There were two main steps in the development of the FLUKA preequilibrium cascade model (PEANUT) by Ferrari and Sala:The first implementation of the FLUKA cascade-preequilibrium model, the "linear" one was finalised in July 1991 [Fer94]. The model, loosely based for the preequilibrium part on the exciton formalism of M. Blann and coworkers called Geometry Dependent Hybrid Model (GDH) [Bla71,Bla72,Bla75,Bla83a,Bla83b] now cast in a Monte Carlo form, was able to treat nucleon interactions at energies between the Coulomb barrier (for protons) or 10-20 MeV (for neutrons) and 260 MeV (the pion threshold). The model featured a very innovative concept, coupling a preequilibrium approach with a classical intranuclear cascade model supplemented with modern quantistic corrections. This approach was adopted for the first time by FLUKA and independently by the LAHET code [Pra89] at LANL. Capture of stopping negative pions, previously very crudely handled (the available alternatives being forced decay or energy deposited on the spot) was also introduced in this framework. This first implementation was called "linear" since in the cascade part refraction and refrection in the nuclear mean field was not yet taken into account, resulting in straight ("linear") paths of particles through the nuclear medium. First order corrections for these effects were anyway implemented on the final state angular distributions. This model immediately demonstrated superb performances when compared with nucleon induced particle production data. Its implementation into FLUKA allowed to overcome some of the most striking limitations of the code and permitted the use of the new neutron cross section library through its ability to produce sound results down to 20 MeV: in this way it opened a huge range of new application fields for the code. However, despite its nice performances, the "linear" cascade-preequilibrium model was always felt by Ferrari and Sala as a temporary solution for the low end side of particle interactions, while waiting for something even more sophisticated. The work on the "full" cascade-preequilibrium, which in the meantime had been called PEANUT (Pre-Equilibrium Approach to Nuclear Thermalisation) started at the end of 1991 and produced the first fully working version by mid-1993. Despite its improved quality this version was not included into any of the general use FLUKA versions until 1995, due to its complexity and the overall satisfactory results of the "linear" one for most applications. Till 1995, the full version was in use only by a few selected groups, including the EET group led by Carlo Rubbia at CERN, which meanwhile decided to adopt FLUKA as their standard simulation tools above 20 MeV, mostly due to the superior performances of PEANUT full version. It would be too long to describe in details all features of this model, which represented a quantum jump in the FLUKA performances and a significant development in the field. Actually, PEANUT combines an intranuclear part and a preequilibrium part (very similar in the "linear" and full versions), with a smooth transition around 50 MeV for secondary nucleons and 30 MeV for primary ones. It included nuclear potential effects (refraction and reflection), as well as quantal effects such as Pauli blocking, nucleon-nucleon correlations, fermion antisymmetrisation, formation zone and coherence length (a new concept introduced by Ferrari-Sala which generalises to low energy and two body scattering the formation zone concept). The model featured a sophisticated pion complex optical potential approach, together with 2 and 3 nucleon absorption processes and took into account the modifications due to the nuclear medium on the pion resonant amplitudes. For all elementary hadron-hadron scatterings (elastic, charge and strangeness exchanges) extensive use was made of available phase-shift analysis. Particle production was described in the framework of the isobar model and DPM at higher energies, using a much extended version of the original HADRIN code from Leipzig, and the FLUKA DPM model at higher energies. In 1995, distinct neutron and proton nuclear densities were adopted and shell model density distributions were introduced for light nuclei. The initial model extended the energy range of the original "linear" one from 260 MeV to about 1 GeV in 1994, with the inclusion of pion interactions. Giant Resonance and Quasideuteron photonuclear reactions were added in 1994 and improved in 2000. In 1996--1997 the emission of energetic light fragments (up to alphas) in the GINC stage emission has been described through the coalescence mechanism. The upper limit of PEANUT was further increased in 1996 to 1.8 GeV for nucleons and pions, and to 0.6 GeV for K+/K0; then again one year later (2.4 GeV for nucleons and 1.6 GeV for pions), and in 2000 (3.5 GeV for both pions and nucleons). In 1998, PEANUT was extended to K- and K0bar's induced interactions. In the 2005 version, all nucleon and pion reactions below 5 GeV/c of momentum are treated by PEANUT, while for kaons and hyperons the upper threshold is around 1.5 GeV (kinetic energy). Since 2005 also anti-nucleon interactions are treated in the PEANUT framework. It is planned to progressively extend PEANUT up to the highest energies by incorporating into its sophisticated nuclear framework the Glauber cascade and DPM part of the high energy model. One of the fall-outs of the work done for ICARUS was the introduction of nucleon decays and neutrino nuclear interactions in 1997 [Cav97], which prompted improvements in PEANUT, for instance concerning Fermi momentum and coherence length. Quasielastic neutrino interactions can be dealt with by PEANUT natively; in 1999, the code was coupled with the NUX neutrino-nucleon interaction code developed by Andre' Rubbia at ETH Zurich to produce full online neutrino-nucleus interactions, including resonance production and deep inelastic scattering. The combined FLUKA(PEANUT)+NUX model gave outstanding results when compared with NOMAD data, therefore giving support to all predictions done for ICARUS. Negative muon capture was also introduced in 1997 due to ICARUS needs. To much surprise, it turned out to be a key factor in the understanding of the unexpected background at the nTOF facility during its initial operation phase in 2001.
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