R. Razavi

The entropy excess and moment of inertia excess ratio with inclusion of statistical pairing fluctuations

The entropy excess of 163Dy compared to 162Dy as a function of nuclear temperature have been investigated using the mean value Bardeen–Cooper–Schrieffer (BCS) method based on application of the isothermal probability distribution function to take into account the statistical fluctuations. Then, the spin cut-off excess ratio (moment of inertia excess ratio) introduced by Razavi [Phys. Rev. C88 (2013) 014316] for proton and neutron system have been obtained and are compared with their corresponding data on the BCS model. The results show that the overall agreement between the BCS model and mean value BCS method is satisfactory and the mean value BCS model reduces fluctuations and washes out singularities. However, the expected constant value in the entropy excess is not reproduced by the mean value BCS method.

Microscopic study of nuclear level densities and thermodynamical properties in 170,171,172Yb

The nuclear level density and entropy were extracted for 170,171,172Yb isotopes within microscopic theory, which includes nuclear pairing interaction. The calculations were based on the modified harmonic oscillator according the Nilsson potential for deformed nuclei. From the level densities, thermodynamical quantities such as temperature and entropy have been deduced. The entropies of the neutron hole and particle are estimated from the entropy difference between the odd-mass and even–even nuclei. The critical temperature of the pair-breaking process is found to be Tc ~ 0.4–0.5 MeV. The results were compared with experimental data obtained by the Oslo group.

Studying temperature dependence of pairing gap parameter in a nucleus as a small superconducting system

In this paper, we have taken the effect of small size of nucleus and static fluctuations into account in the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity calculations of 45Ti nucleus. Thermodynamic quantities of 45Ti have been extracted within the BCS model with the inclusion of the average value of the pairing gap square, extracted by the modified Ginzburg–Landau (MGL) method for small systems. Calculated values of the excitation energy and entropy within the MGL+BCS method improve the extracted results within the usual BCS model and show a smooth behavior around the critical temperature with a very good agreement with the semi-empirical values. The result of using MGL+BCS method for the heat capacity of 45Ti is compared with the corresponding semi-empirical values and the calculated values within the BCS, static path approximation (SPA) and Modified Pairing gap BCS (MPBCS) which is a method that was proposed in our previous publications. Both MGL+BCS and MPBCS avoid the discontinuity of the heat capacity curve, which is observed in the usual BCS method, and lead to an S-shaped curve with a good agreement with the semi-empirical results.

Breaking of Cooper pairs in 108Pd

In this paper, breaking of Cooper pairs in 108Pd is investigated within the canonical ensemble framework and the BCS model. Our results show an evidence of two phase transitions, which are related to neutron and proton systems. Also, with consideration of pairing interaction, the role of neutron and proton systems in entropy, spin cutoff parameter and as a result in the moment of inertia are investigated. The results show minor role for the proton system at low temperatures and approximately equal roles for both neutron and proton systems after the critical temperature. Good agreement was observed between obtained results and the experimental data.

Excited quasiparticles and entropy in 161,162Dy

In this paper, the nuclear level densities of 161,162Dy is studied by the use of a microscopic theory which includes nuclear pairing interaction. It is based on the modified harmonic oscillator model according to the Nilsson potential. The entropy of even–odd and even–even nuclei as a function of nuclear temperature is obtained. The entropy excess of 161Dy is compared with that of 162Dy. It is concluded that the difference is related to the entropy carried by the neutron hole coupled to the even–even core. The numbers of excited quasiparticles are calculated. Good agreement was observed between calculated results and the experimental data.

Temperature dependent pairing gap in nuclei

Based on theoretical models and experimental evidences of a soft phase transition, with significant, but not abrupt changes in pairing gap of nuclei, a modified pairing gap function was suggested and examined for some of nuclei in our previous publications. Now, we propose a new method to extract the parameters, which exist in the modified pairing gap function. This method is based on phenomenological models, which consider the small size of nuclei and the effect of thermal fluctuations, at the vicinity of the critical temperature. The obtained pairing gap function by this method is easily applicable in the microscopic model calculations within an entire temperature range, from zero to temperatures above the critical point.

Studying Nuclear Level Densities of 238U in the Nuclear Reactions within the Macroscopic Nuclear Models

Abstract In this work the nuclear level density parameters of 238U have been extracted in the back-shifted Fermi gas model (BSFGM), as well as the constant temperature model (CTM), through fitting with the recent experimental data on nuclear level densities measured by the Oslo group. The excitation functions for 238U(p,2nα)233Pa, and 238U(p,4n)235Np reactions and the fragment yields for the fragments of the 238U(p,f) reaction have been calculated using obtained level density parameters. The results are compared to their corresponding experimental values. It was found that the extracted excitation functions and the fragment yields in the CTM coincide well with the experimental values in the low-energy region. This finding is according to the claim made by the Oslo group that the extracted level densities of 238U show a constant temperature behaviour.