## 22.3.3 Gamma Decay Gamma decay occurs when an atomic nucleus transitions from a higher energy state to a lower energy state. The process involves the emission of a gamma ray, which is a high-energy photon. Unlike alpha decay and beta decay, gamma decay does not involve the emission of massive particles. ### Gamma Rays Gamma rays are high-energy photons that are a form of electromagnetic radiation. They travel at the speed of light ($c$) and have no charge and negligible mass compared to alpha and beta particles. This nature is similar to other forms of radiation discussed in . ### Nuclear Excited State ($X^*$) Nucleons (protons and neutrons) within an atom can absorb energy, moving to a higher energy state, similar to how electrons behave in . When a nucleus is in such a higher energy state, it is said to be in an excited state, represented as $X^*$. During de-excitation, the nucleus emits gamma radiation to return to a lower, more stable energy state. ### Conservation Laws in Gamma Decay The emission of gamma radiation is subject to specific conservation laws: - Conservation of Energy - Conservation of Momentum - Conservation of Angular Momentum Although no particles with mass are emitted, the emission of gamma radiation results in a change in the nucleus energy state. ### Equation for Gamma Decay The general equation for gamma decay is: $${ }_{Z}^{A} X^{*} \rightarrow{ }_{Z}^{A} X+\gamma$$ Here, $X^{*}$ represents the nucleus in an excited state. The atomic number ($Z$) and mass number ($A$) of the nucleus remain unchanged. **Example:** Gamma ray emission from an excited Cobalt-60 nucleus: $${ }_{27}^{60} \mathrm{Co}^{*} \rightarrow { }_{27}^{60} \mathrm{Co}+\gamma$$ The energy of the emitted gamma photon is equal to the difference between the two nuclear energy levels: $$E_\gamma = h f = \Delta E$$ Since gamma rays are photons, they carry momentum. As explored in , the momentum of a gamma photon is given by: $$p = \frac{h}{\lambda} = \frac{E}{c}$$ ## Comparison of Nuclear Radiations | Property | Alpha | Beta | Gamma | | :--- | :--- | :--- | :--- | | **Charge** | $+2$ | $\pm 1$ | $0$ | | **Mass** | $4$ amu | Very small compared to alpha | Negligible | | **Velocity** | $0.05c-0.1c$ | $0.5c-0.9c$ | $c$ | | **Penetration** | Stopped by few cm of air or paper | Stopped by few mm of aluminum | Stopped by several cm of lead or concrete | | **Typical source** | Radon-222 | Strontium-90 | Cobalt-60 |