Gamma decay
Also called Gamma emission, Gamma ray emission
Gamma decay is the emission of a high-energy photon by an excited nucleus dropping to a lower energy state. Mass number and atomic number both stay the same, so the nucleus is the same nuclide afterwards, only with less energy.
The definition AP Physics 2 gives is narrower than most sources let on. Essential knowledge 15.8.A.2.v: gamma decay occurs after a nucleus has undergone alpha or beta decay, and the excited nucleus decays to a lower energy state by emitting a photon. In this course it is the tidying-up step, not something a stable nucleus does out of nowhere.
Writing the excited nucleus with a star:
A photon has no nucleons and no charge, so it contributes 0 to the mass-number column and 0 to the charge column. Both sides read and both sides read , and that is the entire point of the mode: the nucleus does not become a different element or a different isotope. Only its energy changes.
This is also why the definition of radioactive decay at 15.7.B.1 has two halves. A nucleus transforms into one or more different nuclei, or to a lower energy level of the same nucleus. That second clause is gamma decay, and a question asking which decay leaves the nuclide unchanged is testing that sentence.
The photon energy is the gap between the two nuclear states, found the same way as for an atom, with . Note what the CED does not supply: no nuclear energy level diagram, no typical gamma energies, and nothing about penetration or shielding, none of which appears in any of the four current CEDs.
Topic 15.8 sets it beside the other three modes.