On the Measurement of the $^{13}\mathrm{C}(\alpha, n)^{16}\mathrm{O}$ ${S}$-factor at Negative Energies and its Influence on the $s$-process
| Pub type: | Article |
| Citation: | 0004-637X-777-2-143 |
| Status: | Published |
| Journal: | The Astrophysical Journal |
| Volume: | 777 |
| Year: | 2013 |
| Month: | October |
| Pages: | 143 |
| URL: | http://stacks.iop.org/0004-637... |
| DOI: | 10.1088/0004-637X/777/2/143 |
| Abstract: | The 13 C(α, n ) 16 O reaction is the neutron source for the main component of the s -process, responsible for the production of most of the nuclei in the mass range 90 ##IMG## [http://ej.iop.org/icons/Entities/lsim.gif] {lsim} A ##IMG## [http://ej.iop.org/icons/Entities/lsim.gif] {lsim} 208. This reaction takes place inside the helium-burning shell of asymptotic giant branch stars, at temperatures ##IMG## [http://ej.iop.org/icons/Entities/lsim.gif] {lsim} 10 8 K, corresponding to an energy interval where the 13 C(α, n ) 16 O reaction is effective in the range of 140-230 keV. In this regime, the astrophysical S ( E )-factor is dominated by the –3 keV sub-threshold resonance due to the 6.356 MeV level in 17 O, giving rise to a steep increase in the S -factor. Its contribution is still controversial as extrapolations, e.g., through the R -matrix and indirect techniques such as the asymptotic normalization coefficient (ANC), yield inconsistent results. The discrepancy amounts to a factor of three or more precisely at astrophysical energies. To provide a more accurate S -factor at these energies, we have applied the Trojan horse method (THM) to the 13 C( 6 Li, n 16 O) d quasi-free reaction. The ANC for the 6.356 MeV level has been deduced through the THM as well as the n -partial width, allowing us to attain unprecedented accuracy for the 13 C(α, n ) 16 O astrophysical factor. A larger ANC for the 6.356 MeV level is measured with respect to the ones in the literature, ##IMG## [http://ej.iop.org/images/0004-637X/777/2/143/apj485964ieqn1.gif] {$(\tilde{C}_{\alpha {}^{13}{\rm C}}^{{}^{17}{\rm O}(1/2^+)})^2=7.7 \pm 0.3_{{\rm stat}} \,^{+1.6}_{-1.5} \,_{{\rm norm}}$} fm –1 , yet in agreement with the preliminary result given in our preceding letter, indicating an increase of the 13 C(α, n ) 16 O reaction rate below about 8 × 10 7 K if compared with the recommended values. At ~10 8 K, our reaction rate agrees with most of the results in the literature and the accuracy is greatly enhanced thanks to this innovative approach. |
| Month_val: | 10 |
| Userfields: | issue = {2} |
| Authors: | |
| Added by: | [DGM] |
| Total mark: | 0 |
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