Transcript ppt - Geant4

March 2007
Bremsstrahlung Splitting
Overview
Jane Tinslay, SLAC
Overview & Applications

Biases by enhancing secondary production

Aim to increase statistics in region of interest
while reducing time spent tracking electrons

Useful in radiotheraphy dose calculations
Jane Tinslay, SLAC
2
Bremsstrahlung Splitting Summary
Uniform
Selective
Directional
Multiple
Context
BEAMnrc
Y
Y
Y
Y
EGS4/EGS5/
EGSnrc
Y
N
N
N
Fluka
N
N
N
N
Geant4
Partial
N
N
N
MCNP
N
N
N
N
MCNPX
N
N
N
N
Penelope
N
N
N
N
Jane Tinslay, SLAC
3
EGS4

Implemented as an improvement to EGS4 (~1989)



Do regular electron transport until bremsstrahlung
interaction about to happen
Instead of creating one photon, generate N photons


Developed by A.F. Bielajew et al
Energy and angular distributions sampled N times
Assign secondaries a weight:
1
W  We
N


We = weight of parent electron
Reduce energy of electron by energy of just one photon


Energy conserved on average
Get full energy
 straggling of electron history
Jane Tinslay, SLAC
4

Can gain efficiency by playing Russian Roulette on
products of pair production and compton scattering



Reduces unnecessary electron transport
Keep 1/N charged secondaries with weight increase by factor of
N
All electrons have same weight, all photons have relative weight
of 1/N

Radiotheraphy applications use factors of 5-30 (Bruce
Faddegon)

Others can use factors of 300
Jane Tinslay, SLAC
5
EGSnrc

Same bremsstrahlung splitting as EGS4

Also implements photon Russian Roulette



Define an imaginary plane at depth Z
Define a survival probability factor, RRCUT
Every time a photon is about to cross a given Z
plane, play Russian Roulette

Surviving particles have weight increased by a factor
1/RRCUT
Jane Tinslay, SLAC
6
BEAMnrc Uniform Bremsstrahlung Splitting

Based on EGSnrc version


Uses EGSnrc splitting code
In addition, implements a higher order splitting switch



Splitting not applied to higher-order bremsstrahlung and
annihilation photons unless Russian Roulette turned on
Roulette applied to secondary charged particles arising from split
photons
 Electrons from compton and photoelectric events
 Electrons and positrons from pair production
Saves time by not tracking many higher-order, low weight
photons
Jane Tinslay, SLAC
7
BEAMnrc Selective Bremsstrahlung Splitting



~3-4 times more efficient than uniform bremsstrahlung
splitting
Superseded by directional bremsstrahlung splitting
Aim to preferentially generate photons aimed into in field
of interest



Vary splitting number to reflect the probability a bremsstrahlung
photon will enter a user defined field area
Calculate probability using energy/direction of incident electron
Higher order bremsstrahlung and annihilation photons
split with minimum splitting number provided Russian
Roulette is on
Jane Tinslay, SLAC
8
BEAMnrc Directional Bremsstrahlung Splitting

First Introduced in 2004

Can improve efficiency by factor of 8 relative to selective
bremsstrahlung splitting, up to 20 times higher than
uniform bremsstrahlung splitting

Designed to ensure that all photons in field of interest
have same weight


One of the limitations of selective bremsstrahlung splitting
Reasonably complex algorithm

Can choose to enhance electron contamination statistics through
electron splitting
Jane Tinslay, SLAC
9


Define a field of interest and splitting number
Apply splitting/Roulette in various configurations for :







Bremsstrahlung
Annihilation
Compton
Pair production
Photo electric
Fluorescent
Biasing ensures:




All photons in region of interest have a weight N
Photons outside region of interest have a weight 1
Very little time spent transporting photons not contributing to
fluence in field of interest
Very few electrons with large weight
Jane Tinslay, SLAC
10

To improve contaminant electron statistics, apply
electron splitting

Split only in interesting region

Define splitting and Russian Roulette planes

Apply splitting and roulette such that the number
of electrons is increase in the field of interest

CPU penalty
Jane Tinslay, SLAC
11
References












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
12