THE RGB AND AGB STAR NUCLEOSYNTHESIS IN LIGHT OF THE RECENT 17O(p, α)14N AND 18O(p, α)15N REACTION-RATE DETERMINATIONS

In recent years, the Trojan Horse Method (THM) has been used to investigate the low-energy cross sections of proton-induced reactions on A = 17 and A = 18 oxygen isotopes, overcoming extrapolation procedures and enhancement effects due to electron screening. In particular, the strengths of the 20 keV and 65 keV resonances in the {sup 18}O(p, {alpha}){sup 15}N and {sup 17}O(p, {alpha}){sup 14}N reactions, respectively, have been extracted, as well as the contribution of the tail of the broad 656 keV resonance in the {sup 18}O(p, {alpha}){sup 15}N reaction inside the Gamow window. The strength of the 65 keV resonance in the {sup 17}O(p, {alpha}){sup 14}N reaction, measured by means of the THM, has been used to renormalize the corresponding resonance strength in the {sup 17}O + p radiative capture channel. As a result, more accurate reaction rates for the {sup 18}O(p, {alpha}){sup 15}N, {sup 17}O(p, {alpha}){sup 14}N, and {sup 17}O(p, {gamma}){sup 18}F processes have been deduced, devoid of systematic errors due to extrapolation or the electron screening effect. Such rates have been introduced into state-of-the-art red giant branch and asymptotic giant branch (AGB) models for proton-capture nucleosynthesis coupled with extra-mixing episodes. The predicted abundances have been compared with isotopicmore » compositions provided by geochemical analysis of presolar grains. As a result, an improved agreement is found between the models and the isotopic mix of oxide grains of AGB origins, whose composition is the signature of low-temperature proton-capture nucleosynthesis. The low {sup 14}N/{sup 15}N found in SiC grains cannot be explained by the revised nuclear reaction rates and remains a serious problem that has not been satisfactorily addressed.« les

Trojan Horse Particle Invariance: An Extensive Study

In the last decades, the Trojan Horse method (THM) has played a crucial role for the measurement of several particle (both neutron and charged one) induced cross sections for reactions of astrophysical interest. To better understand its cornerstones and its applications to physical cases, many tests were performed to verify all its properties and the possible future perspectives. The Trojan Horse nucleus invariance proves the relatively simple approach allowed by the pole approximation and sheds light in the involved reaction mechanisms. Here we shortly review the complete work for the binary 2H(d,p)3H, 6Li(d,α)4He, 6Li(p,α)3He, 7Li(p,α)4He reactions, by using the quasi free reactions after break-ups of different nuclides. Results are compared assuming the 6Li and 3He break-up in the case of the d(d,p)t, 6Li(d,α)4He reactions and considering the 2H and 3He break-up for 6Li(p,α)3He, 7Li(p,α)4He reactions. These results, regardless of the Trojan Horse particle or the break-up scheme, confirms the applicability of the standard description of the THM and suggests the independence of binary indirect cross section on the chosen Trojan Horse nuclei for a whole spectra of different cases. This gives a strong basis for the understanding of the quasi-free mechanism which is the foundation on which the THM lies

New direct measurement of the 10 B(p,α) 7 Be reaction with the activation technique

Boron plays an important role in astrophysics and, together with lithium and beryllium, is a probe of stellar structure during the pre-main sequence and main-sequence phases. In this context, the 10 B(p, α ) 7 Be reaction is of particular interest.The literature data show discrepancies in the energy range between 100 keV and 2 MeV. This also poses a normalization problem for indirect data obtained with the Trojan Horse Method.A new measurement of the 10 B(p, α ) 7 Be reaction cross section was performed at Legnaro National Laboratories (LNL). At LNL, the cross section was determined with the activation technique by measuring the activated samples at a low-background counting facility. The analysis of that experiment is now complete and the results are here presented

New measurement of the 10B(p,α0)7Be reaction cross section at low energies and the structure of 11C

We report preliminary results of a new measurement of the 10 B(p,α 0 )7 Be cross section in the 0.6 - 1.0 MeV bombarding energy domain. In this region very few data have been reported in the literature. Excitation functions at both forward and backward angles have been obtained. Angular distributions testify the contribution due to several excited states in the 11 C compound nucleus. The experimental S -factor is about 30% lower than the one reported in the literature and based on activation methods

Assessing the near threshold cross section of the O 17 ( n , α ) C 14 reaction by means of the Trojan horse method

The study of the ^{17}\mathrm{O}(n,\ensuremath{\alpha})^{14}\mathrm{C} reaction has been performed by means of the Trojan horse method (THM) applied to the quasifree ^{2}\mathrm{H}(^{17}\mathrm{O},\ensuremath{\alpha}^{14}\mathrm{C})^{1}\mathrm{H} reaction induced at a beam energy of 43.5 MeV. The THM allowed us to study the 8121-keV $^{18}\mathrm{O}^{*}$ resonant level, for which the previous THM investigation pointed out the ability of the method to overcome the centrifugal barrier suppression effects in the entrance channel. Here, in view of the developments of the method for resonant reactions, the detailed analysis of the performed experiment will be discussed, focusing on the extraction of the 8121-keV resonance strength for which no information is present in scientific literature. Moreover, the experimental results clearly show the excitation of the subthreshold level centered at \ensuremath{-}6 keV in the center-of-mass system, which is fundamental to determine the ^{17}\mathrm{O}(n,\ensuremath{\alpha})^{14}\mathrm{C} reaction rate of astrophysical interest. Finally, a new recommended reaction rate is presented for future astrophysical application

Resonant C-Burning at Astrophysical Energies

n/

10

The 10B(n,α) reaction cross-section is a well-established neutron cross-section standard for incident neutron energies up to 1 MeV. However, above this energy limit there are only scarce direct (n,α) measurements available and these few experimental data are showing large inconsistencies with each other. These discrepancies are reflected in the evaluated data libraries: ENDF/B-VII.1, JEFF-3.1.2 and JENDL-4.0 are in excellent agreement up to 100 keV incident neutrons, whereas the 10B(n,α) data in the different libraries show large differences in the MeV region. To address these inconsistencies, we have measured the cross section of the two branches of the 10B(n,α) reaction for incident neutron energies up to 3 MeV. We present here the 10B(n,α) and the 10B(n,α1γ) reactions cross section data, their branching ratio and the total 10B(n,α) reaction cross section. The measurements were conducted with a dedicated Frisch-grid ionization chamber installed at the GELINA pulsed neutron source of the EC-JRC. We compare our results with existing experimental data and evaluations