Experimental determination of the 3He(& alpha;, & gamma;) 'Be reaction cross section above the 'Be proton separation threshold

Background: The 3He(& alpha;, & gamma; ) 7Be reaction plays a major role both in the big bang nucleosynthesis producing the majority of the primordial 7Li, and in the pp chain of solar hydrogen burning, where it is the branching point between the pp-I and pp-II,-III chains. As a few-nucleon system, this reaction is often used to validate ab initio theoretical calculations and/or test R-matrix theory and code implementations. For the latter, experimental data in an extended energy range is of crucial importance to test the fit and extrapolation capabilities of the different codes.Purpose: The 3He(& alpha;, & gamma; ) 7Be reaction cross section has been measured by several groups up to the first resonance (Ec.m. & AP; 3 MeV) in the reaction. However, only one data set exists above the 7Be proton separation threshold measured in a narrow energy range (Ec.m. = 4.0-4.4 MeV). In this work we extend the available experimental capture cross section database to the energy range of known 7Be levels, where only particle scattering experiments are available for testing the models.Method: The activation method was used for the 3He(& alpha;, & gamma; ) 7Be reaction cross section determination. The experiment was performed using a thin-window gas cell with two high-purity Al foils as entrance and exit windows. The activity of the 7Be nuclei implanted in the exit/catcher foil was measured by detecting the yield of the emitted & gamma; rays using shielded high-purity germanium detectors.Results: New experimental 3He(& alpha;, & gamma; ) 7Be reaction cross section data were obtained for the first time in the Ec.m. = 4.3-8.3 MeV energy region, corresponding to Ex = 5.8-10 MeV excitation energies of 7Be. The new data set with about 0.2 MeV step covers the energy range of known levels and particle separation thresholds. No prominent structures are observer around the 7Be levels.Conclusions: The measured reaction cross section is slowly increasing with increasing energy in the range of Ex = 6-8 MeV from 10 & mu;b to 13 & mu;b. Above the 6Li +p1 threshold, a decrease starts in the cross section trend and reaches a value of about 8 & mu;b around Ex = 10 MeV. The overall structure of the cross section suggest a broad resonance peaking around Ex = 7.5 MeV 7Be excitation energy, with a width of 8 MeV.