Coexistence of Single-Particle, Collective-Quadrupole, andα+C14Molecular-Dipole Degrees of Freedom in

Abstract

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 Triangular D3h Symmetry in C12Marín-Lámbarri1, Bijker2, Freer3 et al. 2014Phys. Rev. Lett.Self Cite204142442View full textAdd to dashboardCite

“…Here the excitation energy in 12 C was calculated by reconstructing E x ( 12 C) from the 8 Be and either the detected (horizontal axis) or undetected (vertical axis) α-particle. The horizontal and vertical loci correspond to 12 C excited states and the weak diagonal loci states in 8 Be.…”

Isoscalar dipole transition as a probe for asymmetric clustering

“…In this section, by using the shell-model and cluster-model wave functions, we estimate the magnitude of the IS dipole transition matrix between the ground and excited 1 − states of 20 Ne and 44 Ti having asymmetric cluster structure with α + 16 O and α + 40 Ca configurations. By assuming that the ground state is described by a shellmodel wave function, we first derive an analytical expression for the transition matrix and show that it is comparable with the Weisskopf estimate.…”

“…In the case of N = Z nuclei, the enhanced electric-dipole (E1) transition was suggested as a probe for such 1 − states [42]. The 1 − cluster states with the α + 14 C configuration in 18 O were experimentally searched by using the E1 strength as a probe [43,44], in addition to the ordinary experimental methods such as α scattering and breakup [45][46][47][48]. The α clustering in the actinides [49][50][51] were also investigated in the same literature.…”

“…For this purpose, by using α + 16 O and α + 40 Ca cluster states in 20 Ne and 44 Ti as examples, we investigate the excitation mechanism of the IS dipole transition from the ground state to the asymmetric cluster states. We first discuss an analytic expression of the IS transition matrix from the shell-model ground state to the cluster states.…”

“…To provide more realistic prediction of the IS dipole transition strength in 20 Ne and 44 Ti, we also performed microscopic nuclear model calculations by using the generator coordinate method with the Brink-Bloch wave function (Brink-Bloch GCM) and antisymmetrized molecular dynamics (AMD) [58,59]. AMD is able to describe the distortion of the clusters and reasonably explains the observed excitation spectra of the ground band and cluster bands for both nuclei.…”