Nuclear Excited States (AQA A Level Physics)

Nuclear Excited States

• A nucleus can exist in an excited state in the same way as an electron

• Once an unstable nucleus has decayed, it may emit any remaining energy in the form of a gamma photon (γ)

  • The emission of a γ photon does not change the number of protons or neutrons in the nucleus, it only allows the nucleus to lose energy

• This happens when a daughter nucleus is in an excited state after a decay

• This excited state is usually very short-lived, and the nucleus quickly moves to its ground state, either directly or via one or more lower-energy excited states

Metastable technetium-99m is a technetium-99 nucleus in an excited energy state

• One common application of this is the use of technetium-99m as a γ source in medical diagnosis

  • The ‘m’ stands for metastable which means the nucleus exists in a particularly stable excited state

• Technetium-99m is the decay product of molybdenum-99, which can be found as a product in nuclear reactors

• The decay of molybdenum-99 is shown below:

• The half-life of molybdenum-99 is 66 hours

  • This is long enough for the sample to be transported to hospitals

  • Subsequently, the technetium-99m can be separated at the hospital

• Technetium-99m has a short half-life of 6 hours

  • This is an adequate timeframe for examining a patient

  • Plus, it is short enough to minimise damage to the patient

Nuclear Energy Level Diagrams

• Nuclear energy levels are similar to electron energy levels

• The nuclear energy level diagram of molybdenum-99 can be represented as follows:

An excited molybdenum-99 nucleus undergoes beta decay into metastable technetium-99m which is a gamma emitter with a half-life of 6 hours

• The decay mode (usually alpha or beta) is represented by a diagonal line

• The excited state, or states, are generally stacked in descending energy order to the right of the decay