EGS code and reaction between electrons and photons

1
EGS code and reaction between electrons and
photons
Japan-Korea Joint Summer School on Radiation
Science and Engineering Kitakyusyu International
Conference Center (15 Jul 2009)

• Y. Namito (KEK)
• Last modified on 2009.7.9

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History of EGS system
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About EGS

• Monte Carlo particle transport simulation code.
• Interaction of electron and photon with matter.
• Energy range 103eV - 1012eV.
• EGS5 Released in 2006. Authors Hirayama,
  Namito, Bielajew, Wilderman, and Nelson.
• Runs on Linux, Cygwin and Windows-PC.
• Combinatorial geometry is available.
• Geometry check program (CGVIEW) is available.
• Separation of geometry and other preparation.
• Transport in EM field.

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Combinatorial Geometry CG
1. Specify BODY using parameters. 2. Specify ZONE
by operation (AND, OR, OUTSIDE) of bodies. 3.
Specify material for ZONE
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User Control data
Information Extracted from Shower
USER CODE
MAIN
HOWFAR
AUSGAB
PEGS5
HATCH
SHOWER
ELECTR
PHOTON
BLOCK SET
MSCAT
COMPT
ANNIH
PAIR
EGS CODE
BLOCK DATA
BHABHA
PHOTO
MOLLER
BLOCK DATA ATOM
BREMS
UPHI
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g
Electron
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Photon Monte Carlo Simulation
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Photon Interaction with Matter
scattered photon
positron
e
?
photon g
photon g
j
nucleus
electron
e
e
electron
Compton scattering
Pair Production
photo-electron
photon g
e
e
L
e
e
nucleus
K
e
e
e
Atom
e
Photoelectric effect
Rayleigh scattering
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Pair Production
sketch
Feynman diagram

• Interact in the field of a nucleus
• Annihilate and produce e - e- pair
• triplet distribution ignored,
• incl. in total spair

• PHOTX CS
• default qm0c2/k0
• Realistic angle. dist. optional

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Pair Production (Cont)
Electron-positron pair production cross section
Electron energy dist of Pair Production for 5.11
MeV g
log k _at_ k?8
Scale as Z(Z1)
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Compton scattering
k0 me k E-
Klein- Nishina ds
e-, E-
?, k
Time
Place
?, k0
e-, me
Feynman diagram
scattered photon, k
photon, k0
?
j
e
electron, Ee, v
sketch
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Compton scattering (Cont)
Optional treatment in egs5
const_at_k?0 (e- is free)

• Binding effect (0 _at_ k?0)
• Doppler Broadening
• e- pre-collision motion
• Linearly polarized photon scattering

1/k _at_ k?8
Scale like Z
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Double Differential Compton Cross Section
Binding effect
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Set up of Experiment
Z
Y
Target
40 keV g

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Cu,40 keV(EGS4LPDBEGS5)
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Effect of Doppler to Ge detector response
500 keV
100 keV
Compton edge
Back scat. Peak
Back scat. Peak
Compton edge
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Example of Compton and Auger electron spectrum
e-Tlt10 ?E3
?
Guadala,LandPrices exp
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Photoelectric effect
k0 EN E- EN
e-, E-
Atom, En
s?Z4/E3
?, k0
Atom, EN
Scale like Z4 ?Z4.6
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Photoelectric effect (Cont)
q0! (Realistic dist. optional)
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Relaxation of atom (option in egs5) - Fluorescent
X ray and Auger electron from K and L shell
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Photon spectrum from Pb target EGS4 (General
Treatment of PE) EGS5
EGS5 H
EGS5 V
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Rayleigh Scattering

• elastic process
• independent atom approx.

Scale as Z2
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Rayleigh Scattering (Cont)
Optional treatment in egs5

• Interference effect between nearby atoms
• Linearly polarized photon scattering

sin2f
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Components of sg in C
Diag.
Radiation Therapy
HEP
Compton plateau
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Components of sg in Pb
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Total photon S vs g-energy
photoelectric region
Ek
Compton plateau
Z independent
pair region
30 diff _at_ 3 keV
H2 is the best g attenuator for this energy
region
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End of Photon Monte Carlo Simulation
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Electron Monte Carlo Simulation
- interaction - approximations - transport
methods
5mm
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Electron interaction with matter
electron
electron
electron
e
e
e
electron
nucleus
e
1. Electron scattering by nucleus (Rutherford
scattering) Change direction
2. Inelastic scattering of electron and
electron Loose energy
electron
electron
e
e
nucleus
e
Brems. X-ray
Brems. X-ray
3. Generation of bremsstrahlung x ray
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Condensed Random Walk
g
d
g
g
In Reality, mean free path is in nm or mm unit.
d
d
d
e-
d
g
d
g
d
Continuous slowing down
e-
dray, brems Treated only if, 2nd particle
energy gt threshold
g
d
Multiple scattering
M.S. Angle qms(E,Z,t) Moliere theory GS theory
e-
g
d
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How do we treat both hard interaction and
continuous approximation consistently?
Use Threshold energy (AE, AP) by Users choice

• Hard interaction Discrete sampling
• large ?E Moller/Bhabha (2nd particle energygtAE)
• large ?E bremsstrahlung (photon energygtAP)
• annihilation in flight at rest
• Soft interaction
• small DE Moller/Bhabha Energy
• atomic excitation
  Absorption
• soft bremsstrahlung
• multiple e Coulomb scattering

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Hard Interaction
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Bremsstrahlung
electron
electron

• Z2 scaling
• 3 body angular distn ignored
• Z2 ?Z(Zx(Z))
• lt50 MeV Normalize to ICRU-37
• gt50 MeV ERL
• Migdal ignored gt10 GeV
• TF screening

e
e
Brems. X-ray
nucleus
e
Brems. X-ray
E0E k

• e- , e treated as same
• e not deflected

Feynman diagram
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Example of brems photon spectrum
1/k divergence
Electron energy E05 MeV
qgme/E0
Z2 scaling
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Moller
Bhabha
e-, E1
e-, E2
e-, E1
e, E2
e-, E1
e, E2

e-, E1
e-, E2
e-, E1
e, E2
e-, E1
e, E2
identical particles - threshold 2(AE-RM)
different particles - threshold AE-RM
Optional treatment in EGS5 - K-X ray production
in Moller (Electron Impact Ionization)

• goes like 1/v2
• scale like Z
• Target e- is free

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Annihilation

• in flight and at rest
• e e- ? n?(ngt2) ignored
• e e- ??N ignored
• at ECUT e annihilates
• Residual drift is ignored
• no binding

annihilation g-ray
e
e
electron
positron
annihilation g-ray

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Statistically grouped interactions(Soft
Interaction)

• Continuous energy loss
• Multiple scattering

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Continuous energy loss

• collisional energy loss (e different)
• Bethe-Bloch theory density effect
• well-above K shell energy
• many electron atoms ?Zav
• radiative energy loss (e treated same)
• integration of bremsstrahlung cross sections
• same approximations
• e, e- treated as identical

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Density effect
Reduction of the collision stopping power due to
the polarization of the medium by the incident
electron.
nucleus
Large polarization in Conductor (ex. Carbon)
Small polarization in Rare Gas (ex. Ar)
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Density effect (2)
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Density effect in egs5

• Berger, Seltzer, and Sternheimer
• Parameters for 278 materials
• Sternheimer and Peierls
• general treatment
• Less precise, Needs only Z and r

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Electron stopping power (unrestricted)
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Energy absorption

• energy absorption for e transport of t
• Mean energy loss from Gaussian distribution
• Needs Landaus distribution for thin geometry
• Absorption Dose (Gy) Energy absorption (J) /
  mass(kg)

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Multiple Scattering
Z
Z
Z
e -
Z
t
q
Z
Z
Z
f(q)? after path length t

• Fermi-Eyges theory
• Goudsmit-Saunderson theory EGS5
• Molieres small angle large pathlength
  theoryEGS5

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Moliere theory (Middle precision, Middle
restriction, Simple)

• Convert scattering angle
• Q (E,Z,t) to reduced angle q
• Use single set of f(n)(q) ? Simple
• Good for small angle (lt20o)
• Needs long t (gt100 elastic mfp)

Goudsmit-Saunderson (GS) theory (High precision,
Little restriction, Cumbersome)

• Expand scattering CS by Legendre function
• Coefficient f (E, Z, t, q) ? Need large Data
  Base
• Good for all scattering angle without
  restriction

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Concept figure for single scattering and
multiple scattering
Single scattering Cross section Rutherford
scattering Mott scattering
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Electron transport in EGS5

• Elastic scattering cross section
• Rutherford CS(Default)(EGS4)
• Coulomb interaction between nucleus and electron.
  Nucleus is treated as a point.
• Mott CS
• Consider spin relativistic effect
• Multiple scattering
• Moliere theory (Default)(EGS4)
• Goudsmit-Saunderson theory (GS)
• Transport mechanics inside m.s. step
• Dual Hinge

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Transport Mechanics inside step
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Transport mechanics inside m.s. step of EGS5 (1)
EGS4

• Developed at U.Mich and U.Barcelona

1.Sampling m.s. step s (straight step
size) 2.Evaluate curved length (t), scattering
angle (t ) and lateral displacement (Dx2?y2)
EGS5

• Sampling multiple scattering hinge point inside
  curved length t
• 2.Change electron direction at that point based
  on m.s. model

lt t/s gt and lt?x2?y2gt are adequately
calculated in this hinge model as long as energy
loss is ignored.
Multiple scattering random hinge
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Transport mechanis inside m.s. hinge in EGS5 (2)

• Instead of hinge model of zt and (1-z)t, hinge
  model based on scattering strength is used.
  zK1(t) and (1-z)K1(t).
• To account for energy loss.
• Introduce Energy loss hinge to simplify
  integral of G1 to evaluate K1.
• Energy is constant between energy loss hinge.
• Introduce Characteristic dimension to make
  setting of adequate step length easy.

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Simple
Accurate
Class II (EGS,Penelope) Energy loss with
correlation
Class I (ITS,MCNP) Energy loss without correlation
EE0-DE(t) EdepDE(t) - Ed
EE0 - t LcolAE - Ed Edept LcolAE

• DE(t) energy loss sampled from energy loss
  distribution
• (Straggling considered)
• LcolAE restricted stopping power for 2nd
  particle (ltAE)

t Fixed length (Function of Max energy) _at_ITS,
Variable _at_ EGS, Penelope
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Comparison of Electron transport model
Adopted as electron transport of MCNP
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g
Electron
Photon and electron interact with
Whole One Atom, Electron, and Nucleus
Exception - Density effect - Interference in
Rayleigh scattering
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Complement

• Electron impact ionzation
• Shielding of a,b,g ray

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Electron Impact Ionization (EII)
e-
e-
K-X
N
K-X
Brem.?
N
N
Brems. ? Photoelectric
EII
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Dick et al (1973)s exp set up
10 keV3 MeV e-
Prop, NaI
Al,Ti,Cu,Ag,Au
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K X-ray yield for Cu
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• Paper Aluminum Plate Lead Block

• aray
• ßray
• ?ray

• Penetration of radiation

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• Paper Aluminum Plate Lead Block

• aray
• ßray
• ?ray

• Low Z
• Long range

• Middle Z
• Middle range

• HighZ
• Short MFP

• Implicit feeling

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CSDA range of a and b ray
(Almost) independent of Z
a
b
Large Iav
Small Iav
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Total photon S vs g-energy
photoelectric region
Ek
Compton plateau
Z independent
pair region
30 diff _at_ 3 keV
H2 is the best g attenuator for this energy
region
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• Penetration of radiation

• Paper Aluminum Plate Lead Block

• aray
• ßray
• ?ray

In reality, a ray and b ray range (g/cm2) or g
ray MFP is (almost) independent of Z!
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End of Electron Monte Carlo Simulation