THE NEUTRINO-PROCESS

As the core of a massive star collapses to form a neutron star, the flux of neutrinos in the overlying shells of heavy elements becomes so great that, despite the small cross section, substantial nuclear transmutation is induced. Neutrinos, especially the higher energy μ- and τ-neutrinos, excite heavy elements and even helium to particle unbound levels. The evaporation of a single neutron or proton, and the back reaction of these nucleons on other species present, significantly alters the outcome of traditional nucleosynthesis calculations leading to a new process: ν-nucleosynthesis. Modifications to traditional hydrostatic and explosive varieties of helium, carbon, neon, oxygen, and silicon burning are considered. The results show that a large number of rare isotopes, including many of the odd-Z nuclei from boron through copper, owe much of their present abundance in nature to this process. Specific nuclei due almost entirely to the ν-process are 7Li, 11B, 19F, 138La, and 180Ta. Significant amounts of 10B, 15N, 22Na, 26Al, 31P, 35Cl, 39,40,41K, 45Sc, 47,49Ti, 50,51V, 55Mn, 59Co, and 63Cu are also produced, so much so that, within the uncertainties of the model, these nuclei also might owe their origin predominantly to the ν-process. Neutrino-induced production of 11B argues against the existence of an unobserved low-energy component of cosmic rays, frequently invoked in spallation scenarios to account for the observed isotopic ratio of boron. Despite our success in producing many intermediate-mass isotopes, we find that the recently suggested neutrino-induced r-process in the helium shell is quite small in any of the realistic scenarios we explored.