Low Energy Electromagnetic Physics - MicroElec

The Geant4-MicroElec extension of the Geant4 Monte Carlo simulation toolkit, developed by CEA, aims at modeling the effect of ionizing radiation in highly integrated microelectronic components. It describes the transport and generation of very low energy electrons by incident electron, proton and heavy ions in silicon.

1. Software

The Geant4-MicroElec extension is fully included in the Geant4 public distribution since the 9.6 beta version (released 29 June 2012). It is located in the $G4INSTALL/source/processes/electromagnetic/lowenergy directory. It requires specific data files, included in the G4EMLOWX.Y set of data files.

2. Physics

Geant4-MicroElec physics processes and models can simulate step-by-step interactions of particles in silicon down to the eV scale.

a. Design of physics process and model classes

All Geant4-MicroElec classes have been developed from Geant-DNA ones, with only minor modifications. The design follows the software design of the electromagnetic Physics developments of the Geant4 toolkit. A physical interaction is described by a process class. A single process can handle several models (for eg. theoretical, semi-empirical, …) which are registered to the process class. All Geant4-MicroElec process classes derive from the G4VEmProcess abstract base class, which derives from the G4VDiscreteProcess class, since all Geant4-MicroElec processes are discrete processes. They use the naming format: G4MicroElecXXX where XXX is the process name. This is the name that will be displayed when using the Geant4 User Interface command /tracking/verbose 1. In these process classes, several methods are implemented :
  • G4MicroElecXXX::InitialiseProcess(...) : process initialisation
  • G4MicroElecXXX::PrintInfo(...) : verbose
All Geant4-MicroElec model classes derive from the G4VEmModel abstract base class. They use the naming format: G4MicroElecXXXModel. For now, there is only one model for each implemented process. Several methods are implemented:
  • G4MicroElecXXXModel::Initialise(...) : model initialisation
  • G4MicroElecXXXModel::CrossSectionPerVolume(...) : computation of model total cross section multiplied by material density (equivalent to inverse mean free path)
  • G4MicroElecXXXModel::SampleSecondaries(...) : computation of model final state
These classes are further detailed in the Processes section.

b. Available physics processes and models

This section describes the list of physical interactions per particle type that can be modeled using the Geant4-MicroElec extension. The corresponding process classes, model classes, low energy limit applicability of models, high energy applicability of models, energy threshold below which the incident particle is killed (stopped and the kinetic energy is locally deposited) are listed in the table below. All models are interpolated. For now, they are valid for silicon only (use the « G4_Si » Geant4-NIST material). Please refer to the publication section of this web site for more details about the models.

The usage of these classes is described in the Physics List section.

Particle Interaction Process class Model class Min. energy Max. energy Kill
Electron Elastic scattering G4MicroElecElastic G4MicroElecElasticModel 5 eV 100 MeV 16.7 eV (*)
  Ionisation G4MicroElecInelastic G4MicroElecInelasticModel 16.7 eV 100 MeV -
Proton and heavy ions Ionisation G4MicroElecInelastic G4MicroElecInelasticModel 50 keV/u 10 GeV/u -

(*) because of the low energy limit applicability of the inelastic model.

c. Physics List

A dedicated Physics constructor is to be developed. In the meantime, the user will have to write his/her own PhysicsList class based on Geant4-MicroElec physics processes and models. To do so, the user can follow the example of the G4EmDNAPhysics Physics constructor (located in the $G4INSTALL/source/physics_lists/constructors/electromagnetic directory), using previous MicroElec processes and models instead of DNA ones.

d. Miscellaneous

This section explains several additional features available to users for the development of Geant4-MicroElec based applications.

Material definition

Geant4-MicroElec Physics models can currently only be used in silicon defined in Geant4 as the NIST material G4_Si. To do so, one may use the following lines in the DetectorConstruction class :

// Si is defined from NIST material database G4NistManager * man = G4NistManager::Instance(); G4Material * Si = man->FindOrBuildMaterial("G4_Si");

// Default materials in setup. SiMaterial = Si;

How to kill particles below a given energy threshold for faster performance ?

In case the user needs to kill particles with energies below a selected energy threshold value, one may instantiate a G4UserLimits object in the DetectorConstruction class and define the process G4UserSpecialCuts in the user PhysicsList class for the affected particles. All details are given in the Geant4 User's Guide For Application Developers.

For example, in order to kill all electrons below 50 eV, one may use the following lines. All electron tracks below 50 eV will be killed and electrons will deposit locally their total energy:

- in the DetectorConstruction class, one can choose to apply this limit to the World volume: #include "G4UserLimits.hh" ... logicWorld->SetUserLimits(new G4UserLimits(DBL_MAX,DBL_MAX,DBL_MAX,50*eV));

- in the PhysicsList class: #include "G4UserSpecialCuts.hh" ... if (particleName == "e-") { ... pmanager->AddDiscreteProcess(new G4UserSpecialCuts()); ... }

How to combine Geant4-MicroElec processes with other Geant4 processes?

Refer to the Microelectronics example.

3. User examples

Refer to the Microelectronics example.

4. Related publications and conference presentations

A. Valentin, M. Raine and J.-E. Sauvestre, "Inelastic cross-sections of low-energy electrons in silicon for the simulation of heavy ion tracks with the GEANT4-DNA toolkit", presented at Nuclear Science Symposium and Medical Imaging Conference, Knoxville, Tennessee, USA, 2010. http://dx.doi.org/10.1109/NSSMIC.2010.5873720

A. Valentin, M. Raine, J.-E. Sauvestre, M. Gaillardin and P. Paillet, "Geant4 physics processes for microdosimetry simulation: very low energy electromagnetic models for electrons in silicon", Nuclear Instruments and Methods in Physics Research B, vol. 288, pp. 66 - 73, 2012. http://dx.doi.org/10.1016/j.nimb.2012.07.028

A. Valentin, M. Raine, M. Gaillardin and P. Paillet, "Geant4 physics processes for microdosimetry simulation: very low energy electromagnetic models for protons and heavy ions in silicon", Nuclear Instruments and Methods in Physics Research B, vol. 287, pp. 124 - 129, 2012. http://dx.doi.org/10.1016/j.nimb.2012.06.007

M. Raine, A. Valentin, M. Gaillardin and P. Paillet, "Improved simulation of ion track structures using new Geant4 models - Impact on the modeling of advanced technologies response ", IEEE Transactions on Nuclear Science, vol. 59, pp. 2697 - 2703, 2012. http://dx.doi.org/10.1109/TNS.2012.2220783

M. Raine, M. Gaillardin and P. Paillet, "Geant4 physics processes for silicon microdosimetry simulation: Improvements and extension of the energy-range validity up to 10 GeV/nucleon", Nuclear Instruments and Methods in Physics Research B, vol. 325, pp. 97 - 100, 2014. http://dx.doi.org/10.1016/j.nimb.2014.01.014