Progress report on research project 'Improvement and testing practical expressions for E1 strength functions and vibrational enhancement of nuclear level densities within semiclassical approach'

Full text: Numerical studies performed during period under report indicate that the calculations of the radiative strength functions within the closed-form models give similar results in a range of gamma-ray energies around the GDR peak. However the results within MLO and EGLO models are different from SLO model calculations in the low energy region. In particular, for Eγ =7 MeV, the calculated RSF values within SLO model are about two times greater comparing to the ones obtained for MLO and EGLO models (excluding the atomic mass range A Α is an element of 150-180 for EGLO model). There is a substantial distinction for some nuclei between the calculation results obtained in microscopic QRPA(RIPL2) model and semiphenomenological MLO approach and experimental data, but for QRPA(RIPL3) this discrepancy is smaller. It should be stressed that the RSF within closed-form approaches are in a rather good agreement with experimental data for gamma-ray energies near GDR peak. Note that in contrast to closed-form methods, all deformed nuclei are considered to be prolate ones in the QRPA. The overall comparison of the calculations within different models and experimental data showed that MLO and GFL provide the most reliable simple methods for determining the E1 radiative strength functions over a relatively wide energy interval ranging from zero to above the GDR peak. The MLO and GFL are not time consuming calculational routes and can be recommended for general use; both of them can be used to predict the photo-absorption cross-sections and to extract the GDR parameters from the experimental data for nuclei of middle and heavy weights. It should be noted that a collisional component of the GFL damping width can become negative in some deformed nuclei. It seems that the semi-microscopic model with moving surface (MSA) and microscopic QRPA(RIPL3) are more adequate for estimation of the RSF in spherical nuclei if reliable values of the GDR parameters are not available. Response function (RF) method was also considered and tested to describe the effect of the vibrational states on nuclear level density. The method allows to take into account the damping of vibrational states in a standard semi classical way. The calculation of the vibrational 2+ x 3- enhancement factor (K) within the RF method and different phenomenological approaches were compared. Calculations within the RF approach agree better with that ones within the methods of boson partition function with average occupation numbers (BAN) and boson partition function with damped occupation numbers (DN). They are also in rather close agreement with the results of a finite temperature extension of IBM. The calculations demonstrate rather strong dependence of the vibrational enhancement factor K on vibrational state damping. Using boson partition function with average occupation numbers (BAN) is the best method to calculate the vibrational enhancement factor when N/Z dependence of asymptotic level density parameter is taken into account. a-tilde = (0.1286A - 0.04872A2/3) / exp(0.0002986(N - Z )2). Method of boson partition function with damping occupation numbers (DN) is the best one without allowance for N/Z dependence of asymptotic level density parameter a-tilde = 0.103A - 0.105A2/3 (RIPL2). (author)