What is Radioactivity?

Radioactivity is a process in which unstable atomic nuclei release subatomic particles. This phenomenon is observed in heavy elements such as uranium. It was Henri Becquerel, a French scientist, to observe the blackening of a photographic plate due to uranium salts when placed in the dark. Becquerel, the SI unit for radioactivity has been named in his honor.

Unstable atomic nuclei spontaneously decompose to form stable nuclei. This decomposition of unstable nuclei of heavy elements is known as radioactivity. The process of decomposition leads to emission of different kinds of particles as also emission of energy. The nuclei of unstable atoms break down due to their instability.

Their breakdown is known as decay. It results in the formation of stable nuclei accompanied by radiation of energy. Nuclear fission can also result in radioactivity. Fission, either spontaneous or one which is initiated in a nuclear reactor, releases energy when heavy nuclei split.

The atomic nuclei that tend to decompose to form stable atoms and emit energy, are called radioactive elements.

Radioactive elements such as uranium, potassium, and thorium as also the isotopes of elements like carbon, bismuth, and strontium decay to form lighter atoms. The energy they release during this process is in the form of fast-moving particles and high-energy waves. Radioactivity is a random process. The time taken for half of the nuclei of the atoms of radioactive elements to decay, is known as the half-life of that element.

The other quantities associated with the measurement of radioactive decay, are mean lifetime and decay constant. Mean lifetime is the average lifetime of a radioactive particle while decay constant in the inverse of mean lifetime. The half-life, decay constant, and mean lifetime are constants associated with radioactive elements. The time-variable quantities associated with radioactivity are total activity, number of particles, and specific activity.

The total activity is the number of decays an object undergoes per second while the specific activity of an element is the number of decays per second per amount of unit mass of a substance. The number of particles is the measurement of the total number of particles in a given radioactive sample.

There are three main types of natural radioactivity namely, alpha, beta, and gamma.

• The alpha radiation consists of a stream of positively charged particles, which are actually helium nuclei as they have an atomic mass of four and a positive charge of two units. When a radioactive nucleus emits an alpha particle, the mass number of the nucleus decreases by four units and the atomic number decreases by two.

• The emission of beta rays from a radioactive nucleus results in an increase of one unit in the atomic number of the element. However, its mass number remains unchanged. Beta radiation is the emission of a stream of electrons. Thus, a beta particle is actually an electron.

• Gamma rays are photons of short wavelengths and they carry high energies. A gamma ray emitted from an atomic nucleus changes neither the atomic number of the element nor its atomic mass. The emission of gamma rays often accompanies alpha and beta radiations. On gamma radiation, the unstable nucleus converts into one with a lower and more stable energy state.

• Radioisotopes find application in tracers. Tracers are radioactive elements that are used to follow the pathway of reactants through a chemical reaction. These elements are traced to find their course in a chemical reaction. 

Tracers are used in the medical field and in the study of plants and animals. For example, Iodine-131 is used to study the functioning of the thyroid gland.

• Nuclear power stations commonly use uranium atoms as a fuel to produce energy. The heat released during the process of nuclear fission is used to generate steam, which in turn rotates the turbine to produce electric energy.

• Sterilization of food and medical instruments harnesses radioactivity. By subjecting the instruments or the food materials to concentrated radiations, the microorganisms causing contamination can be killed. Radioactivity can also be used in the testing and inspection of materials.

Radioactivity is the transition from the 'unstable to the 'stable'. It can prove hazardous if radioactive reactions are not controlled. But controlled radioactivity has indeed proved to be beneficial to mankind.