Plutonium-239 is a radioactive isotope that emits α particles of energy 5.1 MeV and decays
to form a radioactive isotope of uranium. This isotope of uranium emits α particles of
energy 4.5 MeV to form an isotope of thorium which is also radioactive.
Which of the two radioactive isotopes, Plutonium-239 or the uranium isotope has the longer half life? Give a reason for your answer.