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Register a new userInvestigation of alpha-induced reactions on 130Ba and 132Ba and their importance for the synthesis of heavy p nuclei
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Captures of alpha particles on the proton-richest Barium isotope, 130Ba, have been studied in order to provide cross section data for the modeling of the astrophysical gamma process. The cross sections of the 130Ba(alpha,gamma)134Ce and 130Ba(alpha,n)133Ce reactions have been measured with the activation technique in the center-of mass energy range between 11.6 and 16 MeV, close above the astrophysically relevant energies. As a side result, the cross section of the 132Ba(alpha,n)135Ce reaction has also been measured. The results are compared with the prediction of statistical model calculations, using different input parameters such as alpha+nucleus optical potentials. It is found that the (alpha,n) data can be reproduced employing the standard alpha+nucleus optical potential widely used in astrophysical applications. Assuming its validity also in the astrophysically relevant energy window, we present new stellar reaction rates for 130Ba(alpha,gamma)134Ce and 132Ba(alpha,gamma)136Ce and their inverse reactions calculated with the SMARAGD statistical model code. The highly increased 136Ce(gamma,alpha)132Ba rate implies that the p nucleus 130Ba cannot directly receive contributions from the Ce isotopic chain. Further measurements are required to better constrain this result.
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Cross sections for the 168Yb(alpha,gamma)172Hf and 168Yb(alpha,n)171$Hf reactions were measured by means of the activation method using alpha particles with energies between 12.9 MeV and 15.1 MeV. The spectroscopy of the gamma rays emitted by the reaction products was performed using three different HPGe detector types, namely clover-type high-purity germanium detectors, a low-energy photon spectrometer detector, and a coaxial high-purity germanium detector. The results were compared to Hauser-Feshbach statistical model calculations. Within certain assumptions, astrophysical conclusions could be drawn concerning the production of the p nucleus 168Yb. The data in this work can serve as a contribution to the current very fragmentary experimental data base for charged-particle induced reactions. In addition, the absolute intensity for nine gamma-ray transitions following the electron capture decay of 171Hf could be derived.
(Shorten version of the PRC abstract) Alpha-induced reactions on 127I have been studied using the activation technique in order to provide cross section data for the modeling of the astrophysical gamma process. The relative intensity of the 536.1 keV gamma transition was measured precisely, its uncertainty was reduced from 13% to 4%. By measuring the yield of the characteristic X-rays, the cross sections of the 127I(alpha,gamma)131Cs reaction have been determined for the first time close to the astrophysically relevant energy region, at energies 9.50 < Ec.m. < 15.15$ 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.
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.
The aim of the present work is to measure the $^{121}$Sb($alpha,gamma$)$^{125}$I, $^{121}$Sb($alpha$,n)$^{124}$I, and $^{123}$Sb($alpha$,n)$^{126}$I reaction cross sections. The $alpha$-induced reactions on natural and enriched antimony targets were investigated using the activation technique. The ($alpha$,$gamma$) cross sections of $^{121}$Sb were measured and are reported for first time. To determine the cross section of the $^{121}$Sb($alpha$,$gamma$)$^{125}$I, $^{121}$Sb($alpha$,n)$^{124}$I, and $^{123}$Sb($alpha$,n)$^{126}$I reactions, the yields of $gamma$-rays following the $beta$-decay of the reaction products were measured. For the measurement of the lowest cross sections, the characteristic X-rays were counted with a LEPS (Low Energy Photon Spectrometer) detector. The cross section of the $^{121}$Sb($alpha$,$gamma$)$^{125}$I, $^{121}$Sb($alpha$,n)$^{124}$I and $^{123}$Sb($alpha$,n)$^{126}$I reactions were measured with high precision in an energy range between 9.74 MeV to 15.48 MeV, close to the astrophysically relevant energy window. The results are compared with the predictions of statistical model calculations. The ($alpha$,n) data show that the $alpha$ widths are predicted well for these reactions. The ($alpha$,$gamma$) results are overestimated by the calculations but this is due to the applied neutron- and $gamma$ widths. Relevant for the astrophysical reaction rate is the $alpha$ width used in the calculations.While for other reactions the $alpha$ widths seem to have been overestimated and their energy dependence was not described well in the measured energy range, this is not the case for the reactions studied here. The result is consistent with the proposal that additional reaction channels, such as Coulomb excitation, may have led to the discrepancies found in other reactions.
One of the few p nuclei with an odd number of protons is 113In. Reaction cross sections of 113In(alpha,gamma)117Sb and 113In(alpha,n)116Sb have been measured with the activation method at center-of-mass energies between 8.66 and 13.64 MeV, close to the astrophysically relevant energy range. The experiments were carried out at the cyclotron accelerator of ATOMKI. The activities were determined by off-line detection of the decay gamma rays with a HPGe detector. Measured cross sections and astrophysical S factor results are presented and compared with statistical model calculations using three different alpha+nucleus potentials. The comparison indicates that the standard rates used in the majority of network calculations for these reactions were too fast due to the energy dependence of the optical alpha potential at low energy.
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