Existence of nuclear excited states; γ ray emission; application eg use of…

The idea

After alpha or beta decay, a daughter nucleus may be left with excess energy: it is in a nuclear excited state, shown as a higher line on an energy level diagram. It de-excites by emitting a gamma photon, a high-frequency electromagnetic quantum with no mass or charge, so proton and nucleon numbers do not change. Gamma emission usually follows other decay rather than occurring alone. Technetium-99m is a metastable excited state of technetium-99: it decays by gamma emission with a half-life of about six hours, and the gamma photons penetrate tissue, so a detector outside the body can map uptake in organs such as the heart, bones, kidneys and thyroid. The short half-life limits patient dose, and the gamma energy is suitable for detection. A tracer should also be non-toxic and have a known biological pathway.

What earns the marks

• States that a nucleus can exist in excited states after alpha or beta decay, with energy above the ground state.
• Describes gamma emission as a photon emitted when the nucleus drops to a lower energy state, with no change in proton or nucleon number.
• Explains that gamma photons are electromagnetic radiation with no mass and no charge, so they are highly penetrating.
• Describes technetium-99m as a metastable gamma-emitting source used as a medical tracer.
• Explains that the gamma radiation is detected outside the body to reveal the distribution of the tracer in an organ.
• Links the roughly six-hour half-life to a useful balance between sufficient activity for imaging and a limited radiation dose to the patient.
• Recognises that a suitable tracer must also be non-toxic and follow a known biological pathway to the target organ.

Common mistakes and how to put them right

• Saying gamma emission changes the element.

  Put it right by stating that gamma emission alters neither proton number nor nucleon number, so the nuclide stays the same.

• Confusing gamma rays with alpha or beta particles.

  Put it right by describing gamma as uncharged electromagnetic radiation with no mass, unlike charged, massive alpha and beta emissions.

• Claiming technetium-99m is a stable or permanent source.

  Put it right by describing it as metastable, decaying by gamma emission with a half-life of about six hours.

• Treating the gamma photons as the imaging agent itself.

  Put it right by explaining that the tracer is taken up by the organ and the emitted gamma photons are detected externally.

Exam tips

• Use the term metastable when describing technetium-99m, and quote its half-life as approximately six hours.
• When explaining medical use, structure the answer as uptake, gamma emission, external detection and image formation.
• State explicitly that gamma emission leaves proton and nucleon numbers unchanged, since this is a common discriminator in mark schemes.
• Link the short half-life to patient safety and to practical imaging time rather than listing it without comment.
• Mention one named organ or application, such as heart, bone or kidney imaging, to make the application concrete.