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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

SLIDE 1

Bremsstrahlung Splitting Overview

Jane Tinslay, SLAC

March 2007

SLIDE 2

Jane Tinslay, SLAC 2

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

SLIDE 3

Jane Tinslay, SLAC 3

Bremsstrahlung Splitting Summary

Y Y Y Y BEAMnrc N N N Partial Geant4 N N N N Penelope N N N N MCNPX N N N N MCNP N N N N Fluka N N N Y EGS4/EGS5/ EGSnrc Multiple Context Directional Selective Uniform

SLIDE 4

Jane Tinslay, SLAC 4

EGS4

 Implemented as an improvement to EGS4 (~1989)

 Developed by A.F. Bielajew et al

 Do regular electron transport until bremsstrahlung

interaction about to happen

 Instead of creating one photon, generate N photons

 Energy and angular distributions sampled N times

 Assign secondaries a weight:

 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

W = We 1 N

SLIDE 5

Jane Tinslay, SLAC 5  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

f 1/N

 Radiotheraphy applications use factors of 5-30 (Bruce

Faddegon)

 Others can use factors of 300

SLIDE 6

Jane Tinslay, SLAC 6

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

SLIDE 7

Jane Tinslay, SLAC 7

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

SLIDE 8

Jane Tinslay, SLAC 8

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

f 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

SLIDE 9

Jane Tinslay, SLAC 9

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

SLIDE 10

Jane Tinslay, SLAC 10  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

SLIDE 11

Jane Tinslay, SLAC 11

 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

f electrons is increase in the field of interest

 CPU penalty

SLIDE 12

Jane Tinslay, SLAC 12

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. Fernadez- Varea, Josep Sempau, Facultat de Fisica (ECM) , Universitat de Barcelona