Fission fragments have an excess of neutrons
Newly born fission products inherit the neutron: proton ratio present in their parent nucleus. On the map of nuclei, the fragments of uranium fission are located on the straight line which links the fissile uranium nucleus to the origin. In the example shown above, we can see that Br 90 and Xe 143 are both in the blue area of nuclei with an excess of neutrons and thus capable of emitting beta-negative radiation. The two fragments will finally join the black area of stable (or quasi-stable) nuclei through a chain of beta decays. The insert shows the path undertaken by the fragment of bromine 90 as it decays into a stable nucleus of zirconium 90.