18.16.3} Evaporation/Fission and Fermi Break-Up

 Evaporation was initially implemented in FLUKA in 1990-1991 by Ferrari and
 Sala through an extensively modified version of the original Dresner's model
 based on Weisskopf's theory [Dre61]. Relativistic kinematics was substituted
 to the original one; fragmentation of small nuclei was also introduced,
 although initially only in a rough manner. The mass scale was changed to a
 modern one and the atomic masses were updated from a recent compilation.
 Improvements included also a more sophisticated treatment of nuclear
 level densities, now tabulated both with A and Z dependence and with
 the high temperature behaviour suggested by Ignatyuk [Ign75].
 A brand new model for gamma production from
 nuclear deexcitation was added, with a statistical treatment of E1, E2
 and M1 transitions and accounting for yrast line and pairing energy.
 This "initial capability" evaporation was used together with the first
 stage improved high energy hadron generator and the HILO library for the
 very first calculations carried out in 1990 for the LHC detector radiation
 environment. Later, in 1991, with the introduction of the "linear"
 preequilibrium model, a full model coverage down to 20 MeV was available
 and the new neutron cross section library developed together with
 ENEA-Bologna [Cuc91] started to be used.

 In 1993 the RAL high-energy fission model by Atchison [Atc80],
 kindly provided by R.E. Prael as implemented in the LAHET code,
 was included after some extensive modifications to remove some
 unphysical patches which the presence of a preequilibrium stage had
 now made unnecessary. The model was further developed and improved along
 the years and little is now left of the original implementation.
 Competition between evaporation and fission in
 heavy materials was implemented. This development was set off by a
 collaboration on energy amplifiers with C. Rubbia's group at CERN.
 Eventually, Ferrari joined that group in 1998.

 In 1995, a newly developed Fermi Break-up model, with a maximum of 6
 bodies in the exit channel, was introduced by Ferrari and Sala
 to describe the deexcitation of light nuclei (A =< 17). This
 development provided better multiplicities of evaporated neutrons
 and distributions of residual nuclei. The deexcitation gamma
 generation model was improved and benchmarked in the following year.

 A completely new evaporation treatment was developed by Ferrari and Sala
 in 1996 and 1997 in substitution of the improved Dresner model. This new
 algorithm adopted a sampling scheme for the emitted particle spectra which no
 longer made any Maxwellian approximation, included sub-barrier
 effects and took the full energy dependence of the nuclear level
 densities into account. Gamma competition was introduced too.
 These physics improvements allowed a much more accurate description
 of the production of residual nuclei. A refinement of this new
 package took place in 2000/2001. The production of fragments up to mass 24
 has been tentatively included around 2003 and subsequently developed
 and benchmarked [Bal04] and is now available in the
 distributed version as an option to be activated by the user.

 Further improvements in the Fermi Break-up model, the evaporation, and in
 the fission model have been developed by Ferrari and Sala for the FLUKA
 4th generation, starting with fluka2020.0.

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