Resources | Subject Notes | Physics
This section explains the three main types of nuclear emission: alpha decay, beta decay, and gamma decay. We will focus on understanding their relative ionising effects, considering both kinetic energy and electric charge.
Alpha decay occurs when an unstable nucleus emits an alpha particle, which is equivalent to a helium nucleus ($^4He$).
Alpha particles are relatively heavy and have a positive charge (+2). This makes them highly ionising.
Kinetic Energy: Alpha particles are emitted with high kinetic energy. This high energy allows them to readily knock electrons off atoms in the surrounding material, creating positive ions and free electrons. The higher the kinetic energy, the greater the ionising effect.
Electric Charge: The +2 charge of the alpha particle significantly contributes to its ionising power. The positive charge attracts the negatively charged electrons in atoms, leading to their removal.
Beta decay occurs when a neutron in the nucleus transforms into a proton, emitting an electron (beta particle) and an antineutrino.
There are two main types of beta decay: beta-minus ($^14C \rightarrow ^14N + e^- + \bar{\nu}_e$) and beta-plus ($^22Na \rightarrow ^22Ne + e^+ + \nu_e$).
Beta particles are electrons (beta-minus decay) or positrons (beta-plus decay). They are much lighter and have a single negative (beta-minus) or positive (beta-plus) charge.
Kinetic Energy: Beta particles are emitted with varying kinetic energies, typically lower than alpha particles. Their ionising effect is therefore less pronounced than that of alpha particles.
Electric Charge: The charge of a beta particle is either -1 or +1. While they do attract electrons, their lower mass means they are less effective at removing electrons compared to the heavier, more charged alpha particles.
Gamma decay occurs when an unstable nucleus emits a high-energy photon, known as a gamma ray ($ \gamma $). This does not change the number of protons or neutrons in the nucleus.
Gamma rays are electromagnetic radiation and have no mass or charge. They are highly penetrating and can ionise matter through three main processes:
Kinetic Energy: Gamma rays carry a significant amount of energy, which can be transferred to electrons and atoms, leading to ionisation.
Electric Charge: Since gamma rays have no charge, they do not directly attract or repel charged particles. Their ionising effect is solely due to their high energy.
| Emission Type | Relative Kinetic Energy | Relative Electric Charge | Relative Ionising Effect |
|---|---|---|---|
| Alpha ($^4He$) | High | $ +2 $ | Highest |
| Beta ($e^- \text{ or } e^+$) | Medium | $ -1 \text{ or } +1 $ | Medium |
| Gamma ($\gamma$) | High | 0 | Lower than Beta, but higher than Alpha |
Conclusion: Alpha particles have the highest ionising effect due to their high mass, high charge, and high kinetic energy. Gamma rays have a lower ionising effect than alpha and beta particles because they are uncharged and have lower mass, although they carry high energy. Beta particles have an ionising effect intermediate between alpha and gamma decay.