Isotopes of plutonium
Pu-240, Pu-241 and Pu-242

Isotope
Thermal neutron cross section (barns)
Thermal neutron cross section (barns)
Decay Mode
Half-life
Isotope
Capture
Fission
Decay Mode
Half-life
238U
2.683
0.000
α
4.468 x 109 years
239U
20.57
14.11
β
23.45 minutes
239Np
77.03
–
β
2.356 days
239Pu
270.7
747.9
α
24,110 years
240Pu
287.5
0.064
α
6,561 years
241Pu
363.0
1012
β
14.325 years
242Pu
19.16
0.001
α
373,300 years

Question: Which two isotopes of plutonium have the longest half-lives?
Answer: Pu-242, with a half-life of 373,300 years, and Pu-239, with a half-life of 24,110 years.