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Register a new userSpectroscopic factor of the 1+, 25Al(p,g)26Si resonance at Ex=5.68 MeV
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Nuclear shell model predictions for the proton spectroscopic factor of the 1+, Ex = 5.68 MeV level in 26Si are about fifty times smaller than the value suggested by the measured (a,3He) cross section for the Ex = 5.69 MeV mirror level in 26Mg, assuming purely single-particle transfer. Given that the 5.69 MeV level has been very weakly, if it all, populated in previous studies of the simpler 25Mg(d,p) reaction, it is unclear if the (a,3He) result is a true single-particle spectroscopic factor. If we assume the (a,3He) result, the thermonuclear rate of the 25Al(p,g)26Si reaction would increase by factors of 6 - 50 over stellar temperatures of T = 0.05 - 0.2 GK. We examine the implications of this enhanced rate for model predictions of nucleosynthesis in classical nova explosions.
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We examine the impact of the strength of the E_R = 127 keV, 26Al(p,g)27Si resonance on 26Al production in classical nova explosions and asymptotic giant branch (AGB) stars. Thermonuclear 26Al(p,g)27Si reaction rates are determined using different assumed strengths for this resonance and representative stellar model calculations of these astrophysical environments are performed using these different rates. Predicted 26Al yields in our models are not sensitive to differences in rates determined using zero and a commonly stated upper limit corresponding to wg_UL = 0.0042 micro-eV for this resonance strength. Yields of 26Al decrease by 6% and, more significantly, up to 30%, when a strength of 24 x wg_UL = 0.1 micro-eV is assumed in the adopted nova and AGB star models, respectively. Given that the value of wg_UL was deduced from a single, background-dominated 26Al(3He,d)27Si experiment where only upper limits on differential cross sections were determined, we encourage new experiments to confirm the strength of the 127 keV resonance.
The neutron capture cross section of 14C is of relevance for several nucleosynthesis scenarios such as inhomogeneous Big Bang models, neutron induced CNO cycles, and neutrino driven wind models for the r process. The 14C(n,g) reaction is also important for the validation of the Coulomb dissociation method, where the (n,g) cross section can be indirectly obtained via the time-reversed process. So far, the example of 14C is the only case with neutrons where both, direct measurement and indirect Coulomb dissociation, have been applied. Unfortunately, the interpretation is obscured by discrepancies between several experiments and theory. Therefore, we report on new direct measurements of the 14C(n,g) reaction with neutron energies ranging from 20 to 800 keV.
The 15N(p,g)16O reaction represents a break out reaction linking the first and second cycle of the CNO cycles redistributing the carbon and nitrogen abundances into the oxygen range. The reaction is dominated by two broad resonances at Ep = 338 keV and 1028 keV and a Direct Capture contribution to the ground state of 16O. Interference effects between these contributions in both the low energy region (Ep < 338 keV) and in between the two resonances (338 <Ep < 1028 keV) can dramatically effect the extrapolation to energies of astrophysical interest. To facilitate a reliable extrapolation the 15N(p,g)16O reaction has been remeasured covering the energy range from Ep=1800 keV down to 130 keV. The results have been analyzed in the framework of a multi-level R-matrix theory and a S(0) value of 39.6 keV b has been found.
New experimental data for the inclusive reactions (p,xp) and (p,xd) on isotopes of the nuclei $^{90,92}$Zr and $^{92}$Mo, have been measured at E$_{p}$=30.3 MeV, which has not been investigated in detail so far. We show the extension of the pre-equilibrium reactions to this energy region and interpret the results of these experiments. Moreover, we display the mechanism of the reaction and the level of energy-dependence. The adequacy of the theoretical models in explaining the measured experimental data is also discussed. In our theoretical analysis, the contributions of multi-step direct and compound processes in the formation of cross-sections are determined and we assert that the traditional frameworks are valid for the description of the experimental data.
It is shown that a Coulomb suppression of the stellar enhancement factor occurs in many endothermic reactions at and far from stability. Contrary to common assumptions, reaction measurements for astrophysics with minimal impact of stellar enhancement should be preferably performed for those reactions instead of their reverses, despite of their negative reaction Q-value. As a demonstration, the cross section of the astrophysically relevant 85Rb(p,n)85Sr reaction has been measured by activation between 2.16<=E_{c.m.}<= 3.96 MeV and the astrophysical reaction rates at p-process temperatures for (p,n) as well as (n,p) are directly inferred from the data. Additionally, our results confirm a previously derived modification of a global optical proton potential. The presented arguments are also relevant for other alpha- and proton-induced reactions in the p-, rp-, and nu-p-processes.
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