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18.13} Electron and photon transport (EMF)


 The original EGS4 implementation in FLUKA was progressively modified,
 substituded with new algorithms  and increasingly integrated with the
 hadronic and the muon components of FLUKA, giving rise to a very different
 code, called EMF (Electro-Magnetic-Fluka). In 2005, the last remaining EGS
 routine has been eliminated, although  some of the structures still remind
 of the original EGS4 implementation.

 The main developments were made according to the following sequence.

 The Ferrari-Sala multiple scattering algorithm was the first major
 addition in 1989. It has already been described elsewhere since it was
 applied to hadrons and muons as well. Following its implementation, the
 whole electron/positron transport algorithm had to be reworked from scratch
 in order to comply with the features (initial and final step deflections,
 complex boundary crossing algorithm) of the new model.

 In 1990, the treatment of photoelectric effect was completely
 changed. Shell-by-shell cross sections were implemented, the photoelectron
 angular distribution [Sau31] was added, taking into
 account the fine structure of the edges, and production of
 fluorescence X-rays was implemented.

 Many new features were added in 1991. The emission angle of
 pair-produced electron and positrons and that of bremsstrahlung
 photons, which were only crudely approximated in the original EGS4
 code, were now sampled from the actual physical distributions.

 The full set of the electron-nucleus and electron-electron
 bremsstrahlung cross sections, differential in photon energy and
 angle, published by Seltzer and Berger for all elements up to 10 GeV [Sel86]
 was tabulated in extended form and introduced into the code
 together with a brand new sampling scheme by Fasso` and Ferrari.
 The energy mesh was concentrated, especially near the photon spectrum tip,
 and the maximum energy was extended to higher energies. The
 Landau-Pomeranchuk-Migdal effect [Lan53,Lan53a,Mig56,Mig57] for
 bremsstrahlung and the Ter-Mikaelyan polarisation effect [Ter54]
 (suppressing soft photon emission) were implemented.

 Positron bremsstrahlung was treated separately, using below 50 MeV the
 scaling function for the radiation integral given by Kim [Kim86] and
 differential cross sections obtained by fitting proper analytical
 formulae to numerical results of Feng et al.  The photon angular
 distribution was obtained by sampling the emission angle from the
 double differential formula reported by Koch and Motz [Koc59], fully
 correlated with the photon energy sampled from the Seltzer-Berger

 The Compton effect routines were rewritten in 1993 by Ferrari and Luca
 Cozzi (University of Milan), including binding effects.  At the end of
 the same year, the effect of photon polarisation was introduced for
 Compton, Rayleigh and photoelectric interactions by Ferrari.

 In 1993 and 1994, A. Fasso` and A. Ferrari implemented photonuclear
 reactions over the whole energy range, opening the way to the use of
 Monte Carlo in the design of electron accelerator shielding [Fas94]. Giant
 Dipole Resonance, Delta Resonance and high-energy photonuclear total
 cross sections were compiled from published data [Fas98] (further updated in
 2000 and 2002), while the quasideuteron cross section was calculated
 according to the Levinger model, with the Levinger's constant taken
 from Tavares et al. [Tav92], and the damping factor according to Chadwick et
 al. [Cha91]. The photon interaction with the nucleus was handled in the
 frame of the FLUKA hadronic event generators PEANUT and DPM (see below).

 In 1995, a single Coulomb scattering option was made available for
 electrons and positrons by Ferrari and Sala. The aim of this option
 was mainly to eliminate some artefacts which affected the angular
 distributions of charged particles crossing a boundary, but it
 turned out very useful also to solve some problems at very low
 electron energy or with materials of low density (gases).
 In the same year, the electron transport algorithm was reworked once
 more by Ferrari and Sala introducing an adaptive scheme which "senses" close
 boundaries in advance and automatically adapts the step length depending on
 their distance. Also in 1995 Ferrari discovered that the EGS4
 implementation of M{ö}ller and Bhabha scattering, still used at that time in
 FLUKA, was flawed. The bug was duly reported to the EGS4 authors who took
 corrective actions on their own code, while Ferrari developed a new algorithm
 for M{ö}ller and Bhabha scattering for FLUKA.

 In 1997 mutual polarisation of photons emitted in positron annihilation at
 rest was introduced by Ferrari.

 Cherenkov photon production and optical photon transport was implemented
 in 1999 by Ferrari. In 2002 scintillation photon production was added as well.

 In 1998-2001 an improved version of the Ferrari-Sala multiple scattering
 model was developed, introducing further refinements and the so called
 "polygonal" step approach. This version is presently fully tested offline
 and will be soon introduced into the production code.

 In 2005, the need for an external data preprocessor has been eliminated,
 integrating all the needed functionalities into the FLUKA initialization stage.
 At the same time, data from the EPDL97 [EPDL97] photon data library have become
 the source for pair production, photoelectric and total coherent cross section
 tabulations, as well as for atomic form factor data.

 At the same time, Rayleigh scattering has been reworked from scratch by
 Ferrari with a novel approach, and the photoeletric interaction model have been
 further improved with respect to the 1993 Ferrari-Sala approach, extending
 it among the others down to a few eV's.

 Finally the energy sampling for pair production have been completely
 reworked by Ferrari using a vastly superior approach, which can distinguish
 between interactions in the nuclear or electron field, and properly
 sample the element in a compound or mixture on which the interaction is going
 to occur. Thew algorithm is also capable of producing meaningful results for
 photon energies close to thresholds where several corrections are important
 and the symmetry electron/positron is broken, insimilar fashion to the
 bremsstrahlung case.

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