Cecilio Angulo

Isotopic dependence of electron screening in fusion reactions

The fusion reactions6Li(p, α)3He,6Li(d, α)4He, and7Li(p, α)4He have been studied over the c.m. energy rangeE=10 to 1450 keV. Each reaction involved the use of hydrogen projectiles and LiF solid targets as well as Li projectiles and hydrogen molecular gas targets. In all cases the effects of electron screening on the low-energy fusion cross sections (exponential enhancement) have been observed; the effects are somewhat stronger in the case of atomicp ord projectiles compared to the case of molecularH2 orD2 gas targets. If isotopic effects on electron screening are negligible, all three reactions should exhibit the same enhancements for each set of experimental techniques. The measurements confirmed this expectation to a large extent.

Test for isotopic dependence of electron screening in fusion reactions

Abstract The fusion reactions 6Li(p, α) 3He, 6Li(d, α) 4He and 7Li(p, α) 4He have been studied over the CM energy range E = 10–1004 keV. The effects of electron screening have been observed in all three reactions exhibiting the same enhancements for a given experimental technique. The deduced values for the screening potential energy are significantly larger compared to values calculated via the united atom test. This difference between observation and expectation is not understood at present.

The effects of electron screening and resonances in (p, α) reactions on10B and11B at thermal energies

The fusion reactions10B(p, α)7Be and11B(p, α)8Be have been studied over the c.m. energy range E=17 to 134 keV using intense proton beams and thick solid targets. In the case of11B(p, α)8Be the low-energy data in terms of the astrophysical S(E) factor show an exponential enhancement (up to a factor of 1.9) due to the effects of electron screening, where the deduced screening potential is larger than expected. In the case of10B(p, α)7Be the low-energy data exhibit an enhancement by more than a factor of 200, which cannot be explained by the effects of electron screening. The enhancement arises here from the high-energy tail of an expecteds-wave resonance at ER=10 keV. The results offer an improved prospect for this reaction as advanced fuel in future fusion reactors than previously envisioned.

The S(E) factor of7Li(p, ?)8Be and consequences for S(E) extrapolation in7Be(p, ?0)8B

Excitation functions and forward-backward anisotropies have been measured for the7Li(p, γ)8Be capture reaction over the proton energy rangeEp=100 to 1500 keV, using a 4π summing crystal and Ge(Li) detectors, respectively. The data show at all energies the presence of El and M1 capture amplitudes arising from the direct capture (DC) process and theER=441 and 1030 keV resonances, respectively. Due to the observed DC process, the present data increase significantly the reaction rates (up to a factor of 110) compared to values given in the compilation. The data and their analyses remove the recent criticism on DC model calculations, which had implied a significant reduction in the extrapolated S(E) factor for7Be(p,γ)B and thus in the predicted flux of high-energy solar neutrinos; thus, the solar neutrino problem is still with us.

AstrophysicalS(E) factor of10B(p, α1)7Be at low energies

The absolute cross section of the fusion reaction10B(p, α1γ)7has been measured at the c.m. energy range E=48 to 159 keV via the 429 keV secondary γ-ray transition. The studies involved high proton currents and thick solid10Btargets. The resulting cross sections have the same energy dependence as those of the10B(p,α0)7Bereaction but they are lower by a factor 600.