At small neutron flux, as in a nuclear reactor, a single neutron is captured by a nucleus. For example, when natural gold (Au) is irradiated by neutrons (n), the isotope Au is formed in a highly excited state, and quickly decays to the ground state of Au by the emission of gamma rays (𝛾). In this process, the mass number increases by one. This is written as a formula in the form Au + n → Au + γ, or in short form Au(n,γ) Au. If thermal neutrons are used, the process is called thermal capture.