## Alpha Decay Alpha decay is a type of radioactive decay in which an unstable atomic nucleus becomes more stable by emitting an alpha particle. This process typically occurs in heavy nuclei that have excess protons and neutrons. The emission reduces the nuclear size and restores greater stability to the nucleus. ### Definition of Alpha Decay Alpha decay is a nuclear transformation in which an atomic nucleus emits an alpha particle. An alpha particle is identical to a helium nucleus, consisting of **two protons** and **two neutrons**. ### Impact on the Parent Nucleus When a nucleus undergoes alpha decay, the following changes occur: - The atomic number ($Z$) **decreases by two**. - The mass number ($A$) **decreases by four**. The parent nucleus loses two protons and two neutrons, which are carried away by the alpha particle. This shift moves the element two positions back in the periodic table. ### General Equation for Alpha Decay The general equation representing alpha decay is: $$ { }_{Z}^{A} X \rightarrow{ }_{Z-2}^{A-4} Y+{ }_{2}^{4} \mathrm{He}+Q $$ Where: - ${ }_{Z}^{A} X$ is the **parent nucleus**. - ${ }_{Z-2}^{A-4} Y$ is the **daughter nucleus**. - ${ }_{2}^{4} \mathrm{He}$ is the **alpha particle** (helium-4 nucleus). - $Q$ represents the **energy released** (Q-value) during the decay. The Q-value represents the disintegration energy, which is the difference between the rest mass energy of the parent nucleus and the final products. Alpha decay is only energetically possible if the mass of the parent nucleus is greater than the sum of the masses of the daughter nucleus and the alpha particle. ## Example: Alpha Decay of Uranium-238 The alpha decay of Uranium-238 is represented as: $$ { }_{92}^{238} \mathrm{U} \rightarrow{ }_{90}^{234} \mathrm{Th}+{ }_{2}^{4} \mathrm{He}+Q $$ In this reaction: - Uranium-238 (parent) contains 92 protons and 146 neutrons. - Thorium-234 (daughter) contains 90 protons and 144 neutrons. - The emitted alpha particle contains 2 protons and 2 neutrons. This process releases energy $Q$ primarily as the kinetic energy of the alpha particle. However, due to the conservation of momentum, the daughter nucleus also recoils slightly, meaning the alpha particle carries away most, but not all, of the Q-value energy. ### Properties of Alpha Particles Alpha particles are relatively heavy and carry a $+2e$ charge. Due to their mass and charge, they have high ionizing power but low penetrating power compared to beta or gamma radiation.