Bremsstrahlung Splitting Overview Jane Tinslay, SLAC Overview - PowerPoint PPT Presentation

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 Multiple Uniform Selective Directional Context BEAMnrc Y Y Y Y EGS4/EGS5/ Y N N N EGSnrc 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)  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: 1 W = W e N  W e = 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. Fernadez- Varea, Josep Sempau, Facultat de Fisica (ECM) , Universitat de Barcelona Jane Tinslay, SLAC 12