In general, gamma generation by low-energy neutrons (but not gamma transport) is treated in the frame of a multigroup scheme too. A downscattering matrix provides the probability, for a neutron in a given energy group, to generate a photon in each of a number of gamma energy groups (42 in the FLUKA library), covering the range from 1 keV to 50 MeV. With the exception of a few important gamma lines, such as the 2.2 MeV transition of Deuterium and the 478 keV photon from 10B(n,alpha) reaction, the actual energy of the generated photon is sampled randomly in the energy interval corresponding to its gamma group. Note that the gamma generation matrix does not include only capture gammas, but also gammas produced in other inelastic reactions such as (n,n'). For a few elements (Cd, Xe, Ar), for which evaluated gamma production cross sections could not be found, a different algorithm, based on published energy level data, has been provided to generate explicitly the full cascade of monoenergetic gammas [Fas01b]. In all cases, the generated gammas are transported in the same way as all other photons in FLUKA, using continuous cross sections and an explicit and detailed description of all their interactions with matter, allowing for the generation of electrons, positrons, and even secondary particles from photonuclear reactions.
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