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Register a new userExperimental study of the astrophysical gamma-process reaction 124Xe(alpha,gamma)128Ba
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The synthesis of heavy, proton rich isotopes in the astrophysical gamma-process proceeds through photodisintegration reactions. For the improved understanding of the process, the rates of the involved nuclear reactions must be known. The reaction 128Ba(g,a)124Xe was found to affect the abundance of the p nucleus 124Xe. Since the stellar rate for this reaction cannot be determined by a measurement directly, the aim of the present work was to measure the cross section of the inverse 124Xe(a,g)128Ba reaction and to compare the results with statistical model predictions. Of great importance is the fact that data below the (a,n) threshold was obtained. Studying simultaneously the 124Xe(a,n)127Ba reaction channel at higher energy allowed to further identify the source of a discrepancy between data and prediction. The 124Xe + alpha cross sections were measured with the activation method using a thin window 124Xe gas cell. The studied energy range was between E = 11 and 15 MeV close above the astrophysically relevant energy range. The obtained cross sections are compared with statistical model calculations. The experimental cross sections are smaller than standard predictions previously used in astrophysical calculations. As dominating source of the difference, the theoretical alpha width was identified. The experimental data suggest an alpha width lower by at least a factor of 0.125 in the astrophysical energy range. An upper limit for the 128Ba(g,a)124Xe stellar rate was inferred from our measurement. The impact of this rate was studied in two different models for core-collapse supernova explosions of 25 solar mass stars. A significant contribution to the 124Xe abundance via this reaction path would only be possible when the rate was increased above the previous standard value. Since the experimental data rule this out, they also demonstrate the closure of this production path.
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The cross sections of the 162Er(a,g,)166Yb and 162Er(a,n)165Yb reactions have been measured for the first time. The radiative alpha capture reaction cross section was measured from Ec.m. = 16.09 down to Ec.m. = 11.21 MeV, close to the astrophysically relevant region (which lies between 7.8 and 11.48 MeV at 3 GK stellar temperature). The 162Er(a,n)165Yb reaction was studied above the reaction threshold between Ec.m. = 12.19 and 16.09 MeV. The fact that the 162Er(a,g)166Yb cross sections were measured below the (a,n) threshold at first time in this mass region opens the opportunity to study directly the a-widths required for the determination of astrophysical reaction rates. The data clearly show that compound nucleus formation in this reaction proceeds differently than previously predicted.
Rare information on photodisintegration reactions of nuclei with mass numbers $A approx 160$ at astrophysical conditions impedes our understanding of the origin of $p$-nuclei. Experimental determination of the key ($p,gamma$) cross sections has been playing an important role to verify nuclear reaction models and to provide rates of relevant ($gamma,p$) reactions in $gamma$-process. In this paper we report the first cross section measurements of $^{160}$Dy($p,gamma$)$^{161}$Ho and $^{161}$Dy($p,n$)$^{161}$Ho in the beam energy range of 3.4 - 7.0 MeV, partially covering the Gamow window. Such determinations are possible by using two targets with various isotopic fractions. The cross section data can put a strong constraint on the nuclear level densities and gamma strength functions for $A approx$ 160 in the Hauser-Feshbach statistical model. Furthermore, we find the best parameters for TALYS that reproduce the A $thicksim$ 160 data available, $^{160}$Dy($p,gamma$)$^{161}$Ho and $^{162}$Er($p,gamma$)$^{163}$Tm, and recommend the constrained $^{161}$Ho($gamma,p$)$^{160}$Dy reaction rates over a wide temperature range for $gamma$-process network calculations. Although the determined $^{161}$Ho($gamma$, p) stellar reaction rates at the temperature of 1 to 2 GK can differ by up to one order of magnitude from the NON-SMOKER predictions, it has a minor effect on the yields of $^{160}$Dy and accordingly the $p$-nuclei, $^{156,158}$Dy. A sensitivity study confirms that the cross section of $^{160}$Dy($p$, $gamma$)$^{161}$Ho is measured precisely enough to predict yields of $p$-nuclei in the $gamma$-process.
The astrophysical capture process $alpha+d$ $rightarrow$ $^6$Li + $gamma$ is studied in a three-body model. The initial state is factorized into the deuteron bound state and the $alpha+d$ scattering state. The final nucleus $^6$Li(1+) is described as a three-body bound state $alpha+n+p$ in the hyperspherical Lagrange-mesh method. The contribution of the E1 transition operator from the initial isosinglet states to the isotriplet components of the final state is estimated to be negligible. An estimation of the forbidden E1 transition to the isosinglet components of the final state is comparable with the corresponding results of the two-body model. However, the contribution of the E2 transition operator is found to be much smaller than the corresponding estimations of the two-body model. The three-body model perfectly matches the new experimental data of the LUNA collaboration with the spectroscopic factor 2.586 estimated from the bound-state wave functions of $^6$Li and deuteron.
(Shorten version of the PRC abstract) Alpha-induced reactions on 127I have been studied using the activation technique in order to provide cross section data for the modeling of the astrophysical gamma process. The relative intensity of the 536.1 keV gamma transition was measured precisely, its uncertainty was reduced from 13% to 4%. By measuring the yield of the characteristic X-rays, the cross sections of the 127I(alpha,gamma)131Cs reaction have been determined for the first time close to the astrophysically relevant energy region, at energies 9.50 < Ec.m. < 15.15$ MeV. The 127I(alpha,n)130Cs reaction was studied in the range 9.62 < Ec.m. < 15.15 MeV by measuring the yield of the 536.1 keV gamma-ray and at the lower part of this energy range by counting the characteristic X-rays. A comparison of the resulting cross sections to predictions of statistical model calculations confirmed the predictions of the astrophysically relevant averaged alpha width. Nevertheless, the newly derived stellar reaction rates at gamma process temperatures for 127I(alpha,gamma)131$Cs and its reverse reaction are factors 4-10 faster than previous calculations, due to improvements in the reaction model.
In the model calculations of heavy element nucleosynthesis processes the nuclear reaction rates are taken from statistical model calculations which utilize various nuclear input parameters. It is found that in the case of reactions involving alpha particles the calculations bear a high uncertainty owing to the largely unknown low energy alpha-nucleus optical potential. Experiments are typically restricted to higher energies and therefore no direct astrophysical consequences can be drawn. In the present work a (p,alpha) reaction is used for the first time to study the alpha-nucleus optical potential. The measured 64Zn(p,alpha)61Cu cross section is uniquely sensitive to the alpha-nucleus potential and the measurement covers the whole astrophysically relevant energy range. By the comparison to model calculations, direct evidence is provided for the incorrectness of global optical potentials used in astrophysical models.
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