geant4

This directory includes example applications to demonstrate the usage of
different variance reduction techniques supported in Geant4, or possible
from the user applications.

General remark to variance reduction
------------------------------------
The tools provided for importance sampling (or geometrical splitting and
Russian roulette) and for the weight window technique require the user to
have a good understanding of the physics in the problem. This is because
the user has to decide which particle types have to be biased, define the
cells (physical volumes, replicas) and assign importances or weight
windows to that cells. If this is not done properly it can not be
expected that the results describe a real experiment. The examples given
here only demonstrate how to use the tools technically. They don't intend
to produce physical correct results.

General remark to scoring
-------------------------
A interface G4VScorer is provided for the user. The user may create his
own class to perform the desired scoring. The user defined class
therefore should inherit from the interface G4VScorer.
An example of an implementation of a scorer is G4Scorer
which may be found in source/event.
The scoring in these examples is done with a G4Scorer.
The variance reduction techniques and scoring do not support all options
of the Geant4 geometry. It only supports physical volumes and simple
replicas.
To identify a physical volume (or replica) objects of the class
G4GeometryCell are used. Scoring is done according to these
cells and importance values or the weight windows may be assigned to
them.
When scoring is done in a parallel geometry special action has to be taken
to prevent counting of "collisions" with boundaries of the mass geometry
as interactions. This is differently handled when scoring is done in the
mass geometry.

--> G4GeometryCell of the parallel geometry must not share boundaries with
the world volume! <--

Known problems
--------------
In the following scenario it can happen that a particle is not
biased and it's weight is therefore not changed even if it crosses
a boundary where biasing should happen.
Importance and weight window sampling create particles on boundaries
between volumes. If the GPIL method of a physical process returns
0 as step length for a particle on a boundary and if the PostStepDoIt of
that process changes the direction of the particle to go back in the
former volume the biasing won't be invoked.
This will produce particles with weights that do not correspondent to the
importance of the current volumes.

Further information:
--------------------
Short description of importance sampling and scoring:
http://dressel.home.cern.ch/dressel/biasscore/Sampling.html

Example B01
===========

The example uses importance sampling or the weight window technique
according to an input parameter. It uses scoring in both cases.
Importance values or weight windows are defined according to the mass
geometry. In this example the weight window technique is configured such
that it behaves equivalent to importance sampling: The window is actually
not a window but simply the inverse of the importance value and only
one energy region is used that covers all energies in the problem.
The user may change the weight window configuration by changing the
initialization of the weight window algorithm in example,cc.
Different energy bounds for the weight window technique may be specified
in B01DetectorConstruction.

The executable takes one optional argument: 0 or 1. Without argument or
with argument: 0, the importance sampling is applied with argument: 1,
the weight window technique is applied.

Example B02
===========

This example uses a parallel geometry to define G4GeometryCell objects
for scoring and importance sampling. In addition it customizes
the scoring. In this example one scorer creates a histogram.

Compiling and running
---------------------
To compile this example you need AIDA 3.0 installed. To link
and run it you need a AIDA compliant analysis package. The
GNUmakefile of this example shows how to use Anaphe as analysis
package.
The following list shows what to do if you are at CERN using rehat73
and gcc-3.2 and the tcsh:
1) set the following variables (for example):
setenv ANAPHETOP /afs/cern.ch/sw/lhcxx
(or if installed in opt: /opt/Anaphe/5.0.5)
setenv AIDA_DIR /afs/cern.ch/sw/contrib/AIDA/3.0/src/cpp
(or if installed in opt: /opt/AIDA/3.0.0/3.0.0/AIDA/3.0/src/cpp)

2) Source the script:
source $ANAPHETOP/share/LHCXX/5.0.5/scripts/setupAnaphe.csh

This should set the LD_LIBRARY_PATH and the PATH environment to find the
lizrd executable and Anaphe libraries and header files.
It also sets the environment variable PLATF.

3) and add to lirary path the following directories:
setenv LD_LIBRARY_PATH ${LD_LIBRARY_PATH}:${G4WORKDIR}/lib/${G4SYSTEM}:${G4WORKDIR}/tmp/${G4SYSTEM}/exampleB02

Now you should be able to run gmake and to run exampleB02.

The example stores the plot in the file b02.hbook.
To look at the histogram using lizard you also may use Anaphe 5.0.5
http://anaphe.web.cern.ch/anaphe/index2.html.

Files in B02:
B02plot.py: a simple script that may be executed from a lizzard session
to show the plot created by running the exampleB02.

Example B03
===========

This example uses Geant4 and in particular importance sampling and
scoring through python. It creates a simple histogram. It's ment
to demonstrate how to use a customized scorer and importance sampling
in combination with a scripting language, python.
Geant4 code is executed from a python session. Therefore, swig is used
to create python shadow classes and to generate the code necessary to
use the Geant4 libraries from a python session.
For simplicity it is recommended to use Anaphe. Anaphe comes with
python and swig installations.
See http://anaphe.web.cern.ch/

Building, compiling and running
-------------------------------

Follow similar steps as for example B02. If you already have
the environment set up for example B02 jump to point 3 below:

1) set the following variables (for example):
setenv ANAPHETOP /afs/cern.ch/sw/lhcxx
(or if installed in opt: /opt/Anaphe/5.0.5)
setenv AIDA_DIR /afs/cern.ch/sw/contrib/AIDA/3.0/src/cpp
(or if installed in opt: /opt/AIDA/3.0.0/3.0.0/AIDA/3.0/src/cpp)

2) Source the script:
source $ANAPHETOP/share/LHCXX/5.0.5/scripts/setupAnaphe.csh

3) and add to lirary path the following directories:
setenv LD_LIBRARY_PATH ${LD_LIBRARY_PATH}:${G4WORKDIR}/lib/${G4SYSTEM}:${G4WORKDIR}/tmp/${G4SYSTEM}/exampleB03
====
setenv PYTHONPATH $LD_LIBRARY_PATH

4) setenv PYTHON_VERSION 2.2

You may run gmake now.
You should be able to execute the file B03RunApplication.py from your
shell or from a lizard session now.
The histogram is displayed only if the script is beeing ran
from lizard.

Files in B03;
B03Application.py: Is a example class utilizing importance sampling
and scoring using python.
B03RunApplication.py: Is a python script running the example.
It may be executed from the shell or in a python or lizard
session. In case it is executed from lizard it creates a plot
with a histogram.
B03App.py: Is created by swig using swig.