geant4 low energy electromagnetic physics working group 1 low energy electromagnetic physics geant4...

Geant4 Low Energy Electromagnetic Physics Working Group1

Low Energy Electromagnetic Physics

Geant4 tutorialSLAC

on behalf of the Geant4 Low Energy Electromagnetic Physics Working group


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


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]


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


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


Geant4 Low Energy Electromagnetic Physics Working Group

Physics models 1/6

Livermore models

6


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

7


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

8


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

9


Electron models

Bremsstrahlung Parametrisation from EEDL 16 parameters

Ionisation Parametrisation using 5 parameters

by shell

10


Available Livermore models

11

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


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


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


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


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


Physics models 2/6

Penelope models

16


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


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±



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


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)


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


Physics models 3/6

Ions

22


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



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.


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


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


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)


Physics models 4/6

Geant4-DNA

28


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 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


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


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()); ... }


Physics models 5/6

Atomic de-excitation

33


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


Physics models 6/6

Data handling and interpolation

35


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


How to implement a Physics list ?

37


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



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



Advanced examples

40


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


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


Examples in the bio-medical physics field

Brachytherapy Hadrontherapy Human_phantom Medical_linac Purging_magnet Microbeam Microdosimetry Nanobeam


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>


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


Documentation

46


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


Low Energy WG CERN TWiki

https://twiki.cern.ch/twiki/bin/view/Geant4/LowEnergyElectromagneticPhysicsWorkingGroup


EM Physics CERN TWiki

https://twiki.cern.ch/twiki/bin/view/Geant4/ElectromagneticPhysics


CERN Geant4 TWiki

https://twiki.cern.ch/twiki/bin/view/Geant4/WebHome


Medical physics CERN TWiki

https://twiki.cern.ch/twiki/bin/view/Geant4/Geant4MedicalPhysics


Back-up Slides



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



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




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



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)

geant4 low energy electromagnetic physics working group 1 low energy electromagnetic physics geant4...

Documents

photon models

livermore model classes

physics quantities

physics list

process classnaming

certain energy rangesaccording

low energies

selected model