Development of a Liquid Scintillator and of Data ... - Borexino - Infn

1 Solar Neutrinos
Figure 1.1: The pp-cycle is the dominant energy generation mechanism in the sun. It is subdivided
into four chains: pp-I 85 %, pp-II 14 %, pp-III 0.016 %, pp-IV . The reactions, in
which neutrinos are emitted, are framed.
(2) The sun is in hydrostatic and thermal equilibrium. This means that the hydrostatic pressure
resulting from thermonuclear fusion counterbalances the gravity, and that the energy produced
by nuclear reactions balances the total energy loss, which is the sum of the radiative
energy flux (luminosity) and the energy carried away by neutrinos.
(3) Energy transport from the solar center, where nuclear fusion takes place (Ö Ê¬),
outwards to the surface occurs, apart from neutrinos, primarily by electromagnetic radiation
and, for Ö Ê¬, additionally by convection.
(4) Initially, the chemical composition of the sun was homogeneous. The local abundances
of individual isotopes in the solar interior have slowly changed with time due to nuclear fusion,
while in the surface layers the initial elemental abundances are preserved.
The boundary conditions for the model are the sun’s known characteristics: the solar radius,
mass, surface temperature, luminosity and age. Other measured input data are nuclear reaction
cross sections, radiative opacities and isotopic abundances.
The system of differential equations for the solar structure and evolution is solved numerically.
It yields, among other things, predictions for the solar core temperature Ì , the temperature
and density profile inside the sun, and the rates at which the various nuclear fusion reactions
contribute to the He generation, hence the solar neutrino spectrum and flux. The solar neutrino
spectrum according to the SSM [Bah98] is shown in fig. 1.2 and the expected neutrino fluxes
on earth for the individual neutrino branches are listed in table 1.1.
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