3.8.1.4
Gamma emission releases energy without changing the nucleus composition
Nuclear Structure & Radiation — AQA A-Level Physics
- After alpha or beta decay, the daughter nucleus is often in an excited state with excess energyThe capacity to do work. Measured in joules (J)..
- The nucleus de-excites by emitting a gamma photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency..
- No change in A, Z, or N. Only the energyThe capacity to do work. Measured in joules (J). state changes.
- Nuclear energyThe capacity to do work. Measured in joules (J). levels are analogous to electron energy levels. Excited states are represented on nuclear energy levelA discrete amount of energy that an electron in an atom can have. Electrons can only exist at specific energy levels, not between them. diagrams.
- Metastable states (e.g. technetium-99m) are particularly long-lived excited states.
Worked Example
Molybdenum-99 undergoes beta-minus decay to form metastable technetium-99m, which then emits gamma radiation to reach the ground stateThe lowest energy level of an atom. The state in which all electrons are in their lowest possible energy levels.. Write both decay equations.
Show Solution
1
Write the beta-minus decay of Mo-99
$${}^{99}_{42}\text{Mo} \rightarrow {}^{99\text{m}}_{43}\text{Tc} + {}^{0}_{-1}\beta + \bar{\nu}_e$$
2
Write the gamma emission from Tc-99m
$${}^{99\text{m}}_{43}\text{Tc} \rightarrow {}^{99}_{43}\text{Tc} + \gamma$$
The 'm' denotes the metastable excited state. Half-lifeThe time taken for half the number of radioactive nuclei in a sample to decay, or for the activityThe number of nuclear decays per unit time. Measured in becquerels (Bq), where 1 Bq = 1 decay per second. to halve. of Tc-99m is 6 hours.
Answer
Mo-99 → Tc-99m + β⁻ + ν̄ₑ, then Tc-99m → Tc-99 + γ