---------------------------------- Transport of neutrons with energies lower than a certain energy is performed in FLUKA by a multigroup algorithm. The energy boundary below which multigroup transport takes over depends in principle on the cross section library used. This energy is 20 MeV for the 260-group library which is distributed with the code. * Start_Devel_seq For the ENEA 72-group library the energy boundary is 19.6 MeV. * End_Devel_seq In FLUKA, internally there are two neutron energy thresholds: one for high-energy neutrons and one for low-energy neutrons. The high-energy neutron threshold represents in fact the energy boundary between continuous and discontinuous neutron transport. Starting from this release, the former is computed automatically on the basis of the user inputs with options PART-THR and/or LOW-BIAS. Please note that PART-THR no longer controls the transition energy between model and group tretaments for neutrons, but rather it sets the actual lower threshold for neutron transport, regardless if it falls in the group regime or not. The multi-group technique, widely used in low-energy neutron transport programs, consists in dividing the energy range of interest into a given number of intervals ("energy groups"). Elastic and inelastic reactions are simulated not as exclusive processes, but by group-to-group transfer probabilities forming a so-called "downscattering matrix". The scattering transfer probability between different groups is represented by a Legendre polynomial expansion truncated at the (N+1)th term, as shown in the equation: Sigma_s(g-->g',mu) = Sum_{i=0,N}(2i+1)/(4pi) P_i(mu) Sigma_i(g-->g') where mu = Omega.Omega' is the scattering angle and N is the chosen Legendre order of anisotropy. The particular implementation used in FLUKA has been derived from that of the MORSE program [Emm75] (although the relevant part of the code has been completely rewritten). In the FLUKA neutron cross section library, the energy range up to 20 MeV is divided into 260 energy groups of approximately equal logarithmic width (31 of which are thermal). * Start_Devel_seq In the old ENEA cross section library, the energy range up to 19.6 MeV is divided into 72 energy groups of approximately equal logarithmic width (only one of which is thermal). * End_Devel_seq The angular probabilities for inelastic scattering are obtained by a discretisation of a P5 Legendre polynomial expansion of the actual scattering distribution which preserves its first 6 moments. The generalised Gaussian quadrature scheme to generate the discrete distribution is rather complicated: details can be found in the MORSE manual [Emm75]. The result, in the case of a P5 expansion, is a set of 6 equations giving 3 discrete polar angles (actually angle cosines) and 3 corresponding cumulative probabilities. In the library, the first cross section table for an isotope (isotropic term P_0) contains the transfer probabilities from each group g to any group g': Sum_{g-->g'}/Sum_g, where Sum_g is the sum over all the g' (including the "in-scattering" term g' = g). The next cross section table provides the P_1 term for the same isotope, the next the P_2 multigroup cross sections, etc.