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In the last example, nickel, which has 28 protons and 38 neutrons, emits a beta particle, or electron, and becomes copper, which has 29 protons and 37 neutrons. This is still too high a neutron-to-proton ratio, so the copper nucleus emits another electron and becomes a stable zinc nucleus, which has 30 protons and 36 neutrons. Note that each beta decay is a diagonal step one place up and to the left on the stability plot, and that this step brings the nucleus closer to the region of stability. |
Nuclei with too many protons,
above and to the left of the stable zone on the graph, can
decay by two routes that lead to the same place. In some cases
a positron is emitted from the nucleus, and one proton changes
into a neutron. A positron is a particle with the mass of
an electron but with a +1 charge, and is represented by |
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or sometimes by 
. Examples of 
