Low Energy Electromagnetic Physics

1
Low Energy Electromagnetic Physics
Geant4 tutorialSLAC on behalf of the Geant4
Low Energy Electromagnetic Physics Working group
1
2
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

2
3
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

3
4
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)

4
5
How to extract Physics ?

• Thanks to this new software design
• Possible 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-gtGetDensity()
• G4double massSigma emCalculator.ComputeCrossSect
  ionPerVolume(energy,particle,procName,material)/d
  ensity
• G4cout ltlt G4BestUnit(massSigma, "Surface/Mass")
  ltlt G4endl
• A good example G4INSTALL/examples/extended/elect
  romagnetic/TestEm14
• Look in particular at the RunAction.cc class

5
6
Physics models 1/6
Livermore models
6
7
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 Z1 to Z100
• Atomic relaxation Z gt 5 (EADL transition data)
• Data tables are interpolated by Livermore model
  classes
• To compute total cross section and final state

7
8
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
9
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
10
Electron models

• Bremsstrahlung
• Parametrisation from EEDL
• 16 parameters
• Ionisation
• Parametrisation using 5 parameters by shell

10
11
Available Livermore models
11
12
Eg. of validation of Livermore models
Photo-electric Hydrogen (tag 9.2-3)
Photo-electric Neon (tag 9.2-3)
Gamma Conversion Lead (tag 9.2-3)
Electron Range (tag 9.2-4)
12
13
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. A452298-305,2000 (Pair
  production)
• Currently available Compton and Rayleigh

13
14
Eg. polarized Compton cross section

• The Klein Nishina cross section

where h?0 energy of the incident photon h?
energy of the scattered photon ? angle between
the two polarization vectors
15
Polarized Compton simulation

Comparison between the theoretical rates of
intensities with that obtained from Geant4 for
100 keV, 1 MeV and 10 MeV.
15
16
Physics models 2/6
Penelope models
16
17
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

17
18
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 G4P
enelopeGammaConversionModel G4PenelopePhotoElectri
cModel
?-rays
G4PenelopeAnnihilationModel G4PenelopeBremsstrahlu
ngModel G4PenelopeIonisationModel
e
Penelope models are the only low-energy ones
available for e in G4
18
19
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)

19
20
Penelope verification and validation
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)
20
21
Doppler broadening in Compton scattering
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
Au, 50 keV ?-ray
21
22
Physics models 3/6
Ions
22
23
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 d-rays
• Inherently also govern the discrete energy loss
  of ions
• (Note d-rays are only produced above a given
  threshold)
• Primarily of interest for
• Medical applications
• Space applications

23
24
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 Tcut (where Tcut
  denotes the lower production threshold of d-rays)
• Restricted stopping powers are calculated
  according to (T kinetic energy per nucleon)
• T lt TL Free electron gas model
• TL T TH Interpolation of tables or
  parameterization approach
• T gt TH Bethe formula (using an effect. charge)
  high order corr.

24
25
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

25
26
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

26
27
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 model
• Materials 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)

27
28
Physics models 4/6
Geant4-DNA
28
29
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)
  

29
30
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

30
31
Physics models in Geant4 DNA
31
32
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-gtSetUserLimits(new G4UserLimits(DBL_MAX
  ,DBL_MAX,DBL_MAX,9eV))
• In the PhysicsList class
• include "G4UserSpecialCuts.hh"
• ...
• if (particleName "e-")

32
33
Physics models 5/6
Atomic de-excitation
33
34
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 2010

34
35
Physics models 6/6
Data handling and interpolation
35
36
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

37
How to implement a Physics list ?
37
38
Physics lists

• A user can
• build his/her own Physics list in his/her
  application
• or use already available Low Energy
  Electromagnetic Physics lists
• 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
• If you prefer to use the available Physics lists,
  these are named as
• G4EmLivermorePhysics
• G4EmLivermorePolarizedPhysics
• G4EmPenelopePhysics
• G4EmDNAPhysics

38
39
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 PhysicsListConstructPart
  icle() method
• Eventually set your production cuts
• The source code for these Physics lists is
  available in the following directory
  G4INSTALL/source/physics_list/builders

39
40
Advanced examples
40
41
Advanced examples status 1/2

• 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

42
Example status 2/2

• HEP
• Space science/astrophysics
• Medical physics
• Microdosimetry
• Detector technologies

43
Examples in the bio-medical physics field

• Brachytherapy
• Hadrontherapy
• Human_phantom
• Medical_linac
• Purging_magnet
• Microbeam
• Microdosimetry
• Nanobeam

44
Implementation of physics models
Inside the examples, we show different solutions
to implement physics models

• The Local physics models these are
  constructed directly by the User
• The Physics Lists using the macro command
  /physics/addPhysicsltname of the listgt
  (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 ltname of the
  reference listgt

45
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
46
Documentation
46
47
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/wor
  king_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

47
48
Low Energy WG CERN TWiki
https//twiki.cern.ch/twiki/bin/view/Geant4/LowEne
rgyElectromagneticPhysicsWorkingGroup
48
49
EM Physics CERN TWiki
https//twiki.cern.ch/twiki/bin/view/Geant4/Electr
omagneticPhysics
49
50
CERN Geant4 TWiki
https//twiki.cern.ch/twiki/bin/view/Geant4/WebHom
e
50
51
Medical physics CERN TWiki
https//twiki.cern.ch/twiki/bin/view/Geant4/Geant4
MedicalPhysics
51
52
Back-up Slides
53
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)

54
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

55
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

56
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)