Physics¶
Space radiation sources range from very low to very high energy and their interactions with the spacecraft sensitive devices and the shielding structures include both electromagnetic and hadronic processes. The GRAS tool includes therefore all the physics models available within Geant4, with the aim of giving a complete coverage of the main interaction mechanisms for trapped, solar and cosmic radiation in the spacecraft materials.
GRAS offers two ways to build Physics List : in the first combining it from components (constructors) using UI commands (see subsections below), in the second reference Physics List provided with Geant4 source code can be instantiated and if desired subsequently extended/modified.
The user can specify his physics list with the Command:
/gras/physics/addPhysics “physics_module_of_choice”
A list of the available modules can be obtained with the command
/gras/physics/list.
A definition of the available modules is provided in the subsections below. A list of the inserting modules, after the assembly, can be obtained with the command
/gras/physics/describe
Reference Physics Lists¶
All the Geant4 Reference Physics Lists are available to be used with the
/gras/physics/addPhysics command within GRAS software. The Geant4
Reference Physics Lists can be combined with an extension (see EM physics constructors) specifying the ElectroMagnetic (EM) physics
desired.
Note: If no EM physics is specified with the Geant4 reference
physics list then the GRAS default EM physics will be used
G4EmStandardPhysics_option4 and not the one defined within the
reference physics list.
Within GRAS the QGSP_QMD_HP reference physics list is also
available.
Electromagnetic physics¶
The user can select from the Geant4 reference electromagnetic physics lists (note: the names used can also be the ones defined in BROKEN LINK) as well as the GRAS specific options:
em_standard_space: a physics list suited for space applications in general, but especially tuned for X-ray astronomy mission development simulations.
em_standard_remizovich & firsov: dedicated physics lists for low energy proton grazing incident simulations.
em_standardNR
rmc_em_standard: see section blow.
Script examples:
/gras/physics/addPhysics em_standard | em_standard_opt1 | em_standard_opt2 | em_standard_opt3 | em_standard_space | em_standard_remizovich | em_standardSS | em_standardWVI | em_standardNR | rmc_em_standard | em_livermore | em_penelope | firsov
Hadronic physics¶
The user can include hadronic models for low, mid and high-energy particles. The low and mid energy hadronic models available are the Binary intra-nuclear cascade model, which describes the nuclear interactions of protons and neutrons with energy up to 10 GeV and pions up to 1 GeV, and the alternative Bertini cascade model, which describes proton, neutron and pion and kaon interactions up to 10 GeV. The Quark Gluon String (QGS) model, valid up to 100 TeV, covers higher energies and allows the simulation of a significant part of the protons in the wide Cosmic Ray spectrum.
A recent extension is also included of the Binary Cascade model for light ions with energy below 10 GeV/nucleon. However, its use for ions of higher atomic number and weight is not recommended. As an alternative, the recent Geant4 implementation 11 of the Abration/Ablation models 1111 is available. For higher energies, an extension of the QGS model to ions 11 is under development within the Geant4 collaboration and will be included as soon as it becomes public, due to its relevance to both SEE studies and estimates of the radiation effects on manned space flight.
The Radioactive Decay Module can also be added to the physics list as an additional module
Script example:
/gras/physics/addPhysics elastic /gras/physics/addPhysics binary_had /gras/physics/addPhysics binary_ion /gras/physics/addPhysics decay /gras/physics/addPhysics stopping /gras/physics/addPhysics gamma_nuc /gras/physics/addPhysics raddecay
An interface to some of the Geant4 reference physics lists is also provided. In this case, only the hadronic component of the packages is used, and the user can also select the electromagnetic package (standard / low energy).
Script examples:
/gras/physics/addPhysics LHEP /gras/physics/addPhysics QGSP /gras/physics/addPhysics QGSP_BIC /gras/physics/addPhysics QGSP_BERT /gras/physics/addPhysics QGSP_BERT_HP
In addition, GRAS offers the possibility to use a GEANT4 interface to the PHITS 11 models for HZE ions 1111. Both the PHITS code and the GEANT4-PHITS interface have to be obtained privately from the respective authors.
Script example:
/gras/physics/addPhysics jqmd_ion
Control of the Reverse MC physics¶
A physics list has been added in GRAS for the reverse MC simulation. It takes into account at this time the following reverse and forward processes : continuous gain/loss of energy for e-, e-ionisation, e- bremstrahlung, photoelectric effect , Compton scattering. It is selected with the GRAS command
/gras/physics/addPhysics rmc_em_standard
For testing purpose the different processes considered in this physics list can be switch on and off by using some commands in the directory /adjoint_physics/. By default all processes are taken into account except for ion ionisation. Reverse continuous and discrete e- ionisation and protons are always considered. The minimum and maximum energies of the reverse/adjoint models can be set by the commands SetEminForAdjointModels and SetEmaxForAdjointModels in the directory /adjoint_physics/. For computation efficiency it is recommended to set these minimum and maximum energies to the minimum and maximum energies of the external source. Reverse ion ionisation can be taken into account for one type of ion only (proton is not considered as ion but as a separate type of particle.) by using the command /adjoint_physics/UseAdjointIon and DefineAdjointIon. By default the ion ionisation is not considered. A list of types of adjoint primary particles that will be considered in the simulation is built during the initialisation of the physics an depends on the adjoint physics type of processes selected by the user. For example if only the ionisation for e- and proton are considered only adjoint e- and proton will be in the list of adjoint primary. While if the reverse bremsstrahlung is also selected, the adjoint gamma will also be added in the list of primaries. By default all type of adjoint primary contained in the primary list after the physics initilisation, wil be taken into account in the simulation, but the user can unselect or select separetly each of them by using the commands /adjoint/ConsiderAsPrimary and NeglectAsPrimary.
Cuts and step limits¶
The user can select the Geant4 production cuts 11 and an optional limitation to the maximum step size via UI scripts.
- Script example:
- ::
/gras/physics/setCuts 0.1 mm /gras/physics/stepMax 1.0 mm
When using the Low Energy EM processes, the user can in principle set production cuts in range corresponding to cuts in energy as low as 250 eV (or even 100 eV for the Penelope models). A default lower limit of 990 eV is however used in the Geant4 kernel, and to enable lower cut values the user must change the range of the cuts with explicit instructions. GRAS offers the UI Command:/gras/physics/productionCutsEnergyRange [low] [high] for this purpose.
Script example:
/gras/physics/productionCutsEnergyRange 250 eV 100 GeV
Cuts and step limits by region¶
The user can define Geant4 “regions” via UI scripts, and assign cuts and step size limits by region. A region must be defined and volumes can be assigned to regions by name. Cuts and step limits can then be applied to regions.
Script example:
/gras/physics/regionStepMax ABSORBER 0.5 mm /gras/physics/region/addRegion ABSORBER /gras/physics/region/addVolumeToRegion volume_name ABSORBER /gras/physics/region/setRegionCut ABSORBER all 0.01 mm
Physics utilities¶
The user can get some insight into the physics models via dedicated physics utilities UI commands. The code is implemented in the GRASPhysicsUtil class.
Print the dE/dx for a set of kinetic energy points in a given range, particle type, material
/gras/physics/util/printDEDXTable kinEnergyMin kinEnergyMax kineticEnergyUnit nPoints particle material
Print the EM XS for a set of kinetic energy points in a given range, particle type, process, material [, region]
/gras/physics/util/printEMCrossSectionTable kinEnergyMin kinEnergyMax kineticEnergyUnit nPoints particle process material [region]
Print the EM diff XS for a given prim Energy for a set of secondary kinetic energy points in a given range, particle type, process, material [, region]
/gras/physics/util/printDiffCrossSectionTable kinEnergyPrimary kinEnergySecondMin kinEnergySecondMax kineticEnergyUnit nPoints particle process material [region]
Print the Hadronic Elastic and Inelastic XS for a set of kinetic energy points for a given target element, energy range, particle
/gras/physics/util/printHadronicCrossSectionTable kinEnergyMin kinEnergyMax kineticEnergyUnit nPoints element particle [Z] [A]
Example:
/gras/physics/util/printHadronicCrossSectionTable 1 100000 MeV 51 Si proton /gras/physics/util/printHadronicCrossSectionTable 1 100000 MeV 51 Si GenericIon 82 208 /gras/physics/util/printHadronicCrossSectionTable 1 100000 MeV 51 Si GenericIon 54 131
GRAS/trunk/r2242
