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تسجيل مستخدم جديدThe $^{22}$Ne($alpha$,n)$^{25}$Mg neutron source: latest experimental results and prospects
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Claudio Ugalde
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The current status of the reaction rate of $^{22}$Ne($alpha$,n)$^{25}$Mg is summarized. Among the latest new results, probably the most relevant is the conclusion that the E$_x$=11.15 MeV state in $^{26}$Mg has a non-natural parity, so it does not contribute to the rates of the $alpha$ + $^{22}$Ne reactions. However, it may be possible that other neighboring states contribute to the neutron yield at stellar temperatures. Here we make an account of some of the experimental work in the literature that is relevant to this state. Indeed, it would have been possible to avoid the controversy regarding this state before it even started.
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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.
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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.
This paper examines the $^{18}$Ne($alpha, p_{0}$)$^{21}$Na cross-section relevant in X-ray bursts. The study was performed with the K600 magnetic spectrometer in coincidence with the CAKE, a silicon detector array, at iThemba LABS in Cape Town, South
Africa. A 100-MeV proton beam was impinged on a $^{24}$Mg target to study the $^{24}$Mg($p,t$)$^{22}$Mg reaction. The triton ejectiles were momentum-analysed with the magnetic spectrometer and proton decays from the $^{22}$Mg recoil nucleus to the ground state of $^{21}$Na and various excited states thereof were detected with the CAKE. In doing so, we were able to compare our results to previous direct and indirect measurements of the $^{18}$Ne($alpha, p$)$^{21}$Na reaction.
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