Consider the compound nucleus uranium 239 formed by the absorption of a neutron by U-238. For the neutron to induce fission the sum of the binding and kinetic energy transferred to the U-239 compound nucleus must exceed its fission activation energy. From the liquid drop model the activation energy of U-239 is 7 MeV; the difference between the binding energies of U-238 and U-239 is 5.5 MeV, Thus the kinetic energy of the incoming neutron must be at least 1.5 MeV. In practice, arising from imperfections in the liquid drop model, it is found that the threshold value for the neutron energy is 1.1 MeV.

Materials, such as U-238, which may undergo fission following absorption of fast neutrons of a few MeV kinetic energy are called fissionable materials.

 

(ci Toe oscillations may be so severe as to cause a partial separation of the compound nucleus К trie increase m internal energy is not in excess o< the fission excitation energy the compound nucleus «мі reve’t to a spherical shape and dispose of the excess energy by radiation emission

 

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id) if me excitation energy is exceeded fission may occur ana me t«o parts, the fission fragments, will fly apart at hign speed under the influence of the strong electrostatic repulsion Some prompt neutrons wilt oe released at fission.

te) More prompt neutrons are emitted by the fission fragments thus reducing then excess energy These appear about 10" seconds after the

neutron absorption ■ fl Further reduction m excess energy occurs by

emi$$,on of prompt gamma radiation

gi Finally the ‘ission fragments are at rest’ alter _ndergomg collisions wiin neighbouring nuclei The ‘ragmenis are highly radioactive and will decay о у emiss-on ol beta and gamma radiations and possibly delayed neutrons.

Fig, 1.7 The fission event

the inducement of an external disturbance — this is called spontaneous fission. It is a quantum mechani­cal effect and may be understood by referring to the Theory of Quantum Mechanics. The rate of spontane­ous fission is very low. For example U-238 has a half life for spontaneous fission of 1016 years or 26 fissions/gram/hour. Corresponding values for U-235, and Pu-240 are 1 and 106 fissions per gram/hour.

3.2.2 Fissile materials

Consider the compound nucleus U-236 formed from the absorption of a neutron by U-235. In this case the U-236 fission activation energy is 6.5 MeV where­as the difference in binding energies between U-235 and U-236 is 6.8 MeV. Thus neutrons of any kinetic energy may induce fission following absorption in U-235.

Materials, such as U-235, which may be induced to fission by the release of neutron binding energy only are called fissile materials. U-235 is the only naturally occurring fissile material.