Bremsstrahlung Splitting Overview

Jane Tinslay, SLAC

March 2007

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

Jane Tinslay, SLAC 3

Bremsstrahlung Splitting Summary

Uniform Selective DirectionalMultiple Context

BEAMnrc Y Y Y Y

EGS4/EGS5/

EGSnrcY 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 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

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

of 1/N

Radiotheraphy applications use factors of 5-30 (Bruce Faddegon)

Others can use factors of 300

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

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

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Superseded by directional bremsstrahlung splitting Aim to preferentially generate photons aimed into in field

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 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

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

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 of electrons is increase in the field of interest CPU penalty

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