Epithermal flux

The Li concentration in atoms per g was converted to ng g natural Li value and the results are presented in Table 2. The six samples were from three canisters, all of which experienced predominantly thermal fluxes. Two of the canisters (containing samples 30006, 30010, 35031 and 35038) were from below the core with an epithermal flux value twice that of the other canister (samples 30114 and 30115) which was positioned at the side of the core. The calculated Li concentrations are generally consistent for all samples apart from specimen 35031 where the result was affected by contamination with " C. The average value for the predicted natural Li concentration in RPV steel, excluding the result for specimen 35031, was 0.3 0.1 ng g ... 

Instrumental Neutron Activation Analysis (INAA) was applied to the determination of the platinum metals as part of a study of their uptake, accumulation and toxicity in plants. Long irradiations are described for iridium, osmium and ruthenium determinations in specially grown plant materials. An interference in the determination of platinum in plant matrices by this method is reported also. Short irradiations, utilising thermal and epithermal fluxes are investigated for rhodium and palladium determinations, with further studies using cyclic activation analysis. 

The epithermal flux distribution ipsiE) can be approximated by a 1/E energy dependence above some lower cutoff energy of ijkT, and it can be normalized to the integrated thermal flux by a factor Then... 

This equation, first written by Simonits et al. (1980), has the same structure as the equation used with the classical relative method, but contains additional correction factors because the analyte nuclide is different from the monitor nuclide. The subscript m refers to the monitor, /is the ratio of thermal flux to epithermal flux, and Qo is the ratio of the resonance integral to the thermal neutron cross section (see Eq. (30.9)). 

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 assumption that the activation cross section varies with neutron energy as 1/v in the thermal neutron region is valid for most (n,y) reactions. The two reactions that deviate the most from the 1/v assumption are Lu(n,y) Lu (typically +0.4%/K) and Eu(n,y) Eu (typically —0.1%/K). The reaction rates for these two reactions, relative to a monitor reaction like Au(n,y) Au, will depend on the thermal neutron temperature in the irradiation channel used. A new set of equations, the Westcott formalism (Westcott 1955), was developed to account for these cases and used the Westcott g T ) factor, which is a measure of the variation of the effective thermal neutron activation cross-section relative to that of a 1/v reaction. In the modified Westcott formalism, the following differences are also included the Qo(a) value of the Hogdahl formalism is replaced by the So(< ) value, and the thermal to epithermal flux ratio, f, is replaced by the modified spectral index, r a) TJTo). To use this formalism with the kg method (De Corte et al. 1994), it is necessary to measure the neutron temperature, r , for each irradiation and a Lu temperature monitor should be irradiated. The Westcott formalism needs to be implemented only when analyzing for Lu and Eu. There are several other non-1/v nuclides Rh, In, Dy, Ir, and Ir, but for these the error... 

Users of the kg method must determine the thermal neutron flux, the ratio of thermal flux to epithermal flux for the irradiation channel used (/), the shape parameter (a), and detection efficiencies for all counting geometries used. Since the calculations for the derivation and the use of these parameters are extensive, a software package is needed. Some new users begin a collaboration with one of the many laboratories using the Icq method and adopt their methods of calculation and their software. Two complete software packages are now readily available to all potential users. The KAYZERO/SOLCOI Software (Van Sluijs et al. 1997), now called KayWin, performs the calculations in the well-understood classical manner. It is widely used... 

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, / ... 

Production of in the ORNL-HFIR hydraulic tube irradiation facility. Irradiations were performed in hydraulic tube no. 1 at 85 MW power. The thermal neutron flux at this position is 9.0 X lO n cm s with a thermal to epithermal flux ratio of 40... 

The lattice parameters were all determined from foil activation measurements made in the central cells of the full critical lattice loadings in the Process Development Pile (PDP). Thermal and epithermal flux distributions were determined from bare and cadmium covered irradiations of Cu, Mn, and U-235 foils placed In both the hiel and moderator. 

The ratio of thermal flux to epithermal flux should be minimized to reduce damage to the scalp. A target value for the ratio is 0.05. 

Epithermal or resonance neutrons have energies varying from 0.1 MeV down to about 0.5 eV. In the ideal case the epithermal flux epi is inversely proportional to the neutron... 

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 ... 

When a foil is irradiated at a location where there is an appreciable epithermal flux present, the activity observed may have to be corrected to determine the portion of the induced activity due only to the thermal flux. The activity of the foil due to thermal neutrons and corrected for the selfshielding of the foil for thermal neutrons can be expressed as... 

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