arXiv subjects

E. Koshchiy

Publications and source records attributed to E. Koshchiy.

21 records · Page 2Linked to original sources

arXiv2015

New $α$ ANC measurement of the 1/2$^+$ state in $^{17}$O at 6.356 MeV, that dominates the $^{13}$C($α$,n)$^{16}$O reaction rate at temperatures relevant for the s-process

Background: Accurate knowledge of the $^{13}$C($α$,$n$)$^{16}$O reaction cross section is important for the understanding of the s-process in AGB stars, since it is considered to be the main source of neutrons. The sub-threshold 1/2$^+$ state at excitation energy of 6.356 MeV in $^{17}$O has a strong influence on the reaction cross section at energies relevant for astrophysics. Several experiments have been performed to determine the contribution of this state to the $^{13}$C($α,n)^{16}$O reaction rate. Nevertheless, significant discrepancies between different measurements remain. Purpose: The aim of this work is to investigate these discrepancies. Method: An 8 MeV $^{13}$C beam (below the Coulomb barrier) was used to study the $α$-transfer reaction $^6$Li($^{13}$C,$d$)$^{17}$O. Results: The squared Coulomb modified ANC of the 1/2$^+$ state in $^{17}$O measured in this work is $(\tilde C^{^{17}\text{O}(1/2+)}_{α-^{13}\text{C}})^2=3.6\pm0.7\hspace{0.2cm}\text{fm}^{-1}$. Conclusions: Discrepancy between the results of $α$-transfer experiments have been resolved. However, some discrepancy with the most recent measurement using the Trojan Horse method remains.

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arXiv2014

Constraining the 6.05 MeV 0$^+$ and 6.13 MeV 3$^-$ cascade transitions in the $^{12}$C($α$,$γ$)$^{16}$O reaction using the Asymptotic Normalization Coefficients

Background. The $^{12}$C$(α,γ)^{16}$O reaction plays a fundamental role in astrophysics and needs to be known with accuracy better than 10%. Cascade $γ$ transitions through the excited states of $^{16}$O are contributing to the uncertainty. Purpose. To constrain the contribution of the 0$^+$ (6.05 MeV) and 3$^-$ (6.13 MeV) cascade transitions. Method. The $α$ Asymptotic Normalization Coefficient for these states were measured using the $α$-transfer reaction $^{12}$C$(^6$Li,$d)^{16}$O at sub-Coulomb energies. Results. The 0$^+$ and 3$^-$ cascade transitions contribution was found to be 1.96$\pm$0.3 keV b and 0.12$\pm$0.04 keV b for destructive interference of the direct and resonance capture and 4.36$\pm$0.45 keV b and 1.44$\pm$0.12 keV b for constructive interference respectively. Conclusions. The combined contribution of the 0$^+$ and 3$^-$ cascade transitions to the reaction at 300 keV does not exceed 4%. Significant uncertainties have been dramatically reduced.

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arXiv2014

$α$-cluster ANCs for nuclear astrophysics

Background. Many important $α$-particle induced reactions for nuclear astrophysics may only be measured using indirect techniques due to small cross sections at the energy of interest. One of such indirect technique, is to determine the Asymptotic Normalization Coefficients (ANC) for near threshold resonances extracted from sub-Coulomb $α$-transfer reactions. This approach provides a very valuable tool for studies of astrophysically important reaction rates since the results are practically model independent. However, the validity of the method has not been directly verified. Purpose. The aim of this letter is to verify the technique using the $^{16}$O($^6$Li,$d$)$^{20}$Ne reaction as a benchmark. The $^{20}$Ne nucleus has a well known $1^-$ state at excitation energy of 5.79 MeV with a width of 28 eV. Reproducing the known value with this technique is an ideal opportunity to verify the method. Method. The 1$^-$ state at 5.79 MeV is studied using the $α$-transfer reaction $^{16}$O($^6$Li,$d$)$^{20}$Ne at sub-Coulomb energies. Results. The partial $α$ width for the $1^-$ state at excitation energy of 5.79 MeV is extracted and compared with the known value, allowing the accuracy of the method to be evaluated. Conclusions. This study demonstrates that extracting the Asymptotic Normalization Coefficients using sub-Coulomb $α$-transfer reactions is a powerful tool that can be used to determine the partial $α$ width of near threshold states that may dominate astrophysically important nuclear reaction rates. \end{description}

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