THE SCREENED NUCLEAR-REACTION RATES AND THE SOLAR-NEUTRINO PUZZLE

This paper reexamines the nuclear reaction rates used for hydrogen bur ning in the classical framework of stellar evolution. The screening ef fect is discussed, and the formalisms generally used appear not accura te enough for the determination of the solar neutrino fluxes. So we an alyze screening with the more realistic potential suggested by Mitler. We compare the first-order description which allows factorization of the screening correction with a complete calculation in which an ellip soidal charge distribution around reacting ions is introduced. Since t he radial deformation of the charge distribution appears to have a sma ll effect and is of the same order as and of opposite sign to the dyna mical effect of the surrounding ions, we conclude that the factorized prescription of Mitler seems really easy to implement and appropriate within 2% accuracy for the whole hydrogen-burning phase and stellar ma sses between 0.6 and 20 M.. It leads to an enhancement factor of the r eaction rates located between those of Salpeter weak screening and Gra boske et al, intermediate screening. Then we consider each reaction ra te of the p-p chain, and it appears that two reactions may still be mo dified significantly beyond the ''conventional error bar'': the electr onic capture on Be-7 and the proton interaction on the same nucleus, b oth of which influence the B-8 neutrino flux. Considering reasonable v ariations of the nuclear reaction rates of the p-p chains, compatible with the present experimental and theoretical situation, the solar neu trino discrepancy between experiments and predictions could be conside rably reduced.