1. Implicit Capture Overview Jane Tinslay, SLAC March 2007

2. Jane Tinslay, SLAC2 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

3. Jane Tinslay, SLAC3 Implicit Capture Summary Implicit CaptureMultiple Context EGS4/EGS5/EGSn rc/BEAMnrc NN FlukaYY Geant4NN MCNP/MCNPXYY PenelopeNN

4. Jane Tinslay, SLAC4 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, P bias with associated weight correction:

5. Jane Tinslay, SLAC5 If P Bias = 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

6. Jane Tinslay, SLAC6 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 P bias 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

7. Jane Tinslay, SLAC7 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

8. Jane Tinslay, SLAC8 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