12.4 Radioactivity

12.4 Radioactivity

It has been observed that the nuclei whose atomic numbers are greater than $82$ are found naturally unstable, and these nuclei spontaneously emit radiations. Such nuclei are called radioactive and the emission of radiation is known as natural radioactivity. These radiations are of three types, $\alpha$, $\beta$ and $\gamma$-radiations. Unstable isotopes can also be produced artificially in the laboratory by nuclear reactions. This occurs when a stable element is bombarded with high-energy particles, such as neutrons, protons, alpha particles, or gamma rays, causing it to become unstable and emit radiation. This is called artificial radioactivity and radioactive isotopes are named radioisotopes or radionuclides.

The $\alpha$-particles, $\beta$-particles and $\gamma$-radiations are traversed differently when passed through the electric field as shown in Fig. 12.4. It is seen that $\alpha$-particles deflect towards the negative terminal of the electric field, showing that they have a positive charge. The $\beta$-particles are emitted at high speeds, typically a few percent of the speed of light. However, $\alpha$-particles can travel only several centimetres in the air due to their large mass. The $\beta$-particles deflect towards the positive terminal of the electric field, showing they have a negative charge. The deflection of $\beta$-particles is more than the $\alpha$-particles, proving that they are lighter particles than $\alpha$-particles. The $\beta$-particles are fast-moving electrons and move with speeds up to $0.9995$ of the speed of light. The $\gamma$-radiations passed through the electric field without deflection, showing that they have no charge. The $\gamma$-radiations are electromagnetic radiations which consist of photons. They move with the speed of light with the highest penetrating power but the lowest ionization power.

The process of emitting $\alpha$-particles, $\beta$-particles and $\gamma$-radiations from the nucleus is called $\alpha$-decay, $\beta$-decay and $\gamma$-decay, respectively, and are discussed below.