Transcript ppt - Geant4

March 2007
Interaction Forcing Overview
Jane Tinslay, SLAC
Applications

Save computing time tracking particles which
don’t interact by forcing interaction within a
specific geometry

Improve statistics when interactions are rare

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In thin slabs
 E.g bremsstrahlung photons from electrons in a
thin foil
Low density
Jane Tinslay, SLAC
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Interaction Forcing Overview
Forced interaction
in some form
Multiple Context
BEAMnrc
Y
Y
EGS4/EGS5/EGSnrc
Y
N(?)
Fluka
N
N
Geant4
N
N
MCNP
Y
Y
MCNPX
Y
Y
Penelope
Y
Y
Jane Tinslay, SLAC
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EGS4/EGS5/EGSnrc
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First implemented as an improvement to EGS4
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Described in:
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Applies to photon interactions only
Think EGSnrc and EGS5 pick it up from underlying EGS4
Variance Reduction Techniques, D.W.O. Rogers and A.F.
Bielajew (Monte Carlo Transport of Electrons and Photons.
Editors Nelso, Jankins, Rindi, Nahum, Rogers. 1988)
Probability distribution for a photon interaction given by:
p()  e  d

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Where  is the distance in mean free paths

0 <=  < infinity
Jane Tinslay, SLAC
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
In analogue simulation sample  according to:
   ln(1  )



Where random number, 0<=  <1
Since  extendsto infinity, photon may not interact
Force the photon to interact by modifying the probability
distribution
p()d 
e  d


e
 '
d'
0


Where = total # mean free paths along direction of motion of
photon to end of interesting geometry
I.e,  is restricted
to the range 0<=  < 


Sample  from:

Apply weight

   ln(1  (1 e  ))
W '  W(1 e  )
Jane Tinslay, SLAC
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BEAMnrc



Implemented for photons
Different from other EGS based Monte Carlos
Force a photon to interact at a specific interaction #



Define an interaction # at which to stop forcing
When a photon is forced to interact in a geometry, split it
into scattered photon and an unscattered photon

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Scattered weight = probability of interaction
Unscattered weight = remaining weight
Forcing parameters passed onto secondary photons

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Defaults to 1
Secondary photons forced if parent photon didn’t get forced
enough times
Useful with bremsstrahlung splitting
Jane Tinslay, SLAC
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Penelope

Again, slightly different method

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Not limited to photons
Artificially increase interaction probability of process of
interest
Replace mean free path,  by shorter one ’

Specify forcing factor to scale mean free path

F

Weight secondaries:


'
W' 
W
F
Only modify parent state with probability 1/F

Jane Tinslay, SLAC
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MCNP


Use EGS4 sampling method while splitting particle into
collided and uncollided parts in specified cells
Collided part

Sample distance to interaction in cell (same as EGS4)
x
1
ln(1  (1 e d ))

d = distance to cell boundary
  = total cross section

Assign weight
(same as EGS4)



W '  W 1 e d


Can spend too much time tracking forced collision produces specify fraction, f, of time collided part is produced & modify

weight
'
d

W  W 1 e
/ f
Jane Tinslay, SLAC
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
Uncollided part

Assigned weight (same as EGS4)
W '  We d

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Collided parts can
 have small weights



Transport to cell boundary with no interaction
Option to do regular sampling in cell after forced collision
Or apply weight cutoff or weight window and allow multiple
forced collisions
Seems to apply to all available particles
Jane Tinslay, SLAC
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References
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BEAMnrc Users Manual, D.W.O. Rogers et al. NRCC Report PIRS-0509(A)revK (2007)
The EGS4 Code System, W. R. Nelson and H. Hirayama and D.W.O. Rogers, SLAC-265,
Stanford Linear Accelerator Center (1985)
History, overview and recent improvements of EGS4, A.F. Bielajew et al., SLAC-PUB-6499 (1994)
THE EGS5 CODE SYSTEM, Hirayama, Namito, Bielajew, Wilderman, Nelson SLAC-R-730
(2006)
The EGSnrc Code System, I. Kawrakow et al., NRCC Report PIRS-701 (2000)
Variance Reduction Techniques, D.W.O. Rogers and A.F. Bielajew (Monte Carlo Transport of
Electrons and Photons. Editors Nelso, Jankins, Rindi, Nahum, Rogers. 1988)
NRC User Codes for EGSnrc, D.W.O. Rogers, I. Kawrakow, J.P. Seuntjens, B.R.B. Walters and E.
Mainegra-Hing, PIRS-702(revB) (2005)
http://www.fluka.org/course/WebCourse/biasing/P001.html
http://www.fluka.org/manual/Online.shtml
http://geant4.web.cern.ch/geant4/UserDocumentation/UsersGuides/ForApplicationDeveloper/html/
Fundamentals/biasing.html
MCNPX 2.3.0 Users Guide, 2002 (version 2.5.0 is restricted)
PENELOPE-2006: A Code System for Monte Carlo Simulation of Electron and Photon Transport,
Workshop Proceedings Barcelona, Spain 4-7 July 2006, Francesc Salvat, Jose M. FernadezVarea, Josep Sempau, Facultat de Fisica (ECM) , Universitat de Barcelona
Jane Tinslay, SLAC
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