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Interaction Forcing Overview

March 2007. Interaction Forcing Overview. Jane Tinslay, SLAC. Applications. Save computing time tracking particles which don’t interact by forcing interaction within a specific geometry Improve statistics when interactions are rare In thin slabs

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Interaction Forcing Overview

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  1. March 2007 Interaction Forcing Overview Jane Tinslay, SLAC

  2. Applications • Save computing time tracking particles which don’t interact by forcing interaction within a specific geometry • Improve statistics when interactions are rare • In thin slabs • E.g bremsstrahlung photons from electrons in a thin foil • Low density Jane Tinslay, SLAC

  3. Interaction Forcing Overview Jane Tinslay, SLAC

  4. EGS4/EGS5/EGSnrc • First implemented as an improvement to EGS4 • Applies to photon interactions only • Think EGSnrc and EGS5 pick it up from underlying EGS4 • Described in: • 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) • Probability distribution for a photon interaction given by: • Where  is the distance in mean free paths • 0 <=  < infinity Jane Tinslay, SLAC

  5. In analogue simulation sample  according to: • Where random number, 0<=  <1 • Since  extends to infinity, photon may not interact • Force the photon to interact by modifying the probability distribution • Where = total # mean free paths along direction of motion of photon to end of interesting geometry • I.e,  is restricted to the range 0<=  <  • Sample  from: • Apply weight Jane Tinslay, SLAC

  6. BEAMnrc • Implemented for photons • Different from other EGS based Monte Carlos • Force a photon to interact at a specific interaction # • Defaults to 1 • Define an interaction # at which to stop forcing • When a photon is forced to interact in a geometry, split it into scattered photon and an unscattered photon • Scattered weight = probability of interaction • Unscattered weight = remaining weight • Forcing parameters passed onto secondary photons • Secondary photons forced if parent photon didn’t get forced enough times • Useful with bremsstrahlung splitting Jane Tinslay, SLAC

  7. Penelope • Again, slightly different method • Not limited to photons • Artificially increase interaction probability of process of interest • Replace mean free path,  by shorter one ’ • Specify forcing factor to scale mean free path • Weight secondaries: • Only modify parent state with probability 1/F Jane Tinslay, SLAC

  8. MCNP • Use EGS4 sampling method while splitting particle into collided and uncollided parts in specified cells • Collided part • Sample distance to interaction in cell (same as EGS4) • d = distance to cell boundary •  = total cross section • Assign weight (same as EGS4) • Can spend too much time tracking forced collision produces -specify fraction, f, of time collided part is produced & modify weight Jane Tinslay, SLAC

  9. Uncollided part • Assigned weight (same as EGS4) • Transport to cell boundary with no interaction • Collided parts can have small weights • Option to do regular sampling in cell after forced collision • Or apply weight cutoff or weight window and allow multiple forced collisions • Seems to apply to all available particles Jane Tinslay, SLAC

  10. 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

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