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BEAMnrc Selective Bremsstrahlung Splitting ~3-4 times more efficient than uniform bremsstrahlung splitting Superseded by directional bremsstrahlung splitting Aim to preferentially generate photons aimed into in field of interest Vary splitting number to reflect the probability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
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