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 //------------------------------- -*- C++ -*- -------------------------------//
 // Copyright Celeritas contributors: see top-level COPYRIGHT file for details
 // SPDX-License-Identifier: (Apache-2.0 OR MIT)
 //---------------------------------------------------------------------------//
 //! \file celeritas/phys/PhysicsOptions.hh
 //---------------------------------------------------------------------------//
 #pragma once
 
 #include "corecel/Types.hh"
 #include "celeritas/Quantities.hh"
 #include "celeritas/Types.hh"
 #include "celeritas/Units.hh"
 
 namespace celeritas
 {
 //---------------------------------------------------------------------------//
 /*!
  * Particle-dependent physics configuration options.
  *
  * These parameters have different values for light particles
  * (electrons/positrons) and heavy particles (muons/hadrons).
  *
  * Input options are:
  * - \c min_range: below this value, there is no extra transformation from
  *   particle range to step length.
  * - \c max_step_over_range: at higher energy (longer range), gradually
  *   decrease the maximum step length until it's this fraction of the tabulated
  *   range.
  * - \c lowest_energy: tracking cutoff kinetic energy.
  * - \c displaced: whether MSC lateral displacement is enabled for e-/e+
  * - \c range_factor: used in the MSC step limitation algorithm to restrict the
  *   step size to \f$ f_r \cdot max(r, \lambda) \f$ at the start of a track or
  *   after entering a volume, where \f$ f_r \f$ is the range factor, \f$ r \f$
  *   is the range, and \f$ \lambda \f$ is the mean free path.
  * - \c step_limit_algorithm: algorithm used to determine the MSC step limit.
  *
  * NOTE: min_range/max_step_over_range are not accessible through Geant4, and
  * they can also be set to be different for electrons, mu/hadrons, and ions
  * (they are set in Geant4 with \c G4EmParameters::SetStepFunction()).
  */
 struct ParticleOptions
 {
     using Energy = units::MevEnergy;
 
     //!@{
     //! \name Range calculation
     real_type min_range;
     real_type max_step_over_range;
     //!@}
 
     //!@{
     //! \name Energy loss
     Energy lowest_energy;
     //!@}
 
     //!@{
     //! \name Multiple scattering
     bool displaced;
     real_type range_factor;
     MscStepLimitAlgorithm step_limit_algorithm;
     //!@}
 
     //! Default options for light particles (electrons/positrons)
     static ParticleOptions default_light()
     {
         ParticleOptions opts;
         opts.min_range = real_type{1} * units::millimeter;
         opts.max_step_over_range = 0.2;
         opts.lowest_energy = ParticleOptions::Energy{0.001};
         opts.displaced = true;
         opts.range_factor = 0.04;
         opts.step_limit_algorithm = MscStepLimitAlgorithm::safety;
         return opts;
     };
 
     //! Default options for heavy particles (muons/hadrons)
     static ParticleOptions default_heavy()
     {
         ParticleOptions opts;
         opts.min_range = 0.1 * units::millimeter;
         opts.max_step_over_range = 0.2;
         opts.lowest_energy = ParticleOptions::Energy{0.001};
         opts.displaced = false;
         opts.range_factor = 0.2;
         opts.step_limit_algorithm = MscStepLimitAlgorithm::minimal;
         return opts;
     };
 };
 
 //---------------------------------------------------------------------------//
 /*!
  * Physics configuration options.
  *
  * Input options are:
  * - \c fixed_step_limiter: if nonzero, prevent any tracks from taking a step
  *   longer than this length.
  * - \c min_eprime_over_e: energy scaling fraction used to estimate the maximum
  *   cross section over the step in the integral approach for energy loss
  *   processes.
  * - \c linear_loss_limit: if the mean energy loss along a step is greater than
  *   this fractional value of the pre-step kinetic energy, recalculate the
  *   energy loss.
  * - \c lambda_limit: limit on the MSC mean free path.
  * - \c safety_factor: used in the MSC step limitation algorithm to restrict
  *   the step size to \f$ f_s s \f$, where \f$ f_s \f$ is the safety factor and
  *   \f$  s \f$ is the safety distance.
  * - \c secondary_stack_factor: the number of secondary slots per track slot
  *   allocated.
  * - \c disable_integral_xs: for particles with energy loss processes, the
  *   particle energy changes over the step, so the assumption that the cross
  *   section is constant is no longer valid. By default, many charged particle
  *   processes use MC integration to sample the discrete interaction length
  *   with the correct probability. Disable this integral approach for all
  *   processes.
  */
 struct PhysicsOptions
 {
     //!@{
     //! \name Range calculation
     real_type fixed_step_limiter{0};
     //!@}
 
     //!@{
     //! \name Energy loss
     real_type min_eprime_over_e{0.8};
     real_type linear_loss_limit{0.01};
     //!@}
 
     //!@{
     //! \name Multiple scattering
     real_type lambda_limit{real_type{1} * units::millimeter};
     real_type safety_factor{0.6};
     //!@}
 
     //!@{
     //! \name Particle-dependent parameters
     ParticleOptions light{ParticleOptions::default_light()};
     ParticleOptions heavy{ParticleOptions::default_heavy()};
     //!@}
 
     real_type secondary_stack_factor{3};
     bool disable_integral_xs{false};
 };
 
 //---------------------------------------------------------------------------//
 }  // namespace celeritas

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