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 \page ExampleelectronScattering2 Example electronScattering2
 
  Electron scattering benchmark variant 2 (Joseph Perl)
 
 The two examples, electronScattering and electronScattering2 demonstrate how two alternate
 approaches can be taken to simulating the same electron scattering benchmark experiment.
 
 ElectronScattering2 simulates the experiment with a minimum of user code, whereas the other
 version of this example shows how to do more of the work directly as the user.
 
 In ElectronScattering2:
 - Primary generation relies on the G4GeneralParticleSource.
 - Material definitions are taken from the NIST database.
 - Visualization uses the standard G4VisExective.
 - Scoring is handled by the ready-made Geant4 scorer, G4PSCellFlux.
 
 Both electronScattering and electronScattering2 simulate a precision electron scattering benchmark
 experiment performed at NRCC Canada and published as:
 "Measurement of multiple scattering of 13 and 20 MeV electrons by thin foils,"
 by C. K. Ross, M. R. McEwen, A. F. McDonald, C. D. Cojocaru and B. A. Faddegon,
 Med. Phys. Volume 35, Issue 9, pp. 4121-4131 (September 2008), DOI: 10.1118/1.2968095
 
 The experiment represents the most accurate benchmark of its type.
 
 A schematic of the experimental setup is shown in NRCC_electron_scat.pdf
 
 The experimental data was itself published, as document number E-MPHYA6-35-034809 at http://www.aip.org/pubservs/epaps.html
 
 The testing of three Monte Carlo simulation codes against this experimental benchmark has been
 publshed as:
 "The accuracy of EGSnrc, Geant4 and PENELOPE Monte Carlo systems for the simulation of electron 
 scatter in external beam radiotherapy,"
 by Bruce A Faddegon, Iwan Kawrakow, Yuri Kubyshin, Joseph Perl, Josep Sempau and Laszlo Urban,
 Phys. Med. Biol. 54 (2009) 6151-6163, DOI:10.1088/0031-9155/54/20/008 
 
 This present example, electronScattering2, was the Geant4 code used for this publication.
 
         
 ## GEOMETRY DEFINITION
  
  The geometry is described in the enclosed documument: NRCC_electron_scat.pdf
  - A "World" volume contains an Exit Window, Primary Foil, Monitor Chamber and Helium Bag.
  - The Helium Bag contains a Gas volume, which in turn contains 2 aluminium Rings
  - A scoring cylinder is defined near the end of the World.
         
  The default geometry is constructed in the DetectorConstruction class.
  The parameters of the Primary Foil (material and thickness) are defined from macro commands.
  
 
 ## PHYSICS LIST
 
  Physics lists are based on modular design. Several modules are instantiated:
  1. Transportation
  2. EM physics
  3. Decays
  4. StepMax - for step limitation
 
  EM physics builders are from the G4 kernel physics_lists subdirectory.
   - "emstandard_opt0" recommended standard EM physics for LHC
   - "emstandard_opt1" best CPU performance standard physics for LHC
   - "emstandard_opt2" similar fast simulation
   - "emstandard_opt3" best standard EM options - analog to "local" above
   - "emstandard_opt4" best current advanced EM options standard + lowenergy
   - "emstandardSS"  standard EM physics and single scattering model
   - "emstandardWVI" standard EM physics and WentzelVI multiple scattering
   - "emstandardGS"  standard EM physics and Goudsmit-Saunderson multiple scatt.
   - "emlivermore"  low-energy EM physics using Livermore data
   - "empenelope"   low-energy EM physics implementing Penelope models
   - "emlowenergy"  low-energy EM physics implementing experimental
                    low-energy models
 
  Physics lists and options can be (re)set with UI commands
 
  Note that options set through G4EmProcessOptions are global, eg for all particle types.
  In G4 builders, it is shown how to set options per particle type.
  
         
 ## PRIMARY GENERATOR
  
  All of the work of the primary event generation is deferred to the G4GeneralParticleSource.
  The specific characteristics of the source are configured from the macro file, using
  /gps commands.
  - Energy is set to either 13 or 20 MeV.
  - Particle type is set to electron.
  - Direction is set exactly orthogonal to the scattering foil.
  - Source type is set to beam.
  - Spatial distribution of the beam is set as a circle gassian sigma_r of 0.042 cm
 
  
 ## VISUALIZATION
  
  Visualization is controlled by the standard G4VisExecutive.
  The standard set of visualization options is supported.
         
 
 ## SCORING
         
  All of the work of the scoring is done by the standard Geant4 scorers, G4PSCellFlux and G4PSPopulation.
  - Flux and population are scored in concentric rings at the scoring surface.
  - Flux is corrected for ring area just before output (as PSCellFlux assumes the area is the complete
  area of the face of the cylinder, not the area of the one ring).
 
  Because the geometry may need to be updated after various /primfoil commands,
  ElectronBenchmarkDetector is written in such a way that the sensitive detector can have its scorer ring
  logical volume updated after geometry changes. After any geometry change, ConstructSDandField is again
  called, setting the sensitive detector to use the logical volume of the updated scorer ring.
  The same sensitive detector is reused, but the manner in which the sensitive detector pointer
  is stored for this reuse is complicated by the fact that this example may be run in multi-threaded mode
  (in which each worker thread has its own sensitive detector).
  We therefore store the sensitive detector pointer in a G4Cache rather than in a direct pointer.
 
 ## MACROS
 
  Macros are provided for all of the 37 different combinations of scattering foils and beam energies
  that were used in the actual experiment.
  Separate directories of these macros are provided for the different EM Physics options, Opt0, 1, 2 and 3.
 
         
 ## HOW TO START
  
  - Execute electornScattering2 in 'batch' mode from macro files:
 ```
 % ./electronScattering2 <macro_file> <starting_seed> <output_file>
 ```
 
    for example, to run the Al1_13MeV case with physics option 2 and a starting random seed of 1:
 ```
 % ./electronScattering2 macros/Opt2/Al1_13MeV.mac 1  output/Opt2/Al1_13MeV_1
 ```
 
  - If macro_file is not specfied, the job starts and then just waits for interactive commands
  
  - If starting_seed is not specified, it defaults to 1.
    Precision benchmark studies often involve running multiple long batch jobs and then combining the results.
    If you plan to combine more than one job with the same primary foil and energy, be sure to use different
    random seeds for each of the jobs.
 
  - If output_file is not specified, it defaults to output.csv in the current directory
 
  - To simplify batch submission of large numbers of jobs, a shell script is given called run.csh.
    The above submission could be done, for example, by:
 ```
 run.csh Opt2/Al1_13MeV 1
 ```
 
  - To run a basic visualization job, use vis.mac, as in:
 ```
 % ./electronScattering2 macros/vis.mac
 ```
 
 ## OUTPUT
         
   Results are given as comma-separated values (csv) files.
   - The first column gives the CellFlux for all particles.
   - The second column gives the CellFlux counting only electrons.
   - The third column gives the population for all particles.
   - The fourth column gives the populatin counting only electrons.
 
 ## MULTITHREADING
 
   This example supports multi-threading.
   To run in multi-threaded mode, build your Geant4 with the cmake option: <br>
   -DGEANT4_BUILD_MULTITHREADED=ON
   and then set the desired number of threads by inserting the /run/numberOfThreads command
   into your macro file just before /run/initialize, as in:
 ```
 /run/numberOfThreads 8
 ```
   You may also find the output more readable if you use the following command to make output from
   each thread be collected together:
 ```
 /control/cout/useBuffer
 ```
   The visualization macro, /macros/vis.mac demonstrates use of the above two commands.
   Output from the multiple worker threads is merged by the Merge method at the end of ElectronRun.cc

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