4.2.6Atomic Structure
Gamma Decay — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 17 of 23
Section 1 of 7What gamma decay is
Excess energy
- Excess energy After some decays a nucleus is left with more energy than it needs to be stable.
- It is in an excited state.
- It gets rid of the extra energy by emitting a gamma ray.
A gamma ray
- A gamma ray Gamma radiation is a high-frequency, high-energy electromagnetic wave.
- It has no mass and no charge.
Structure unchanged
- Structure unchanged No protons or neutrons are lost or changed, so the nucleus keeps the same number of each.
Section 2 of 7Effect on the nucleus
| Quantity | Effect of gamma emission | Why |
|---|---|---|
| Mass number | No change | No particles with mass are emitted |
| Atomic number | No change | No protons are gained or lost |
| Element | Stays the same | The number of protons is unchanged |
| Energy of nucleus | Decreases | Energy is carried away by the gamma ray |
Section 3 of 7Gamma emission in a sequence
1Unstable nucleus
- Unstable nucleus The original nucleus decays by alpha or beta emission.
2Excited daughter
- Excited daughter The new nucleus is formed with surplus energy.
3Gamma ray emitted
- Gamma ray emitted The surplus energy leaves as a gamma ray.
4Stable arrangement
- Stable arrangement The nucleus has less energy and is now more stable.
Section 4 of 7Showing gamma decay in an equation
1
Write the nucleus before the decay, for example uranium-238 as 23892U.
2
Write the same nucleus after the decay, still 23892U, with the gamma ray shown as the symbol for gamma (a Greek letter) with 0 and 0 for its numbers.
3
Check that nothing has changed in the mass number or the atomic number.
4
Remember the energy released is not shown by numbers in the equation, but it is the whole point of the process.
Section 5 of 7Where gamma radiation shows up
Gamma rays
Alongside alpha
- Alongside alpha Many alpha emitters release a gamma ray at the same moment, so exam questions may list both.
Alongside beta
- Alongside beta The same applies to beta decay: the daughter nucleus often de-excites by gamma emission.
Very penetrating
- Very penetrating Gamma rays pass through most materials, and only thick lead or concrete reduces them significantly.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Gamma ray | A high-energy electromagnetic wave emitted by an excited nucleus. |
| Excited nucleus | A nucleus that has more energy than it needs to be stable. |
| Mass number | The total number of protons and neutrons in a nucleus. |
| Atomic number | The number of protons in a nucleus. |
| Electromagnetic radiation | Waves of electric and magnetic fields, from radio waves to gamma rays. |
Section 7 of 7Exam technique
1
Claiming gamma emission turns the nucleus into a different element - it does not, because the numbers do not change.
2
Giving a gamma ray a mass number of 4 or a charge; it has neither mass nor charge.
3
Thinking gamma decay happens on its own all the time - it usually follows an alpha or beta decay.
4
Writing that a gamma ray is a particle; it is electromagnetic radiation, not a particle with mass.
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