International Reactor Dosimetry File 2002 - IAEA Publications

cross-sections, and as a consequence the uncertainties in the evaluated crosssection
values are reduced.
Additional information was obtained from theoretical model calculations
for the excitation functions of the dosimetry reactions 47 Ti(n,x) 46 Sc m+g ,
48 Ti(n,x) 47 Sc, 49 Ti(n,x) 48 Sc, 139 La(n,γ) 140 La, 186 W(n,γ) 187 W, 204 Pb(n,n¢) 204 Pb m and
237 Np(n,f).
The optical statistical method was used in order to obtain a theoretical
description of the excitation functions of the above mentioned reactions, taking
into account the contribution of the direct, pre-equilibrium and statistical
equilibrium processes to the different outgoing channels. Cross-sections were
calculated using modified versions of GNASH [2.4] and STAPRE [2.5]. The
principal difference between the original GNASH code [2.6] and this modified
version is that the latter contains a subroutine for calculation of the width
fluctuation correction. Calculations of penetrability coefficients for neutrons
were performed using the generalized optical model, which permits estimation
of the cross-sections for the direct excitations of collective low lying levels; the
ECIS coupled channel deformed optical model code was used for these calculations
[2.7]. The optical coefficients of proton and alpha particle penetrabilities
were determined using the SCAT2 code [2.8].
Modified GNASH was used to calculate the cross-sections from 1 keV to
20 MeV for the 139 La(n,γ) 140 La and 186 W(n,γ) 187 W reactions. The same data for
the 47 Ti(n,x) 46 Sc m+g , 48 Ti(n,x) 47 Sc, 49 Ti(n,x) 48 Sc and 204 Pb(n,n¢) 204 Pb m reactions
were obtained from threshold to 20 MeV, and the results of the STAPRE calculations
were used as supplementary information for the 237 Np(n,f) cross-section
evaluation between 10 and 20 MeV.
Evaluations of the excitation functions for the dosimetry reactions were
carried out using prepared input data, within the framework of the generalized
least squares method. The rational function was used as a model function [2.9],
and calculations of the recommended cross-section data and the related
covariance uncertainty matrices were performed using PADE-2 [2.10].
The multi-level Breit–Wigner (MLBW) resonance parameters used for
the calculation of the excitation functions in the resolved resonance region of
the 139 La(n,γ) 140 La, 186 W(n,γ) 187 W and 237 Np(n,f) reactions were evaluated on
the basis of data given in the compilations of Mughabghab et al. [2.11] and
Sukhoruchkin et al. [2.12]. Radiative capture cross-sections for 139 La and 186 W
nuclei in the unresolved resonance region were evaluated on the basis of calculations
performed using EVPAR [2.13].
Three block matrices give the uncertainties in the evaluated excitation
function for the 139 La(n,γ) 140 La and 186 W(n,γ) 187 W reactions. The first and
second matrices describe the cross-section uncertainty in the resolved
resonance region, while the third block matrix defines the uncertainty of the
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