10.3.1} Gamma generation

 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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