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.
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