Program - Brookhaven National Laboratory

Prompt fission neutron spectra (PFNS) have proven to have a significant effect in some critical benchmarks,
in some cases as important as cross-section uncertainties. Therefore, a precise determination of
uncertainties in PFNS is largely desired. Existing PFNS evaluations in nuclear data libraries relied so far
almost exclusively on the Los Alamos model [1]. However, deviations of evaluated data from available
experiments have been noticed at both low and high neutron emission energies. Existing experimental
database for PFNS in EXFOR has been revisited and covariance information of selected experimental sets
have been proposed [2]. New experimental measurements have been recently published. The use of the
Bayesian Kalman filter has successfully proven to be a powerful tool to improve and refine evaluations by
fitting cross-section data. It therefore represents a natural path for fitting fission spectra using all available
experimental data. The goal is to derive realistic values for PFNS model parameters and corresponding
uncertainties and correlations. The present work describes the effort of integrating Kalman and EMPIRE
codes in such a way to allow for parameter fitting of PFNS models. The first results are shown for the
major actinides for two different models (Kornilov [3] and Los Alamos [1]). This represents the first step
towards consistent fitting of both cross-section and fission spectra data considering both microscopic and
integral experimental data.
[1] D. G. Madland and J. R. Nix, Nucl. Sci. Eng., 81, 213 (1982) [2] IAEA Summary Report, Consultant’s
Meeting, INDC(NDS)-0540, 2008 [3] N. V. Kornilov, A. B. Kagalenko, and F.-J. Hambsch, Physics of
Atomic Nuclei, 62, 173 (1999)
PR 112
Study of the Channel Spin Dependence of nu for Pu-239
G. Noguere, O. Serot, E. Fort, C. De Saint Jean, CEA, DEN, DER, SPRC, Cadarache, F-13108
Saint-Paul-lez-Durance, France. L. Leal, Oak Ridge National Laboratory, P.O.Box 2008, Oak Ridge, TN,
USA, 37831.
The description of fluctuations of the various fission quantities in the resonance range, such as the number
of prompt neutron emitted per fission, is a longstanding problem difficult to solve experimentally. For
Pu-239, the fluctuations in nu have been widely accepted since the pioneer works in the late 60. However,
only two data sets reported by Frehaut et al. (1973) and Gwin et al. (1984) were used to investigate such
fluctuations below few ten’s of eV. A phenomenological description proposed by Shackleton (1974) was
already applied on Pu-239 by Fort in 1988. The formalism takes into account the channel spin dependence
(s=0 and s=1) for the (n,f) and (n,gf) processes. In practice, the fission cross section has to be decomposed
in four components. Effects of the two-step (n,gf) process can be included as a corrective term. The weight
of each component was determined using the Pu-239 resonance parameters recommended in the existing
Evaluated Neutron Data File. In the present work, an improved Neutron Resonance Shape Analysis was
applied. The methodology consists to extract simultaneously the resonance parameters together with
the weight of each component by including in the fitting procedure all the cross section data and neutron
multiplicities. Thanks to this approach, the smallest fission widths for J=1+ resonances will provide reliable
partial widths for the (n,gf) reaction. Such results are crucial to validate the theoretical calculation of the
partial width for the (n,gf) reaction in the J=0+ resonances. The analysis is performed with the nuclear
data code CONRAD. For the prior information, we use the latest Pu-239 resonance parameters established
in the frame of the WPEC/SG-34. The CONRAD results (posterior model parameters and covariance
matrix) provide accurate information to adress the impact of the (n,gf) reaction on the average radiation
width.
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