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Register a new userGamma-ray emission in alpha-particle reactions with C, Mg, Si, Fe
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Cross sections for the strongest gamma-ray emission lines produced in alpha-particle reactions with C, Mg, Si, Fe have been measured in the range E_alpha = 50 - 90 MeV at the center for proton therapy at the Helmholtz-Zentrum Berlin. Data for more than 60 different gamma-ray lines were determined, with particular efforts for lines that are in cross section compilations/evaluations with astrophysical purpose, and where data exist at lower projectile energies. The data are compared with predictions of a modern nuclear reaction code and cross-section curves of the latest evaluation for gamma-ray line emission in accelerated-particle interactions in solar flares.
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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 astrophysical 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.
Gamma-ray excitation functions have been measured for 30, 42, 54 and 66 MeV proton beams accelerated onto C + O (Mylar), Mg, Si, and Fe targets of astrophysical interest at the separate-sector cyclotron of iThemba LABS in Somerset West (Cape Town, South Africa). A large solid angle, high energy resolution detection system of the Eurogam type was used to record Gamma-ray energy spectra. Derived preliminary results of Gamma-ray line production cross sections for the Mg, Si and Fe target nuclei are reported and discussed. The current cross section data for known, intense Gamma-ray lines from these nuclei consistently extend to higher proton energies previous experimental data measured up to Ep ~ 25 MeV at the Orsay and Washington tandem accelerators. Data for new Gamma-ray lines observed for the first time in this work are also reported.
The $^{24}$Mg($alpha,gamma$)$^{28}$Si reaction influences the production of magnesium and silicon isotopes during carbon burning and is one of eight reaction rates found to significantly impact the shape of calculated X-ray burst light curves. The reaction rate is based on measured resonance strengths and known properties of levels in $^{28}$Si. The $^{24}$Mg($alpha,gamma$)$^{28}$Si reaction rate has been re-evaluated including recent additional indirect data. The reaction rate is substantially unchanged from previously calculated rates, especially at astrophysically important temperatures. Increases in the reaction rate could occur at lower temperatures due to as-yet unmeasured resonances but these increases have little astrophysical impact. The $^{24}$Mg($alpha,gamma$)$^{28}$Si reaction rate at temperatures relevant to carbon burning and Type I X-ray bursts is well constrained by the available experimental data. This removes one reaction from the list of eight previously found to be important for X-ray burst light curve model-observation comparisons.
$gamma$-ray production cross sections have been measured in proton irradiations of N, Ne and Si and $alpha$-particle irradiations of N and Ne. In the same experiment we extracted also line shapes for strong $gamma$-ray lines of $^{16}$O produced in proton and $alpha$-particle irradiations of O. For the measurements gas targets were used for N, O and Ne and a thick foil was used for Si. All targets were of natural isotopic composition. Beams in the energy range up to 26 MeV for protons and 39 MeV for $alpha$-particles have been delivered by the IPN-Orsay tandem accelerator. The $gamma$ rays have been detected with four HP-Ge detectors in the angular range 30$^{circ}$ to 135$^{circ}$. We extracted 36 cross section excitation functions for proton reactions and 14 for $alpha$-particle reactions. For the majority of the excitation functions no other data exist to our knowledge. Where comparison with existing data was possible usually a very good agreement was found. It is shown that these data are very interesting for constraining nuclear reaction models. In particular the agreement of cross section calculations in the nuclear reaction code TALYS with the measured data could be improved by adjusting the coupling schemes of collective levels in the target nuclei $^{14}$N, $^{20,22}$Ne and $^{28}$Si. The importance of these results for the modeling of nuclear $gamma$-ray line emission in astrophysical sites is discussed.
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 strongly 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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