) ^ 10~2 Weisskopf units (W.u.) and B(E2) ^ 20 W.u. These data suggest the existence of an a + 14 C dipole band in 18 0 similar to those discussed recently by Iachello and Jackson.PACS numbers: 21.60. Gx, 21.10.Pc, 23.20.Ck, 27.20.+n The A = 18 system is an attractive one for study of nuclear structure 1 in that it gives access to both charge symmetry and charge independence of the nucleon-nucleon interaction, as well as interplay of the single-particle and deformed collective-quadrupole degrees of freedom. 2 " 4 Indeed, the coexistence of core-excitation deformed states [e.g., the four-particle, two-hole (4p-2h) 0 2 + state at 3.63 in 18 0] and simple twoparticle shell-model states is now well established in 18 0.Recently, it has been suggested 5 that certain nuclei may display an altogether new collective degree of freedom. When the nucleus can be described as a dinuclear molecular system, as this suggestion implies, the relevant degree of freedom is the separation vector of the nuclear centers and the pertinent variables are the length of this vector and two of the three Euler angles which define its spatial orientation. The dinuclear molecular system can be described by a classical geometrical description 6 as well as a group-theoretical algebraic picture. 5 In this latter case the molecular spectra are considered to be generated by one S boson and three P u (-1 ^ /! ^1) bosons, the generators of U(4). Two quite distinct physical situations are possible. In the first, the participant nuclei do not themselves deform and only the length of the separation vector undergoes oscillation while the entire system can rotate about its center of mass. Such motion would be expected to lead to a conventional vibration-rotation spectra involving rotational bands having spin sequences 0 + , 1", 2 + , 3", 4 + , etc.; additionally it would be expected that enhanced collective El intraband transitions would be observed in non-self-conjugate systems. 7 In the second physical situation the participant nuclei interpenetrate as the separation vector oscillates. Such motion can give rise to a spectrum having equidistant multiplets of dipole vibrational character and again enhanced collective El transitions (between states of different multiplets) are predicted in non-self-conjugate systems. 7 Recently this enhancement of the radiative widths of transitions linking molecular states has been examined in a model-independent fashion 6 and sum rules have been derived for El, E2, and E3 transitions. These sum rules together with the usual Wigner limits of reduced widths for particle decay of the presumed molecular states provide an effective signature for such structure as well as a measure of the degree of collective enhancement. They a...

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...

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