Low Energy Electromagnetic Physics

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Low Energy Electromagnetic Physics

• http//www.ge.infn.it/geant4/lowE
• Maria Grazia Pia
• INFN Genova
• on behalf of Geant4 Low Energy Electromagnetic
  Working Group
• Monte Carlo 2005
• Chattanooga, 18-21 April 2005

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Low Energy Electromagnetic Physics

• A set of processes extending the coverage of
  electromagnetic interactions in Geant4 down to
  low energy
• 250/100 eV (in principle even below this limit)
  for electrons and photons
• down to approximately the ionisation potential of
  the interacting material for hadrons and ions
• Processes based on detailed models
• shell structure of the atom
• precise angular distributions
• Specialised models depending on particle type
• data-driven models based on the Livermore
  Libraries for e- and photons
• analytical models for e, e- and photons
  (reengineering Penelope into Geant4)
• parameterised models for hadrons and ions
  (Ziegler 1977/1985/2000, ICRU49)
• original model for negative hadrons

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Low energy e/g models in
Cosmic rays, jovian electrons
were triggered by astrophysics requirements
X-Ray Surveys of Asteroids and Moons
Solar X-rays, e, p
Geant3.21
ITS3.0, EGS4
Courtesy SOHO EIT
Geant4
Induced X-ray line emission indicator of target
composition (100 mm surface layer)
C, N, O line emissions included
Courtesy ESA Space Environment Effects Analysis
Section
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Dark matter searches
From deep underground to galaxies
From crystals to human beings
Radiobiology
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Software Process

• A rigorous approach to software engineering
• in support of a better quality of the software
• especially relevant in the physics domain of
  Geant4-LowE EM
• several mission-critical applications (space,
  medical)

A life-cycle model that is both iterative and
incremental
Spiral approach
Collaboration-wide Geant4 software process,
tailored to the specific projects

• Public URD
• Full traceability through UR/OOD/implementation/te
  st
• Testing suite and testing process
• Public documentation of procedures
• Defect analysis and prevention
• etc.

• Huge effort invested into SPI
• started from level 1 (CMM)
• in very early stages chaotic, left to heroic
  improvisation

current status
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User requirements
Various methodologies adopted to capture URs
User Requirements

• Elicitation through interviews and surveys
• useful to ensure that UR are complete and there
  is wide agreement
• Joint workshops with user groups
• Use cases
• Analysis of existing Monte Carlo codes
• Study of past and current experiments
• Direct requests from users to WG coordinators

Posted on the WG web site
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OOAD
Technology as a support to physics
Rigorous adoption of OO methods ? openness to
extension and evolution
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Data Management
Very important domain physics models based on
the use of evaluated databases
Intelligent data know how to handle themselves
through algorithm objects e.g. interpolation
algorithms encapsulated in objects (to let them
vary and be interchangeable) Composite pattern to
treat different physical entities
transparently (e.g. whole atom and atom with
shell structure)
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Testing
Integrated with development (not something to do
at the end)

• Suite of unit tests (at least 1 per class)
• Cluster testing
• 3 integration/system tests
• Suite of physics tests (in progress with
  publications)
• Regression testing
• Testing process
• Testing requirements
• Testing procedures
• etc.
• Physics validation

XP practice write a test before writing the
code recommended to WG developers!
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Photons and electrons processes based on the
Livermore library

• Based on evaluated data libraries from LLNL
• EADL (Evaluated Atomic Data Library)
• EEDL (Evaluated Electrons Data Library)
• EPDL97 (Evaluated Photons Data Library)
• especially formatted for Geant4 distribution
  (courtesy of D. Cullen, LLNL)
• Validity range 250 eV - 100 GeV
• The processes can be used down to 100 eV, with
  degraded accuracy
• In principle the validity range of the data
  libraries extends down to 10 eV
• Elements Z1 to Z100
• Atomic relaxation Z gt 5 (transition data
  available in EADL)

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Calculation of cross sections
Interpolation from the data libraries
E1 and E2 are the lower and higher energy for
which data (s1 and s2) are available
Mean free path for a process, at energy E
ni atomic density of the ith element
contributing to the material composition
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Compton scattering
Klein-Nishina cross section

• Energy distribution of the scattered photon
  according to the Klein-Nishina formula,
  multiplied by scattering function F(q) from
  EPDL97 data library
• The effect of scattering function becomes
  significant at low energies
• suppresses forward scattering
• Angular distribution of the scattered photon and
  the recoil electron also based on EPDL97

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Rayleigh scattering

• Angular distribution F(E,q)1cos2(q)?F2(q)
• where F(q) is the energy-dependent form factor
  obtained from EPDL97
• Further improvements to the current
  implementation of the angular distribution
  foreseen

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Photoelectric effect

• Cross section
• Integrated cross section (over the shells) from
  EPDL interpolation
• Shell from which the electron is emitted selected
  according to the detailed cross sections of the
  EPDL library
• Final state generation
• Direction of emitted electron direction of
  incident photon
• Deexcitation via the atomic relaxation
  sub-process
• Initial vacancy following chain of vacancies
  created
• Improved angular distribution in preparation

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g conversion

• The secondary e- and e energies are sampled
  using Bethe-Heitler cross sections with Coulomb
  correction
• e- and e assumed to have symmetric angular
  distribution
• Energy and polar angle sampled w.r.t. the
  incoming photon using Tsai differential cross
  section
• Azimuthal angle generated isotropically
• Choice of which particle in the pair is e- or e
  is made randomly

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Polarisation
Cross section
x
Scattered Photon Polarization
250 eV -100 GeV
x
f
hn
?
A

• ? Polar angle
• ? Azimuthal angle
• ? Polarization vector

hn0
Low Energy Polarised Compton
q
z
a
O
C
y
More details talk on Geant4 Low
Energy Electromagnetic Physics
Other polarised processes under development
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Electron Bremsstrahlung

• Parameterisation of EEDL data
• 16 parameters for each atom
• At high energy the parameterisation reproduces
  the Bethe-Heitler formula
• Precision is 1.5
• Plans
• Systematic verification over Z and energy

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Bremsstrahlung Angular Distributions
Three LowE generators available in
GEANT4 G4ModifiedTsai, G4Generator2BS and
G4Generator2BN G4Generator2BN allows a correct
treatment at low energies (lt 500 keV)
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Electron ionisation

• Parameterisation based on 5 parameters for each
  shell
• Precision of parametrisation is better then 5
  for 50 of shells, less accurate for the
  remaining shells
• Work in progress to improve the parameterisation
  and the performance

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Processes à la Penelope

• The whole physics content of the Penelope Monte
  Carlo code has been re-engineered into Geant4
  (except for multiple scattering)
• processes for photons release 5.2, for
  electrons release 6.0
• Physics models by F. Salvat et al.
• Power of the OO technology
• extending the software system is easy
• all processes obey to the same abstract
  interfaces
• using new implementations in application code is
  simple
• Profit of Geant4 advanced geometry modeling,
  interactive facilities etc.
• same physics as original Penelope

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Hadrons and ions

• Variety of models, depending on
• energy range
• particle type
• charge
• Composition of models across the energy range,
  with different approaches
• analytical
• based on data reviews parameterisations
• Specialised models for fluctuations
• Open to extension and evolution

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Transparency of physics, clearly exposed to users
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Hadron and ion processes
Variety of models, depending on energy range,
particle type and charge

• Density correction for high energy
• Shell correction term for intermediate energy
• Spin dependent term
• Barkas and Bloch terms
• Chemical effect for compound materials
• Nuclear stopping power

Positive charged hadrons

• Bethe-Bloch model of energy loss, E gt 2 MeV
• 5 parameterisation models, E lt 2 MeV
• based on Ziegler and ICRU reviews
• 3 models of energy loss fluctuations

Positive charged ions

• Effective charge model
• Nuclear stopping power

• Scaling
• 0.01 lt b lt 0.05 parameterisations, Bragg peak
• based on Ziegler and ICRU reviews
• b lt 0.01 Free Electron Gas Model

Negative charged hadrons

• Parameterisation of available experimental data
• Quantum Harmonic Oscillator Model

• Model original to Geant4
• Negative charged ions required, foreseen

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Some results protons
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Positive charged ions

• Scaling
• 0.01 lt b lt 0.05 parameterisations, Bragg peak
• based on Ziegler and ICRU reviews
• b lt 0.01 Free Electron Gas Model

• Effective charge model
• Nuclear stopping power

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Models for antiprotons

• ? gt 0.5 Bethe-Bloch formula
• 0.01 lt ? lt 0.5 Quantum harmonic oscillator model
• ? lt 0.01 Free electron gas model

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See next talk
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Geant4 validation vs. NIST database

• All Geant4 physics models of electrons, photons,
  protons and a compared to NIST database
• Photoelectric, Compton, Rayleigh, Pair Production
  cross-sections
• Photon attenuation coefficients
• Electron, proton, a stopping power and range
• Comparison of Geant4 Standard and Low Energy
  Electromagnetic packages against NIST reference
  data
• document the respective strengths of Geant4
  electromagnetic models
• Quantitative comparison
• Statistical goodness-of-fit tests
• See talk by B. Mascialino on Wednesday

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Electrons dE/dx
Ionisation energy loss in various
materials Compared to Sandia database More
systematic verification planned
Also Fe, Ur
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Photons, evidence of shell effects
Photon transmission, 1 mm Pb
Photon transmission, 1 mm Al
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Electrons, transmitted
20 keV electrons, 0.32 and 1.04 mm Al
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The problem of validation finding reliable data
Note Geant4 validation at low energy is not
always easy experimental data often exhibit large
differences!
Backscattering low energies - Au
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Applications

• A small sample in the next slides
• no time to show all!
• various talks at this conference concerning
  Geant4 Low Energy Electromagnetic applications
• Many valuable contributions to the validation of
  LowE physics from users all over the world
• excellent relationship with our user community

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Homogeneous Phantom
P. Rodrigues, A. Trindade, L.Peralta, J. Varela,
LIP

• Simulation of photon beams produced by a Siemens
  Mevatron KD2 clinical linear accelerator
• Phase-space distributions interface with GEANT4
• Validation against experimental data depth dose
  and profile curves

Preliminary!
LIP Lisbon
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LINAC for IMRT
Kolmogorov-Smirnov Test p-value1
Kolmogorov-Smirnov Test p-value0.1-0.9
M.Piergentili, INFN Genova
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Dosimetry Superficial brachytherapy
Dosimetry Interstitial brachytherapy
Dosimetry Endocavitary brachytherapy
MicroSelectron-HDR source
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Hadrontherapy beam line at INFN-LNS, Catania
G.A.P. Cirrone, G. Cuttone, INFN LNS
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Bepi Colombo Mission to Mercury
Study of the elemental composition of Mercury by
means of X-ray fluorescence and PIXE Insight
into the formation of the Solar System
(discrimination among various models)
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Shielding in Interplanetary Space Missions
Aurora Programme
ESA REMSIM Project
Dose in astronaut resulting from Galactic Cosmic
Rays
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DNA
http//www.ge.infn.it/geant4/dna/
Study of radiation damage at the cellular and DNA
level in the space radiation environment (and
other applications)
More at next Monte Carlo conference!
5.3 MeV ? particle in a cylindrical volume The
inner cylinder has a radius of 50 nm
Prototyping

• Relevance for space astronaut and airline pilot
  radiation hazards, biological experiments
• Also in radiotherapy, radiobiology...

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Conclusions

• New physics domain in HEP simulation
• Wide interest in the user community
• A wealth of physics models
• A rigorous approach to software engineering
• Significant results from an extensive validation
  programme
• A variety of applications in diverse domains