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Low-Energy Neutron Treatment in FLUKA 7th FLUKA Course NEA, Sept 29- Oct 3, 2008 Sep 29-Oct 3rd FLUKA course NEA: Low Energy Neutrons 1 Part 1 Description of the Low Energy Neutron Library Sep 29-Oct 3rd FLUKA course NEA: Low Energy Neutrons 2 Introduction z Transport and interactions of neutrons with energies below 20 MeV are handled by a dedicated library z Neutron interactions at higher energy are handled by FLUKA nuclear models z In FLUKA we call neutrons below 20 MeV low energy neutrons z The low energy neutron library uses a multigroup approach z About 230 materials at different temperatures are available z The library handles also gamma generation, energy deposition by kerma factors, residual nuclei production, secondary neutrons, fission neutrons z For some isotopes/materials: self shielding, molecular binding, correlated gamma generation, point-wise transport Sep 29-Oct 3rd FLUKA course NEA: Low Energy Neutrons 3 Group Transport Technique z In neutron transport codes in general two approaches used: point-wise (“continuous” cross sections) and group-wise transport z Point-wise follows cross section precisely but is can be time and memory consuming z Group approach is widely used in neutron transport codes because it is fast and gives good results for most applications z Group-wise transport: energy range of interest is divided into a given number of discrete intervals: energy groups z Elastic and inelastic reactions are not simulated as exclusive processes but by a group to group transfer probability: downscattering matrix z Downscattering means that a neutron in a given group undergoes a scattering event where it loses energy. Afterwards it is in a group of lower energy. If the neutron does not lose enough energy to be in an other group it stays in the same group (in-scattering). z In thermal region neutrons can gain energy, which is taken into account by an upscattering matrix which contains the transfer probability to be scattered into a group of higher energy Sep 29-Oct 3rd FLUKA course NEA: Low Energy Neutrons 4 The FLUKA Low Neutron Library z FLUKA uses the group transport technique z Number of groups: 260 of approximately equal logarithmic with, the actual energies limits of each group can be found in the manual (or can be printed to *.out file), NB the group with the highest energy has the number 1, the group with the lowest energy has number 260 z Thermal groups: 31, with 30 upscattering groups z Energy range of library: 0.01 meV - 20 MeV z Based on recent versions of evaluated nuclear data files: ENDF/B- VI.8, ENDF/B-VII.0, Jendl-3.3, Jeff-3.1,… z About 230 isotopes/materials available z Almost all materials available at 2 temperatures: 87K, 296K z Some also at 4K and 120K z NB: Because of the group technique the energy of a neutron below 20 MeV is only defined within the accuracy of the groups Sep 29-Oct 3rd FLUKA course NEA: Low Energy Neutrons 5 Gamma generation z Gamma generation from (n,xγ) reactions is done for the elements where data is available in the evaluated nuclear data files (c.f. manual for a complete list) z Gamma generation is done also by a multigroup scheme z Number of groups: 42, NB the number of gamma groups is different from the number of neutron groups! z Energy range: 1keV - 50 MeV, NB the gamma energy range is different from the neutron energy range z The actual energy of the generated photon is sampled randomly in the energy interval corresponding to its gamma group, exception: 2.2 MeV transition of deuterium and 478 keV photon from 10B(n,γ) and gamma cascades from Cd(n, γ) and Xe(n, γ) z Capture gammas as well as gammas from inelastic reactions like (n,n’) are included z The neutron library only creates gammas, the transport is done by the EMF module (like all other gammas in FLUKA) Sep 29-Oct 3rd FLUKA course NEA: Low Energy Neutrons 6 Energy deposition z Energy deposition by neutrons below 20 MeV is estimated by means of kerma factors z For some materials with gamma production the kerma values of some groups (mainly at high energies) are problematic (c.f manual). The reason is inconsistent data in the evaluated data files. Effort was addressed to apply corrections to improve the situation but it there are still some materials with problematic kermas. z The user should check carefully the results of simulations with these materials. However, the effect should vanish in a typical simulation. Sep 29-Oct 3rd FLUKA course NEA: Low Energy Neutrons 7 Secondary and Fission Neutrons z Neutrons from (n,xn) reactions are taken into account implicitly

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