Gamma (γ) decay: the nucleus sheds extra energy as light
No particle changes and the element stays the same. An "excited" nucleus settles into a calmer arrangement and gets rid of the difference as a single very-high-energy photon.
6028Ni*→6028Ni+γ
The * means "excited." Same 28 protons, same 32 neutrons, just less energy afterward.
What's an "excited" nucleus?
Remember the energy ladders from the beta-minus chapter. After an α or β decay, the new nucleus often lands with a proton or neutron on a higher rung than it needs, like a ball that bounced onto a shelf instead of the floor. Within about a trillionth of a second it rearranges to a lower rung, and the energy difference leaves as a photon. Because protons carry charge, rearranging them shakes the electromagnetic field, and a photon is exactly a packet of that shaking.
It's the same physics as a neon sign glowing when its atoms' electrons drop between levels. Nuclear energy gaps are about a million times bigger (MeV instead of eV), so instead of visible light you get gamma rays.
Step through cobalt-60's decay chain
Step 1Cobalt-60 (27 protons, 33 neutrons) is neutron-heavy. On the energy diagram (right) it sits 2.82 MeV above nickel-60's ground state. The blue dot tracks where the nucleus is.
Step 2It β⁻ decays to nickel-60, but lands on an excited level 2.51 MeV above the bottom. Only 0.32 MeV went to the electron and antineutrino. The rest is still trapped in the nucleus.
Step 3Within about a trillionth of a second the nucleons rearrange to a lower level and a 1.17 MeV gamma ray carries off the difference.
Step 4It drops once more, to the ground state, emitting a 1.33 MeV gamma ray. Through both gamma steps the nucleus stayed 28 protons + 32 neutrons. Only energy left.
Step 5Calm, stable nickel-60. Gamma energies are exact (they're the gaps between fixed levels), so this 1.17 + 1.33 MeV pair is cobalt-60's fingerprint. Detectors identify isotopes this way, and Co-60's gammas are used in cancer radiotherapy and food sterilization.
Two variations: some excited nuclei get "stuck" for hours (technetium-99m, used in tens of millions of medical scans a year, waits 6 hours on average). And sometimes the nucleus hands its energy directly to one of its own inner electrons and ejects it, instead of making a photon ("internal conversion").