Plutonium-242

242Pu's gamma ray emissions are also weaker than those of the other isotopes.

The odd-mass isotopes 239Pu and 241Pu have about a 3/4 chance of undergoing fission on capture of a thermal neutron and about a 1/4 chance of retaining the neutron and becoming the following isotope.

Even then, there is a chance either of those two fissile isotopes will absorb the fourth neutron instead of fissioning, becoming curium-246 (on the way to even heavier actinides like californium, which is a neutron emitter by spontaneous fission and difficult to handle) or becoming 242Pu again, so the mean number of neutrons absorbed until fission is even higher than 4.

However, 242Pu's low cross section means that relatively little of it is transmuted during one cycle in a thermal reactor.

No fission products have a half-lifein the range of 100 a–210 ka ... ... nor beyond 15.7 Ma[6] 242Pu alpha decays into uranium-238, before continuing along the uranium series.