Radioactivity | Decay of Alpha, Beta-particles and Gamma-rays

The phenomenon of spontaneous decay of a nucleus accompanied by the emission of alpha-particles, beta-particles, or gamma-rays is known as radioactivity. It could be either natural or artificial.

U_{92}^{238}\rightarrow He_2^4+Th_{90}^{234}

Radioactivity was first discovered by Henry Becquerel in 1896 accidentally in the same way as X-rays were discovered by Roentgen in 1895. When, French scientist Henry Becquerel took a photographic plate wrapped in black paper and placed a uranium salt on it and exposed it to sunlight. On developing the photographic plate he found it to be foggy or darkened indicating that the uranium salt has emitted radiations in the presence of sunlight, which could penetrate into black paper and effect the photographic plate. The radioactive radiation were called Becquerel rays.

(i) Alpha Decay

In this process, parent nucleus disintegrates to give a daughter nucleus and helium nucleus or an alpha-particle. Mass number of the daughter nucleus decreases by four units and atomic number decreases by two units. A typical example of this decay mode is

U_{92}^{238}\rightarrow He_2^4+Th_{90}^{234}

Thus, daughter nucleus is shifted in periodic table by 2 unit in backward direction.

(ii) Beta Decay

In this process, parent nucleus disintegrates to give a daughter nucleus and a Beta-particles. Beta-decay is classified into three categories.

a) Electron emission or β--decay

Here, parent nucleus disintegrates to give a daughter nucleus, a Beta-particles and a new particle called antineutrino. Mass number of the daughter nucleus remains the same and atomic number increases by one unit.

C_6^{12}\rightarrow N_7^{14}+\beta^{-}+\overline\nu

b) Positron Emission or β+-decay

In this process, parent nucleus disintegrates to give a daughter nucleus, a Beta-particles and a new particle called neutrino. Mass number of the daughter nucleus remains the same and atomic number increases by one unit.

Na_{11}^{22}\rightarrow Ne_{10}^{22}+\beta^++\nu

c) Electron Capture

Here the parent nucleus captures one of the orbital electrons with the emission of a neutrino. Mass number of the daughter nucleus remains the same and atomic number increases by one unit.

Mn_{25}^{54}+\beta^{-}\rightarrow Cr_{24}^{54}+\nu

Thus, daughter nucleus is shifted in periodic table by 2 unit in forward direction.

(iii) Gamma-Decay

Alpha and beta decays of a radioactive nucleus leave the daughter nucleus in an excited state. If the excitation energy available with the daughter nucleus is not sufficient for further particle emission, it loses its energy by emitting electromagnetic radiations, also known as Gamma-rays. Mass and charge of the daughter nucleus remains the same as before the emission of Gamma-rays.

Ba_{56}^{137}\rightarrow Ba_{56}^{137}+\gamma