Paraskevi Demetriou

Microscopic nuclear level densities for practical applications

Abstract New nuclear level densities based on the microscopic statistical model are proposed for practical applications. The statistical calculations are performed using the deformed Hartree–Fock–BCS predictions of the ground-state structure properties. The microscopic model includes a consistent treatment of the shell effects, pairing correlations, deformation effects and collective excitations. It predicts the experimental neutron resonance spacings with a degree of accuracy comparable to that of the phenomenological back-shifted Fermi-gas-type formulae. The microscopic level densities are renormalized to the existing experimental data, namely the s-wave neutron resonance spacings and the cumulative number of low-lying levels. Level densities for more than 8000 nuclei are made available in a table format for practical applications.

The r-process nucleosynthesis: a continued challenge for nuclear physics and astrophysics

The identification of the astrophysical site and the specific conditions in which r-process nucleosynthesis takes place remain unsolved mysteries of astrophysics. The present paper emphasizes some important future challenges faced by nuclear physics in this problem, particularly in the determination of the radiative neutron capture rates by exotic nuclei close to the neutron drip line and the fission probabilities of heavy neutron-rich nuclei. These quantities are particularly relevant to determine the composition of the matter resulting from the decompression of initially cold neutron star matter. New detailed r-process calculations are performed and the final composition of ejected inner and outer neutron star crust material is estimated. We discuss the impact of the many uncertainties in the astrophysics and nuclear physics on the final composition of the ejected matter. The similarity between the predicted and the solar abundance pattern for A ≥ 140 nuclei as well as the robustness of the prediction with varied input parameters makes this scenario one of the most promising that deserves further exploration.

Proton induced reaction cross section measurements on Se isotopes for the astrophysical p process

As a continuation of a systematic study of reactions relevant to the astrophysical p process, the cross sections of the Se-74,Se-76(p,gamma) Br-75,Br-77 and Se-82(p,n) Br-82 reactions have been measured at energies from 1.3 to 3.6 MeV using an activation technique. The results are compared to the predictions of Hauser-Feshbach statistical model calculations using the NON-SMOKER andMOST codes. The sensitivity of the calculations to variations in the optical proton potential and the nuclear level density was studied. Good agreement between theoretical and experimental reaction rates was found for the reactions Se-74(p,gamma) Br-75 and Se-82(p,n) Br-82 .

Proton capture cross section of Sr isotopes and their importance for nucleosynthesis of proton-rich nuclides

The (p,gamma) cross sections of three stable Sr isotopes haven been measured in the astrophysically relevant energy range. These reactions are important for the p process in stellar nucleosynthesis and, in addition, the reaction cross sections in the mass region up to 100 are also of importance concerning the rp process associated with explosive hydrogen and helium burning. It is speculated that this rp process could be responsible for a certain amount of p nuclei in this mass reg,ion. The (p, gamma) cross sections of Sr-84,Sr-86,Sr-87 isotopes were determined using an activation technique. The measurements were carried out at the 5 MV Van de Graaff accelerator of the ATOMKI, Debrecen. The resulting cross sections are compared with the predictions of statistical model calculations. The predictions are in good agreement with the experimental results for Sr-84(p,gamma) Y-85 whereas the other two reactions exhibit differences that increase with mass number. The corresponding astrophysical reaction rates have also been computed.

Proton and alpha-particle capture reactions at sub-Coulomb energies relevant to the p process

Several cross-section measurements of proton as well as α-particle capture reactions in the Se–Sb region have been carried out at sub-Coulomb energies with the aim to obtain global input parameters for Hauser–Feshbach (HF) calculations. Some of the results are compared with HF calculations using various optical model potentials and nuclear level densities.

Cross sections of (p,γ) reactions of N=50 nuclei relevant to p-process

The proton capture reactions 89Y(p,γ)90Zr and 88Sr(p,γ)89Y have been investigated at energies Ep = 1.5-3.5 MeV via an array of 4 HPGe detectors of ≈100% rel. efficiency. The resulting cross sections as well as the existing data on the 90Zr(p,γ)91Nb and 92Mo(p,γ)93Tc reactions are compared with improved statistical model calculations carried out using the MOST code.

Calculations of Fission Properties Using Microscopic Models for Astrophysics Applications

Potential energy surfaces obtained with a Skyrme‐Hartree‐Fock‐Bogoliubov method in a multidimensional deformation space are analysed in order to obtain the fission properties of nuclei relevant to the r‐process nucleosynthesis. Static fission paths consisting of single‐, double‐, and triple‐humped barriers are determined using the method of steepest descent. The heights and widths of the individual barriers are estimated by fitting them with inverted parabolas. Spontaneous‐fission half‐lives, neutron‐induced fission and β‐delayed neutron emission and fission rates are subsequently calculated using the Hill‐Wheeler fission transmission coefficients. The nuclear level densities at ground‐state and saddle‐point deformations are obtained from statistical calculations using the same microscopic Hartree‐Fock‐Bogoliubov model. The β‐delayed emission rates are based on the β‐strength functions given by the gross theory. The results of these global default calculations are first compared with existing experimental...

Improved global α-optical model potential at low energies

Abstract We propose a new global α-nucleus optical model potential that takes into account the strong energy dependence and nuclear structure effects that characterize the alpha-nucleus interaction. The potential consists of a double-folded real part and a Woods-Saxon imaginary part with a Fermi-type energy dependence. The influence of purely volume absorption, volume plus surface absorption, as well as the dispersive relation is investigated. Three potentials are considered that are able to reproduce the bulk of existing experimental data at low energies. The need for more experimental data in the mass region A ⋍ 100 and A ⋍ 200 is stressed.

A systematic study of proton capture reactions in the SeSb region at energies relevant to the p process

Abstract A systematic investigation of (p,γ) cross sections of nuclei from Se to Sb is presented. In-beam cross section measurements were carried out at E p = 1.4–5 MeV by using an array of 4 HPGe detectors of 100% relative efficiency shielded with BGO crystals. The S factors obtained are compared with the predictions of the statistical model code MOST.

Studying [sup 12]B via [sup 8]Li-α resonant scattering

The inverse kinematics thick target scattering method (TTIK) has been used to study the 8Li elastic scattering on 4He in order to investigate 8Li‐α cluster configurations in excited states of 12B. A 8Li beam, provided by the radioactive beam facility EXCYT, at Ebeam = 30.6 MeV, passing through helium thick target, continuosly decreases its energy inducing elastic scattering starting from the initial energy down to zero. Four ΔE‐E double stage silicon detector telescopes were used to detect the recoil α‐particles coming from the scattering. Event by event time measurement between beam particles passing through a MCP detector and α‐particles impinging on ΔE stage allows elastic from inelastic events discrimination, thus representing an improvement of the TTIK method. In this paper the used experimental technique and some preliminary results will be briefly described.