Assertion: plutonium's isotope | /r/HasProperty | within thermal neutron spectrum
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text: Plutonium-240, an even-mass isotope is, within the thermal neutron spectrum, a fertile material like uranium-238, becoming fissile plutonium-241 on neutron capture; however, the even-mass plutonium-242 not only has a low neutron capture cross section within the thermal spectrum, it also requires 3 neutron captures before becoming a fissile nuclide.