1.4 The fission event

Nature always strives to be in the most stable state possible. An example of this is radioactivity by means of which nuclei achieve a stable combination of pro­tons and neutrons.

Figure 1.3, the binding energy curve, is a stability graph and shows that the medium elements are more stable than the light or heavy elements. In fact, iron is the most stable of all. One would perhaps expect all elements to strive to ‘become’ iron, the heavier elements by fission and the lighter elements by fu­
sion with other light elements. Such an event would of course be accompanied by the spontaneous release of energy, reflecting the more stable state of iron. The reason why this does not happen is that each element has an associated energy barrier, which gives it its degree of stability; a barrier which must be overcome before fission or fusion can occur. For fis­sion this energy barrier is referred to as the fission activation energy. It follows that a disturbance to a nucleus can induce a fissioning event only if the nucleus has been ‘disturbed’ by at least its activation energy. Figure 1.7 depicts this representation of in­duced fission. It is based on the ‘liquid drop’ model for fission, a theory which historically yielded a large part of our understanding of the fission process.

The disturbance here is the absorption of a neutron. Fission may also be induced by other particles — and by gamma rays — but neutrons are absorbed much more readily because of the absence of any electrostatic forces. We shall confine ourselves to neutron-induced nuclear fission reactions.

In Fig 1.7 the terms prompt neutrons and prompt gamma are used to contrast with the delayed neu­trons and gammas emitted in the radioactive decay of the primary fission fragments. The prompt neu­trons appear about 10" 14 s after the absorption of the bombarding neutron whereas delayed neutrons are subject to the half-lives of the radioactive fragments and may thus appear seconds, minutes or even hours after the fissioning event. In Fig 1.7, for the sake of clarity a total of five neutrons is shown. In practice this many would be rare and the release of two or three neutrons is much more likely.

The significance of the fission event lies in the amount of energy released. The fission fragments and the other particles, collectively known as the fission products, have less total mass than the original target nucleus and bombarding neutron. As was shown in Section 1.3.3 of this chapter, the energy equivalent to the ‘lost mass’ is about 200 MeV. Contrast this with the few eV released in chemical reactions, for example the burning of coal or oil. The fissioning of 1 g of U-235, say, is equivalent to burning 2.5 tonnes of coal or 1 MW days of heat energy.

To complete this section of the fission event, spon­taneous fission must be mentioned. Nuclei of some of the heaviest elements may undergo fission without

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potential to undergo fission; however the energy bar­rier that needs to be exceeded before fission can occur is impossibly high for all but the heavier elements. It is only for mass numbers greater than about 230 that the fission activation energy may be less than 10 MeV or so.