Plutonium 240
240Pu is a fertile isotope because its fission cross-section is very low in comparison with fissile isotopes. Radiative capture of a neutron leads to the formation of fissile 241Pu, similar to 238U, which radiative capture leads to the formation of fissile 239Pu.
It is a manufactured isotope and can be found in irradiated uranium fuel or spent uranium fuel. Isotope 240Pu is formed in a nuclear reactor from fissile isotope 239Pu. Absorption of resonance or thermal neutron by the 239Pu nucleus yields 240Pu. Trace amounts can be found in nature. The isotope 240Pu is in a decay chain of the primordial isotope 244Pu. These nuclei are present in the unalterable proportions of the radioactive equilibrium between 244Pu and 240Pu.
240Pu has relatively high radiative capture cross-section (about 290 barns for thermal neutrons).240Pu decays via alpha decay into 236U with half-life of 6560 years.
Fissile / Fertile Material Cross-sections. Comparison of total fission cross-sections.Source: JANIS (Java-based Nuclear Data Information Software); ENDF/B-VII.1
Plutonium 240. Comparison of total fission cross-section and cross-section for radiative capture. Source: JANIS (Java-based Nuclear Data Information Software); ENDF/B-VII.1
Source of data: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library