Alpha decay takes place frequently in massive nuclei that have too large a proton-to-neutron ratio. As shown in Figure 4.8, the helium-4 (4 2He) nucleus is quite stable. Physicists suggest that an unstable massive nucleus sometimes rearranges its excess protons to form a very stable configuration, the alpha particle. This alpha particle then moves around within the parent nucleus trying to “tunnel” out by quantum mechanical processes.
Skipping a great deal of the complex nuclear physics that goes on within the nucleus, it is sufficient to mention here that the alpha particle eventually manages to slip away from the attractive zone of the strong nuclear force. Once the alpha particle tunnels out of the nuclear attractive zone, it finds itself suddenly ejected out of the nucleus by the ever-lurking electrostatic forces present there due to the combined repulsive influence of all the other protons. Equation 4.14 describes the alpha decay reaction for plutonium-239 (239 94Pu)—the human-made, transuranium radioactive isotope of special interest as a fuel both in nuclear weapons and in fast neutron spectrum nuclear reactors.
The parent nuclide (plutonium-239) and the daughter nuclide (uranium235) are different elements, so the alpha decay process converts one chemical element into another. We call this process a nuclear transmutation. Alpha decay is very important in applied nuclear technology. Equation 4.15 provides the general nuclear reaction description of the alpha particle emission process during which an original parent nuclide (P) transforms into its daughter nucleus (D).
In alpha decay, the atomic number changes, so the parent atoms and the daughter atoms are different elements and, therefore, have different chemical properties. Consider the decay of the radioactive isotope polonium-210 (210 84Po) by the emission of an alpha particle. Scientists use the following nuclear reaction to describe the event: Since the polonium-210 nucleus has 84 protons and 126 neutrons, the ratio of protons to neutrons is Z/N = 84/126 or 0.667. The nucleus of the lead206 (206 82Pb) nucleus has 82 protons and 124 neutrons, which means the ratio of protons to neutrons is now Z/N = 82/122, or 0.661. This small change in the proton-to-neutron (Z/N) ratio is sufficient to put the nucleus in a more stable state and brings the lead-206 nucleus into the region of stable nuclei, as shown in Figure 4.7. Polonium was the first element discovered by Marie Curie in 1898 as she searched for the cause of radioactivity in pitchblende.
Polonium-210 has a half-life of 138.38 days and emits a 5.3 MeV alpha particle as it undergoes radioactive decay. Because of this relatively short half-life and high-energy alpha particle, nuclear technologists regard polonium-210 as an intense, potentially dangerous, alpha emitter that must be handled with extreme care and kept under strict control. Essentially all of the emitted alpha radiation is stopped within the polonium metal and its container. Polonium-210 releases thermal energy at a rate of 140 watts per gram, making it a suitable candidate for portable heat-source applications. For example, a sealed metal capsule containing just half a gram of polonium-210 would reach a temperature above 500°C. In alpha decay, the change in the binding energy of the parent nucleus appears as kinetic energy of the alpha particle and the daughter nucleus.