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Regional Basic Professional Training Course in Korea

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1.2.3.2. Neutron emission <strong>in</strong> fission<br />

❙ 33 ❙<br />

1. Nuclear Reactor Pr<strong>in</strong>ciples<br />

The dependency of the average number of prompt neutrons emitted by fission with<br />

<strong>in</strong>cident neutron energy and with fission<strong>in</strong>g system is illustrated <strong>in</strong> Figure 1.16 (data from<br />

JEF2.2). We see an almost l<strong>in</strong>ear <strong>in</strong>crease of the number of neutrons emitted with the<br />

energy of the <strong>in</strong>com<strong>in</strong>g neutron (about 0.15 additional neutrons per 1 MeV energy<br />

<strong>in</strong>crease). Some structure <strong>in</strong> also observed <strong>in</strong> the resonance range (not visible <strong>in</strong> the graph<br />

due to the use of l<strong>in</strong>ear energy scale) which orig<strong>in</strong>ate from the competition of direct<br />

fission reaction and (n,γf). In this last eventuality, the residual nucleus has less excitation<br />

energy at the fission.<br />

Figure 1.17 shows the variation of the average number of delayed neutrons with <strong>in</strong>cident<br />

neutronenergy. Notice that <strong>in</strong> k<strong>in</strong>etic studies, the pert<strong>in</strong>ent quantity is the fractional<br />

number of delayed neutrons, which is equal to the ratio of the number of delayed<br />

neutrons by the total number of emitted neutrons. At low energy, the number of delayed<br />

is fairly constant. The structure shown at high energy is due to other fission processes <strong>in</strong><br />

which one, two…neutrons are emitted before fission, which changes of course the<br />

compound nucleus undergo<strong>in</strong>g fission.<br />

FIG. 1.16. Average number of prompt neutrons emitted <strong>in</strong> fission with <strong>in</strong>cident neutron energy.

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