Epithermal Flux Spectrum

She third defect mentioned in the previous section for the thermal flux distribution vas the omission of the contribution of the slowlng-dowi neutrons to the thermal spectrum. It must be recognised that the thermal and epithermal flux distributions overlap, it Is here vhere It, 1s most useful to coofblne the two distributions and again introduce the Westcott fhix formulation. Westcott defines a neutron-density distribution per unit speed interval as follows  

For all praotlcal purposes the density distributions can he cut off at 100 kev no virgin fission neutrons are to he Included. 

The quantity f Is the fraction of the total density in the epithermal distribution. The thermal distribution Is given by 

1) Determine the spectral index for the lattice. Usually only r for the fuel Is necessary since most non-l/v absorbers are found In the fuel region of the lattice (For l/v, 75 See Section 2.3.2.1). 

2) Determine the neutron temperature of the Maxwellian portion of the spectrum. 

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In Eq. (30.38), the term Qo(a)//in the factor (1 + Qo( )//) corrects for activation by epithermal neutrons. This factor is the heart of the ko method and was the key innovation that made the method possible. It was necessary to predict, with high accuracy, the relative reaction rates for two different (n,y) reactions in any reactor neutron spectrum. For each reaction, the Qo value, the ratio of the resonance integral to the thermal neutron activation cross section, and / the ratio of thermal flux to epithermal flux for the irradiation channel used, is needed. ... 

The neutron spectrum from a nuclear reactor is typically divided into two components a thermalized flux with a Maxwell energy distribution and an epithermal flux whose energy distribution is proportional to the reciprocal of the neutron energy, / ... 

Another approach to the determination of Li in RPV steel was by neutron activation analysis (NAA) followed by radiochemical analysis for H. This was far more complex than the ICP-MS analysis as it involved the irradiation of the inactive steel specimens in the CONSORT reactor at Ascot and transport of the activated steel to the Springfields Laboratory for radiochemical analysis. Six inactive specimens of RPV steel from TRA were irradiated for 63 hours in a neutron flux comprising flux values of 960 x 10 n cm s 44 X 10 n cm s and 300 x 10 n cm s for thermal, epithermal and fast regions of the energy spectrum respectively. 

If a sample contains N nuclei of a particular stable isotope, the rate of formation of its (n,y) product nuclei is Nc in which is the neutron flux density of the thermal and epithermal neutron flux density in n-cm -s (O = Oth + epi) and a the (n.y) cross section (generally expressed in barns, one barn equalling 10 cm ) of the target nuclei in the neutron spectrum in which the sample is irradiated a = + lo epi/ ) ... 

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