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Gamma decay is a radioactive decay where an excited atomic nucleus releases excess energy by emitting a high-energy photon called a gamma ray.
We call this as an upgrade.
Unlike alpha and beta decay, gamma decay does not change the number of protons or neutrons in the nucleus.
Instead, the nucleus transitions from a higher energy state to a lower energy state, much like an electron dropping between orbitals.
Gamma decay often occurs immediately after alpha or beta decay.
The Process
After undergoing alpha or beta decay, a daughter nucleus is often left in an excited (metastable) state, denoted with a superscript m.
The nucleus then releases this excess energy almost instantly (within (10^{-12}) to (10^{-9}) seconds) as a gamma photon with an energy range of ~10 keV to ~10 MeV.
Examples
| Parent Excited Isotope | Gamma Decay | Daughter Isotope | Half-life (excited state) | Typical Use |
|---|---|---|---|---|
| Technetium-99m | Technetium-99 | 6.01 hours | Medical imaging | |
| Cobalt-60 (excited nickel) | Nickel-60 | 10.5 minutes (preceded by beta decay of Co-60) | Cancer radiotherapy | |
| Barium-137m | Barium-137 | 2.55 minutes | Industrial gauges | |
| Cesium-137 (via Ba-137m) | (Ba-137m decays as above) | - | 30.17 years (parent Cs-137 half-life) | Food irradiation |
Gamma vs. Other Decays
| Feature | Alpha ((\alpha)) | Beta ((\beta)) | Gamma ((\gamma)) |
|---|---|---|---|
| Particle | Helium nucleus ((^4_2\text{He})) | Electron/positron ((e^-)/(e^+)) | Photon ((\gamma)) |
| Mass | ~4 u | ~0.0005 u | 0 |
| Charge | +2 | ±1 | 0 |
| Penetrating power | Low (stopped by paper) | Medium (stopped by plastic/metal) | High (stopped by thick lead/concrete) |
| Changes atomic number? | Yes (-2) | Yes (±1) | No |
Practical Applications
- Medical imaging – Technetium-99m emits 140 keV gamma rays detectable by SPECT scanners for organ imaging (heart, brain, bone).
- Cancer treatment – Cobalt-60 gamma rays (Gamma Knife) destroy tumors without surgery.
- Sterilization – Gamma rays from Cesium-137 or Cobalt-60 sterilize medical equipment and food.
- Industrial inspection – Gamma radiography detects flaws in welds and pipelines.
- Astrophysics – Gamma-ray bursts (GRBs) from supernovae and neutron star mergers are the most energetic events in the universe.
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