biasing/GB03/README.md

0001 \page ExampleGB03 Example GB03
0002
0003 ## Geometry based biasing
0004
0005 This example illustrates a use of generic biasing classes to implement a
0006 technique near to "geometry importance biasing".
0007
0008 The geometry is the same than in EM tests, with the sampling calorimeter
0009 made of a series of layers of absorber and gap.
0010
0011 The biasing applies to neutrons only.
0012
0013 Instead of explicitly assigning "importance" values to the layers, we
0014 split neutrons moving forward and kill the ones moving backward, when they
0015 reach the exit of an absorber volume.
0016
0017 The splitting factor can be controlled by command line, eg:
0018 ```
0019 /GB03/biasing/setSplittingFactor 2
0020 ```
0021
0022 which also determines the killing probability : 1/(splitting factor).
0023
0024 It can be seen than when defining 10 layers (see exampleGB03.in), a
0025 splitting factor 2 works fine : we don't suffer from under- or over-splitting.
0026 If going to 20 layers, then a splitting with a factor 2 is too large,
0027 and the biasing suffers from over-splitting. (And we can not go lower than
0028 "2", which would mean "1" and hence, no biasing...)
0029
0030 To alleviate the over-splitting, we introduce a probability to apply the
0031 splitting (and killing) (this is one solution, others can be considered), that
0032 can be changed as:
0033
0034 ```
0035 /GB03/biasing/setApplyProbability 0.5
0036 ```
0037
0038 With above value, we can see that we recover a satisfactory biasing scheme,
0039 with neutrons penetrating the entire setup, without over-splitting.
0040
0041 The commands in ```/GB03/biasing``` section are available only if application
0042 started with biasing "on".
0043
0044 The classes involved are:
0045
0046 - GB03BOptnSplitOrKillOnBoundary : which is the biasing operation making
0047   the splitting and killing;
0048 - GB03BOptrGeometryBasedBiasing : which is the biasing operator, making
0049   decision to use above operation, and configuring it, passing it the
0050   splitting factor and probability to apply the biasing.
0051
0052 ## HOW TO START ?
0053
0054 The example can be executed in "batch" mode if macro file name is specified
0055 or interactive mode.
0056
0057 To run it:
0058 ```
0059   ./exampleGB03 [-m macro_file] [-b [on|off]]
0060 ```
0061
0062 Without parameters the application runs in interactive mode with biasing "on".
0063
0064 ## THE OUTPUT
0065
0066 During the run the histograms of energy distributions for neutrons and gammas
0067 after the "shield" volume and their positions along x axis are filed and
0068 saved to GB03.root file.
0069
0070 Depending on ```/GB03/verbose``` flag
0071
0072 For *verbose >= 1* all particles penetrating the shield were counted and
0073 printed with their weights and Emin Emax values.
0074
0075 The track #, particle, kinetic energy and position on each step are printed
0076 if *verbose >= 2*.
0077
0078 The particle count per thread is also printed if *verbose >= 3*.