Fission Neutron Yields

The number of neutrons produced in a fission reaction is highly important for fission reactors, explosive fission devices, and the safe storage and processing of fissionable materials. It is an energy-dependent quantity, and it has both prompt and delayed components. Fission neutron yields, or "nu-bar's," are given in three special sections of File 1:
  • MT=452, total neutrons per fission,
  • MT=456, prompt neutrons per fission, and
  • MT=455, delayed neutrons per fission and time constants.

Upon fission, the excited nucleus formed by the target and the projectile breaks up into two roughly equal "fission fragments" and a number of additional neutrons (1, 2, 3, 4, ...). It is the average over all these possible break-up modes that gives the "prompt" nu-bar (e.g., 2.5). Some of the excited fission fragments emit additional neutrons as they decay over the next second or so; these are called "delayed neutrons."

Prompt nu-bar tends to be constant at low energies, and then it gradually increases as the energy increases. In earlier times, this increasing trend was often represented as a simple polynomial (approximately linear) in energy E. More detailed analysis shows more structure associated with second- and third-chance fission processes. Some evaluations even have structure at very low energies. Therefore, most modern evaluations use a detailed tabulated function to represent the energy dependence of the fission nu-bar. The following figure shows a few examples:

Nubar plot

The following example is U-233 from ENDF/V-VI. The "2" in the fourth field on the first line says that the nu-bar data are given in tabulated form. The actual tabulation of total nu-bar vs energy starts on line 4.

    9.223300+4 2.310430+2          0          2          0          09222 1452
    0.000000+0 0.000000+0          0          0          1         109222 1452
            10          2                                            9222 1452
    1.000000-5 2.494700+0 2.530000-2 2.494700+0 3.200000+5 2.494000+09222 1452
    2.000000+6 2.687400+0 4.500000+6 3.052000+0 6.000000+6 3.268000+09222 1452
    6.500000+6 3.340900+0 1.400000+7 4.270400+0 1.500000+7 4.393800+09222 1452
    2.000000+7 5.013500+0                                            9222 1452
                                                                     9222 1  0

The formats for prompt nu-bar and total nu-bar are the same. The format for delayed nu-bar has an additional LIST record giving the decay constants that are used to describe the time history of delayed neutron emission. This will be discussed later.


15 December 2012 T-2 Nuclear Information Service