Giant dipole states and excited dipole states of12C in the continuum approximation

Abstract

Experimental invest igat ion of the giant resonance of 12C by using the riB(p, ~/0)12C and the nB(p, y~)~C* reactions, where the final states of 12C are ground and first 2 + excited states (1), has shown the impor tance of the coupling of surface vibrat ion with the giant dipole states in photonuclear reactions. The main peak of the riB(p, y1)~2C * reaction lies about 4 MeV higher than the corresponding peak of the l~B(p, y0)12C reaction (2), in agreement with the energy of the 2 + s tate of 1~C, but, the giant resonance based on the excited s tate having a broader width than the one for the ground state, the observed correlations between the excita t ions functions for the two reactions and the peak at 25.5 MeV, which is not reproduced by the calculations of, e.g., VINH-MAU and B~owN (3) and •ARANGONI and SARUIS (4) and GILL]~T and VIN~-MAu (5) in the lp lh model, suggest the break-down of the adiabat ic approximation and the presence of strong coupling between the dipole and quadrupole modes influencing the observed structure. To explain these features KA~I1MURA et al. (2) performed a Tamm-Dancoff type calculation using a representat ion of bound states in an infinite potent ia l well. Considering tha t some of the single-particle s tates having posit ive energy should be t rea ted as scattering resonances ra ther than discrete levels, we have developed a generalization of the coupled-channel approach of Buck and HILL (~). This involves taking into account the 2p-2h states which are re levant to this par t icu lar reaction. I t is a well-known fact tha t higher configurations can be introduced, at least formally, in the coupled-channel formalism. This leads to a very complex system of coupled integro-different ial equations containing the inverse of a matr ix , singular on isolated points of the real E-axis, Such an approach has never been used in pract ice (7), but simplifications occur in the case of x2C, where the first 2 + excited state, though vibra t ional in character, can be described with good approximat ion b y the simple (lP 89 lp~ 1) configuration (2).