Californium-252 neutron sources. Californium (Cf) is a chemical element that is mainly produced synthetically, but trace quantities are found in nature from neutron capture by uranium atoms. Californium-252 is an isotope of Californium that undergoes spontaneous fission and is used in small sized neutron sources. The world's most compact and intense isotope neutron sources are therefore provided by californium-252. Photographed circa 1972.