Tuesday, July 10, 2012

Morning Talks

Physics I

• slide 10 - can explicitly list particles
• Need to tell the particles that they need to move
• can group EM physics and all other physics if you would like
• slide 12 - while writing methods in Physics list - use physics list helper (new 4.9.5)
  • helps with ordering
  • list processes, it will put them in correct order
  • you do need to instantiate a particle iterator
• ConstructGeneral is "fictitous" - you can make it anything you like
  • need to loop over all particles (not just those that can decay)
  • check to make sure decay process is applicable for that particle
• Cuts
  • final mandatory method you have to implement
  • 0.7*mm recommended for most applications
  • needed for all 4 (gamma, e-, e+, proton) and no others (even neutrons)
  • can change cut for individual particles (don't have to be the same)
  • if you use a standard package, it wll be set

• Modular physics lists
  • some nice features, like certain things automatic
  • basically a bookkeeping device for EM vs hadronic processes
  • for example proton, electron, decay physics all separate
  • still have to do setCuts, but it is simpler

• Prepackaged physics lists - the "easy" way to do it
• adding hadronic physics is more involved than EM physics
  • need to use models
  • for example - for neutrons - there is a basic model, then there are HP_neutron model
  • there are several physics lists which have been validated by experiment
• geant4.cern.ch/support/proc_mod_catalog/physics_lists/physicsLists.html
• really are a "best guess" of the physics needed in a given use case
• may not do exactly what you want
• possible to use prepackaged physics lists and remove one process and replace it

• Reference Physics Lists
  • subset of prepackaged physics lists

• Questions
  • Hadronic - Bertini vs. binary cascade - you have to decide - need to validate
    • Bertini handles more particles
    • Binary better for lower energies
  • photonuclear - EM or Hadronic - both! - see future talk

Physics II

Not much is said that is not on the slides

• EM
  • standard
  • low energy - 100 eV and below
  • optical photon

• Hadronic
  • Pure hadronic
  • Strong radioactive decay
  • Photonuclear
  • Leptonuclear

• Decay and Parameterization
  • weak decay
  • EM decay
  • Parameterized - EM showers

• Processes
  • final state - all the changes you are going to apply to incident particle
  • energy loss, momentum change, generation of secondaries
  • mixed processes are a mixture of discrete, continuous and At Rest

• threshhold is a distance, not an energy - 0.7*mm in PhysicsList
  • different energy production threshholds depending on energy
  • transition between discrete and non-discrete mode - not actually stopped

Geometry II

• Copy number starts from zero and is incremented
• G4PVParameterized
  • GEANT4 doesn't know how to parameterize your volume unless you tell it
  • can't use BREPs solids
  • Can't get copy number of the mother volume
  • using nested parameterizations - can change position but not size using replicas

• G4 Replica
  • slide 18 - Radial axis is called - kRho, not kRaxis

• Parameterized and Replica are different
  • Replica is faster - if they are all the same
  • but if you need gaps in between need to use parameterized

• Note that mother volumes can be tubes with nothing in the center, for example
• will still fill the volume - even if you give an offset

• G4 Division
  • may have gaps in this case at "top" and "bottom"
  • side surfaces should be touching the mother

• G4 Replicated Slice
  • can have gaps between daughters

• Nested parameterization
  • slide 28 - no actual gaps, just for illustration
  • this is the exception to putting parameterized volume inside a replica
    • not changing the size, just the placement

• Assembly volume
  • acts as an envelop for the daughter volumes!

• Touchable

Visualization II

Could use some more notes here

• /vis/open
  • x, y sizes of window and position on screen relatively to top and right hand side
  • Autorefresh false good for getting configuration correct
• slide 10 - simplest is open window, draw detectors
• axes are x, y, z - red, green, blue (I think)
• Draw hits draws all hits in hits collection(s)
• need to use smooth trajectory to get the control points to do the circular arc
• /vis/verbose errors <-- only tell me if I did something stupid
• /vis/verbose warnings <-- tell me you could draw something, even if it is okay for tracking
• OpenGL postscripts good enough for talks; pub quality - use DAWN
• some commands call multiple lower level commands; error messages will come from lower level

Afternoon Talks

Primary Particle

Could probably use some more notes here.

• UserPrimaryGeneratorAction should NOT generat particles, but just control it
  • this method is evoked at each event (no UI commands)

Physics III

• no absolute cutoff - everything tracked to zero energy
• cut 450 keV is fine - but wastes CPU time with delta rays in Pb

EM Physics I

not much is said that is not on the slides

• you have to check the settings for step limits
• example - slide 33

Hands On II

Return to JLAB GEANT4 Workshop, July 9-13, 2012
Return to MOLLER GEANT4 Simulations
Return to MOLLER at 11 GeV E09-005