For the purposes of this problem you may ignore the rest energy of the electrons and their binding energy. Fusion in the sun is a complicated process, but the net effect is the conversion of four protons into an alpha particle, or a helium-4 nucleus. This is what powers the sun. At the earth's orbit we get about from the sun. Imagine a cylindrical water tank of end cross-sectional area and length , with neutrinos passing through the tank in a direction parallel to the axis of the cylinder.
Water has a density of about .
Suppose that fission of a uranium-235 nucleus induced by absorbing a slow neutron ultimately results in two equal nuclei plus two neutrons. The fission products must beta decay until they reach the line of stability on the N-Z plot. Thus, the final state consists of the two free neutrons, two nuclei with the same value of A as the fission products, but with some of the neutrons converted to protons, and the resulting electrons and neutrinos.