Speaker
Description
FLUKA [1,2,3] is a multipurpose Monte Carlo code for the simulation of particle transport and interaction with matter. FLUKA relies on dedicated models for hadronic interactions, with the notable exception of low-energy neutrons (below 20 MeV), for which a group-wise treatment based on evaluated nuclear data has been employed for a sizable list of relevant isotopes. A point-wise treatment was made available only for a few isotopes and channels of practical relevance. The advantages of the group-wise neutron treatment are that it requires a very modest memory footprint and it is considerably faster than other methods, rendering this approach optimal for shielding calculations and deep-penetration problems. However, it suffers from several drawbacks, including insufficient precision in resonance regions, self-shielding issues, and a too coarse discretization of the energy and angular distribution of emitted secondaries. These drawbacks unfortunately limit the use of group-wise neutron transport for several applications.
In this work, a revised point-wise treatment for low-energy neutron transport in FLUKA v4 (distributed by CERN) is described, relying on the processed evaluated neutron cross section libraries that are used by Geant4 [4,5,6]. Special attention is devoted to the implementation decisions adopted in order to ensure energy and momentum conservation at every interaction, to respect the supplied distributions of emitted particles, preserving correlations and avoiding discretizations to the extent possible, while trying to keep the implementation as performant as possible.
Our implementation divides the low-energy neutron interactions into four categories: elastic, inelastic (including 36 individual sub-channels), radiative capture, and fission. All cross sections are Doppler broadened [7] during initialization for the temperatures requested by the user. For thermal neutron energies (En < 4 eV) the implementation accounts for chemical-binding effects. Molecular and crystal-lattice effects are accounted for via an S(α,β) treatment, including coherent and incoherent effects for a few selected materials. Finally, in view of the ongoing evolution of the FLUKA code, the implementation presented here is written as a C++ module that can be easily linked with other codes. Benchmarks against experimental data and other transport codes are provided.
References:
1. http://fluka.cern
2. G. Battistoni, T. Boehlen, F. Cerutti, P.W. Chin, L.S. Esposito, A. Fassò, A. Ferrari, A. Lechner, A. Empl, A. Mairani, A. Mereghetti, P. Garcia Ortega, J. Ranft, S. Roesler, P.R. Sala, V. Vlachoudis, G. Smirnov, "Overview of the FLUKA code", Annals of Nuclear Energy 82, 10-18 (2015).
3. T.T. Bohlen, F. Cerutti, M.P.W. Chin, A. Fassò, A. Ferrari, P.G. Ortega, A. Mairani, P.R. Sala, G. Smirnov, and V. Vlachoudis, "The FLUKA Code: Developments and Challenges for High Energy and Medical Applications", Nuclear Data Sheets 120, 211-214 (2014).
4. E. Mendoza, D. Cano-Ott, T. Koi and C. Guerrero, IEEE Trans. Nucl. Science 61 (2014) 2357.
5. E. Mendoza, D. Cano-Ott, C. Guerrero, and R. Capote, “New Evaluated Neutron Cross Section Libraries for the Geant4 Code”, IAEA technical report INDC(NDS)-0612, Vienna, 2012. Data available online at Geant4.
6. E. Mendoza, D. Cano-Ott, “Update of the Evaluated Neutron Cross Section Libraries for the Geant4 Code”, IAEA technical report INDC(NDS)-0758, Vienna, 2018.
7. D.Cullen, C.Weisbin, “Exact Doppler Broadening of Tabulated Cross Sections”, Nucl Sci and Eng 60, 199-229 (1976)