Fissile isotopes such as 233U, 235U or 239Pu get exposed to a thermal neutron beam. The resulting nuclear chain reaction is controlled by moderators such as graphite, Beryllium or heavy water.
For example Mo-99, source for Tc-99m, is produced by fission of highly enriched uranium (HEU) in a few reactors around the world (monitored and controlled by The International Atomic Energy Agency).

Neutron capture is a process in which a neutron collides with a nucleus and becomes part of this nucleus caused by nuclear forces. It interacts without release of another heavy particle. A gamma ray photon is emitted as an immediate result of the neutron capture process. Through the neutron capture the nucleus becomes a heavier isotope of the same element. The kind of decay depends on the isotope and its stability.
This process is for example part of the neutron activation analysis, in which a sample is positioned in a neutron beam and also used in the 'boron neutron capture therapy'.

See also Thermal Neutrons, Epithermal Neutron, Neutron Activation Analysis, Nuclear Charge Number, Deuteron, Isomeric Transition, Isotones, N P Reaction.