In each fission event the number of neutrons emitted must be an integer. For a very large number of fis­sions of U-235 by thermal neutrons it is known that 2.7% of the fissions give no neutrons, 15.8% give one neutron, 33.9% give two, and so on. The average number of neutrons released per fission is denoted by v and has a value of 2.43 for thermal fission in U-235.

The average number of neutrons released per fis­sion is different for different fissile isotopes. Also, v increases more or less linearly with increasing neutron energy giving an extra neutron for each 7 MeV of neutron kinetic energy.

Table 1.3 gives the values of v for the fission of uranium and plutonium by thermal and 1 MeV neu­trons (or the threshold 1.1 MeV for U-238).

3.3.1 Energy of the neutrons

The neutrons that are emitted at the instant of fission — the prompt neutrons — are found experimentally to have a range of energies. The relative numbers of neutrons of each energy is shown in Fig 1,9.

Fig. 1.9 Energy of the neutrons emitted in fission

This shows a most probable energy of just less than 1 MeV and a long tail on the high energy side, resulting in an average value for the neutron energy of about 2 . MeV. Therefore, although not strictly true for all, the neutrons released at fission may be regarded as being fast neutrons with average energy of 2 MeV.