Interaction Rates of Polyenergetic Neutrons

Equation 4.84 gives the reaction rate for the case of monoenergetic neutrons. In practice, and especially for neutrons produced in a reactor, the flux consists of neutrons that have an energy spectrum extending from = 0 up to some maximum energy F ax-such a case, the reaction rate is written in terms of an average cross section. Let 

4 E) dE = neutron flux consisting of neutrons with kinetic energy between E and E + dE 

Oi(E) = cross section for reaction type i for neutrons with kinetic energy E N = number of targets per m (stationary targets) 

In practice, an average cross section is defined in such a way that, when is multiplied by the total flux, it gives the reaction rate of Eq. 4.86, i.e.. 

The calculation of average cross sections is beyond the scope of this text. The reader should consult the proper books on reactor physics. The main purpose of this short discussion is to alert the reader to the fact that when polyenergetic neutrons are involved, an appropriate average cross section should be used for the calculation of reaction rates. 

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