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تسجيل مستخدم جديدDecay properties of $^{22}mathrm{Ne} + alpha$ resonances and their impact on $s$-process nucleosynthesis
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The astrophysical $s$-process is one of the two main processes forming elements heavier than iron. A key outstanding uncertainty surrounding $s$-process nucleosynthesis is the neutron flux generated by the ${}^{22}mathrm{Ne}(alpha, n){}^{25}mathrm{Mg}$ reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing ${}^{22}mathrm{Ne}(alpha, gamma){}^{26}mathrm{Mg}$ reaction, is not well constrained in the important temperature regime from ${sim} 0.2$--$0.4$~GK, owing to uncertainties in the nuclear properties of resonances lying within the Gamow window. To address these uncertainties, we have performed a new measurement of the ${}^{22}mathrm{Ne}({}^{6}mathrm{Li}, d){}^{26}mathrm{Mg}$ reaction in inverse kinematics, detecting the outgoing deuterons and ${}^{25,26}mathrm{Mg}$ recoils in coincidence. We have established a new $n / gamma$ decay branching ratio of $1.14(26)$ for the key $E_x = 11.32$ MeV resonance in $^{26}mathrm{Mg}$, which results in a new $(alpha, n)$ strength for this resonance of $42(11)~mu$eV when combined with the well-established $(alpha, gamma)$ strength of this resonance. We have also determined new upper limits on the $alpha$ partial widths of neutron-unbound resonances at $E_x = 11.112,$ $11.163$, $11.169$, and $11.171$ MeV. Monte-Carlo calculations of the stellar ${}^{22}mathrm{Ne}(alpha, n){}^{25}mathrm{Mg}$ and ${}^{22}mathrm{Ne}(alpha, gamma){}^{26}mathrm{Mg}$ rates, which incorporate these results, indicate that both rates are substantially lower than previously thought in the temperature range from ${sim} 0.2$--$0.4$~GK.
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The competing $^{22}$Ne($alpha,gamma$)$^{26}$Mg and $^{22}$Ne($alpha,n$)$^{25}$Mg reactions control the production of neutrons for the weak $s$-process in massive and AGB stars. In both systems, the ratio between the corresponding reaction rates stro
ngly impacts the total neutron budget and strongly influences the final nucleosynthesis. The $^{22}$Ne($alpha,gamma$)$^{26}$Mg and $^{22}$Ne($alpha,n$)$^{25}$Mg reaction rates was re-evaluated by using newly available information on $^{26}$Mg given by various recent experimental studies. Evaluations of The evaluated $^{22}$Ne($alpha,gamma$)$^{26}$Mg reaction rate remains substantially similar to that of Longland {it et al.} but, including recent results from Texas A&M, the $^{22}$Ne($alpha,n$)$^{25}$Mg reaction rate is lower at a range of astrophysically important temperatures. Stellar models computed with NEWTON and MESA predict decreased production of the weak branch $s$-process due to the decreased efficiency of $^{22}$Ne as a neutron source. Using the new reaction rates in the MESA model results in $^{96}$Zr/$^{94}$Zr and $^{135}$Ba/$^{136}$Ba ratios in much better agreement with the measured ratios from presolar SiC grains.
The $^{22}$Ne($alpha$,$gamma$)$^{26}$Mg and $^{22}$Ne($alpha$,n)$^{25}$Mg reactions play an important role in astrophysics because they have significant influence on the neutron flux during the weak branch of the s-process. We constrain the astrophys
ical rates for these reactions by measuring partial $alpha$-widths of resonances in $^{26}$Mg located in the Gamow window for the $^{22}$Ne+$alpha$ capture. These resonances were populated using $^{22}$Ne($^6$Li,d)$^{26}$Mg and $^{22}$Ne($^7$Li,t)$^{26}$Mg reactions at energies near the Coulomb barrier. At these low energies $alpha$-transfer reactions favor population of low spin states and the extracted partial $alpha$-widths for the observed resonances exhibit only minor dependence on the model parameters. The astrophysical rates for both the $^{22}$Ne($alpha$,$gamma$)$^{26}$Mg and the $^{22}$Ne($alpha$,n)$^{25}$Mg reactions are shown to be significantly different than the previously suggested values.
We studied $alpha$ cluster states in $^{26}$Mg via the $^{22}$Ne($^{6}$Li,$dgamma$)$^{26}$Mg reaction in inverse kinematics at an energy of $7$ MeV/nucleon. States between $E_x$ = 4 - 12 MeV in $^{26}$Mg were populated and relative $alpha$ spectrosco
pic factors were determined. Some of these states correspond to resonances in the Gamow window of the $^{22}$Ne($alpha$,n)$^{25}$Mg reaction, which is one of the main neutron sources in the astrophysical $s$-process. We show that $alpha$-cluster strength of the states analyzed in this work have critical impact on s-process abundances. Using our new $^{22}$Ne($alpha$,n)$^{25}$Mg and $^{22}$Ne($alpha$,$gamma$)$^{26}$Mg reaction rates, we performed new s-process calculations for massive stars and Asymptotic Giant Branch stars and compared the resulting yields with the yields obtained using other $^{22}$Ne+$alpha$ rates from the literature. We observe an impact on the s-process abundances up to a factor of three for intermediate-mass AGB stars and up to a factor of ten for massive stars. Additionally, states in $^{25}$Mg at $E_x$ $<$ 5 MeV are identified via the $^{22}$Ne($^{6}$Li,$t$)$^{25}$Mg reaction for the first time. We present the ($^6$Li, $t$) spectroscopic factors of these states and note similarities to the $(d,p$) reaction in terms of reaction selectivity.
The $^{22}$Ne(p,$gamma$)$^{23}$Na reaction is the most uncertain process in the neon-sodium cycle of hydrogen burning. At temperatures relevant for nucleosynthesis in asymptotic giant branch stars and classical novae, its uncertainty is mainly due to
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