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Explain their relative ionising effects with reference to: (a) kinetic energy (b) electric charge
There are three main types of nuclear emission:
In alpha decay, the nucleus emits an alpha particle, which is a helium nucleus ($^{4}_{2}He$).
Equation: $^{Z}_{N}A \rightarrow ^{Z-2}_{N-4}A + ^{4}_{2}He$
Ionising Effect: Alpha particles are relatively large and have a positive charge (+2). This makes them highly ionising. When an alpha particle passes through matter, it collides with atoms and strips electrons from them, creating positive ions.
In beta decay, a neutron in the nucleus transforms into a proton, emitting an electron (beta particle, $^{-1}e$) and an antineutrino.
Equation: $^{Z}_{N}A \rightarrow ^{Z+1}_{N-1}A + ^{-1}e + \bar{u}_e$
Ionising Effect: Beta particles are electrons and have a single negative charge (-1). They are less ionising than alpha particles because they are much smaller and interact less with the nuclei of atoms. However, they still cause ionisation by stripping electrons from atoms.
In gamma decay, a nucleus in an excited state releases energy in the form of a high-energy photon, called a gamma ray ($ \gamma $).
Equation: $^{Z}_{N}A^* \rightarrow ^{Z}_{N}A + \gamma$
Ionising Effect: Gamma rays are electromagnetic radiation and do not have any charge. They are the least ionising of the three types of emission. Gamma rays can penetrate matter more easily than alpha or beta particles, but they ionise by interacting with electrons in atoms, causing them to be ejected from their orbits.
The relative ionising effects of alpha, beta, and gamma emissions can be explained by their kinetic energy and electric charge.
| Emission Type | Electric Charge | Relative Kinetic Energy | Relative Ionising Effect |
|---|---|---|---|
| Alpha ($^{4}_{2}He$) | +2 | Low | High |
| Beta ($^{-1}e$) | -1 | Medium | Medium |
| Gamma ($ \gamma $) | 0 | High | Low |
Alpha particles have the highest ionising effect due to their large size and positive charge, which allows them to interact strongly with the nuclei of atoms. Beta particles have a moderate ionising effect. Gamma rays have the lowest ionising effect because they are neutral and can travel further through matter before interacting with atoms.