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We present a method to calculate molecular vibrational densities of states (level densities) with anharmonic vibrations, crucial in many cases for describing thermal properties of molecular species. Coupling of modes is fully implemented and the method eliminates the severe numerical artefacts associated with the commonly used limitation to the second order expansion in quantum numbers with their negative coupling constants. The method proceeds by calculating the canonical partition function perturbatively and applying the inverse Laplace transform. C 7 − is used as a case study. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper proposes that the study of Sturm sequences is invaluable in the numerical computation and theoretical derivation of eigenvalue distributions of random matrix ensembles. We first explore the use of Sturm sequences to efficiently compute histograms of eigenvalues for symmetric tridiagonal matrices and apply these ideas to random matrix ensembles such as the β-Hermite ensemble. Using our techniques, we reduce the time to compute a histogram of the eigenvalues of such a matrix from O(n2+m) to O(mn) time where n is the dimension of the matrix and m is the number of bins (with arbitrary bin centers and widths) desired in the histogram (m is usually much smaller than n). Second, we derive analytic formulas in terms of iterated multivariate integrals for the eigenvalue distribution and the largest eigenvalue distribution for arbitrary symmetric tridiagonal random matrix models. As an example of the utility of this approach, we give a derivation of both distributions for the β-Hermite random matrix ensemble (for general β). Third, we explore the relationship between the Sturm sequence of a random matrix and its shooting eigenvectors. We show using Sturm sequences that assuming the eigenvector contains no zeros, the number of sign changes in a shooting eigenvector of parameter λ is equal to the number of eigenvalues greater than λ. Finally, we use the techniques presented in the first section to experimentally demonstrate a O(log n) growth relationship between the variance of histogram bin values and the order of the β-Hermite matrix ensemble. © SFoCM 2008.

Excitation functions for 232Th(n,γ) and 232Th(n,2n) reactions from reaction threshold to 20 MeV were calculated using TALYS-1.9 nuclear code by invoking suitable options for the level densities, optical model potentials, pre-equilibrium effects and γ-ray strength functions. In earlier studies, theoretical plots for 232Th(n,γ) and 232Th(n,2n) reaction cross-sections were obtained by using EMPIRE 3.2 and TALYS 1.9 codes with default parameters, however none of the reported plots could match with the corresponding experimental cross-sections reported in EXFOR data particularly between 14-20 MeV. The results of the present study reveal that by using a combination of specific input parameters in TALYS 1.9 code, the theoretical evaluation of the cross sections favour a higher pre-equilibrium rate for the harder spectrum. Moreover the estimated cross-sections match fairly well with the corresponding experimental data (EXFOR database) as well as with the evaluated data files (ENDF/VII.0, JENDL-4.0). The results of the present study are important for the validation of nuclear model approaches with increased predictive power for (n,xn) cross-sections and particularly for the application of thorium based fuel in Accelerator-Driven Sub-critical System. • Excitation functions for 232Th(n,2n)231Th and 232Th(n,γ)233Th from reaction threshold to 20 MeV. • Effects of level densities, optical model potentials, pre-equilibrium and gamma-ray strength functions were studied. • Comparison of theoretical plots of cross-section with EXFOR data. • Validation of the predictive power of the nuclear model calculations on the excitation functions. [ABSTRACT FROM AUTHOR]

Spallation nuclear reactions in the range of 0.2 to 1.2 GeV are studied using the CRISP code. A new approach for the deexcitation stage of the compound nucleus was introduced. For the calculations of the level densities, this approach is based on the Back-shifted Fermi gas model (BSFG), which takes into account pairing effects and shell corrections, whereas the calculation of the fission barriers were performed by means of the Extended Thomas-Fermi plus Strutinsky Integral (ETFSI) method, which is a high-speed approximation to the Hartree-Fock method with pairing correlations treated as in the usual BCS plus blocking approach. This procedure is more appropriate to calculate level densities for exotic nuclei. Satisfactory results were obtained and compared with experimental data obtained in the GSI experiments. As another important result, we highlight some directions for the development of a qualitatively superior version of the CRISP code with the implementation of more realistic and suitable physical models to be applied in stable and exotic nuclei that participate in the process. This new version of the code includes several substantial changes in the decay of the hot compound nucleus which allow satisfactory agreement with the experimental data and a reduction of the adjustment parameters. [ABSTRACT FROM AUTHOR]

Abstract: The excitation functions for (n,2n) reactions from reaction threshold to 20MeV on four stable lead isotopes 204Pb, 206Pb, 207Pb and 208Pb were calculated using Talys-1.2 nuclear model code, this essentially involves fitting a set of global parameters. An excellent agreement between the calculated and experimental data is obtained. The level densities and effective imaginary potential were studied to obtain the best fit of the excitation functions. The cross sections were calculated by invoking suitable options for global optical model potential, microscopic level densities from Goriely’s Table, Hauser–Feshbach model for equilibrium calculations, the two-component exciton model by Kalbach for pre-equilibrium calculations in the Talys input file. The results of the present study reveal that the theoretical estimation of the cross sections match fairly well with the experimental data (EXFOR database) as well as with the evaluated data files (ENDF/VII.0, JENDL-4.0, CENDL-3.1). This is of importance to the validation of nuclear model approaches with increased predictive power. [Copyright &y& Elsevier]

Abstract: Observed fission cross section data are reproduced up to within Hauser–Feshbach pre-equilibrium model. Symmetric and asymmetric fission mode partitioning of the observed fission cross section is predicted. Fission barriers of neutron-deficient Th nuclides are compared with the microscopic and macroscopic–microscopic calculations. Nucleon optical model potential for 232Th is obtained. Rigid rotator coupled-channel model reproduces experimental neutron total cross section data up to 200 MeV. [Copyright &y& Elsevier]

The symmetric SL-mode and lumped asymmetric (S1+S2)-mode fission cross sections of 238U(n,f) reaction are calculated up to incident neutron energy of 200 MeV within a Hauser–Feshbach statistical model. For each U nuclide, contributing to the observed fission cross section via 238U(n,xnf) reaction separate outer fission barrier is assumed for the SL-mode fission, while the inner one is assumed to be the same either for the symmetric SL- and lumped asymmetric (S1+S2)-mode fission. Inner saddle is axial asymmetric and mass-symmetric for fissioning U nuclei with number of neutrons N>144, for lower N values axial symmetry is assumed to persist. Axial asymmetry and mass-symmetry are assumed for the outer saddle of the SL-mode, as distinct from asymmetric fission modes, for which axial symmetry and mass-asymmetry are assumed. The SL-mode fission cross section up to emissive threshold is controlled by a rather high outer fission barrier with significant transparency. Damping of collective modes contribution to the level density at intrinsic excitation energies higher than 20–30 MeV for axial symmetric saddle and equilibrium deformations seems to be essential for the 238U(n,F) observed fission cross section description. Ratio of symmetric to all 238U neutron-induced fission events is described assuming damping of triaxial collective modes contribution to the level densities at outer saddles relevant for the SL-mode. High sensitivity of the ratio of symmetric and asymmetric fission cross sections to the relative contributions of emissive fission chances with high and low number of pre-fission (n,xnf) neutrons is observed. [Copyright &y& Elsevier]

The symmetric SL-mode fission cross sections of 235U(n, f) and 238U(n, f) are analyzed up to the emissive fission threshold within a Hauser–Feshbach statistical model. A separate outer fission barrier is assumed for the SL-mode, while the inner one is assumed to be the same for the symmetric SL- and the asymmetric S1- and S2-modes. Axial asymmetry and mass-symmetry is assumed for the outer saddle of the SL-mode, as distinct from the S1- and S2-modes, which are axially symmetric and mass-asymmetric. The observed fission cross section behaviour is modelled using the interplay of few-quasiparticle excitations in the level density of the fissioning and residual nuclei. The SL-mode fission cross section seems to be controlled by a rather high outer fission barrier with significant transparency. [Copyright &y& Elsevier]

Excitation functions for 209Bi(α,2n)211At, 209Bi(α,3n)210At and 209Bi(α,4n)209At reactions were calculated using TALYS-1.95 nuclear code from threshold to 50 MeV by invoking suitable options for level densities, nucleon-nucleus optical model potentials and alpha optical model potentials. Statistical factors were used to verify the quality of matching between theoretical model calculations and the experimental data from the EXFOR database. The TTY of 211At calculated using the excitation function of 209Bi(a,2n)211At reaction is compared with existing experimental studies from literature The results of the present study are important for the validation of nuclear model approaches with increased predictive power for 209Bi(α,xn) reactions for the production of 211At. • Excitation functions for 209Bi(a,2n)211At and 209Bi(a,3n)210Bi from reaction threshold to 50 MeV. • Effects of level densities, nucleon-nucleus optical model potentials and alpha particle optical model potentials. • Validation of the predictive power of the nuclear model calculations on the excitation functions. • Comparison of calculated TTY with existing experimental studies from literature. [ABSTRACT FROM AUTHOR]