Ion parametrized energy loss model

Purpose

Describes the energy loss of ions heavier than Helium due to the interaction with electrons of target atoms or molecules. The model computes:

  • Restricted stopping powers, which determine the continuous energy loss of primary ions along simulation steps
  • Cross sections for -ray production
  • The final state of the projectile (due to the production of delta-rays), as well as the final state of the -rays themselves

Since Geant4 9.3 (December 2009) the model incorporates ICRU 73 stopping data up to 1 GeV/nucleon (see below for details). It is highly recommended to switch to this new version.

Restricted stopping powers

Electronic stopping powers are an important ingredient to determine the continuous energy loss of ions as they slow down inside a medium. Restricted stopping powers account for the fact that the continuous energy loss description is restricted to energies below , where denotes the lower production threshold for -rays.

For an ion with energy , propagating in a material of density , restricted stopping powers are calculated according to

,

where are mass stopping powers obtained by interpolating tabulated values and indicates the mean energy transferred to -electrons per unit path length. If mass stopping powers are not available for a compound material but only for its elemental constituents, Bragg's additivity rule is applied to calculate the first term in the above equation. More details may be found in the Geant4 Physics Reference Manual (version 9.3 or higher).

Since Geant4 9.3, the model incorporates by default ICRU 73 stopping powers up to 1 GeV/nucleon. Revised ICRU 73 tables were considered for a few materials, affecting G4_WATER, G4_WATER_VAPOR, G4_NYLON-6-6 and G4_Cu (we thank P. Sigmund/Univ. of Southern Denmark for providing these tables). Due to their importance in radiotherapy applications, it should be noted that the revised ICRU 73 tables for water are based on model oscillator strength spectra implying a mean excitation energy of 78 eV, compared to 67.2 eV in the original ICRU 73 tables (see P. Sigmund, A. Schinner and H. Paul, "Errata and Addenda for ICRU Report 73", in press). It should be emphasized that mean excitation energies of Geant4 materials are not considered when applying the ICRU 73-based parametrization.

Stopping powers for ion-material pairs, which are not included in ICRU 73, are computed by scaling stopping powers using an effective charge expression.

Note: In order that ICRU 73 stopping powers are applied, materials must have a name matching the name of Geant4 NIST materials (i.e. either pre-defined Geant4 NIST materials are used, or user defined materials are created with the same name as materials in the Geant4 NIST data base, e.g. G4_WATER). For elemental materials this requirement does not apply, i.e. any material name may be used. Also consider, that ICRU 73 stopping powers are not available for all Geant4 NIST materials. All ICRU 73 stopping data sets are included as data files in the ion_stopping_data/icru73 sub-directory of $G4LEDATA.

Design of Physics process and model classes

The model, G4IonParametrisedLossModel, is part of the low-energy package, but follows the current design of the standard electromagnetic (EM) package. The model base class is G4VEmModel; important virtual functions implemented in the derived class are:

  • Initialise: Initialisation of tables
  • CrossSectionPerVolume: Computation of delta-ray production cross sections
  • ComputeDEDXPerVolume: Computation of restricted stopping powers
  • SampleSecondaries: Computation of final state due to delta-ray production

The model is designed to be used with the G4ionIonisation process of the standard EM package and is applicable to G4GenericIon only. Note, that the model is currently (9.3) not activated by default when G4ionIonisation is used (see next section for how to use the model, or use one of the constructors listed in the Physics constructors section).

How to use the model in Geant4 user physics lists

The model can be used as follows:

G4PhysicsListHelper* ph = G4PhysicsListHelper::GetPhysicsListHelper();

G4ionIonisation* ionIoni = new G4ionIonisation();
ionIoni->SetEmModel(new G4IonParametrisedLossModel());

ph->RegisterProcess(ionIoni, particle); // for the current particle

Physics constructors

The ion model is currently included in the G4EmLivermorePhysics, G4EmLivermorePolarizedPhysics, G4EmPenelopePhysics and G4EmStandardPhysics_option3 constructors, available in $G4INSTALL/source/physics_lists/constructors/electromagnetic.

How to access Physics ?

Note, that currently stopping powers calculated by G4IonParametrisedLossModel are not accessible through G4EmCalculator (as an intermediate solution, stopping powers can be retrieved from the model itself, using the ComputeDEDXPerVolume function, however this is only possible once all tables are initialised).

Documentation

Geant4 Physics Reference Manual (see Section 12.10.13 in Manual of Geant4 9.3)


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Topic revision: r9 - 2015-04-28 - SebastienIncerti
 
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