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18.4} Fourth generation (the modern FLUKA, 2019 to today)

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 With the end of the INFN-CERN-Authors collaboration in 2019, the FLUKA
 development continues to be performed exclusively by its surviving legitimate
 Authors, A.Fasso`, A.Ferrari, and P.R.Sala. The original author of FLUKA, 
 Prof. J.Ranft unfortunately died in 2018. A new impetus allowed the new 
 generation of FLUKA to introduce a burst of major improvements and additions.
 Here one can find a short list of the most important new features 
 characterizing the 4th FLUKA generation:
   - The PEANUT model has been significantly improved in the energy range
     from a few GeV's on. Comparisons with experimental data for
     production in the 10-20 GeV range shows noticeable improvements 
     in the predicted distributions, particularly for heavy targets. The 
     introduction of cross section fluctuations now allows better 
     predictions of multiplicity distributions of fast hadrons in the
     multi-GeV regime, with impact on computed calorimeter resolutions;
   - Electro-nuclear reactions have been implemented by Fasso`, Ferrari,
     and Sala (see the PHOTONUC card);
   - Muon-nuclear reactions have been extended by Ferrari down to virtual 
     photon energies in the quasi-deuteron and Giant Dipole Resonance regions 
     with significant improvements in the predictions of cosmogenics backgrounds
   - Electro-Magnetic-Dissociation has been improved by Ferrari and
     Sala introducing also the Electric Quadrupole (E2) contribution, 
     of particular importance at the lowest energies;
   - An extended and improved nuclear database has been introduced by 
     Ferrari and Sala allowing better treatment of unstable nuclei 
     near the neutron and proton drip lines.
     Masses, decay channels and branching ratios have been extensively
     revised. Many more isomers are now included in the database:
     conventionally FLUKA considers "isomers" all excited states with
     half-lives in excess of 1 microsecond;
   - The inclusion of many more isomers made the old, 3rd generation
     FLUKA, crude, 50-50% estimate for isomer vs ground state production rather
     questionable. An evaluation of isomer production "a priori" during the 
     nuclear model evolution has been therefore implemented. While still 
     rudimentary, it is already significantly better than the old ansatz.
     Together with the isomer production data available in the neutron 
     cross section file (see below), it significantly improveed isomer 
     production  predictions. It should be noted that isomers, when produced,
     are allowed to fly until stopping, since their half-life is such
     that they will hardly decay before stopping in most practical problems;  
   - A deuteron pre-formation production mechanism by light nuclei has been
     implemented by Ferrari and Sala, resulting in much better predictions
     of excitation functions of reactions like (p,d)/(p,pn), (n,d)/(n,np) on 
     light nuclei at low and intermediate energies;
   - Direct reactions to IAS (Isobar Analogue State) have ben implemented
     by F.Salvat-Pujol and Ferrari for a few isotopes of particular importance
     as neutron producing targets, resulting in major improvements in the 
     prediction of neutron spectra at forward angles at low to medium energies;
   - Full account for discrete levels, out of the (IAEA) Ripl-3 library,
     has been implemented by Sala in every nuclear reaction step/generator;
   - A simplified model for angular momentum barriers has been developed by
     Ferrari and Sala inside the Fermi break-up de-excitation model 
     whenever emission cannot occur with L=0. Major improvements in residual 
     nuclei predictions for virtual or real photons on light targets in the 
     GDR region had been observed thanks to this extension;
   - The high energy fission model has vastly been improved, and it is now 
     able to provide reasonable predictions for photofission as well, as well
     as for fragment mass distribution for neutron and proton induced
     fission;
   - Fission fragment yields by low energy neutrons (neutrons below 20 MeV)
     are now based on recent evaluations, Endf/b-VIIIr0, Jeff-3.3, and 
     Jendl-4.0;
   - Group-wise neutron cross sections for several isotopes had been updated 
     with more recent evaluations, mostly Endf/b-VIIIr0. A few extra 
     isotopes/elements sections have been added;
   - Group-wise neutron cross sections now contain the information for 
     production of isomers, out of the European Activation File. Therefore 
     isomer versus ground state production by low energy neutrons is no longer 
     based on the "old", crude, estimate 50-50%, but it is rather calculated
     according to the evaluated values when available;
   - (Anti)Neutrino cross sections have been revised and improved by 
     Ferrari, Sala, and G.Smirnov, particularly at high energies,
     and for charm production ;
   - A preequilibrium step, based on the PEANUT one, has been introduced by
     Ferrari in the rQMD event generator. Together with other improvements 
     this results in significantly better reproduction of ion-ion experimental
     data, particularly towards the bottom energy range of application of rQMD;
   - A preequilibrium step, based on the PEANUT one, has been introduced by
     the BME event generator when the projectile-target/pre-fragment
     combination is not one of those pre-calculated in the BME database.
     This addition results in significant improvements particularly for 
     (very) light projectiles on heavy targets. There are still some 
     weaknesses for intermediate-to-heavy projectiles on 
     intermediate-to-heavy targets, with BME somewhat overpredicting 
     ejectile high energy tails;
   - Alpha-Nucleus cross sections for light nuclei have been updated
     by G.Arico` and Ferrari according to recent expreimental data, 
     bringing better agreement with measured attenuation curves and Bragg peaks;
   - A special treatment has been implemented by Sala for some exo-energetic,
     low energy reaction cross sections, in particular p-11B, Alpha-18O,
     Alpha-17O, Alpha-13C;
   - 
Default
damage threshold energies when asking DPA estimates are now available for all elements. The user can still override them by inputting her/his own values. Please note that widely different values are sometimes available in the literature for the same element, hence care must be used when using these default values. Also they apply to elements, for compounds/mixtures no specific default is available - Explicit emission of Alpha particles is now included in the radioactive decay part, for isotopes whose alpha decay energies and branchings are known; - Fully correlated pointwise transport of low energy neutrons is now available as an alternative to the standard group-wise treatment (Fasso`, Ferrari, and M.C.Morone). This feature represents a major improvement in FLUKA capabilities, it provides both the "continuous" transport of neutrons, and the generation of interaction products with a mixed data and model driven treatment fully conserving energy and momentum event-by-event. When neutron pointwise transport is activated, it is possible to set-up estimators with equidistant (linear or logarithmic) energy intervals, without the usual group-wise structure; - Transport in electric field has been implemented in vacuum and gas (Ferrari and M.Santana). The user routine, elefld.f, is available for providing non-uniform fields. Time-varying fields can also be implemented through this user routine. Combined electric and magnetic fields are supported. Transport is performed according to a Runge-Kutta treatment. In case of transport in gas, single scattering is automatically activated; - Transport in non uniform magnetic fields through the same Runge-Kutta algorithm is available as an option for vacuum and gas regions (it is used by default if an electric field is also present). The Runge-Kutta algorithm is significantly more accurate than the traditional one for the same CPU time, or it is faster for the same accuracy; - A new model for coherent elastic scattering of hadrons on nuclei has been introduced (Ferrari and Sala). A general model from combining black disk scattering and grayness has been derived for FLUKA. Parameters of the model for protons and neutrons up to 200 MeV have been fitted to distributions available in ENDF/B-8R0 and JENDL40-HE. Experimental data have been used to set the model parameters above 1 GeV. Scaling and interpolation of parameters are used for combinations where no data is available. The new algorithm is applied to protons and neutrons up to 200 MeV, and to all hadrons from 1 GeV upwards. For 4He, 12C, 16O, 208Pb, the model is applied for protons and neutron scattering over the whole energy range; - Quasi-elastic interactions (above few GeV) are now properly accounted for using PEANUT (Ferrari and Sala). Quasi-elastic are interactions where the projectile scatters elastically on one of the target nucleons. Traditionally, those interactions are considered as nonelastic one at low energies. As energy increases, they are less and less experimentally distinguishable from coherent elastic scattering. In FLUKA, high energy quasi-elastic was included in the past in the elastic scattering treatment, without production of secondaries. Starting with FLUKA version 2021, quasi-elastic has its own treatment, including production of secondary particles from nuclear de-excitation; - Excited nuclei with measurable/known mean life no longer de-excite during the nuclear interaction which produced them, but they rather fly until decay according to the level mean life (Ferrari). This has consequences for instance at very high energies, with nuclei potentially decaying far from the production point, and for Doppler broadening of gamma lines; - The nuclear properties database has been updated using nuclear masses from AME 2020. - Cross sections for interactions of very light ions, mass 3 and 4 on hydrogen (and for the inverse kinematic reaction) have been greatly improved (Ferrari); - The possibility to generate and transport optical photon from non-monochromatic scintillation lines is now available (Sala). Suitable user routines are available for providing the spectrum and intensity of scintillation lines; - Decay of the Delta(1232) resonance into photon + nucleon is now simulated Ferrari and Sala). The (small) branching ratio depends on the actual Delta mass; - Completely new cross sections and models for photo-nuclear interactions above 100 MeV have been developed (Fasso` and Ferrari). In the Delta resonance region, an universal cross section curve has been implemented for all nuclei with A>4, extending up to 2-3 GeV: this curve reproduces the experimental data significantly better than the previous parametrization which was 30 years old. Now, photon-nucleus interactions for A>4, and in this energy range, are simulated with PEANUT as such and no longer as pseudo-pi0. In particular, the initial target nucleon is chosen from the nuclear volume using new parametrizations for the photon-nucleon cross sections, accounting for the in-medium broadening of the intermediate Delta resonance, and accounting for the resulting 2 and 3-nucleon absorption channels. MAID07 generated data are now used for the single pion photon-nucleon cross sections and angular distributions. Several other resonant and non resonant photon-nucleon channels have been introduced in order to model multi-pion and strangeness production up to 2-3 GeV. Photon interactions at higher energies are described with PEANUT and no longer with the old interaction model which has been deleted. The procedure is explained the following: a) First the photon is supposed to fluctuate into a vector meson, with probability according to experimental coupling; b) then the selection is made between a (point-like) photon interaction (a volume one with no shadowing), and a vector-meson nucleus like one with shadowing; c) If the interaction is not a point-like one, it is selected whether the interaction proceeds through a diffractive-like coherent pseudo-elastic scattering, or through a non elastic vector meson nucleus one (the latter including the pseudo-quasielastic); d) If the interaction is a point-like one, or a VMD non-elastic one, the vector meson is used as projectile in PEANUT, in the former case with a volume selection of the target nucleon, in the latter with a hadron like (Glauber) interaction. PEANUT can now deal with vector meson resonances as projectiles, and with their (re)interaction and decay inside the nucleus. All this machinery is applied to real as well as virtual photon interactions; - The Photon cross section had been upgraded from EPDL97 to EPICS2017 [EPICS17]; - The Photon and bremsstrahlung cross sections had been extended up to 10^21 eV; - Ortho-Positronium three-gamma annihilation is now included, including the 3-body matrix-element for the decay. The ratio 3/2 photon annihilation is tunable per element and region/material (default 1/378). The decay time constants are also tunable; - The hadronisation model of the high energy generator has been deeply reworked in order to better account for experimental data which somewhat contradict widely accepted assumptions about string fragmentation. In particular, the surprisingly large yield of hard rho0's measured in pion-nucleon and pion-nucleus experiments, and the evolving rho0/Omega/pi0 ratio with Feynmann X could not be reconciled with some critical assumption of all string hadronization/fragmentation models. Those features are important for very high energy cosmic rays, since they can slow down the increase of the EM fraction in air showers and increase their muon content. They are also important for calorimeters, since they can impact the e/h ratio; - All hadron-nucleus cross sections are new and extended up to 1.E20 eV, and all of them are now consistently calculated with PEANUT. The PEANUT Glauber model had been improved and its capability of computing reliable quasi-elastic and absorption cross section extended to very high energies. The computed cross sections had been validated up to the highest cosmic ray data, and by default they substitute the previous FLUKA high-energy hadron-nucleus cross sections which were still based on extrapolations of available experimental data from the 80's; - Coherent elastic scattering of neutrons between 20 and 40 MeV is now treated using ad-hoc Legendre expansions derived from ENDF/B-VIIIr0; - Updated galactic cosmic ray spectra, based on AMS02 data (thanks to Nicola Mazziotta and Pedro de la Torre Luque) are now available: they can be activated by new option in SPECSOUR : sdum = GCR-AMS; - Built-in source routines for neutron spectra from Am-Be, Am-B, 252-Cf sources are now available (see the BEAM card); - Internal conversion decays (IC) are now modeled as such and no longer as (wrongly) gamma emission, including the explicit transport of the excited nuclei to the decay point; - The default DPMJET is now the newest one, 3.19.3.x, the previous version is no longer distributed; - By default intranuclear cascade reinteractions in Dpmjet are now treated by the FLUKA hadronic models; - The sets of fluence-to-dose conversion coefficients from ICRP116 [ICRP116] are now available, in particular ambient dose has been added; * Start_Devel_seq - The interface with the cosmic ray code CORSIKA, version 7, has been completely streamlined and reworked. It is now ready for CORSIKA 8 as well, and it can provide interfacing with the FLUKA routines for other processes beyond hadron-nucleus non-elastic interactions; * End_Devel_seq - A new interface to Ultra High Energy Cosmic Ray generators is now available: at present it supports interfacing to EPOS and Sibyll. The rationale is twofold: a) Despite the FLUKA philosophy of developing/choosing the best model for a given energy range and stick with it, above LHC energies there are little or no direct data available, and therefore the possibility to explore a range of predictions with 3-4 models can be important; b) At the same time, this interface allows FLUKA to run at the highest cosmic ray energies, which of course is an added bonus for the code. - Dose averaged LET maps are now implemented on-line (generalized particle DOSAVLET): they are computed taking care also of particles below threshold in order to minimize whichever possible bias; - The source code has been deeply reorganised. Among others, the old high energy interaction model (eventv/eventvmvax) has been removed, since it is no longer used by any of the interaction routines;

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