Any material that emits neutrons, such as a mixture of radium and beryllium, that can be inserted into a reactor to ensure a neutron flux large enough to be distinguished from background to register on neutron detection equipment.
NEUTRON SOURCE
Meaning of NEUTRON SOURCE in English
NRC Nuclear Energy English Glossary. Английский глоссарий ядерной энергии NRC (Национальный научно исследовательский совет). 2012