Excitation energy for nuclear fission
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The fission of a nucleus basically requires the supply
of a minimum energy. If a neutron is attached to a nucleus an energy
is released consisting of the kinetic energy of the neutron and the
binding energy of this neutron to the nucleus. If this energy is greater
than the excitation energy for a fission of this nucleus, the nucleus
may fission. For U-235 the excitation energy for fission amounts to
5.7 MeV, the binding energy of the attached neutron is 6.5 MeV, so that
even neutrons with very low kinetic energies (e.g. thermal neutrons
with a kinetic energy of only 0.025 eV) may trigger the fission. The
ratios of excitation energy and binding energy are similar for the nuclei
of U-233, Pu-239 and Pu-241. For U-238 and Th-232, on the other hand,
the excitation energy required for fission is 6.5 MeV and thus much
higher than the binding energy of the attached neutron of 4.8 MeV, so
that fission of the nucleus is possible only if the neutron has a kinetic
energy of at least 1.7 MeV. Spontaneous fission is also possible in the case of some very heavy nuclei.
Nucleus of an atom |
Excitation energy for fission MeV |
Binding energy of the last neutron MeV |
Th-232 |
6.5 |
4.8 |
U-233 |
6.2 |
6.8 |
U-235 |
5.7 |
6.5 |
U-238 |
6.5 |
4.8 |
Pu-239 |
5.8 |
6.5 |
Pu-240 |
6.2 |
5.2 |
Pu-241 |
5.6 |
6.3 |
Excitation energy for fission
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11 - 15 March 2018
Munich, Germany
30 September - 04 October 2018
Prague, Czech Republic |
