ON THE MEASUREMENT OF THE¹³C(α,n)¹⁶OS-FACTOR AT NEGATIVE ENERGIES AND ITS INFLUENCE ON THEs-PROCESS

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 A 208. This reaction takes place inside the heliumburning shell of asymptotic giant branch stars, at temperatures 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, (C 17 O(1/2 + ) α 13 C ) 2 = 7.7 ± 0.3 stat +1.6 −1.5 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.

The Trojan Horse Method for nuclear astrophysics and its recent applications

“…However, it must be stressed here that the TH S(E)-factor determination requires a normalization procedure to the available direct measurements performed at higher energies, thus rendering the THM a complementary experimental technique for nuclear astrophysics aimed at reaching the ultra-low energy region of interest for astrophysical applications. THM has been used in studying several problems, ranging from BBN [7], light element burning reactions [8][9][10][11][12], CNO reactions [13][14][15], explosive nucleosynthesis involving RIB [16,17] and removing/producing neutron reactions [18,19].…”

“…More advanced approaches have been introduced in later years, allowing us to justify and generalize Eq. 1, releasing some approximations contained in it (see, for instance, [5,6,15,19]). In order to assess the method, some of the most important sources of uncertainties in a typical THM experiment and/or data analysis need to be underlined:…”

Does the main component of the s-process in AGB stars constrain the neutron source in the 13C-pocket?

“…The second issue is strongly related to the rate of the 13 C(α, n) 16 O reaction, which is diffusely discussed in [10,11]. Entering into the details of its measurements is not the aims of this work.…”

“…However, in addition our scheme now also reproduces the chemical evolution of the Galaxy, an achievement that was previously impossible. The calculations were obtained by using the NEWTON nucleosynthesis code, in which we adopted new reaction rate estimates for the two main neutron sources, ( [11,13], respectively). We also included the most recent cross sections for neutron capture reactions (n, γ), as provided by the KaDONIS database verion 1.0; we used solar abundances as suggested by [12].…”

Measurement of the B10(p,α0)Be7 cross section from 5 keV to 1.5 MeV

“…It has been proved that the HOES cross section is essentially linked to the OES one by the penetration factor of the Coulomb barrier [47][48][49][50], calculated introducing the correct wave number and interaction radius for the p-10 B system. The THM cross section is then obtained through the relation:…”