Geant4 Low Energy Electromagnetic Physics Working Group1
Low Energy Electromagnetic Physics
Geant4 tutorialSLAC
on behalf of the Geant4 Low Energy Electromagnetic Physics Working group
Geant4 Low Energy Electromagnetic Physics Working Group2
Content Context Physics models
Livermore Penelope Ion model Geant4-DNA Atomic de-excitation Data handling and interpolation
How to implement a Physics list ? Advanced examples Documentation
Geant4 Low Energy Electromagnetic Physics Working Group3
Purpose Extend the coverage of Geant4 electromagnetic interactions with matter
photons, electrons, hadrons and ions down to very low energies (sub-keV scale)
Possible domains of applications space science medical physics Microdosimetry …
Choices of Physics models include Livermore library: electrons and photons [250 eV – 1 GeV] Penelope (Monte Carlo): electrons, positrons and photons [250 eV
– 1 GeV] Microdosimetry models (Geant4-DNA project): [7 eV – 10 MeV]
Geant4 Low Energy Electromagnetic Physics Working Group4
Software design
Identical to the one proposed by the Standard EM working group Apply to all Low Energy Electromagnetic classes Allow a coherent approach to the modelling of electromagnetic
interactions
A physical interaction or process is described by a process class Naming scheme : « G4ProcessName » Eg. : « G4Compton » for photon Compton scattering
A physical process can be simulated according to several models, each model being described by a model class
Naming scheme : « G4ModelNameProcessNameModel » Eg. : « G4LivermoreComptonModel » for the Livermore Compton model Models can be alternative and/or complementary in certain energy
ranges
According to the selected model, model classes provide the computation of the process total cross section & the stopping power the process final state (kinematics, production of secondaries…)
Valid from Geant4 9.3 BETA
Geant4 Low Energy Electromagnetic Physics Working Group5
How to extract Physics ?
Thanks to this new software designPossible to retrieve Physics quantities using a G4EmCalculator object
Example for retrieving the total cross section of a process with name procName:
#include "G4EmCalculator.hh" ... G4EmCalculator emCalculator; G4double density = material->GetDensity(); G4double massSigma = emCalculator.ComputeCrossSectionPerVolume
(energy,particle,procName,material)/density; G4cout << G4BestUnit(massSigma, "Surface/Mass") << G4endl;
A good example: $G4INSTALL/examples/extended/electromagnetic/TestEm14Look in particular at the RunAction.cc class
Valid from Geant4 9.3 BETA
Geant4 Low Energy Electromagnetic Physics Working Group
Physics models 1/6
Livermore models
6
Geant4 Low Energy Electromagnetic Physics Working Group
Livermore models Based on publicly available evaluated data tables from LLNL
EADL : Evaluated Atomic Data Library EEDL : Evaluated Electrons Data Library EPDL97 : Evaluated Photons Data Library
Validity range : 250 eV - 100 GeV Processes can be used down to 100 eV, with a reduced accuracy In principle, validity range down to ~10 eV
Included elements from Z=1 to Z=100 Atomic relaxation : Z > 5 (EADL transition data)
Data tables are interpolated by Livermore model classes To compute total cross section and final state
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Geant4 Low Energy Electromagnetic Physics Working Group
Photon models (1) Compton scattering (incoherent)
Scattered photon energy: from Klein Nishina formula Modified by the Hubbel form factor obtained from EPDL97
Angular distributions of scattered photon and recoil electron from EPDL97
Rayleigh scattering (coherent : no energy loss) Angular distribution from Rayleigh formula Include the Hubbel form factor from EPDL97
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Geant4 Low Energy Electromagnetic Physics Working Group
Photon models (2)
Photoelectric effect Cross section integrated over shells and cross section by shell from
EPDL Several angular distribution generators available (naive, Sauter-
Gravila, Gravila) De-excitation : managed by the atomic relaxation process
Initial vacancy and cascade of resulting vacancies
Pair conversion e+ and e- energies computed from Bethe-Heitler formula
Include Coulomb correction Tsai differential cross section for energy and polar angle
computation Polar angular distribtuion: symmetric Azimuthal angle distribution: isotropic
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Geant4 Low Energy Electromagnetic Physics Working Group
Electron models
Bremsstrahlung Parametrisation from EEDL 16 parameters
Ionisation Parametrisation using 5 parameters
by shell
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Geant4 Low Energy Electromagnetic Physics Working Group
Available Livermore models
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PhysicsProcess
ProcessClass
ModelClass
Low EnergyLimit
High EnergyLimit
Gammas
Compton G4ComptonScattering G4LivermoreComptonModel 250 eV 100 GeV
Polarized Compton
G4ComptonScattering G4LivermorePolarizedComptonModel 250 eV 100 GeV
Rayleigh G4RayleighScattering G4LivermoreRayleighModel 250 eV 100 GeV
Polarized Rayleigh
G4RayleighScattering G4LivermorePolarizedRayleighModel 250 eV 100 GeV
Conversion G4GammaConversion G4LivermoreGammaConversionModel 1.022 MeV 100 GeV
Photo-electric G4PhotoElectricEffect G4LivermorePhotoElectricModel 250 eV 100 GeV
Electrons
Ionization G4eIonisation G4LivermoreIonisationModel 250 eV 100 GeV
Bremsstrahlung G4eBremsstrahlung G4LivermoreBremsstrahlungModel 250 eV 100 GeV
Geant4 Low Energy Electromagnetic Physics Working Group12
Photo-electricHydrogen(tag: 9.2-3)
Photo-electricNeon
(tag: 9.2-3)
Gamma Conversion
Lead(tag: 9.2-3)
Electron Range(tag: 9.2-4)
Eg. of validation of Livermore models
Geant4 Low Energy Electromagnetic Physics Working Group13
Polarized Livermore processes
Describe in detail the kinematics of polarized photon interactions
Ad-hoc generation of secondary products(based on differential cross section)
Possible applications of such developments: design of new space missions for the detection of polarized photons
Documentation Nucl. Instrum.Meth. A566: 590-597, 2006 (Photoelectric) Nucl. Instrum.Meth. A512: 619-630, 2003 (Compton and Rayleigh) Nucl.Instrum.Meth. A452:298-305,2000 (Pair production)
Currently available: Compton and Rayleigh
Geant4 Low Energy Electromagnetic Physics Working Group
Eg. polarized Compton cross section
The Klein Nishina cross section:
2
0
020
220 cos42
h
h
h
h
h
hr
4
1
d
d
whereh0 : energy of the incident photonh : energy of the scattered photon : angle between the two polarization vectors
The code properly reproduces polarized photon interactions and also the secondary polarization acquired after a Compton interaction
Geant4 Low Energy Electromagnetic Physics Working Group15
Polarized Compton simulation
0 1 2 3
2
Comparison between the theoretical rates of intensities with that obtained from Geant4 for 100 keV, 1 MeV and 10 MeV.
p=dd
Geant4 Low Energy Electromagnetic Physics Working Group
Physics models 2/6
Penelope models
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Geant4 Low Energy Electromagnetic Physics Working Group17
Penelope physics
Geant4 includes the low-energy models for e± and -rays from the Monte Carlo code PENELOPE (PENetration and Energy LOss of Positrons and Electrons)
Nucl. Instrum. Meth. B 207 (2003) 107
Physics models Specifically developed by the Barcelona group (F. Salvat et
al.) Great care was dedicated to the low-energy description
(atomic effects, fluorescence, Doppler broadening, etc.)
Mixed approach: analytical, parametrized & database-driven applicability energy range: 250 eV 1 GeV
Geant4 Low Energy Electromagnetic Physics Working Group18
Penelope in Geant4 Reliability of the physics models
Extensively tested by the Penelope group itself (several papers) Penelope coding
Original in FORTRAN77 Version 2001 re-engineered in Geant4 (C++)
Corresponding physics models in Geant4:G4PenelopeComptonModel
G4PenelopeRayleighModel
G4PenelopeGammaConversionModel
G4PenelopePhotoElectricModel
G4PenelopeAnnihilationModel
G4PenelopeBremsstrahlungModel
G4PenelopeIonisationModel
Penelope models are the only low-energy ones available for e+ in G4
-rays
e±
Valid from Geant4 9.3 BETA
Geant4 Low Energy Electromagnetic Physics Working Group19
When/how to use Penelope models
Use Penelope models (as an alternative to Livermore or Standard models) when you:
need precise treatment of EM showers and interactions at low-energy (keV scale)
are interested in atomic effects, as fluorescence x-rays, Doppler broadening, etc.
can afford a more CPU-intensive simulation want to cross-check an other simulation (with a different
model) are interested in low-energy positrons (only choice in
Geant4) Do not use when you are interested in EM physics > MeV same results as Standard EM models, performance
penalty
G4 physics list (from version 9.2) including Penelope EM physics: G4EmPenelopePhysics
Geant4 Low Energy Electromagnetic Physics Working Group20
Penelope verification and validation
-ray attenuation coeff in Al
Energy (MeV)
Att
enua
tion
coe
ff. (
cm2 /
g)
NIST data
Penelope
2=15.9
=19
p=0.66
cos
Rayleigh scattering
50 keV -ray in Au
Penelope FORTRAN
G4Penelope
If G4Penelope gives the same results as Penelope-Fortran take for granted the (large) validation work performed by the Penelope group
Additional validation within Geant4 for e± and -rays (all EM models)
Geant4 Low Energy Electromagnetic Physics Working Group21
Doppler broadening in Compton scattering
Au, 50 keV -ray
Compton scattering: electrons bound and not at rest (as assumed for Klein-Nishina) change of angular distribution, reduction of XS
Penelope model includes it (via analytical approach)
Livermore model also deals with Doppler broadening
(EGS database approach)
Good agreement Penelope-Livermore
Standard model includes cross section suppression, but samples final state
according to Klein-Nishina
Geant4 Low Energy Electromagnetic Physics Working Group
Physics models 3/6
Ions
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Geant4 Low Energy Electromagnetic Physics Working Group23
New ion energy loss model Describes the energy loss of ions heavier than Helium due to
interaction with the atomic shells of target atoms
The model computes Restricted stopping powers
Determine the continuous energy loss of ions as they slow down in an absorber (more details on next slides)
Cross sections for the production of δ-rays
Inherently also govern the discrete energy loss of ions
(Note: δ-rays are only produced above a given threshold)
Primarily of interest for Medical applications Space applications
Valid from Geant4 9.3 BETA
Geant4 Low Energy Electromagnetic Physics Working Group24
Ion stopping powers (1/2) Electronic stopping powers: important ingredient to determine the mean
energy loss of ions along simulation steps Impacts the ion range (for example)
Restricted stopping powers: account for the fact that the continuous energy loss description is restricted to energies below T
cut
(where Tcut
denotes the lower production threshold of δ-rays)
Restricted stopping powers are calculated according to (T = kinetic energy per nucleon)
T < TL: Free electron gas model
TL ≤ T ≤ TH: Interpolation of tables or parameterization approach
T > TH: Bethe formula (using an effect. charge) + high order corr.
Geant4 Low Energy Electromagnetic Physics Working Group25
Ion stopping powers (2/2) Parameterization approach
Model incorporates ICRU 73 stopping powers into Geant4
ICRU73 model Covers a large range of ion-material combinations: Li to Ar,
and Fe Stopping powers: based on binary theory Special case: water
Revised ICRU 73 tables of P. Sigmund are used (since Geant4 9.3.b01)
Mean ionization potential of water of 78 eV Current model parameters (Geant4 9.3.b01):
THigh = 10 MeV/nucleon (except Fe ions: TH = 1 GeV/nucleon) TLow = 0.025 MeV/nucleon (lower boundary of ICRU 73 tables)
For ions heavier than Ar Scaling of Fe ions based on effective charge approach
Geant4 Low Energy Electromagnetic Physics Working Group26
How to use the new model ? Model name: G4IonParametrisedLossModel
Designed to be used with G4ionIonisation process (of standard EM package) Not activated by default when using G4ionIonisation Users can employ model by utilizing SetEmModel function of
G4ionIonisation process
Restricted to one Geant4 particle type: G4GenericIon Note: The process G4ionIonisation is also applicable to alpha
particles (G4Alpha) and He3 ions (G4He3), however the model must not be activated for these light ions
Geant4 Low Energy Electromagnetic Physics Working Group27
Using ICRU 73 tables ICRU 73 stopping powers: available for a range of elemental and compound
materials:
To use the ICRU 73 tables for ion modelMaterials must have names of Geant4 NIST materials
Either Geant4 NIST materials are used, or user-specific materials are created with the same name as materials in Geant4 NIST data base.
Note: ICRU 73 stopping powers are not available for all NIST materials.
Available stopping powers can be looked up in the following classes of the Geant4 material sub-package ($G4INSTALL/source/materials):
G4SimpleMaterialStoppingICRU73 (ions up to Ar) G4MaterialStoppingICRU73 (ions up to Ar) G4IronStoppingICRU73 (Fe ions & ions scaled from Fe)
Geant4 Low Energy Electromagnetic Physics Working Group
Physics models 4/6
Geant4-DNA
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Geant4 Low Energy Electromagnetic Physics Working Group29
Geant4 for microdosimetry History : initiated in 2001 by Petteri Nieminen (European Space Agency /
ESTEC) in the framework of the « Geant4-DNA » project
Objective : adapt the general purpose Geant4 Monte Carlo toolkit for the simulation of interactions of radiation with biological systems at the cellular and DNA level (« microdosimetry »)
A full multidisciplinary activity of the Geant4 low energy electromagnetic Physics working group, involving physicists, theoreticians, biophysicsts…
Applications : Radiobiology, radiotherapy and hadrontherapy
(eg. prediction of DNA strand breaks from ionising radiation) Radioprotection for human exploration of Solar system Not limited to biological materials (ex. Silicon)
Geant4 Low Energy Electromagnetic Physics Working Group30
Geant4 for microdosimetry Several models are available for the description of physical processes
involving e-, p, H, He, He+, He++
Include elastic scattering, excitation, ionisation and charge change
For now, these models are valid for liquid water medium only
Models available in Geant4-DNA are published in the literature may be purely analytical or use interpolated cross section
data
They are all discrete processes
Geant4 Low Energy Electromagnetic Physics Working Group31
Physics models in Geant4 DNA
e p H , He+, HeElastic
scattering> 7.4 eV – 10 MeV
Screened Rutherford> 7 eV – 10 MeV
Champion
- - -
Excitation
A1B1, B1A1, Ryd A+B, Ryd C+D,
diffuse bands
7.4 eV – 10 MeVEmfietzoglou
10 eV – 500 keVMiller Green
500 keV – 10 MeVBorn
-
Effective charge scaling from same
models as for proton
Charge Change -
1 keV – 10 MeVDingfelder
1 keV – 10 MeVDingfelder
Ionisation
1b1, 3a1, 1b2, 2a1 + 1a1
12.6 eV – 30 keVBorn
100 eV – 500 keVRudd
500 keV – 10 MeVBorn
100 eV – 100 MeVRudd
• Models in black are analytical• Models in purple use interpolated dataValid from Geant4 9.3 BETA
Geant4 Low Energy Electromagnetic Physics Working Group32
How to set a low energy threshold ?
To kill particles with energies below an energy threshold value Instantiate a G4UserLimits object in the DetectorConstruction class Define the process G4UserSpecialCuts in the PhysicsList class for the affected
particles.
All details are given in the Geant4 User's Guide For Application Developers.
Example: to kill all electrons below 9 eV, see the following lines. All electron tracks below 9 eV will be killed and electrons will deposit locally their
total energy In the DetectorConstruction class, in order to apply this limit to the World volume
#include "G4UserLimits.hh" ... logicWorld->SetUserLimits(new G4UserLimits(DBL_MAX,DBL_MAX,DBL_MAX,9*eV));
In the PhysicsList class
#include "G4UserSpecialCuts.hh" ... if (particleName == "e-") { ... pmanager->AddDiscreteProcess(new G4UserSpecialCuts()); ... }
Geant4 Low Energy Electromagnetic Physics Working Group
Physics models 5/6
Atomic de-excitation
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Geant4 Low Energy Electromagnetic Physics Working Group
Overview Atomic de-excitation initiated by other EM processes
Examples: photo-electric effect, ionisation (PIXE) … Leave the atom in an excited state
EADL data contain transition probabilities radiative: fluorescence non-radiative:
Auger e-: inital and final vacancies in different sub-shells Coster-Kronig e-: identical sub-shell
Atomic de-excitation simulation Undergoing major design iteration To be fully compatible with: Low energy EM package and Standard
package
More in 201034
Geant4 Low Energy Electromagnetic Physics Working Group
Physics models 6/6
Data handling and interpolation
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Geant4 Low Energy Electromagnetic Physics Working Group
Efficiency Optimization in Geant4 data handling and interpolation methods
G4 Low Energy Electromagnetic processes use tabulated data sets to calculate cross sections (in $G4LEDATA)
Data vectors are initialized with the data sets required by each process at the beginning of a simulation
Several types of data interpolation are performed later on data vectors to estimate the cross section values
Logarithmic Data Interpolations log-log interpolation is the most common type of data
interpolations performed by low-energy EM processes semi-log and linear-log interpolations also required, but less often very time-consuming when repeated for every cross section value
calculation past Geant4 log-log interpolation methods required five log10
math operations per iteration
Geant4 Low Energy Electromagnetic Physics Working Group
How to implement a Physics list ?
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Geant4 Low Energy Electromagnetic Physics Working Group38
Physics lists
A user can build his/her own Physics list in his/her application or use already available Low Energy Electromagnetic Physics lists
1. If you choose to build your own Physics list Refer to the Geant4 Low Energy EM working group website,
look at the Processes and Physics lists sections Also you may refer to Geant4 examples
$G4INSTALL/examples/advanced: microdosimetry for Geant4-DNA
1. If you prefer to use the available Physics lists, these are named as: G4EmLivermorePhysics G4EmLivermorePolarizedPhysics G4EmPenelopePhysics G4EmDNAPhysics
Valid from Geant4 9.3 BETA
Geant4 Low Energy Electromagnetic Physics Working Group39
How to use the already available Physics lists ?
These Physics list classes derive from the G4VPhysicsConstructor abstract base class
A good implementation example of PhysicsList class that uses these already available Physics lists is available in $G4INSTALL/examples/extended/electromagnetic/TestEm2
You need to : Create a dynamic Physics List object in the constructor
For eg. emPhysicsList = new G4EmLivermorePhysics(); Delete it in the destructor Define particles in the PhysicsList::ConstructParticle()
method Eventually set your production cuts
The source code for these Physics lists is available in the following directory $G4INSTALL/source/physics_list/builders
Valid from Geant4 9.3 BETA
Geant4 Low Energy Electromagnetic Physics Working Group
Advanced examples
40
Geant4 Low Energy Electromagnetic Physics Working Group
Located in $G4INSTALL/examples/advanced
At the moment, 20 examples are available in Geant4
A web page where the general status of the examples (in terms of compilation and/or run errors with the last Geant4 version) is reported. The page is linked also from the official CERN web page so that users can better understand the status of the examples.
If someone would like to contribute with his/her Geant4 application, can freely contact us
Advanced examples status 1/2
Geant4 Low Energy Electromagnetic Physics Working Group
HEP Space science/astrophysics Medical physics Microdosimetry Detector technologies
Wide experimental coverage:
Coverage of many aspects of Geant4Geometry featuresMagnetic field Physics (EM and hadronic)Biological processesHits & DigisAnalysisVisualisation, UI
Example status 2/2
Geant4 Low Energy Electromagnetic Physics Working Group
Examples in the bio-medical physics field
Brachytherapy Hadrontherapy Human_phantom Medical_linac Purging_magnet Microbeam Microdosimetry Nanobeam
Geant4 Low Energy Electromagnetic Physics Working Group
Implementation of physics models
Inside the examples, we show different solutions to implement physics models
You can have a look inside the PhysicsList of the Hadrontherapy example to see how to implement:
The ‘Local’ physics models: these are constructed directly by the User
The ‘Physics Lists’ using the macro command /physics/addPhysics<name of the list> (G4EmLivermorePhysics and G4EmPenelopePhysics are two examples of physics lists)
The ‘References Physics Lists’ that are already compiled packages containing a full set pf physics models (both for electromagnetic as well as for Hadronic processes). For this use the command /physics/addPackage <name of the reference list>
Geant4 Low Energy Electromagnetic Physics Working Group
Implementation of Physics models This a macro file provided inside the Hadrontherapy example and
tailored for the use with proton beams
# Set the physic models
/physic/addPhysics LowE_Livermore # Electromagnetic model (G4EmLivermorePhysics)
/physic/addPhysics elastic # Hadronic elastic model
/physic/addPhysics binary # Hadronic inelastic model
# Initialisation procedure
/run/initialize
/beam/energy/meanEnergy 62 MeV
/beam/energy/sigmaEnergy 400 keV
/beam/position/Xposition -2600 mm
# Set here the cut and the step max for the tracking.
# Suggested values of cut and step:
/physic/setCuts 0.01 mm
/Step/waterPhantomStepMax 0.01 mm
/run/beamOn 5000
Geant4 Low Energy Electromagnetic Physics Working Group
Documentation
46
Geant4 Low Energy Electromagnetic Physics Working Group47
Low Energy WG Web site
Either from Geant4 web site http://cern.ch/geant4
Who we are Low energy Electromagnetic Physics
or directly http://geant4.web.cern.ch/geant4/collaboration/
working_groups/LEelectromagnetic/ There, links to :
Geant4 Low Energy Electromagnetic Physics working group Twiki pages
Geant4 Electromagnetic Physics TWiki pages Geant4 Standard Electromagnetic Physics working group
pages
Geant4 Low Energy Electromagnetic Physics Working Group48
Low Energy WG CERN TWiki
https://twiki.cern.ch/twiki/bin/view/Geant4/LowEnergyElectromagneticPhysicsWorkingGroup
Geant4 Low Energy Electromagnetic Physics Working Group49
EM Physics CERN TWiki
https://twiki.cern.ch/twiki/bin/view/Geant4/ElectromagneticPhysics
Geant4 Low Energy Electromagnetic Physics Working Group50
CERN Geant4 TWiki
https://twiki.cern.ch/twiki/bin/view/Geant4/WebHome
Geant4 Low Energy Electromagnetic Physics Working Group51
Medical physics CERN TWiki
https://twiki.cern.ch/twiki/bin/view/Geant4/Geant4MedicalPhysics
Geant4 Low Energy Electromagnetic Physics Working Group
Back-up Slides
Geant4 Low Energy Electromagnetic Physics Working Group
Efficiency Optimization in Geant4 data handling and interpolation methods
Streamlining of the G4 logarithmic interpolation methods
math formula used for logarithmic data interpolation redefined
log10 function calls reduced to four per iteration
average speed-up factor of 1.1 (10%) observed in Geant4 medical applications
the performance gain varies significantly depending on the frequency of cross section calculations required
relatively higher gain when voxelized geometries are required (medical applications)
Valid from Geant4 9.2
Geant4 Low Energy Electromagnetic Physics Working Group
Efficiency Optimization in Geant4 data handling and interpolation methods
Revised methods for handling the data retrieved by G4LEDATA data sets
New LoadData methods for all cross section handler classes
The logarithmic values of the data sets are calculated during initialization phase of simulation (negligible performance penalty)
New SetLogEnergiesData methods Both the original data and their calculated logarithmic values
are loaded to separate data vectors during initialization phase
The availability of pre-calculated logarithmic data nearly eliminates the need to perform log10 function calls
(CPU-intensive) every time a cross section value is calculated thus, enhances the computing performance of the low-energy
EM processes, which require logarithmic interpolations often
Valid from Geant4 9.3 BETA
Geant4 Low Energy Electromagnetic Physics Working Group
Efficiency Optimization in Geant4 data handling and interpolation methods
New Calculation methods for the G4 logarithmic interpolation classes to be used together with the new data handling methods new interpolation calculates methods exploit the presence
of both original and logarithmic data vectors perform logarithmic interpolations more efficiently by
directly loading original and pre-calculated logarithmic data
Average speed-up factor of 1.5 (50%) recorded in G4 medical applications
log-log interpolation now requires only a single log10 function call per iteration
similar performance gain for all Livermore, Penelope and Geant4-DNA process models
successfully validated for all the low-energy EM processes
Valid from Geant4 9.3 BETA
Geant4 Low Energy Electromagnetic Physics Working Group
Profiling results for each revision of the data handling and interpolation methods
Total time performance cost of low-EM processes in GATE (Geant4 Application for Tomography Emission) for each revision stage
rev0 → previous implementation rev1 → streamlining of logarithmic interpolation (geant4 9.2) rev3 → new data handling and interpolation methods (geant4 9.2.ref09) reference case → performance cost when standard EM classes are
employed two cases of phantom geometries examined:
NEMA cylindrical phantom (left bars) and NCAT voxelized phantom (right bars)