Collective effects in the level density are not well understood, and including these effects as enhancement factors to the level density does not produce sufficiently consistent predictions of observables. Therefore, collective effects are investigated in the level density parameter instead of treating them as a final factor in the level density. A new Laplacelike formula is proposed for the energy dependence of the level density parameter, including collective effects. A significant improvement has been achieved in agreement between observed and predicted energy levels. This new model can also be used in both structure and reaction calculations of the nuclei far from stability, especially near the drip lines.

In the present study, the effect of the pairing interaction and the isovector correlation between nucleons on the properties of the isobar analog resonances (IAR) in 112−124 Sb isotopes and the isospin admixture in 100−124 Sn isotopes is investigated within the framework of the quasiparticle random phase approximation (QRPA). The form of the interaction strength parameter is related to the shell model potential by restoring the isotopic invariance of the nuclear part of the total Hamiltonian. In this respect, the isospin admixtures in the 100−124 Sn isotopes are calculated, and the dependence of the differential cross section and the volume integral J F for the Sn( 3 He,t)Sb reactions at E( 3 He)= 200 MeV occurring by the excitation of IAR on mass number A is examined. Our results show that the calculated value for the isospin mixing in the 100 Sn isotope is in good agreement with Colo et al.'s estimates (4 − 5%), and the obtained values for the volume integral change within the error range of the value reported by Fujiwara et al. (53±5 MeV fm 3 ). Moreover, it is concluded that although the differential cross section of the isobar analog resonance for the ( 3 He,t) reactions is not sensitive to pairing correlations between nucleons, a considerable effect on the isospin admixtures in N ≈ Z isotopes can be seen with the presence of these correlations.

The ω-meson photoproduction, γ + p → p + ω, is studied in the framework of a model, containing π-meson exchange in t-channel and nucleon-exchange in s-and u-channels. Considering both ωN N -coupling constants in the region of time-like meson four momenta as the free parameters, we find different sets of solutions for these constants from the existing data on the t-dependence of the differential cross sections, dσ(γ + p → p + ω)/dt, in the near threshold region E γ ≤ 2 GeV. These sets of ωN N -coupling constants, corresponding to destructive and constructive π N -interference contributions to dσ/dt can be well distinguished by measurements of beam asymmetry, induced by linear photon polarization.

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