Astrophysical S(E) factor of 3He(3He,2p)4He at solar energies

The 3He(3He,2p)4He reaction has been investigated in the energy range Ec.m. = 17.9 to 342.5 keV. The studies involved high-current accelerators with well-known beam characteristics and windowless gas target systems. At low energies the studies required the measurement of coincidences between the two protons in the exit channel as well as precautions for contributions from cosmic background and electronic noise. The data extend into the thermal energy region of the sun, e.g., σ = 7±2 pb at Ec.m. = 24.5 keV, and upper limits for the reaction yield have been obtained down to Ec.m.≅17.9 keV. No evidence for a suggested low-energy resonance has been found. The astrophysical S(E) factor at zero energy is S(0) = 5.57±0.32 MeV . b. The implications of the data for the solar-neutrino problem are discussed. (orig.)