Radioactive decay
Also called Nuclear decay
Radioactive decay is the spontaneous transformation of a nucleus into one or more different nuclei, or into a lower energy level of the same nucleus. When a given nucleus will decay cannot be predicted, only the probability per unit time.
What turns 15.7.B.1 into a statistics problem is the sub-statement under it. 15.7.B.1.i says the moment an individual nucleus decays is indeterminable, and that decay rates are described using probability. What is predictable is the behavior of a population, which is what half-life measures.
Topic 15.8 sets out four modes.
- Alpha decay: the nucleus ejects an alpha particle, a helium nucleus of two protons and two neutrons. AP Physics 2 considers only He-4.
- Beta-minus decay: a neutron changes to a proton, emitting an electron and an antineutrino.
- Beta-plus decay: a proton changes to a neutron, emitting a positron and a neutrino.
- Gamma decay: after an alpha or beta decay, the excited nucleus drops to a lower energy state by emitting a photon.
Every one of them conserves nucleon number, lepton number and charge (15.8.A.2.i). Those three rules are the entire balancing technique, and gamma decay changes neither nucleon number nor charge, because a photon carries neither.
The Topic 15.8 boundary statement removes a lot of memorization. Students are not expected to know how specific isotopes decay or their half-lives. Neutron emission and electron capture are excluded. So are types of neutrinos, the characteristics distinguishing neutrinos from antineutrinos, and any explanation or application of the weak force.