1. Interaction Forcing Overview Jane Tinslay, SLAC March 2007

2. Jane Tinslay, SLAC2 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

3. Jane Tinslay, SLAC3 Interaction Forcing Overview Forced interaction in some form Multiple Context BEAMnrcYY EGS4/EGS5/EGSnrcYN(?) FlukaNN Geant4NN MCNPYY MCNPXYY PenelopeYY

4. Jane Tinslay, SLAC4 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

5. Jane Tinslay, SLAC5 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

6. Jane Tinslay, SLAC6 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

7. Jane Tinslay, SLAC7 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

8. Jane Tinslay, SLAC8 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

9. Jane Tinslay, SLAC9 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

10. Jane Tinslay, SLAC10 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