Abstract
A theory of photonuclear reactions is formulated using a projection-operator formalism. We obtain a matrix describing a direct photoeffect and a resonance reaction. By introducing doorway and secondary-doorway states, we can conveniently study the structure and energy dependence of the matrix.
The formalism is applied to the analysis of the photonuclear cross sections of . The () and () cross sections are calculated. We consider those channels in which the residual nucleus is left in the ground state () or the third excited state (). In the shell-model formulation, the doorways are taken to be mixtures of 1p-1h states, which are constructed in the Tamm-Dancoff approximation. The secondary doorways are assumed to be 3p-3h states, which are constructed in the interacting-boson approximation of Iachello and Feshbach. By mixing the doorways and the secondary doorways, we obtain a microscopic description of the compound states formed in the reactions. The doorways are shown to be responsible for the gross structure of the giant dipole resonances, while their couplings to the secondary doorways give rise to intermediate structure. A particular model of the 3p-3h states, together with certain simplifications in the description of the reaction, reproduces some of the experimental data (the photodisintegration to the ground state) to a surprising degree of accuracy.
The calculation evidently shows the importance of the 3p-3h admixture in the low-lying odd-parity states of . Our results also give strong support to assigning nature to the resonances at 21.0, 22.3, 23.1, 24.2, 25.2, and 25.6 MeV.
- Received 28 October 1971
DOI:https://doi.org/10.1103/PhysRevC.5.1898
©1972 American Physical Society