Radioactivity

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. 

Nuclei stability

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 binding energy is defined as the amount of energy required to completely separate the nucleons. The 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. 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 orbitals ⁶.

When the mass number increases, the binding energy per nucleon is plotted against the mass number, the graph rises to a peak of 8.8 MeV binding energy per nucleon, which is found in iron group of isotopes such as Iron-56 and Iron-58 also 62-NickelNickel-62. 62-NickelNickel-62 has 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 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 wih 126 and 184 neutrons or 114 protons are also stable ⁸. Thus, the nucleons that deviates from this band of stability towill 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.

Modes of decay

• alpha decay

• beta decay

• electron capture

• spontaneous fission

• isomeric transition

  • gamma (γ) ray emission (gamma decay)

  • internal conversion

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.

History and etymology

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.