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).

Stable elements get activated by neutrons.

A nucleon is a proton or a neutron - part of the nucleus.

The nucleus is the positively charged center of an atom and contains most of the mass. Except for 1H, the atom core consists of a combination of the subatomic particles protons and neutrons.

Nuclear reaction in which a proton enters the nucleus and a neutron is emitted.