Systematic study of neutron capture including the compound, pre-equilibrium, and direct mechanisms

Abstract

The neutron capture reaction is investigated. The three major reaction mechanisms, namely, compound-nucleus capture (CNC), pre-equilibrium capture (PEC), and direct capture (DIC), are considered on the basis of the Hauser-Feshbach model, the exciton model, and potential model, respectively. The three mechanisms are treated simultaneously and consistently, i.e, they are obtained on the basis of the same nuclear ingredients, such as the optical potential and nuclear-level densities. In this framework, the three components are calculated on the same footing and represent partial fluxes of the same total reaction cross section. The total neutron-capture cross sections and astrophysical reaction rates are calculated within the updated modern reaction code TALYS for about 8000 nuclei with 8 Z 110 lying between the proton and neutron drip lines. The nuclear-structure ingredients involved in the calculation are determined from experimental data whenever available and, if not, from global microscopic nuclear models. For the targets with mass number A > 26, a fair agreement between the computed total-capture cross sections and experimental data is found but, for the lightest nuclei, only the predicted DIC cross sections reproduce the experimental results satisfactorily. Significant and even dominant contribution to the total reaction rate comes from the DIC component for neutron-rich nuclei, especially in the Z = 50-70 region. The impact of the newly determined reaction rates on the r process abundances resulting from the ejection of matter in neutrino-driven winds or the decompression of neutron star matter is investigated.

The fundamental role of fission during r-process nucleosynthesis in neutron star mergers

“…This DC proceeds via the excitation of only a few degrees of freedom on much shorter time scale reflecting the time taken by the projectile to travel across the target. This mechanism can be satisfactorily described with the perturbative approach known as the potential model [98][99][100][101]. It is now well accepted that the DC is important, and often dominant at the very low energies of astrophysical interest for light or exotic nuclei systems for which few, or even no resonant states are available.…”

Bazı Tellür İzotoplarının Nötron Yakalama Tesir Kesiti Analizi / Analysis of Some Tellur Isotopes Neutron Capture Cross Sections

“…Bunların yanısıra, nötron yakalama reaksiyonlarının oluşum basamakları olan bileşik, ön-denge ve direk mekanizmalarının teorik analizi yapılmıştır [23]. Bu çalışmada ise bazı tellür izotoplarının nötron yakalama tesir kesiti değerleri farklı mekanizmalar (farklı şiddet fonksiyonları ve nükleer seviye yoğunluğu modelleri) kullanılarak analiz edilmiş ve elde edilen teorik sonuçlar hem birbirleriyle hem de deneysel veriler ile karşılaştırılmıştır.…”

Continuum quasiparticle random-phase approximation for astrophysical direct neutron capture reactions on neutron-rich nuclei

“…For this purpose, we take neutron-rich tin isotopes with mass number A ∼ 140, in which oneneutron separation energy is S 1n ∼ a few MeV, and hence the direct neutron capture is expected to be dominant in the r-process reaction of these isotopes [14,15,21]. Another reason of the choice is that we can assume the spherical shape because of the proton magicity We employ the Skyrme energy density functional model and the effective pairing interaction of the contact type to construct the HFB ground state and the associated selfconsistent mean-field.…”

“…Recently new modes of dipole excitation such as the pygmy resonance and the soft dipole excitation [4][5][6][7] have attracted attentions since influence of the new modes on the r-process nucleosynthesis was pointed out [8]. Motivated with this possibility, microscopic many-body models of electric multipole responses, developed on the basis of the density functional theories, have been applied to the gamma-ray strength function for the compound process calculations [9][10][11][12][13][14].…”