Dipole Molecular Degrees of Freedom in Silicon -28 and OXYGEN-18 and Their Description Within a Four-Dimensional Unitary Group Symmetry
The discovery by Bromley et al., of sharp resonances having very large carbon reduced widths, at low energy in the ('12)C + ('12)C system, led them to suggest that these resonances reflect the existence of nuclear molecular states at high excitations in ('24)Mg. Many studies--both experimental and theoretical--have been carried out subsequently in order to map and explain these resonances. Using an algebraic model developed by Iachello to describe dinuclear configurations, Erb and Bromley reported a very successful analysis of all the ('12)C + ('12)C data. In this model the molecular configuration arises from a new dipole degree of freedom, and the predicted signature of this configurations is the existence of rotation-vibration bands having spin sequences: 0('+), 1('-), 2('+), 3('-)...with enhanced intraband E1 transitions. Negative parity states can only be observed, however, in a non symmetric system; thus, we have undertaken a detailed study of the ('16)O + ('12)C system. It has been found that resonances above the Coulomb barrier cannot be considered as of simple dinuclear character. Consequently, in this work resonances in the ('16)O + ('12)C system were studied at, and below, the Coulomb barrier at energies 7 MeV (LESSTHEQ) E(,CM) 10.4 MeV. Forty -six angular distributions of the ('12)C(('16)O,(alpha)(,0))('24)Mg reaction have been measured. Spin assignments have been made for eleven resonances and elastic and alpha particle reduced partial widths have been extracted for six of them, all of which support molecular dinuclear configuration for these resonances. It has been suggested that such a dipole molecular degree of freedom also may play a role in bound states of nuclei. Such bound molecular states would be expected to involve two relatively stiff, strongly bound, nuclei, as for example: ('8)Be((alpha) + (alpha)), ('20)Ne((alpha) + ('16)O), ('18)O((alpha) + ('14)C), etc. In ('8)Be negative parity ((alpha) + (alpha)) dinuclear states cannot exist, of course, for symmetry reasons, and since E1 transitions are isospin forbidden in first order in ('20)Ne, we chose to study the excitation spectrum of ('18)O and, more specifically, the electromagnetic deexcitation of all the low-lying states of ('18)O. Evidences for molecular configurations in ('18)O were found by the measure of enhanced selective E1 intraband transitions via the ('14)C(('7)Li,t(gamma))('18)O reaction. (Abstract shortened with permission of author.).
- Pub Date:
- HEAVY IONS;
- Physics: Nuclear