THE NEUTRINO-INDUCED NEUTRON SOURCE IN HELIUM SHELL AND R-PROCESS NUCLEOSYNTHESIS

The huge neutrino pulse that occurs during the collapse of a massive s tellar core, is expected to contribute to the origination of a number of isotopes both of light chemical elements and heavy ones. In particu lar, evaporation of neutrons from helium nuclei excited by neutrino-nu clear inelastic collisions, may result in the r-process as it was firs t discussed by Epstein et al. (1988). Here we consider mainly the poss ibility to obtain the considerable amount of neutrons owing to the neu trino breakup of helium nuclei. It is shown that, in general, the heat ing of stellar matter due to the neutrino scattering off electrons and the heat released from the neutrino-helium breakup followed by the th ermonuclear reactions should be taken into account. On the base of kin etic network, using all the important reactions up to Z = 8, the main features and the time-dependent character of the neutrino-driven neutr on flux are investigated. The time-dependent densities of free neutron s produced in helium breakup, Y-n(t), were used to calculate the r-pro cess nucleosynthesis with another full kinetic network for similar to 3200 nuclides. It was found that in the case of metal-deficient stars, Z less than or similar to 0.01 Z., the resulting density of free neut rons seems to be high enough to drive the r-process efficiently under favorable conditions. But it is impossible to obtain a sufficient amou nt of heavy nuclei in neutrino-induced r-process in a helium shell at radii R> R-cr approximate to 10(9)cm. We speculate that to make the ne utrino-induced r-process work efficiently in the shell, one has to inv oke nonstandard presupernova models in which helium hopefully is close r to the collapsed core owing, for instance, to a large scale mixing o r/and rotation and magnetic fields. Apart from this exotic possibility , the neutrino-induced nucleosynthesis in the helium shell is certainl y not strong enough to explain the observed solar r-process abundances .