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Implicit Capture Overview

March 2007. Implicit Capture Overview. Jane Tinslay, SLAC. Overview & Applications. Implicit capture is also known as Survival biasing Non-analogue absorption Absorption by weight reduction Generally applied to photons and neutrons

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Implicit Capture Overview

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  1. March 2007 Implicit Capture Overview Jane Tinslay, SLAC

  2. Overview & Applications • Implicit capture is also known as • Survival biasing • Non-analogue absorption • Absorption by weight reduction • Generally applied to photons and neutrons • Basically bias absorption probability either at collision site or along path length • Useful to apply in highly absorbing media where want to decrease absorption • Also useful where want to increase absorption • E.g, limit thermal neutron scattering in materials with low thermal neutron absorption cross section Jane Tinslay, SLAC

  3. Implicit Capture Summary Jane Tinslay, SLAC

  4. Fluka • Non-analogue absorption derived from MORSE code • Available for neutrons only • Analogue survival probability given by: • absorption = absorption cross section • scattering = scattering cross section • Apply artificial survival probability, Pbias with associated weight correction: Jane Tinslay, SLAC

  5. If PBias = 1, neutron never killed • Biasing applied by default to thermal neutrons • Survival probability set to 0.85 • Limits number of thermal neutron histories • Configurable region by region and as a function of energy • Recommend using weight windows to deal with large weight fluctuations Jane Tinslay, SLAC

  6. MCNP Implicit Capture at Collision • Implicit capture can be applied to photons and neutrons • Same techique applied to different processes • Similar to Fluka method with Pbias always set to 1 • Particle is split into two • Absorbed • No need to track any further • Fraction of incident particle weight and energy deposited in collision cell (question - does Fluka do splitting ?) • Surviving - continue tracking • Default method of capture for: • Simple treatment of photons (also have detailed photon simulation where analogue capture is default) • Neutrons - Russian Roulette also played below a specified weight Jane Tinslay, SLAC

  7. MCNP Implicit Capture Along a Flight Path • Common in astrophysics • Sample distance to scatter rather than distance to collision • Reduce particle weight at scattering point by capture loss • Useful in highly absorbing media when most collisions result in capture • Special form of exponential transform • They recommend using exponential transform instead Jane Tinslay, SLAC

  8. 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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