Last revised by Daniel J Bell on 21 Feb 2024

Radioactivity, also known as radioactive decay, describes the process of spontaneous breakdown of unstable (or radioactive) nuclides, with the formation of daughter nuclei and release of subatomic particles and/or gamma radiation. A single decay (a.k.a. disintegration) refers to the degradation of one nuclide into another. 

A nucleus is considered absolutely stable if it does not decay at all 7 or is relatively stable if it has a half-life of 4.5 billion years (compared to the age of the earth at about 5 billion years) 5.

There are four fundamental forces of nature, namely: electromagnetic, weak, strong, and gravity. Inside a nucleus, the strong force is the principal force that holds the nucleons (protons and neutrons) together while the electromagnetic force acts as a repulsive force due to positive charges of protons. The proportions of the strong and electromagnetic forces affect the binding energy of the nucleus 5.

The nuclear binding energy is defined as the amount of energy required to completely separate the nucleons. The nuclear binding energy is also equal to the mass defect, which is the difference between the measured mass of an atom and the sum of the masses of the nucleons. Nuclear binding energy is equivalent to mass defect due to the mass-energy equivalence as described in theory of relativity. The binding energy of a nucleus is very high, thus it is measured in MeV as opposed to KeV as in electron binding energy 6.

When the mass number increases, the binding energy per nucleon rises to a peak of 8.8 MeV, which is found in iron group of isotopes such as iron-56 and iron-58 and also nickel-62. Nickel-62 has the most tightly bound nucleus. After iron, the binding energy per nucleon reduces gradually 6. Therefore, nucleons that have a higher mass number than iron would decay gradually to form nuclei that are more tightly bound together 5.

When the mass number increases, the ratio of neutrons to protons also increases in order to maintain the nuclei stability. For example, helium-4 has a neutron to proton ratio of 1. For indium-115, the neutron to proton ratio increased to 1.35, while uranium-238 has neutron to proton ratio of 1.59 5. There are several "magic numbers" for protons and neutrons that give stability to the nucleus. For example, nuclei that have either 2, 8, 20, 28, 50, and 82 protons or neutrons are stable. In addition to these, nuclei with 126 and 184 neutrons or 114 protons are also stable 8. Thus, the nucleons that deviates from this band of stability will decay in various modes 5. Alpha decay occurs if there are too many nucleons, beta minus decay occurs when there are too much neutrons and beta plus decay occurs when there are too few neutrons or too much protons. The weak nuclear force mediates the transformation of a proton to a neutron and vice versa in a beta decay.

Radioactive decay is a stochastic process, i.e. it is probabilistic, and it is impossible to foresee which specific nuclei will decay. Nevertheless, it can be predicted with a high degree of confidence the proportion of any sample of radioactive atoms that will decay in a specified period of time.

Henri Becquerel (1852-1908) discovered radioactivity in 1896 when studying potassium uranyl sulfate 2. Further early advances were made by wife and husband, Marie Skłodowska Curie (1867-1934) and Pierre Curie (1859–1906), who were co-awarded the Nobel Prize in Physics with Becquerel in 1903. It was Marie Curie who coined the term radioactivity 4

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