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The coupled channels approach in a shell model intermediate coupling developed earlier is reformulated in such a way that the distortions in the description of the nucleon–nucleus scattering states owing to the centre of mass motion of the nucleus are almost fully corrected.

The coupled channels approach in the shell model intermediate coupling is presented. The problems of orthonormalization of the solutions, fulfillment of boundary conditions, agreement of the effective two-body intercation with the nuclear average field and continuum discretization are considered. It is shown that in light and medium nuclei the description of the structure and decay characteristics of multipole giant resonances can be reduced to the solution of a compact algebraical system of equations. The model is applied to the description of the photonuclear reactions on the 24 Mg, 28 Si and 32 S nuclei. The origin of the gross and intermediate structure of the dipole giant resonance (DGR) as well as the partial decay channels of the DGR are studied. It is found that the 24 Mg DGR splitting results from the nuclear shape deformation.

A simple, but sufficiently realistic, model of nucleon-nucleus scattering with a separable effective interaction is formulated. A modified random-phase approximation (RPA) is used to make allowance for the coupling of single-hole states to other nuclear modes. Nucleon-nucleus scattering is described using the Chew-Low-Wick integral equation. The model permits the nucleon-nucleus S -matrix to be expressed explicitly through the variables obtained either from standard bound-state shell-model calculations or from experimental spectroscopic data. The calculations of the differential cross sections of photonucleon reactions are examined as an example of the application of the model.

A modified RPA suitable for open-shell nuclei is constructed using the Green function method. Unlike the standard RPA, this modification makes allowance for the spread of bound single-particle and single-hole states over more complex configurations which is done due to a more realistic approximation of single-particle Green functions. The calculations of the nucleon-nucleus S -matrix and of the probability amplitudes for the single-particle transitions of the nucleus from the ground state into scattering states are examined. For the separable effective forces these quantities are expressed in closed form.

In the generalized ph basis generated for the realistic nuclear ground state, the nuclear hamiltonian is approximated by an expression containing the separable effective forces and terms which can be estimated from the spectroscopic data. The model hamiltonian obtained is used to describe normal nuclear vibrations. The application of this approach in the shell-model theory of nuclear reactions is discussed. The S -matrix for nucleon-nucleus scattering is calculated explicitly. The 12 C photodisintegration calculation is made as a test example.

Using the semimicroscopic nuclear vibration model and the combined model for giant resonance decay, we have calculated the giant dipole resonance structure and the partial photo-nucleon cross sections for 32S. The theoretical parameters were estimated from the spectroscopic data. The calculation satisfactorily describes the structure and decay characteristics of the 32S giant resonance. It has been shown that the statistical mechanism plays a significant role in the photodisintegration of 32S.

The energy spectra of photoprotons from 26Mg have been measured with bremsstrahlung at five values of the end-point energy Eγmax, namely, at 19.5, 20.5, 22.0, 24.5 and 27.0 MeV. Using the difference spectra technique, we have determined the main decay channels of the states of the giant dipole resonance (GDR) of 26Mg. The photoproton spectra have been used to obtain the cross sections for the formation of the final nucleus of the 26Mg(γ, p)25Na reaction in its various states. The magnitude of the configurational splitting of the GDR of 26Mg has been estimated. Relevant data from other studies have been utilized to calculate the isospin components of the GDR of 26Mg within the framework of a combined model for the decay of the giant resonance, with isospin taken into account. A new isospin interpretation of the cross sectios for the photoproton and photoneutron reactions on the 26Mg nucleus is proposed.

A simple model for decay of the giant dipole resonance is described that takes into account the semidirect, pre-equilibrium, and equilibrium mechanisms of the photonucleon emission. The semidirect photoeffect is treated within the framework of the R -matrix theory in the shell model. The wave function for the doorway dipole state is constructed using the generator procedure that offers the possibility to obtain simple formulas for a calculation of the semidirect component of the photonucleon spectra. The problem of the inclusion of isospin effects is treated. A reasonable agreement between theoretical and experimental photoproton spectra has been obtained.