Proton capture cross sections on neutron-magicSm144at astrophysically relevant energies

Abstract

Background: The $p$ nuclei, which are not produced by neutron capture processes, are present with a typical isotopic abundance of 0.01%--0.3%. Abundance decreases with an increase in atomic number. However, the neutron-magic isotopes of $^{92}\text{Mo}$ and $^{144}\text{Sm}$ exhibit unusually large abundances in comparison. A combination of proton and $\ensuremath{\alpha}$-particle capture reactions and neutron emission reactions are key to understanding this issue. Currently, complex network calculations do not have access to much experimental data, and hence require theoretically predicted reaction rates in order to estimate final abundances produced in nucleosynthesis.Purpose: Few experimental cross sections of $(p$,\ensuremath{\gamma}) reactions on heavy nuclides with mass numbers of 130--150 have been reported. The $^{144}\text{Sm}(p,\phantom{\rule{0.16em}{0ex}}\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{145}\text{Eu}$ reaction is the main destruction pathway for the nucleosynthesis of the $^{144}\text{Sm}$ nuclide. In the present paper, experimental cross sections of the $^{144}\text{Sm}(p,\phantom{\rule{0.16em}{0ex}}\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{145}\text{Eu}$ reaction at a range including astrophysically relevant energies for the $p$ process were determined to compare with theoretical predictions using the Hauser-Feshback statistical model.Methods: The $^{144}\text{Sm}$ was deposited on a high-purity Al foil with the molecular plating method. Stacks consisting of Ta degrader foils, $^{144}\text{Sm}$ targets, and Cu foils used as flux monitors were irradiated with 14.0-MeV proton beams. The $^{144}\text{Sm}(p,\phantom{\rule{0.16em}{0ex}}\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{145}\text{Eu}$ cross sections were determined from the $^{145}\text{Eu}$ activities and the proton fluence estimated from the $^{65}\text{Zn}$ activity in the Cu monitor foil. The proton energies bombarded on each $^{144}\text{Sm}$ target were estimated using srim2013.Results: We determined the $^{144}\text{Sm}(p,\phantom{\rule{0.16em}{0ex}}\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{145}\text{Eu}$ cross sections at proton energies between 2.8 and 7.6 MeV. These energies encompass nucleosynthesis temperatures between 3 and 5 GK. The cross sections at energies higher than 3.8 MeV agreed well with theoretically predicted cross sections using talys using the generalized superfluid (GS) model for level densities. However, calculations using non-smoker overestimated the cross section. When the components of the energy uncertainties in the experimental cross sections were corrected, the cross sections at energies lower than 3.8 MeV showed comparable values with talys but higher than those predicted by both non-smoker and talys.Conclusions: talys using the GS model reproduced well the experimental cross sections without correction of the proton widths at energies between 2.8 and 7.6 MeV. Thus, the reaction rates of $^{144}\text{Sm}(p,\phantom{\rule{0.16em}{0ex}}\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{145}\text{Eu}$ in the stellar environment at 2.5--5 GK estimated with talys corresponded with those by the experimental cross section within 10%. However, the reaction rates depended on the extrapolation of the cross section at energies of 0--2.8 MeV at temperatures of 0.5--2.5 GK. The reaction rate estimated by talys employing the GS model showed an uncertainty within a factor of 2 at 1.5--3.5 GK for nucleosynthesis temperatures of the $p$ nuclei.