Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe (n,g) cross sections of short-living s-process branching points
215
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
An experimental method to determine the (n,g) cross section of short-living s-process branching points using data of the inverse (g,n) reaction is presented. The method was used to observe the branching point nucleus 95Zr because the elemental abundance patterns corresponding to this branching point cannot be reproduced by full stellar models and a possible error source is the neutron capture cross section of 95Zr. The analysis of the experiment is still under progress, we will outline the current status in this manuscript.
rate research
Read More
We have carried out photodisintegration cross-section measurements on 86Kr using monoenergetic photon beams ranging from the neutron separation energy, S_n = 9.86 MeV, to 13 MeV. We combine our experimental 86Kr(g,n)85Kr cross section with results from our recent 86Kr(g,g) measurement below the neutron separation energy to obtain the complete nuclear dipole response of 86Kr. The new experimental information is used to predict the neutron capture cross section of 85Kr, an important branching point nucleus on the abundance flow path during s-process nucleosynthesis. Our new and more precise 85Kr(n,g)86Kr cross section allows to produce more precise predictions of the 86Kr abundance from s-process models. In particular, we find that the models of the s-process in asymptotic giant branch stars of mass < 1.5 Msun, where the 13C neutron source burns convectively rather than radiatively, represent a possible solution for the highest 86Kr/82Kr ratios observed in meteoritic stardust SiC grains.
Photoneutron cross sections were measured for $^{58}$Ni, $^{60}$Ni, $^{61}$Ni, and $^{64}$Ni at energies between the one-neutron and two-neutron thresholds using quasi-monochromatic $gamma$-ray beams produced in laser Compton-scattering at the NewSUBARU synchrotron radiation facility. The new photoneutron data are used to extract the $gamma$-ray strength function above the neutron threshold complementing the information obtained by the Oslo method below the threshold. We discuss radiative neutron capture cross sections and the Maxwellian-averaged cross sections for Ni isotopes including $^{63}$Ni, a branching point nucleus along the weak s-process path. The cross sections are calculated with the experimentally constrained $gamma$-ray strength functions from the Hartree-Fock-Bogolyubov plus quasi-particle-random phase approximation based on the Gogny D1M interaction for both $E1$ and $M1$ components and supplemented with the $M1$ upbend.
The neutron capture cross section of the unstable nucleus 185W has been derived from experimental photoactivation data of the inverse reaction 186W(gamma,n)185W. The new result of sigma = (687 +- 110) mbarn confirms the theoretically predicted neutron capture cross section of 185W of sigma = 700 mbarn at kT = 30 keV. A neutron density in the classical s-process of n_n = (3.8 +0.9 -0.8} * 1e8 cm-3 is derived from the new data for the 185W branching. In a stellar s-process model one finds a significant overproduction of the residual s-only nucleus 186Os.
Direct neutron capture on 62Ni is calculated in the DWBA and the cross sections in the energy range relevant for s-process nucleosynthesis are given. It is confirmed that the thermal value of the capture cross section contains a subthreshold resonance contribution. Contrary to previous investigations it is found that the capture at higher energies is dominated by p-waves, thus leading to a considerably increased cross section at s-process energies and a modified energy dependence.
The nucleosynthesis of elements beyond iron is dominated by the s and r processes. However, a small amount of stable isotopes on the proton-rich side cannot be made by neutron capture and are thought to be produced by photodisintegration reactions on existing seed nuclei in the so-called p process. So far most of the p-process reactions are not yet accessible by experimental techniques and have to be inferred from statistical Hauser-Feshbach model calculations. The parametrization of these models has to be constrained by measurements on stable proton-rich nuclei. A series of (n,$gamma$) activation measurements on p nuclei, related by detailed balance to the respective photodisintegrations, were carried out at the Karlsruhe Van de Graaff accelerator using the $^7$Li(p,n)$^7$Be source for simulating a Maxwellian neutron distribution of kT= 25 keV. We present here preliminary results of our extended measuring program in the mass range between A=74 and A=132, including first experimental (n,$gamma$) cross sections of $^{74}$Se, $^{84}$Sr, $^{120}$Te and $^{132}$Ba, and an improved value for $^{130}$Ba. In all cases we find perfect agreement with the recommended MACS predictions from the Bao et al. compilation.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
