Import Geant4 11.0.0 source tree
This commit is contained in:
Gabriele Cosmo
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///\file "biasing/.README.txt"
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///\brief Examples biasing README page
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/*! \page Examples_biasing Category "biasing"
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\section biasing_s1 B01, B02 and B03
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B01, B02 and B03 applications demonstrate the usage of different variance
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reduction techniques supported in Geant4, or possible from the user
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applications.
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\subsection biasing_sub_11 General remark to variance reduction
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The tools provided for importance sampling (or geometrical splitting and
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Russian roulette) and for the weight window technique require the user to
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have a good understanding of the physics in the problem. This is because
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the user has to decide which particle types have to be biased, define the
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cells (physical volumes, replicas) and assign importances or weight
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windows to that cells. If this is not done properly it can not be
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expected that the results describe a real experiment. The examples given
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here only demonstrate how to use the tools technically. They don't intend
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to produce physical correct results.
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\subsection biasing_sub_12 General remark to scoring
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Scoring is carried out using the built-in Multifunctional detectors. For
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parallel geometries this requires a special scoring physics process.
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See examples/extended/runAndEvent (especailly RE05) for clarification.
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\subsection biasing_sub_13 Known problems - should not happen
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In the following scenario it can happen that a particle is not
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biased and it's weight is therefore not changed even if it crosses
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a boundary where biasing should happen.
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Importance and weight window sampling create particles on boundaries
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between volumes. If the GPIL method of a physical process returns
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0 as step length for a particle on a boundary and if the PostStepDoIt of
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that process changes the direction of the particle to go back in the
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former volume the biasing won't be invoked.
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This will produce particles with weights that do not correspondent to the
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importance of the current volumes.
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\subsection biasing_sub_14 Further information:
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Short description of importance sampling and scoring:
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http://cern.ch/geant4/working_groups/geometry/biasing/Sampling.html
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\subsection biasing_sub_15 Example B01
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The example uses importance sampling or the weight window technique
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according to an input parameter. It uses scoring in both cases.
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Importance values or weight windows are defined according to the mass
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geometry. In this example the weight window technique is configured such
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that it behaves equivalent to importance sampling: The window is actually
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not a window but simply the inverse of the importance value and only
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one energy region is used that covers all energies in the problem.
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The user may change the weight window configuration by changing the
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initialization of the weight window algorithm in example,cc.
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Different energy bounds for the weight window technique may be specified
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in B01DetectorConstruction.
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The executable takes one optional argument: 0 or 1. Without argument or
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with argument: 0, the importance sampling is applied with argument: 1,
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the weight window technique is applied.
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\subsection biasing_sub_16 Example B02
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This example uses a parallel geometry to define G4GeometryCell objects
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for scoring and importance sampling. The output should be equivalent to B01.
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A modular approach is applied to the physicslist and the extension for biasing.
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The parallel geometry is included in this extension.
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\subsection biasing_sub_17 Example B03
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This example uses a parallel geometry to define G4GeometryCell objects
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for scoring and importance sampling. The output should be statistically
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equivalent to B02 (and B01).
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This demonstrates a customised "flat" physics implementation with the addition
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of biasing. Complementary approach to the modular physics lists of B01 and B02
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\section biasing_s2 Generic biasing examples GB01 - GB06
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These examples illustrate the usage of a biasing scheme implemented since
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version Geant4 10.0.
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The scheme is meant to be extensible, not limited to these six examples.
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\link ExampleGB01 Example GB01 \endlink
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This example illustrates how to bias process cross-sections in this scheme.
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\link ExampleGB02 Example GB02 \endlink
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Illustrates a force collision scheme similar to the MCNP one.
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\link ExampleGB03 Example GB03 \endlink
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Illustrates geometry based biasing.
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\link ExampleGB04 Example GB04 \endlink
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Illustrates a bremsstrahlung splitting.
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\link ExampleGB05 Example GB05 \endlink
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Illustrates a "splitting by cross-section" technique: a splitting-based
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technique using absorption cross-section to control the neutron population.
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\link ExampleGB06 Example GB06 \endlink
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Illustrates the usage of parallel geometries with generic biasing.
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\link ExampleGB07 Example GB07 \endlink
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Illustrates the usage of leading particle biasing with generic biasing.
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\section biasing_s3 Reverse MonteCarlo Technique example
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\link ExampleReverseMC01 Example ReverseMC01 \endlink
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Example illustrating the use of the Reverse Monte Carlo (RMC) mode in a Geant4
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application. See details in \link ExampleReverseMC01 Example README page
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\endlink.
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*/
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///\file "biasing/GB01/.README.txt"
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///\brief Example GB01 README page
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/*! \page ExampleGB01 Example GB01
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\section ExampleGB01_s1 Cross-section biasing
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This example illustrates how to bias process cross-sections.
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Generally speaking, the scheme consists of a G4VBiasingOperator that takes
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decisions on what sort of biasing is to be applied. The operator makes these
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decision on requests of the G4BiasingProcessInterface process. This process
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wraps an actual physics process and asks to the operator about what sort of
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biasing it should apply. This operator selects G4VBiasingOperation objects that
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implement the actual biasing content.
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In the present case, the G4VBiasingOperation objects are
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- G4BOptnChangeCrossSection
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instances. This class is defined in processes/biasing/generic.
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A first operator is defined to handle the case of one particle:
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- GB01BOptrChangeCrossSection .
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The change of cross-section is generally speaking a change of process occurence.
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G4BOptnChangeCrossSection objets are then selected in the method:
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- G4VBiasingOperation* GB01BOptrChangeCrossSection::ProposeOccurenceBiasingOperation(...)
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To allow this same cross-section change to be applied to several particle
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types, an other operator is defined
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- GB01BOptrMultiParticleChangeCrossSection
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which holds one GB01BOptrChangeCrossSection per particle type, and which
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delegates then everything to it.
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The geometry is simple : a single volume to which an instance of
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GB01BOptrMultiParticleChangeCrossSection is attached to.
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The wrapping of physics processes by G4BiasingProcessInterface processes
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is simply handled by the G4GenericBiasingPhysics physics constructor, as shown
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in the main program (see exampleGB01.cc).
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Then, at whatever level (stepping action, or sensitive detector) the
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statistical weight of the track can be obtained as:
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\verbatim
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w = track->GetWeight() ;
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\endverbatim
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*/
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Example GB01 : cross-section biasing
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------------------------------------
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This example illustrates how to bias process cross-sections.
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Generally speaking, the scheme consists of a G4VBiasingOperator that takes
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decisions on what sort of biasing is to be applied. The operator makes these
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decision on requests of the G4BiasingProcessInterface process. This process
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wraps an actual physics process and asks to the operator about what sort of
|
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biasing it should apply. This operator selects G4VBiasingOperation objects that
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implement the actual biasing content.
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In the present case, the G4VBiasingOperation objects are
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G4BOptnChangeCrossSection
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instances. This class is defined in processes/biasing/generic.
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A first operator is defined to handle the case of one particle:
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GB01BOptrChangeCrossSection .
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The change of cross-section is generally speaking a change of process occurence.
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G4BOptnChangeCrossSection objets are then selected in the method:
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G4VBiasingOperation* ProposeOccurenceBiasingOperation(...)
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of the GB01BOptrChangeCrossSection operator.
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To allow this same cross-section change to be applied to several particle
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types, an other operator is defined
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GB01BOptrMultiParticleChangeCrossSection
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which holds one GB01BOptrChangeCrossSection per particle type, and which
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delegates then everything to it.
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The geometry is simple : a single volume to which an instance of
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GB01BOptrMultiParticleChangeCrossSection is attached to.
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The wrapping of physics processes by G4BiasingProcessInterface processes
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is simply handled by the G4GenericBiasingPhysics physics constructor, as shown
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in the main program.
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Then, at whatever level (stepping action, or sensitive detector) the
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statistical weight of the track can be obtained as:
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w = track->GetWeight() ;
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///\file "biasing/GB02/.README.txt"
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///\brief Example GB02 README page
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/*! \page ExampleGB02 Example GB02
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\section ExampleGB02_s1 Force collision biasing
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This example illustrates how to make a force collision biasing in a way
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that is essentially the same than the MCNP one.
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Generally speaking, the scheme consists of a G4VBiasingOperator that takes
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decisions on what sort of biasing is to be applied. The operator makes these
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decision on requests of the G4BiasingProcessInterface process. This process
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wraps an actual physics process and asks to the operator about what sort of
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biasing it should apply. This operator selects G4VBiasingOperation objects that
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implement the actual biasing content.
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In the present case, we make use of the biasing operator
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- G4BOptrForceCollision
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that implements an "a la MCNP" force collision scheme for one particle type.
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This operator is defined in processes/biasing/generic. It is a non-trivial
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operator.
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It starts by "splitting" the track at the volume entrance. Then this
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track is forced to fly through the volume with no interaction. The
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G4OptnForceFreeFlight biasing operation is used for that.
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The second copy is then forced to interact within the volume, which is
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handled by the G4BOptnForceCommonTruncatedExp operation : it is common as it
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takes care of several processes by itself, and it applies a truncated
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exponential law : ie and exponential law limited to the [0,L] range, L being
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the volume width along the track flight.
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To allow several particle types to undergo this force interaction scheme,
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an other operator is defined
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- GB02BOptrMultiParticleForceCollision
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which holds one G4BOptrForceCollision per particle type, and which
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delegates then everything to it.
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The geometry is simple : a single volume to which an instance of
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GB02BOptrMultiParticleForceCollision is attached to.
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The wrapping of physics processes by G4BiasingProcessInterface processes
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is simply handled by the G4GenericBiasingPhysics physics constructor, as shown
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in the main program.
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Then, at whatever level (stepping action, or sensitive detector) the
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statistical weight of the track can be obtained as:
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\verbatim
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w = track->GetWeight() ;
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\endverbatim
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*/
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Example GB02 : force collision biasing
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--------------------------------------
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This example illustrates how to make a force collision biasing in a way
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that is essentially the same than the MCNP one.
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|
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Generally speaking, the scheme consists of a G4VBiasingOperator that takes
|
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decisions on what sort of biasing is to be applied. The operator makes these
|
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decision on requests of the G4BiasingProcessInterface process. This process
|
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wraps an actual physics process and asks to the operator about what sort of
|
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biasing it should apply. This operator selects G4VBiasingOperation objects that
|
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implement the actual biasing content.
|
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|
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In the present case, we make use of the biasing operator
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G4BOptrForceCollision
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that implements an "a la MCNP" force collision scheme for one particle type.
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This operator is defined in processes/biasing/generic. It is a non-trivial
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operator.
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It starts by "splitting" the track at the volume entrance. Then this
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track is forced to fly through the volume with no interaction. The
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G4OptnForceFreeFlight biasing operation is used for that.
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The second copy is then forced to interact within the volume, which is
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handled by the G4BOptnForceCommonTruncatedExp operation : it is common as it
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takes care of several processes by itself, and it applies a truncated
|
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exponential law : ie and exponential law limited to the [0,L] range, L being
|
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the volume width along the track flight.
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|
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To allow several particle types to undergo this force interaction scheme,
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an other operator is defined
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GB02BOptrMultiParticleForceCollision
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which holds one G4BOptrForceCollision per particle type, and which
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delegates then everything to it.
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The geometry is simple : a single volume to which an instance of
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GB02BOptrMultiParticleForceCollision is attached to.
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The wrapping of physics processes by G4BiasingProcessInterface processes
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is simply handled by the G4GenericBiasingPhysics physics constructor, as shown
|
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in the main program.
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|
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Then, at whatever level (stepping action, or sensitive detector) the
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statistical weight of the track can be obtained as:
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w = track->GetWeight() ;
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///\file "biasing/GB03/.README.txt"
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///\brief Example GB03 README page
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/*! \page ExampleGB03 Example GB03
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\section ExampleGB03_s1 Geometry based biasing
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This example illustrates a use of generic biasing classes to implement a
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technique near to "geometry importance biasing".
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The geometry is the same than in EM tests, with the sampling calorimeter
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made of a series of layers of absorber and gap.
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The biasing applies to neutrons only.
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Instead of explicitely assigning "importance" values to the layers, we
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split neutrons moving forward and kill the ones moving backward, when they
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reach the exit of an absorber volume.
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The splitting factor can be controlled by command line, eg:
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\verbatim
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/GB03/biasing/setSplittingFactor 2
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\endverbatim
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which also determines the killing probability : 1/(splitting factor).
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It can be seen than when defining 10 layers (see exampleGB03.in), a
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splitting factor 2 works fine : we don't suffer from under- or over-splitting.
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If going to 20 layers, then a splitting with a factor 2 is too large,
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and the biasing suffers from over-splitting. (And we can not go lower than
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"2", which would mean "1" and hence, no biasing...)
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To alleviate the over-splitting, we introduce a probability to apply the
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splitting (and killing) (this is one solution, others can be considered), that
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can be changed as:
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\verbatim
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/GB03/biasing/setApplyProbability 0.5
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\endverbatim
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With above value, we can see that we recover a satisfactory biasing scheme,
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with neutrons penetrating the entire setup, without over-splitting.
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The classes involved are:
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- GB03BOptnSplitOrKillOnBoundary : which is the biasing operation making
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the splitting and killing;
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- GB03BOptrGeometryBasedBiasing : which is the biasing operator, making
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decision to use above operation, and configuring it, passing it the
|
||||
splitting factor and probability to apply the biasing.
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|
||||
*/
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Example GB03 : geometry based biasing
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||||
-------------------------------------
|
||||
|
||||
This example illustrates a use of generic biasing classes to implement a
|
||||
technique near to "geometry importance biasing".
|
||||
|
||||
The geometry is the same than in EM tests, with the sampling calorimeter
|
||||
made of a series of layers of absorber and gap.
|
||||
|
||||
The biasing applies to neutrons only.
|
||||
|
||||
Instead of explicitely assigning "importance" values to the layers, we
|
||||
split neutrons moving forward and kill the ones moving backward, when they
|
||||
reach the exit of an absorber volume.
|
||||
|
||||
The splitting factor can be controlled by command line, eg:
|
||||
|
||||
/GB03/biasing/setSplittingFactor 2
|
||||
|
||||
which also determines the killing probability : 1/(splitting factor).
|
||||
|
||||
It can be seen than when defining 10 layers (see exampleGB03.in), a
|
||||
splitting factor 2 works fine : we don't suffer from under- or over-splitting.
|
||||
If going to 20 layers, then a splitting with a factor 2 is too large,
|
||||
and the biasing suffers from over-splitting. (And we can not go lower than
|
||||
"2", which would mean "1" and hence, no biasing...)
|
||||
|
||||
To alleviate the over-splitting, we introduce a probability to apply the
|
||||
splitting (and killing) (this is one solution, others can be considered), that
|
||||
can be changed as:
|
||||
|
||||
/GB03/biasing/setApplyProbability 0.5
|
||||
|
||||
With above value, we can see that we recover a satisfactory biasing scheme,
|
||||
with neutrons penetrating the entire setup, without over-splitting.
|
||||
|
||||
|
||||
The classes involved are:
|
||||
|
||||
- GB03BOptnSplitOrKillOnBoundary : which is the biasing operation making
|
||||
the splitting and killing;
|
||||
- GB03BOptrGeometryBasedBiasing : which is the biasing operator, making
|
||||
decision to use above operation, and configuring it, passing it the
|
||||
splitting factor and probability to apply the biasing.
|
||||
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||||
@@ -0,0 +1,41 @@
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||||
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||||
///\file "biasing/GB04/.README.txt"
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||||
///\brief Example GB04 README page
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||||
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||||
/*! \page ExampleGB04 Example GB04
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||||
\section ExampleGB04_s1 bremsstrahlung splitting
|
||||
|
||||
This example illustrates the use of the generic biasing classes to create
|
||||
a bremsstrahlung splitting technique.
|
||||
Note that the EM package also offers a bremsstrahlung splitting, that is
|
||||
built-in to the package.
|
||||
|
||||
|
||||
- GB04BOptnBremSplitting
|
||||
The bremsstrahlung splitting is implemented in the GB04BOptnBremSplitting
|
||||
class (BOptn = Biasing Operation), which acts on the final state creation
|
||||
of the bremsstrahlung process.
|
||||
|
||||
|
||||
- GB04BOptrBremSplitting
|
||||
Decisions when to apply the GB04BOptnBremSplitting biasing operation are
|
||||
taken by the GB04BOptrBremSplitting (BOptr = Biasing Operator) operator. This
|
||||
one also configures the biasing operation, setting it the splitting factor
|
||||
and its behavior regarding electrons to be biased : only the primary one, or
|
||||
all, and only the first bremsstrahlung operation, or all. These are
|
||||
controled by this specific example commands:
|
||||
\verbatim
|
||||
/GB04/biasing/setSplittingFactor [N splitting]
|
||||
/GB04/biasing/biasPrimaryOnly [true/false]
|
||||
/GB04/biasing/biasOnlyOnce [true/false]
|
||||
\endverbatim
|
||||
|
||||
The geometry is minimal : a single volume to which an instance of
|
||||
GB04BOptrBremSplitting is attached to.
|
||||
|
||||
The wrapping of physics processes by G4BiasingProcessInterface processes
|
||||
is simply handled by the G4GenericBiasingPhysics physics constructor, as shown
|
||||
in the main program.
|
||||
|
||||
*/
|
||||
@@ -0,0 +1,36 @@
|
||||
Example GB04 : bremsstrahlung splitting
|
||||
---------------------------------------
|
||||
|
||||
This example illustrates the use of the generic biasing classes to create
|
||||
a bremsstrahlung splitting technique.
|
||||
Note that the EM package also offers a bremsstrahlung splitting, that is
|
||||
built-in to the package.
|
||||
|
||||
|
||||
GB04BOptnBremSplitting
|
||||
|
||||
The bremsstrahlung splitting is implemented in the GB04BOptnBremSplitting
|
||||
class (BOptn = Biasing Operation), which acts on the final state creation
|
||||
of the bremsstrahlung process.
|
||||
|
||||
|
||||
GB04BOptrBremSplitting
|
||||
|
||||
Decisions when to apply the GB04BOptnBremSplitting biasing operation are
|
||||
taken by the GB04BOptrBremSplitting (BOptr = Biasing Operator) operator. This
|
||||
one also configures the biasing operation, setting it the splitting factor
|
||||
and its behavior regarding electrons to be biased : only the primary one, or
|
||||
all, and only the first bremsstrahlung operation, or all. These are
|
||||
controled by this specific example commands:
|
||||
|
||||
/GB04/biasing/setSplittingFactor [N splitting]
|
||||
/GB04/biasing/biasPrimaryOnly [true/false]
|
||||
/GB04/biasing/biasOnlyOnce [true/false]
|
||||
|
||||
|
||||
The geometry is minimal : a single volume to which an instance of
|
||||
GB04BOptrBremSplitting is attached to.
|
||||
|
||||
The wrapping of physics processes by G4BiasingProcessInterface processes
|
||||
is simply handled by the G4GenericBiasingPhysics physics constructor, as shown
|
||||
in the main program.
|
||||
@@ -0,0 +1,53 @@
|
||||
|
||||
///\file "biasing/GB05/.README.txt"
|
||||
///\brief Example GB05 README page
|
||||
|
||||
/*! \page ExampleGB05 Example GB05
|
||||
|
||||
\section ExampleGB05_s1 Splitting by cross-section
|
||||
|
||||
This example illustrates a technique that uses physics cross-sections to
|
||||
determine the splitting [killing] rate in a shielding problem. This technique
|
||||
is supposed to be an invention, and this example here is not optimized. The
|
||||
technique is applied here to neutrons.
|
||||
|
||||
In the classical treatment of the shielding problem, the shield is divided
|
||||
in slices at the boundaries of which particles are splitted[killed] if moving
|
||||
forward[backward]. In the present technique, we collect the cross-section of
|
||||
"absorbing/destroying" processes : decay, capture, inelastic. We then use the
|
||||
generic biasing facilities to create an equivalent of a spitting process, that
|
||||
has a "cross-section" which is the sum of the previous ones. This process is
|
||||
competing with other processes, as a regular one. The occurence of this process
|
||||
is hence the same than the "absorbing/destroying" processes together. When this
|
||||
process wins the competition, it splits the track, with a splitting factor 2 (ie
|
||||
the original track is kept and a copy of it is created). This splitting is hence
|
||||
occuring at the same rate than the absorption, resulting in an expected
|
||||
maintained (unweighted) flux.
|
||||
|
||||
|
||||
The geometry is made of a single block of concrete it. Behind it (in the +z
|
||||
direction) a thin empty volume is placed to print out the particles which are
|
||||
exiting the shield.
|
||||
|
||||
As in any generic biasing use, a biasing operator (taking decisions on what
|
||||
biasing to apply) and a biasing operation (applying these decisions) are defined.
|
||||
These are:
|
||||
GB05BOptrSplitAndKillByCrossSection for the operator,
|
||||
GB05BOptnSplitAndKillByCrossSection for the operation.
|
||||
|
||||
The operator is created in the detector construction, and receives here the
|
||||
names of the absorbing/destroying processes to counterbalance for.
|
||||
At tracking time, it collects the up to date cross-section of these processes
|
||||
in the ProposeNonPhysicsBiasingOperation(...) method, and passes the sum to the
|
||||
GB05BOptnSplitAndKillByCrossSection operation.
|
||||
|
||||
The operation uses the cross-section (interaction length) to sample the
|
||||
distance to "interaction" with a classical exponential. If it wins the race
|
||||
(ie it proposes the smallest of the interaction distances among all processes)
|
||||
its GenerateBiasingFinalState(...) method is called, and it applies splitting
|
||||
or killing (Russian roulette) if the track moves forward or backward.
|
||||
|
||||
|
||||
*/
|
||||
|
||||
|
||||
@@ -0,0 +1,47 @@
|
||||
Example GB05: splitting by cross-section
|
||||
----------------------------------------
|
||||
|
||||
This example illustrates a technique that uses physics cross-sections to
|
||||
determine the splitting [killing] rate in a shielding problem. This technique
|
||||
is supposed to be an invention, and this example here is not optimized. The
|
||||
technique is applied here to neutrons.
|
||||
|
||||
In the classical treatment of the shielding problem, the shield is divided
|
||||
in slices at the boundaries of which particles are splitted[killed] if moving
|
||||
forward[backward]. In the present technique, we collect the cross-section of
|
||||
"absorbing/destroying" processes : decay, capture, inelastic. We then use the
|
||||
generic biasing facilities to create an equivalent of a spitting process, that
|
||||
has a "cross-section" which is the sum of the previous ones. This process is
|
||||
competing with other processes, as a regular one. The occurence of this process
|
||||
is hence the same than the "absorbing/destroying" processes together. When this
|
||||
process wins the competition, it splits the track, with a splitting factor 2 (ie
|
||||
the original track is kept and a copy of it is created). This splitting is hence
|
||||
occuring at the same rate than the absorption, resulting in an expected
|
||||
maintained (unweighted) flux.
|
||||
|
||||
|
||||
The geometry is made of a single block of concrete it. Behind it (in the +z
|
||||
direction) a thin empty volume is placed to print out the particles which are
|
||||
exiting the shield.
|
||||
|
||||
As in any generic biasing use, a biasing operator (taking decisions on what
|
||||
biasing to apply) and a biasing operation (applying these decisions) are defined.
|
||||
These are:
|
||||
GB05BOptrSplitAndKillByCrossSection for the operator,
|
||||
GB05BOptnSplitAndKillByCrossSection for the operation.
|
||||
|
||||
The operator is created in the detector construction, and receives here the
|
||||
names of the absorbing/destroying processes to counterbalance for.
|
||||
At tracking time, it collects the up to date cross-section of these processes
|
||||
in the ProposeNonPhysicsBiasingOperation(...) method, and passes the sum to the
|
||||
GB05BOptnSplitAndKillByCrossSection operation.
|
||||
|
||||
The operation uses the cross-section (interaction length) to sample the
|
||||
distance to "interaction" with a classical exponential. If it wins the race
|
||||
(ie it proposes the smallest of the interaction distances among all processes)
|
||||
its GenerateBiasingFinalState(...) method is called, and it applies splitting
|
||||
or killing (Russian roulette) if the track moves forward or backward.
|
||||
|
||||
|
||||
|
||||
|
||||
@@ -0,0 +1,108 @@
|
||||
|
||||
///\file "biasing/GB06/.README.txt"
|
||||
///\brief Example GB06 README page
|
||||
|
||||
/*! \page ExampleGB06 Example GB06
|
||||
|
||||
\section ExampleGB06_s1 Parallel geometries with generic biasing
|
||||
|
||||
|
||||
This example demonstrates the use of parallel geometries in generic biasing,
|
||||
on a classical shield problem, using geometry-based importance biasing.
|
||||
|
||||
\subsection ExampleGB06_sub_s1 Geometry and activation of navigation in parallel world:
|
||||
|
||||
|
||||
The geometry is made of two parts:
|
||||
- the mass (standard) geometry, which is made of a single block of
|
||||
concrete ; this is implemented in GB06DetectorConstuction ;
|
||||
- a parallel geometry, in which a series of slices is defined, these
|
||||
slices being created using a replica volume ; this is implemeted in
|
||||
GB06ParallelGeometryForSlices, which derives from the base class
|
||||
G4VUserParallelWorld .
|
||||
|
||||
The navigation in the parallel geometry is activated for neutrons. This is
|
||||
done in the main program exampleGB06.cc. The activation is made using the
|
||||
facilities of the G4GenericBiasingPhysics class, as:
|
||||
|
||||
\verbatim
|
||||
biasingPhysics->AddParallelGeometry("neutron",
|
||||
"parallelWorldForSlices");
|
||||
\endverbatim
|
||||
|
||||
where the first name is for the particle type to be aware of the parallel word,
|
||||
the second argument is the name of the parallel world.
|
||||
|
||||
When checking the process list of neutrons (/particle/select neutron and
|
||||
then /particle/process dump ) a new process, `biasingLimiter', is visible. This
|
||||
process handles the step limitation in the parallel geometry. This process can
|
||||
handle several parallel geometries, these being passed to the process as
|
||||
biasingPhysics->AddParallelGeometry("neutron", "parallelWorld1") ,
|
||||
biasingPhysics->AddParallelGeometry("neutron", "parallelWorld2") , etc.
|
||||
|
||||
The geometry-based importance technique utilizes only splitting and killing,
|
||||
hence techniques which are "non-physics biasing" techniques, in the sense they
|
||||
don't modify the behavior of physics processes. For this reason, only a process
|
||||
making the interface between the tracking and the biaising is inserted in the
|
||||
physics list, the physics processes themselves being untouched, this is made as:
|
||||
|
||||
\verbatim
|
||||
biasingPhysics->NonPhysicsBias("neutron");
|
||||
\endverbatim
|
||||
|
||||
Finally, the volume (ie the slice) importances are defined in a simple
|
||||
"importance map" that is created in the GB06ParallelGeometryForSlices class, this
|
||||
map associating a replica number to a volume importance. The map is hold by the
|
||||
biasing operator.
|
||||
|
||||
|
||||
\subsection ExampleGB06_sub_s2 Biasing classes:
|
||||
|
||||
As usual, with the generic biasing scheme, a biasing operator and a biasing
|
||||
operation are defined, these are, respectively the
|
||||
- GB06BOptrSplitAndKillByImportance and
|
||||
- GB06BOptnSplitAndKillByImportance
|
||||
|
||||
classes. The operator here only handles one particle type. In the StartRun()
|
||||
method, it configures the biasing operation GB06BOptnSplitAndKillByImportance
|
||||
passing it the information related to the parallel geometry, and passing it the
|
||||
importance map.
|
||||
|
||||
The biasing operation GB06BOptnSplitAndKillByImportance applies a classical
|
||||
importance-based geometry technique, with spliting / killing at the slice
|
||||
bondaries. Splitting is made if the track goes from a smaller importance to a
|
||||
volume of larger importance, and killing (Russian roulette) is applied in the
|
||||
other case.
|
||||
|
||||
The particularity of this biasing operation is its handling of the parallel
|
||||
geometry information. It has to get by itself geometry information that, in the
|
||||
case of information of the mass geometry, are provided in the G4StepPoint objects
|
||||
(pre step point, post step point) of the G4Step. Here, in the
|
||||
DistanceToApplyOperation(...), which is called at the beginning of the step, it
|
||||
gets a "snapshot" of the geometry state keeping a G4TouchableHistoryHandle. Then
|
||||
in the GenerateBiasingFinalState, which is called at the end of the step, it gets
|
||||
the new geometry state, with an other G4TouchableHistoryHandle. For a step that
|
||||
ends on the boundary, this last touchable history will logically point to the
|
||||
next volume. In this case, the biasing is applied, and the importances are
|
||||
obtained from the replica numbers taken from the two touchable histories, and
|
||||
then from the importance map.
|
||||
|
||||
\subsection ExampleGB06_sub_s3 Output
|
||||
|
||||
A simple sensitive detector is defined (GB06SD) and is attached to a thin
|
||||
volume ("meas.logical") placed after the concrete shield. This sensitive
|
||||
detector simply prints the information (particle type, kinetic energy, etc,
|
||||
and weight) of particles leaving the shield.
|
||||
|
||||
|
||||
\subsection ExampleGB06_sub_s4 Known problems
|
||||
|
||||
In exampleGB06.in the neutron killer process, nKiller, is de-activated
|
||||
(process that kills neutrons after some time), for two reasons. First, killing
|
||||
neutrons in a shield problem is not desirable because neutrons may fly for long
|
||||
time before leaving the shield, and hence must be accounted for. Second, if
|
||||
nKiller is left active, an exception message about a spurious displacement by
|
||||
1e-7mm will appear sometimes : this happens when a neutron is killed on a volume
|
||||
boundary, and the navigation "sees" a (tiny) displacement, that should not exist.
|
||||
|
||||
*/
|
||||
@@ -0,0 +1,100 @@
|
||||
Example GB06: parallel geometries with generic biasing
|
||||
------------------------------------------------------
|
||||
|
||||
|
||||
This example demonstrates the use of parallel geometries in generic biasing,
|
||||
on a classical shield problem, using geometry-based importance biasing.
|
||||
|
||||
1) Geometry and activation of navigation in parallel world:
|
||||
--------------------------------------------------------
|
||||
|
||||
The geometry is made of two parts:
|
||||
- the mass (standard) geometry, which is made of a single block of
|
||||
concrete ; this is implemented in GB06DetectorConstuction ;
|
||||
- a parallel geometry, in which a series of slices is defined, these
|
||||
slices being created using a replica volume ; this is implemeted in
|
||||
GB06ParallelGeometryForSlices, which derives from the base class
|
||||
G4VUserParallelWorld .
|
||||
|
||||
The navigation in the parallel geometry is activated for neutrons. This is
|
||||
done in the main program exampleGB06.cc. The activation is made using the
|
||||
facilities of the G4GenericBiasingPhysics class, as:
|
||||
|
||||
biasingPhysics->AddParallelGeometry("neutron",
|
||||
"parallelWorldForSlices");
|
||||
|
||||
where the first name is for the particle type to be aware of the parallel word,
|
||||
the second argument is the name of the parallel world.
|
||||
|
||||
When checking the process list of neutrons (/particle/select neutron and
|
||||
then /particle/process dump ) a new process, `biasingLimiter', is visible. This
|
||||
process handles the step limitation in the parallel geometry. This process can
|
||||
handle several parallel geometries, these being passed to the process as
|
||||
biasingPhysics->AddParallelGeometry("neutron", "parallelWorld1") ,
|
||||
biasingPhysics->AddParallelGeometry("neutron", "parallelWorld2") , etc.
|
||||
|
||||
The geometry-based importance technique utilizes only splitting and killing,
|
||||
hence techniques which are "non-physics biasing" techniques, in the sense they
|
||||
don't modify the behavior of physics processes. For this reason, only a process
|
||||
making the interface between the tracking and the biaising is inserted in the
|
||||
physics list, the physics processes themselves being untouched, this is made as:
|
||||
|
||||
biasingPhysics->NonPhysicsBias("neutron");
|
||||
|
||||
Finally, the volume (ie the slice) importances are defined in a simple
|
||||
"importance map" that is created in the GB06ParallelGeometryForSlices class, this
|
||||
map associating a replica number to a volume importance. The map is hold by the
|
||||
biasing operator.
|
||||
|
||||
|
||||
2) Biasing classes:
|
||||
----------------
|
||||
|
||||
As usual, with the generic biasing scheme, a biasing operator and a biasing
|
||||
operation are defined, these are, respectively the
|
||||
|
||||
GB06BOptrSplitAndKillByImportance and
|
||||
GB06BOptnSplitAndKillByImportance
|
||||
|
||||
classes. The operator here only handles one particle type. In the StartRun()
|
||||
method, it configures the biasing operation GB06BOptnSplitAndKillByImportance
|
||||
passing it the information related to the parallel geometry, and passing it the
|
||||
importance map.
|
||||
|
||||
The biasing operation GB06BOptnSplitAndKillByImportance applies a classical
|
||||
importance-based geometry technique, with spliting / killing at the slice
|
||||
bondaries. Splitting is made if the track goes from a smaller importance to a
|
||||
volume of larger importance, and killing (Russian roulette) is applied in the
|
||||
other case.
|
||||
The particularity of this biasing operation is its handling of the parallel
|
||||
geometry information. It has to get by itself geometry information that, in the
|
||||
case of information of the mass geometry, are provided in the G4StepPoint objects
|
||||
(pre step point, post step point) of the G4Step. Here, in the
|
||||
DistanceToApplyOperation(...), which is called at the beginning of the step, it
|
||||
gets a "snapshot" of the geometry state keeping a G4TouchableHistoryHandle. Then
|
||||
in the GenerateBiasingFinalState, which is called at the end of the step, it gets
|
||||
the new geometry state, with an other G4TouchableHistoryHandle. For a step that
|
||||
ends on the boundary, this last touchable history will logically point to the
|
||||
next volume. In this case, the biasing is applied, and the importances are
|
||||
obtained from the replica numbers taken from the two touchable histories, and
|
||||
then from the importance map.
|
||||
|
||||
3) Output:
|
||||
-------
|
||||
|
||||
A simple sensitive detector is defined (GB06SD) and is attached to a thin
|
||||
volume ("meas.logical") placed after the concrete shield. This sensitive
|
||||
detector simply prints the information (particle type, kinetic energy, etc,
|
||||
and weight) leaving the shield.
|
||||
|
||||
|
||||
4) Known problems:
|
||||
---------------
|
||||
|
||||
In exampleGB06.in the neutron killer process, nKiller, is de-activated
|
||||
(process that kills neutrons after some time), for two reasons. First, killing
|
||||
neutrons in a shield problem is not desirable because neutrons may fly for some
|
||||
time before leaving the shield, and hence must be accounted for. Second, if
|
||||
nKiller is left active, an exception message about a spurious displacement by
|
||||
1e-7mm will appear sometimes : this happens when a neutron is killed on a volume
|
||||
boundary, and the navigation "sees" a (tiny) displacement, that should not exist.
|
||||
@@ -0,0 +1,88 @@
|
||||
|
||||
///\file "biasing/GB07/.README.txt"
|
||||
///\brief Example GB07 README page
|
||||
|
||||
/*! \page ExampleGB07 Example GB07
|
||||
|
||||
\section ExampleGB07_s1 Leading particle biasing
|
||||
|
||||
This example illustrates how to use the leading particle biasing option.
|
||||
|
||||
It uses the G4BOptnLeadingParticle biasing operation located in:
|
||||
|
||||
source/processes/biasing/generic ,
|
||||
|
||||
and defines the following biasing operation to handle it:
|
||||
|
||||
GB07OptrLeadingParticle.
|
||||
|
||||
As a reminder, the generic biasing scheme consists of a G4VBiasingOperator
|
||||
that takes decisions on what sort of biasing technique to be applied. The
|
||||
techniques are called biasing operations, represented by the G4VBiasingOperation
|
||||
class. The operator is attached to a logical volume in which the biasing must
|
||||
happen. Decisions are made on requests of the G4BiasingProcessInterface process
|
||||
that messages the operator when the track is travelling in the volume. To equip
|
||||
the phyics list with this process, the G4GenericBiasingPhysics physics
|
||||
constructor is used. In this example, several processes -to which the technique
|
||||
is applied- are wrapped by this process to control their final state production
|
||||
for applying the biasing technique.
|
||||
|
||||
\section ExampleGB07_s2 Geometry
|
||||
|
||||
The geometry is simply :
|
||||
- a volume in which the biasing occurs and to which an instance of
|
||||
GB07OptrLeadingParticle is attached,
|
||||
- a thin volume placed after the above volume, that is used to tally the
|
||||
particles exiting biasing volume.
|
||||
- a sensitive detector is attached to the thin volume to simply print the
|
||||
particles entering here. In particular the statistical weight is printed,
|
||||
this one is obtained by:
|
||||
|
||||
\verbatim
|
||||
w = track->GetWeight() ;
|
||||
\endverbatim
|
||||
|
||||
\section ExampleGB07_s3 Biasing configuration
|
||||
|
||||
The particle types and processes under the leading particle biasing are
|
||||
visible in the main program exampleGB07.cc, these are:
|
||||
|
||||
pi+ and pi-, inelastic process,
|
||||
proton and anti-proton, inelastic process,
|
||||
neutron, inelastic and capture processes,
|
||||
anti-neutron, inelastic process,
|
||||
gamma, conversion and photonNuclear processes,
|
||||
electron, electronNuclear process,
|
||||
positron, annihilation and positronNuceal processes,
|
||||
pi0, decay process.
|
||||
|
||||
For the inelastic and lepto/gamma-nuclear processes, leading particle is applied in a rather
|
||||
classical way:
|
||||
- keep the leading particle,
|
||||
- keep one particle of each species (particles and anti-particles are considered a one
|
||||
species, and all hadrons with Z>=2 are counted as one species too).
|
||||
|
||||
For e+, e-, gamma and pi0 processes (which means in practice main conversion, annihililation
|
||||
and pi0 decay processes), the leading particle is kept, and the companion track(s) is(are) randomly
|
||||
kept/killed under a Russian roulette, with a 2/3 killing probabilty. See
|
||||
GB07BOptrLeadingParticle::StartTracking( ... ) for this killing probability setting.
|
||||
|
||||
\section ExampleGB07_s4 Running the program:
|
||||
|
||||
The program can be run in batch or interactive mode and has the following options:
|
||||
|
||||
- batch mode:
|
||||
\verbatim
|
||||
./exampleGB07 [-m macro ] [-b biasing {'on' = default,'off'}]
|
||||
\endverbatim
|
||||
or
|
||||
\verbatim
|
||||
./exampleGB07 [macro.mac]
|
||||
\endverbatim
|
||||
|
||||
- interactive mode:
|
||||
\verbatim
|
||||
./exampleGB07 [-b biasing {'on' = default,'off'}]
|
||||
\endverbatim
|
||||
|
||||
*/
|
||||
@@ -0,0 +1,84 @@
|
||||
Example GB07 : leading particle biasing
|
||||
----------------------------------------
|
||||
|
||||
This example illustrates how to use the leading particle biasing option.
|
||||
|
||||
It uses the G4BOptnLeadingParticle biasing operation located in:
|
||||
|
||||
source/processes/biasing/generic ,
|
||||
|
||||
and defines the following biasing operation to handle it:
|
||||
|
||||
GB07OptrLeadingParticle.
|
||||
|
||||
As a reminder, the generic biasing scheme consists of a G4VBiasingOperator
|
||||
that takes decisions on what sort of biasing technique to be applied. The
|
||||
techniques are called biasing operations, represented by the G4VBiasingOperation
|
||||
class. The operator is attached to a logical volume in which the biasing must
|
||||
happen. Decisions are made on requests of the G4BiasingProcessInterface process
|
||||
that messages the operator when the track is travelling in the volume. To equip
|
||||
the phyics list with this process, the G4GenericBiasingPhysics physics
|
||||
constructor is used. In this example, several processes -to which the technique
|
||||
is applied- are wrapped by this process to control their final state production
|
||||
for applying the biasing technique.
|
||||
|
||||
|
||||
Geometry:
|
||||
---------
|
||||
|
||||
The geometry is simply :
|
||||
- a volume in which the biasing occurs and to which an instance of
|
||||
GB07OptrLeadingParticle is attached,
|
||||
- a thin volume placed after the above volume, that is used to tally the
|
||||
particles exiting biasing volume.
|
||||
- a sensitive detector is attached to the thin volume to simply print the
|
||||
particles entering here. In particular the statistical weight is printed,
|
||||
this one is obtained by:
|
||||
|
||||
w = track->GetWeight() ;
|
||||
|
||||
|
||||
Biasing configuration:
|
||||
----------------------
|
||||
|
||||
The particle types and processes under the leading particle biasing are
|
||||
visible in the main program exampleGB07.cc, these are:
|
||||
|
||||
pi+ and pi-, inelastic process,
|
||||
proton and anti-proton, inelastic process,
|
||||
neutron, inelastic and capture processes,
|
||||
anti-neutron, inelastic process,
|
||||
|
||||
gamma, conversion and photonNuclear processes,
|
||||
electron, electronNuclear process,
|
||||
positron, annihilation and positronNuceal processes,
|
||||
|
||||
pi0, decay process.
|
||||
|
||||
For the inelastic and lepto/gamma-nuclear processes, leading particle is applied in a rather
|
||||
classical way:
|
||||
- keep the leading particle,
|
||||
- keep one particle of each species (particles and anti-particles are considered a one
|
||||
species, and all hadrons with Z>=2 are counted as one species too).
|
||||
For e+, e-, gamma and pi0 processes (which means in practice main conversion, annihililation
|
||||
and pi0 decay processes), the leading particle is kept, and the companion track(s) is(are) randomly
|
||||
kept/killed under a Russian roulette, with a 2/3 killing probabilty. See
|
||||
GB07BOptrLeadingParticle::StartTracking( ... ) for this killing probability setting.
|
||||
|
||||
|
||||
Running the program:
|
||||
--------------------
|
||||
|
||||
The program can be run in batch or interactive mode and has the following options:
|
||||
|
||||
in batch:
|
||||
./exampleGB07 [-m macro ] [-b biasing {'on' = default,'off'}]
|
||||
or
|
||||
./exampleGB07 [macro.mac]
|
||||
interactive:
|
||||
./exampleGB07 [-b biasing {'on' = default,'off'}]
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
@@ -0,0 +1,149 @@
|
||||
|
||||
Examples for event biasing: B01, B02 and B03
|
||||
--------------------------------------------
|
||||
|
||||
B01, B02 and B03 applications 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
|
||||
-------------------------
|
||||
Scoring is carried out using the built-in Multifunctional detectors. For
|
||||
parallel geometries this requires a special scoring physics process.
|
||||
See examples/extended/runAndEvent (especailly RE05) for clarification.
|
||||
|
||||
Known problems - should not happen
|
||||
----------------------------------
|
||||
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://cern.ch/geant4/working_groups/geometry/biasing/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.
|
||||
|
||||
A modular approach is applied to the physicslist and the extension for biasing.
|
||||
|
||||
Example B02
|
||||
===========
|
||||
|
||||
This example uses a parallel geometry to define G4GeometryCell objects
|
||||
for scoring and importance sampling. The output should be equivalent to B01.
|
||||
|
||||
A modular approach is applied to the physicslist and the extension for biasing.
|
||||
The parallel geometry is included in this extension.
|
||||
|
||||
Example B03
|
||||
===========
|
||||
|
||||
This example uses a parallel geometry to define G4GeometryCell objects
|
||||
for scoring and importance sampling. The output should be statistically
|
||||
equivalent to B02 (and B01).
|
||||
|
||||
This demonstrates a customised "flat" physics implementation with the addition
|
||||
of biasing. Complementary approach to the modular physics lists of B01 and B02
|
||||
|
||||
|
||||
___________________________________________________________________________
|
||||
|
||||
|
||||
Generic biasing examples GB01 - GB06
|
||||
------------------------------------
|
||||
|
||||
These examples illustrate the usage of a biasing scheme implemented since
|
||||
version Geant4 10.0.
|
||||
The scheme is meant to be extensible, not limited to these six examples.
|
||||
|
||||
Example GB01:
|
||||
=============
|
||||
|
||||
This example illustrates how to bias process cross-sections in this scheme.
|
||||
|
||||
|
||||
Example GB02:
|
||||
=============
|
||||
|
||||
Illustrates a force collision scheme similar to the MCNP one.
|
||||
|
||||
|
||||
Example GB03:
|
||||
=============
|
||||
|
||||
Illustrates geometry based biasing.
|
||||
|
||||
|
||||
Example GB04:
|
||||
=============
|
||||
|
||||
Illustrates a bremsstrahlung splitting.
|
||||
|
||||
|
||||
Example GB05:
|
||||
=============
|
||||
|
||||
Illustrates a "splitting by cross-section" technique: a splitting-based
|
||||
technique using absorption cross-section to control the neutron population.
|
||||
|
||||
|
||||
Example GB06:
|
||||
=============
|
||||
|
||||
Illustrates the usage of parallel geometries with generic biasing.
|
||||
|
||||
Example GB07:
|
||||
=============
|
||||
|
||||
Illustrates the usage of leading particle biasing with generic biasing.
|
||||
|
||||
|
||||
___________________________________________________________________________
|
||||
|
||||
|
||||
Reverse MonteCarlo Technique example: ReverseMC01
|
||||
-------------------------------------------------
|
||||
|
||||
Example ReverseMC01
|
||||
===================
|
||||
|
||||
Example illustrating the use of the Reverse Monte Carlo (RMC) mode in a Geant4
|
||||
application. See details in ReverseMC01/README.
|
||||
|
||||
@@ -0,0 +1,487 @@
|
||||
|
||||
///\file "biasing/ReverseMC01/.README.txt"
|
||||
///\brief Example ReverseMC01 README page
|
||||
|
||||
/*! \page ExampleReverseMC01 Example ReverseMC01
|
||||
|
||||
This example illustrates the use of Reverse Monte Carlo in Geant4.
|
||||
|
||||
\section ReverseMC01_author Author
|
||||
|
||||
This example code and the adjoint classes in the G4 toolkit have been developed by L.Desorgher (SpaceIT GmbH)
|
||||
under the ESA contract 21435/08/NL/AT. For any (reasonable) question you may contact the author
|
||||
at the following email address : desorgher@spaceit.ch
|
||||
|
||||
|
||||
|
||||
\section ReverseMC01_abstract Abstract
|
||||
|
||||
This is the README file for the first G4 example illustrating the use of the Reverse Monte Carlo (RMC) mode in a Geant4
|
||||
application. The Reverse Monte Carlo method is also known as the Adjoint Monte Carlo (AMC) method and
|
||||
in this document we will alternate both Reverse and Adjoint terms.
|
||||
|
||||
\section ReverseMC01_other_doc Other documentation
|
||||
|
||||
See also the section 3.7.3 Adjoint/Reverse Monte carlo in the
|
||||
Geant4 User guide for application developers.
|
||||
|
||||
|
||||
\section ReverseMC01_s1 Definition of Reverse/Adjoint Monte Carlo
|
||||
|
||||
When the sensitive part of a detector is small compared to its entire size and to the size of the
|
||||
external extended primary particle source, a lot of computing time is spent during a normal Monte Carlo run
|
||||
in the simulation of particle showers that are not contributing to the detector signal.
|
||||
In such particular case the Reverse Monte Carlo (RMC) method, also known as the
|
||||
Adjoint Monte Carlo method, can be used.
|
||||
In this method particles are generated in or on the external surface of the sensitive volume
|
||||
of the instrument and then are tracked backward in the geometry till they reach the source surface,
|
||||
or exceed an energy threshold. During the reverse tracking reverse reactions are applied to the particles.
|
||||
|
||||
|
||||
|
||||
\section ReverseMC01_s2 The Reverse Monte Carlo mode in Geant4 (since G4.9.3 release)
|
||||
|
||||
(See also the section 3.7.3 Adjoint/Reverse Monte carlo in the
|
||||
Geant4 User guide for application developers.)
|
||||
|
||||
Different G4Adjoint classes have been implemented into the Geant4
|
||||
toolkit to run an adjoint/reverse simulation in a Geant4 application.
|
||||
In this implementation an adjoint run is divided in a succession
|
||||
of alternative adjoint and forward tracking of adjoint and normal particles.
|
||||
One Geant4 event treats one of this tracking phase.
|
||||
|
||||
|
||||
\subsection ReverseMC01_sub_s2_1 Reverse tracking phase
|
||||
|
||||
Adjoint particles (adjoint_e-, adjoint_gamma,...) are generated one by one on the so called
|
||||
adjoint source with random position, energy (1/E distribution) and direction. The adjoint
|
||||
source is the external surface of a user defined volume or of a user defined sphere. The
|
||||
adjoint source should contain one or several sensitive volumes and should be small
|
||||
compared to the entire geometry. The user can set the minimum and maximum energy of the
|
||||
adjoint source. After its generation the adjoint primary particle is tracked backward in
|
||||
the geometry till a user defined external surface (spherical or boundary of a volume)
|
||||
or is killed before if it reaches a user defined upper energy limit that represents the
|
||||
maximum energy of the external source. During the reverse tracking, reverse processes take
|
||||
place where the adjoint particle being tracked can be either scattered or transformed in
|
||||
another type of adjoint particle. During the reverse tracking the
|
||||
G4AdjointSimulationManager replaces the user defined primary, run, stepping, ... actions,
|
||||
by its own actions.
|
||||
|
||||
\subsection ReverseMC01_sub_s2_2 Forward tracking phase:
|
||||
|
||||
When an adjoint particle reaches the external surface its weight, type, position,
|
||||
and direction are registered and a normal primary particle with a type equivalent
|
||||
to the last generated adjoint primary is generated with the same energy,
|
||||
position but opposite direction and is tracked in the forward direction
|
||||
in the sensitive region as in a forward MC simulation.
|
||||
During this forward tracking phase the event, stacking, stepping, tracking actions defined
|
||||
by the user for its general forward application are used.
|
||||
By this clear separation between adjoint and forward tracking phases, the code of the
|
||||
user developed for a forward simulation should be only slightly
|
||||
modified to adapt it for an adjoint simulation. Indeed the computation of the signal
|
||||
is done by the same user actions or analysis classes that the one used in the forward
|
||||
simulation mode. Before the G4.10.0 release the reverse and forward tracking mode
|
||||
took place in separated events. Since the G4.10.0 release,
|
||||
in order to preapre to the migration of the
|
||||
ReverseMC to the G4 Multiple Threading mode, the reverse and forward tracking
|
||||
phase of corresponding adjoint and forward primaries have been merged in the same
|
||||
event.
|
||||
|
||||
|
||||
\subsection ReverseMC01_sub_s2_3 Reverse Processes
|
||||
|
||||
During the reverse tracking phase reverse processes act on the adjoint particles.
|
||||
The Reverse processes that are available at the moment in Geant4 are the:
|
||||
- Reverse discrete Ionization for e-, proton and ions
|
||||
- Continuous gain of energy by ionization and bremsstrahlung for e- and by ionization for protons and ions
|
||||
- Reverse discrete e- bremsstrahlung
|
||||
- Reverse photoelectric effect
|
||||
- Reverse Compton scattering
|
||||
- Approximated multiple scattering (MS) (see section 5.3)
|
||||
|
||||
|
||||
It is important to note that the electromagnetic reverse processes are cut dependent
|
||||
as their equivalent forward processes. The implementation of the reverse processes is
|
||||
based on the forward processes
|
||||
implemented in the G4 standard electromagnetic package.
|
||||
|
||||
|
||||
\subsection ReverseMC01_sub_s2_4 Remark on Nb of adjoint particle types and Nb of G4 events considered in an adjoint simulation
|
||||
|
||||
The list of type of adjoint and forward particles that are generated on the adjoint source
|
||||
and considered in the simulation is a function of the adjoint processes declared in the
|
||||
physics list. For example if only the e- and gamma electromagnetic processes are considered
|
||||
, only adjoint e- and adjoint gamma will be considered as primaries. In this case an
|
||||
adjoint event will be divided in two G4 events. The first event will consist
|
||||
into the coupled reverse and forward tracking of an adjoint e- and its equivalent
|
||||
forward e-, while the second events will process the reverse and forward trackings
|
||||
of corresponsing adjoint and forward primary gamms. In this case a
|
||||
run of 100 adjoint events will consist into 200 Geant4 events. If the proton ionization is
|
||||
also considered adjoint and forward protons are also generated as primaries
|
||||
and 300 Geant4 events are processed for 100 adjoint events.
|
||||
|
||||
\subsection ReverseMC01_sub_s2_5 Modifications to bring in a existing G4 application to use the Reverse MC method
|
||||
|
||||
(for more details see also the section 3.7.3 Adjoint/Reverse Monte carlo in the
|
||||
Geant4 User guide for application developers.)
|
||||
|
||||
Due the clear separation between the reverse and forward tracking phase only few modifications are needed
|
||||
to an existing Geant4 application in order to adapt it for the use of the reverse simulation mode.
|
||||
Except in the physics list where all the reverse processes and their forward equivalent
|
||||
have to be declared, the principal code modifications are needed only in the analysis phase at the end
|
||||
of the forward tracking where computed signals have to be multiplied by the weight
|
||||
of the last reverse tracks and then normalized to different user defined spectra and angular distribution representing
|
||||
the external source.
|
||||
The weight of the adjoint tracks is computed by the G4Adjoint classes and the user needs
|
||||
only to multiply them to the primary differential, directional spectrum of its choice.
|
||||
The adjoint weight a the end of tracks can be also registered if needed in answer matrices.
|
||||
|
||||
More precisely, in order to be able to use the Reverse MC method in his simulation, the user should modify
|
||||
its code as such:
|
||||
|
||||
- Adapt its physics list to use Reverse Processes for adjoint particles. An example of such physics list is provided in an extended
|
||||
example.
|
||||
- Create an instance of G4AdjointSimManager somewhere in the main () code.
|
||||
|
||||
- Modify the analysis part of the code to normalize the signal computed during the forward phase to the weight of the last adjoint particle
|
||||
that reaches the external surface. This is done by using the following method of G4AdjointSimManager:
|
||||
- G4int GetIDOfLastAdjParticleReachingExtSource()
|
||||
- G4ThreeVector GetPositionAtEndOfLastAdjointTrack(){ return last_pos;}
|
||||
- G4ThreeVector GetDirectionAtEndOfLastAdjointTrack(){ return last_direction;}
|
||||
- G4double GetEkinAtEndOfLastAdjointTrack(){ return last_ekin;}
|
||||
- G4double GetEkinNucAtEndOfLastAdjointTrack(){ return last_ekin_nuc;}
|
||||
- G4double GetWeightAtEndOfLastAdjointTrack(){return last_weight;}
|
||||
- G4double GetCosthAtEndOfLastAdjointTrack(){return last_cos_th;}
|
||||
- G4String GetFwdParticleNameAtEndOfLastAdjointTrack(){return last_fwd_part_name;}
|
||||
- G4int GetFwdParticlePDGEncodingAtEndOfLastAdjointTrack(){return last_fwd_part_PDGEncoding;}
|
||||
- G4int GetFwdParticleIndexAtEndOfLastAdjointTrack().
|
||||
|
||||
In order to have a code working for both forward and adjoint simulation mode, the extra code needed in user actions for the adjoint
|
||||
simulation mode can be separated to the code needed only for the normal forward simulation by using the following method:
|
||||
- G4bool GetAdjointSimMode() that return true if an adjoint simulation is running and false if not!
|
||||
|
||||
|
||||
|
||||
\section ReverseMC01_s3 exampleRMC01
|
||||
|
||||
The example RMC01 illustrates how to modify a G4 application in order to use
|
||||
both forward and reverse MC modes in the same code.
|
||||
|
||||
|
||||
\subsection ReverseMC01_sub_s3_1 Geometry
|
||||
|
||||
The following simple geometry is considered:
|
||||
- sensitive Silicon cylinder at the center of an Aluminum spherical shielding with 10 cm Radius.
|
||||
- two 0.5mm thick Tantalum plates set horizontally above and below the Sensitive Cylinder
|
||||
|
||||
The free parameters of the geometry that can bes set by the user are:
|
||||
- the thickness of the Aluminum shielding
|
||||
- the height of the sensitive Si cylinder
|
||||
- the radius of the sensitive Si cylinder
|
||||
|
||||
|
||||
|
||||
\subsection ReverseMC01_sub_s3_2 Physics
|
||||
|
||||
The physical processes considered are:
|
||||
- Reverse and forward discrete Ionization for e- and proton
|
||||
- Continuous gain and loss of energy by ionization and bremsstrahlung for e- and by ionization for protons
|
||||
- Reverse and forward discrete e- bremsstrahlung
|
||||
- Reverse and forward photoelectric effect
|
||||
- Reverse and forward Compton scattering
|
||||
- Reverse and forward Multiple scattering
|
||||
|
||||
These processes are implemented in the class G4AdjointPhysicsList distributed with the example. The G4AdjointPhysicsMessenger allows the user
|
||||
to switch on/off some processes for testing purpose. By default all processes cited above are considered except the proton ionization that
|
||||
has to be specifically switch on in the macro file by the user.
|
||||
|
||||
|
||||
|
||||
\subsection ReverseMC01_sub_s3_3 Analysis and output of the code
|
||||
|
||||
The example computes the energy deposited in the sensitive Si cylinder and the current of e-, protons, and gamma
|
||||
entering this cylinder.
|
||||
The Hits are registered in the sensitive detector class RMC01SD that is a typical G4 sensitive detector class
|
||||
used in a forward simulation and is not modified at all
|
||||
for the adjoint simulation mode.
|
||||
The analysis of the registered hits during forward events is done by the RMCO1AnalysisManager.
|
||||
That is the class that illustrates how to adapt an analysis code of a fwd simulation in order to use it also for
|
||||
an adjoint simulation.
|
||||
In this class during a forward simulation the method EndOfEventForForwardSimulation is used at the end of an event
|
||||
while during an adjoint simulation at the end of fwd tracking event the method EndOfEventForAdjointSimulation is called.
|
||||
By looking at the source of RMCO1AnalysisManager and more particularly to its method EndOfEventForAdjointSimulation the user will
|
||||
learn how to adapt its G4 analysis code for an adjoint simulation.
|
||||
|
||||
The outputs of an adjoint simulation are:
|
||||
|
||||
- The total energy deposited and particle current entering the sensitive cylinder normalized
|
||||
automatically to a user defined primary spectrum(exponential or power law) .These results are stored in the files:
|
||||
- Adj_Edep_vs_EkinPrim.txt
|
||||
- Adj_ElectronCurrent.txt
|
||||
- Adj_GammaCurrent.txt
|
||||
- Adj_ProtonCurrent.txt
|
||||
- ConvergenceOfAdjointSimulationResults.txt:
|
||||
The total normalized edep and its relative error registered every 5000 adjoint events
|
||||
|
||||
|
||||
- The answer matrix of the energy deposited and particles current on the sensitive cylinder in function of primary energy of e-, gamma and
|
||||
protons. These results are stored in the files Adj********_Answer.txt
|
||||
|
||||
|
||||
|
||||
The outputs of a forward simulation are:
|
||||
- The mean energy deposited and particle current entering the sensitive cylinder per event.
|
||||
These results are stored in the files:
|
||||
- Fwd_Edep_vs_EkinPrim.txt
|
||||
- Fwd_ElectronCurrent.txt
|
||||
- Fwd_GammaCurrent.txt
|
||||
- Fwd_ProtonCurrent.txt
|
||||
|
||||
|
||||
|
||||
\subsection ReverseMC01_sub_s3_4 Run macrofiles
|
||||
|
||||
The following example run macro files are distributed with the code:
|
||||
- run_adjoint_simulation_electron.mac and run_adjoint_simulation_proton.mac for adjoint simulations
|
||||
|
||||
- run_forward_simulation_electron.mac and run_forward_simulation_proton.mac for forward simulations
|
||||
|
||||
|
||||
\subsection ReverseMC01_sub_s3_5 Comparison of adjoint and forward simulation results
|
||||
|
||||
It is the responsibility of the user to select in the macro file the same external spectrum
|
||||
for both the forward and adjoint simulations and to normalize the per event results of the forward simulation
|
||||
to the fluence considered in the adjoint simulation.
|
||||
|
||||
For the macro files that are provided with the examples it consists into multiplying the forward results by pi*100.
|
||||
This normalization factor is explained by the following:
|
||||
|
||||
- For the forward simulation the results are given per number of events. It corresponds
|
||||
to a normalization to a fluence of 1 particle emanating from the external source.
|
||||
|
||||
- In run_fwd_simulation.mac the source is set on a sphere of 10 cm radius (see /gps commands in
|
||||
macrofile).Therefore the omnidirectional fluence for the fwd simulation is 1./(pi*R^2) with R=10cm.
|
||||
|
||||
- The adjoint results are normalized to a fluence of 1/cm2.
|
||||
(See command /RMC01/analysis/SetExponentialSpectrumForAdjointSim in macrofile)
|
||||
|
||||
- In conclusion to compare the adjoint and forward results, the forward results should
|
||||
be multiplied by pi*R^2/cm2= pi*100.
|
||||
|
||||
|
||||
|
||||
\section ReverseMC01_s4 Control of the adjoint simulation and the RMC01 code by G4 macro UI commands
|
||||
|
||||
Different G4 macro UI commands are provided to control the RMC01 example and the adjoint simulation.
|
||||
Some macro commands are provided within the geant4 toolkit and appears in a G4 application when the singleton
|
||||
class G4AdjointSimManager is called somewhere in the code, the other macro commands are
|
||||
declared in the code distributed within the example.
|
||||
|
||||
|
||||
\subsection ReverseMC01_sub_s4_1 G4UI commands in the directory /adjoint
|
||||
|
||||
The macro command directory /adjoint appears in a user application when the singleton
|
||||
class G4AdjointSimManager is called somewhere in the code.
|
||||
It allows to control the adjoint source, the external source and start an adjoint simulation.
|
||||
|
||||
The command to start an adjoint run is:
|
||||
|
||||
- /adjoint/start_run nb \n
|
||||
Start an adjoint simulation with a number of events given by nb. It is important to note that the total number of events in the sense of G4
|
||||
will be nb*2*nb_primary_considered (see 3.4.)
|
||||
|
||||
|
||||
The commands to control the adjoint source are:
|
||||
|
||||
- /adjoint/DefineSphericalAdjSource R X Y Z unit_length \n
|
||||
The adjoint source is set on a sphere with radius R and centered on position (X,Y,Z)
|
||||
|
||||
- /adjoint/DefineSphericalAdjSourceCenteredOnAVolume phys_vol_name R unit_length \n
|
||||
The external source is set on a sphere with radius R and with its center position located at the center of the
|
||||
the physical volume specified by the name phys_vol_name.
|
||||
- /adjoint/DefineAdjSourceOnExtSurfaceOfAVolume phys_vol_name \n
|
||||
The external surface is set as the external boundary of a the physical volume with name phys_vol_name
|
||||
|
||||
- /adjoint/SetAdjSourceEmin Emin energy_unit \n
|
||||
Set the minimum energy of the external source
|
||||
|
||||
- /adjoint/SetAdjSourceEmax Emax energy_unit \n
|
||||
Set the maximum energy of the external source
|
||||
|
||||
- /adjoint/ConsiderAsPrimary particle_name \n
|
||||
The type of particle specified by "particle_name" will be added in the list of primary adjoint particles.
|
||||
The list of candidates depends on the reverse physics processes considered in the simulation. At the most the
|
||||
potential candidates are (e-, gamma, proton , ion). For this example only e-, gamma, proton
|
||||
can be chosen. As the proton ionization is not considered by default, the default list of particles is
|
||||
[e-,gamma]. To have also the proton as candidate the proton ionization should
|
||||
be switch on (/adjoint_physics/UseProtonIonisation true).
|
||||
|
||||
- /adjoint/NeglectAsPrimary particle_name \n
|
||||
|
||||
The type of particle specified by "particle_name" will be removed from the list of primary adjoint particles.
|
||||
The list of candidates depends on the reverse physics processes considered in the simulation. At the most the
|
||||
potential candidates are (e-, gamma, proton , ion). For this example only e-, gamma, proton
|
||||
can be chosen. As the proton ionization is not considered by default, the default list of particles is
|
||||
[e-,gamma].To have also the proton as candidate the proton ionization should
|
||||
be switch on (/adjoint_physics/UseProtonIonisation true).
|
||||
|
||||
|
||||
The commands to control the external source are:
|
||||
|
||||
- /adjoint/DefineSphericalExtSource R X Y Z unit_length:\n
|
||||
The external source is set on a sphere with radius R and centered on position (X,Y,Z)
|
||||
|
||||
- /adjoint/DefineSphericalExtSourceCenteredOnAVolume phys_vol_name R unit_length\n
|
||||
The external source is set on a sphere with radius R and with its center position located at the center of the
|
||||
the physical volume specified by the name phys_vol_name.
|
||||
|
||||
- /adjoint/DefineExtSourceOnExtSurfaceOfAVolume phys_vol_name \n
|
||||
The external surface is set as the external boundary of a the physical volume with name phys_vol_name
|
||||
|
||||
- /adjoint/SetExtSourceEmax Emax energy_unit \n
|
||||
Set the maximum energy of the external source. An adjoint track will be stop when a an adjoint particle get an energy higher than this maximum energy.
|
||||
|
||||
|
||||
|
||||
\subsection ReverseMC01_sub_s4_2 G4UI commands in the directory /adjoint_physics
|
||||
|
||||
These commands allow to control the electromagnetic processes that will be considered in the simulation.
|
||||
|
||||
The processes that can be used are:
|
||||
- Reverse and forward e- continuous and discrete Ionization. Always switch on
|
||||
- Reverse and forward e- Bremsstrahlung. Switch on by default
|
||||
- Reverse and forward Compton scattering. Switch on by default
|
||||
- Reverse and forward photo electric effect. Switch on by default
|
||||
- Reverse and forward photo electric effect. Switch on by default
|
||||
- Reverse and forward multiple scattering. Switch on by default
|
||||
- Reverse and forward proton continuous and discrete Ionization. Switch off by default
|
||||
- Forward e-e+ pair production. Switch off by default.
|
||||
|
||||
|
||||
The commands that can be used to switch on of these processes are:
|
||||
|
||||
- /adjoint_physics/UseProtonIonisation true/false \n
|
||||
Switch on/off the reverse and forward proton ionization. Off by default.
|
||||
|
||||
- /adjoint_physics/UseBremsstrahlung true/false \n
|
||||
Switch on/off the reverse and forward e- bremsstrahlung. On by default.
|
||||
|
||||
- /adjoint_physics/UseCompton true/false \n
|
||||
Switch on/off the Compton scattering. On by default.
|
||||
|
||||
|
||||
- /adjoint_physics/UseMS true/false \n
|
||||
Switch on/off the multiple scattering. On by default.
|
||||
|
||||
|
||||
- /adjoint_physics/UseEgainElossFluctuation true/false \n
|
||||
Switch on/off the fluctuation in the continuous energy loss/gain. On by default. Only for test purpose.
|
||||
|
||||
- /adjoint_physics/UsePEEffect true/false \n
|
||||
Switch on/off the photo electric effect. On by default.
|
||||
|
||||
|
||||
- /adjoint_physics/UseGammaConversion true/false \n
|
||||
Switch on/off the forward e-e+ pair production from gamma. Off by default. When On all the e+
|
||||
electromagnetic physics is considered.
|
||||
|
||||
|
||||
The user can also fix the maximum energy Emax and minimum energy Emin of the adjoint physical processes used
|
||||
in the simulation. The adjoint process will be applied to particles within the energy range [Emin, Emax]
|
||||
and will produce adjoint secondary only in this energy range. It is recommended to fix Emin to the minimum
|
||||
energy of the adjoint source and fix Emax to the maximum energy of the external source.
|
||||
The commands controlling Emin and Emax are:
|
||||
|
||||
- /adjoint_physics/SetEminForAdjointModels Emin Energy_unit \n
|
||||
Set the minimum energy of the adjoint processes/models.
|
||||
|
||||
- /adjoint_physics/SetEmaxForAdjointModels Emin Energy_unit \n
|
||||
Set the maximum energy of the adjoint processes/models.
|
||||
|
||||
|
||||
\subsection ReverseMC01_sub_s4_3 G4UI commands in the directory /RMC01
|
||||
|
||||
|
||||
Commands/RMC01/geometry/ to control the geometry:
|
||||
|
||||
- /RMC01/geometry/SetSensitiveVolumeHeight H length_unit \n
|
||||
Set the height H of the Si sensitive cylinder.
|
||||
|
||||
|
||||
- /RMC01/geometry/SetSensitiveVolumeRadius R length_unit \n
|
||||
Set the radius R of the Si sensitive cylinder.
|
||||
|
||||
- /RMC01/geometry/SetShieldingThickness D length_unit \n
|
||||
Set the thickness D of the aluminum shielding.
|
||||
|
||||
Commands /RMC01/analysis/ to control the primary spectrum used for the normalization of the
|
||||
adjoint simulation results and fix the expected precision of the computed Edep:
|
||||
|
||||
- /RMC01/analysis/SetPowerLawPrimSpectrumForAdjointSim particle_name F F_unit alpha Emin Emax E_unit \n
|
||||
Set the primary spectrum to which the adjoint simulation results will be normalised to a power law
|
||||
spectrum E^(-alpha) of particle defined by particle_name, with an omnidirectional fluence F, and
|
||||
energy range [Emin,Emax]. The fluence unit candidates for F_unit are [1/cm2, 1/m2, cm-2, m-2].
|
||||
|
||||
|
||||
- /RMC01/analysis/SetExponentialSpectrumForAdjointSim particle_name F F_unit E0 Emin Emax E_unit \n
|
||||
Set the primary spectrum to which the adjoint simulation results will be normalised to an exponential
|
||||
spectrum exp(-E/E0) of particle defined by particle_name, with an omnidirectional fluence F, and
|
||||
energy range [Emin,Emax]. The fluence unit candidates for F_unit are [1/cm2, 1/m2, cm-2, m-2].
|
||||
|
||||
|
||||
|
||||
- /RMC01/analysis/SetExpectedPrecisionOfResults precision \n
|
||||
Set the expected precision in % for the computed energy deposited in the sensitive volume
|
||||
for both the forward and adjoint simulation case. When the relative statistical error
|
||||
of the computed energy deposited reach this precision the run is aborted and the results are registered.
|
||||
Otherwise the run continue till the nb of events specified by the user are processed. By default the precision is set
|
||||
to 0. meaning that the run will not be aborted in this case.
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
\section ReverseMC01_s5 Known issues
|
||||
|
||||
\subsection ReverseMC01_sub_s5_1 Rare too high weight in the adjoint simulation
|
||||
|
||||
In rare cases an adjoint track may get a much too high weight when reaching the external source.
|
||||
While this happen not often it may corrupt the simulation results significantly. The reason of this high weight is
|
||||
the joint use at low e- and gamma energy of both the photoelectric and bremsstrahlung processes.
|
||||
Unfortunately we still need some investigations to remove this problem at the level of physical processes.
|
||||
However this problem can be solved at the level of event action in the user code by adding a test on the adjoint
|
||||
weight. Such test has been implemented in the example RMC01.
|
||||
In this implementation an event is rejected when the relative error of the computed normalised edep
|
||||
increase during one event by more than 50% when the precision is already below 10%.
|
||||
|
||||
|
||||
\subsection ReverseMC01_sub_s5_2 Limitation of the reverse bremsstrahlung
|
||||
|
||||
The difference between the differential cross sections used in the adjoint and forward bremsstrahlung
|
||||
models is the source of a higher flux of >100 keV gamma in the reverse simulation compared to the forward simulation.
|
||||
The adjoint processes/models should make use of the direct differential cross section to sample
|
||||
the adjoint secondaries and compute the adjoint cross section.
|
||||
The differential cross section used in G4AdjointeBremstrahlungModel is obtained by the numerical derivation
|
||||
over the cut energy of the direct cross section provided by G4eBremsstrahlungModel.
|
||||
This would be a correct procedure if the distribution of secondary in G4eBremsstrahlungModel
|
||||
would match this differential cross section. Unfortunately it is not the case as independent parameterization are used
|
||||
in G4eBremsstrahlungModel for both the cross sections and the sample of secondary. (It means that in the forward case
|
||||
if one would integrate the effective differential cross section considered in the simulation we would not find back
|
||||
the used cross section).
|
||||
In the future we plan to correct this problem by using an extra weight correction factor after the occurrence of a reverse
|
||||
bremsstrahlung. This weight factor should be the ratio between the differential CS used in the adjoint simulation and the
|
||||
one effectively used in the forward processes. As it is impossible to have access to the forward differential CS
|
||||
in G4eBremsstrahlungModel we are investigating the feasibility to use the differential CS considered in
|
||||
G4Penelope models.
|
||||
|
||||
|
||||
\subsection ReverseMC01_sub_s5_3 Limitation of the reverse multiple scattering
|
||||
|
||||
For the reverse multiple scattering we are using the same models than for the forward case.
|
||||
This approximation makes that the discrepancy between the adjoint and forward
|
||||
simulation cases can get to a level of ~ 10-15% relative differences in the test cases that we have considered.
|
||||
In the future we plan to improve the adjoint multiple scattering models by forcing the computation of
|
||||
multiple scattering effect at the end of an adjoint step.
|
||||
|
||||
*/
|
||||
@@ -0,0 +1,542 @@
|
||||
Example1 for Reverse Monte Carlo
|
||||
--------------------------------
|
||||
|
||||
|
||||
Author
|
||||
------
|
||||
This example code and the adjoint classes in the G4 toolkit have been developed by L.Desorgher (SpaceIT GmbH)
|
||||
under the ESA contract 21435/08/NL/AT. For any (reasonable) question you may contact the author
|
||||
at the following email address : desorgher@spaceit.ch
|
||||
|
||||
|
||||
Abstract
|
||||
--------
|
||||
This is the README file for the first G4 example illustrating the use of the Reverse Monte Carlo (RMC) mode in a Geant4
|
||||
application. The Reverse Monte Carlo method is also known as the Adjoint Monte Carlo (AMC) method and
|
||||
in this document we will alternate both Reverse and Adjoint terms.
|
||||
|
||||
Other documentation
|
||||
-------------------
|
||||
See also the section 3.7.3 Adjoint/Reverse Monte carlo in the
|
||||
Geant4 User guide for application developers.
|
||||
|
||||
|
||||
Table of Contents:
|
||||
-----------------
|
||||
|
||||
1.Definition of Reverse/Adjoint Monte Carlo
|
||||
|
||||
2.The Reverse Monte Carlo mode in Geant4 (since G4.9.3 release)
|
||||
2.1. Reverse tracking phase
|
||||
2.2. Forward tracking phase
|
||||
2.3. Reverse processes
|
||||
2.4. Remark on Nb of adjoint particle types and G4 events considered in an adjoint simulation
|
||||
2.5. Modifications to bring in a existing G4 application to use the Reverse MC method
|
||||
|
||||
3.exampleRMC01
|
||||
3.1. Geometry
|
||||
3.2. Physics
|
||||
3.3. Analysis and output of the code
|
||||
3.4. Run macrofiles
|
||||
3.5. Comparison of adjoint and forward simulation results. Normalization!
|
||||
|
||||
4.Control of the adjoint simulation and the RMC01 code by G4 macro UI commands
|
||||
4.1. G4UI commands in the directory /adjoint
|
||||
4.2. G4UI commands in the directory /adjoint_physics
|
||||
4.3. G4UI commands in the directory /RMC01
|
||||
|
||||
5. Known issues
|
||||
5.1. Rare too high weight in the adjoint simulation
|
||||
5.2. Limitation of the reverse bremsstrahlung
|
||||
5.3.Limitation of the reverse multiple scattering
|
||||
|
||||
|
||||
|
||||
1. Definition of Reverse/Adjoint Monte Carlo
|
||||
-----------------------------------------
|
||||
-----------------------------------------
|
||||
When the sensitive part of a detector is small compared to its entire size and to the size of the
|
||||
external extended primary particle source, a lot of computing time is spent during a normal Monte Carlo run
|
||||
in the simulation of particle showers that are not contributing to the detector signal.
|
||||
In such particular case the Reverse Monte Carlo (RMC) method, also known as the
|
||||
Adjoint Monte Carlo method, can be used.
|
||||
In this method particles are generated in or on the external surface of the sensitive volume
|
||||
of the instrument and then are tracked backward in the geometry till they reach the source surface,
|
||||
or exceed an energy threshold. During the reverse tracking reverse reactions are applied to the particles.
|
||||
|
||||
|
||||
|
||||
2. The Reverse Monte Carlo mode in Geant4 (since G4.9.3 release)
|
||||
----------------------------------------------------------------
|
||||
----------------------------------------------------------------
|
||||
(See also the section 3.7.3 Adjoint/Reverse Monte carlo in the
|
||||
Geant4 User guide for application developers.)
|
||||
|
||||
Different G4Adjoint classes have been implemented into the Geant4
|
||||
toolkit to run an adjoint/reverse simulation in a Geant4 application.
|
||||
In this implementation an adjoint run is divided in a succession
|
||||
of alternative adjoint and forward tracking of adjoint and normal particles.
|
||||
One Geant4 event treats the reverse tracking of an adjoint primary particle
|
||||
and its secondaries, and the forward tracking of a primary particle euqivalent
|
||||
to the adjoint primary as well as its secondaries.
|
||||
|
||||
|
||||
2.1. Reverse tracking phase:
|
||||
-------------------------
|
||||
|
||||
Adjoint particles (adjoint_e-, adjoint_gamma,...) are generated one by one on the so called
|
||||
adjoint source with random position, energy (1/E distribution) and direction. The adjoint
|
||||
source is the external surface of a user defined volume or of a user defined sphere. The
|
||||
adjoint source should contain one or several sensitive volumes and should be small
|
||||
compared to the entire geometry. The user can set the minimum and maximum energy of the
|
||||
adjoint source. After its generation the adjoint primary particle is tracked backward in
|
||||
the geometry till a user defined external surface (spherical or boundary of a volume)
|
||||
or is killed before if it reaches a user defined upper energy limit that represents the
|
||||
maximum energy of the external source. During the reverse tracking, reverse processes take
|
||||
place where the adjoint particle being tracked can be either scattered or transformed in
|
||||
another type of adjoint particle. During the reverse tracking the
|
||||
G4AdjointSimulationManager replaces the user defined primary, run, stepping, ... actions,
|
||||
by its own actions.
|
||||
|
||||
2.2. Forward tracking phase:
|
||||
--------------------------
|
||||
|
||||
When an adjoint particle reaches the external surface its weight, type, position,
|
||||
and direction are registered and a normal primary particle with a type equivalent
|
||||
to the last generated adjoint primary is generated with the same energy,
|
||||
position but opposite direction and is tracked in the forward direction
|
||||
in the sensitive region as in a forward MC simulation.
|
||||
During this forward tracking phase the event, stacking, stepping, tracking actions defined
|
||||
by the user for its general forward application are used.
|
||||
By this clear separation between adjoint and forward tracking phases, the code of the
|
||||
user developed for a forward simulation should be only slightly
|
||||
modified to adapt it for an adjoint simulation. Indeed the computation of the signal
|
||||
is done by the same user actions or analysis classes that the one used in the forward
|
||||
simulation mode. Before the G4.10.0 release the reverse and forward tracking mode
|
||||
took place in separated events. Since the G4.10.0 release,
|
||||
in order to prepare to the migration of the
|
||||
ReverseMC to the G4 Multiple Threading mode, the reverse and forward tracking
|
||||
phase of corresponding adjoint and forward primaries have been merged in the same
|
||||
event.
|
||||
|
||||
|
||||
2.3. Reverse Processes:
|
||||
---------------------
|
||||
|
||||
During the reverse tracking phase reverse processes act on the adjoint particles.
|
||||
The Reverse processes that are available at the moment in Geant4 are the:
|
||||
- Reverse discrete Ionization for e-, proton and ions
|
||||
- Continuous gain of energy by ionization and bremsstrahlung for e- and by ionization for protons and ions
|
||||
- Reverse discrete e- bremsstrahlung
|
||||
- Reverse photoelectric effect
|
||||
- Reverse Compton scattering
|
||||
- Approximated multiple scattering (MS) (see section 5.3)
|
||||
|
||||
For the gamma reverse physics an adjoint gamma reverse forced interaction process has been implemented
|
||||
since GEANT4.10.3. THis process splits a new created gamma in two tracks.
|
||||
The first tracks is used to force a free flight of the adjoint gamma through the geometry.
|
||||
The second track is used to force a reverse bremsstrahlung or a reverse compton at some random
|
||||
position along the free flight track.
|
||||
|
||||
It is important to note that the electromagnetic reverse processes are cut dependent
|
||||
as their equivalent forward processes. The implementation of the reverse processes is
|
||||
based on the forward processes
|
||||
implemented in the G4 standard electromagnetic package.
|
||||
|
||||
|
||||
2.4. Remark on Nb of adjoint particle types and Nb of G4 events considered in an adjoint simulation:
|
||||
---------------------------------------------------------------------------------
|
||||
|
||||
The list of type of adjoint and forward particles that are generated on the adjoint source
|
||||
and considered in the simulation is a function of the adjoint processes declared in the
|
||||
physics list. For example if only the e- and gamma electromagnetic processes are considered
|
||||
, only adjoint e- and adjoint gamma will be considered as primaries. In this case an
|
||||
adjoint event will be divided in two G4 events. The first event will consist
|
||||
into the coupled reverse and forward tracking of an adjoint e- and its equivalent
|
||||
forward e-, while the second events will process the reverse and forward trackings
|
||||
of corresponsing adjoint and forward primary gammas. In this case a
|
||||
run of 100 adjoint events will consist into 200 Geant4 events. If the proton ionization is
|
||||
also considered adjoint and forward protons are also generated as primaries
|
||||
and 300 Geant4 events are processed for 100 adjoint events.
|
||||
|
||||
2.5. Modifications to bring in a existing G4 application to use the Reverse MC method
|
||||
--------------------------------------------------------------------------------
|
||||
(for more details see also the section 3.7.3 Adjoint/Reverse Monte carlo in the
|
||||
Geant4 User guide for application developers.)
|
||||
|
||||
Due the clear separation between the reverse and forward tracking phase only few modifications are needed
|
||||
to an existing Geant4 application in order to adapt it for the use of the reverse simulation mode.
|
||||
Except in the physics list where all the reverse processes and their forward equivalent
|
||||
have to be declared, the principal code modifications are needed only in the analysis phase at the end
|
||||
of the forward tracking where computed signals have to be multiplied by the weight
|
||||
of the reverse tracks that have reached the external surface of the simulatrion
|
||||
and then normalized to different user defined spectra and angular distribution representing
|
||||
the external source.
|
||||
The weight of the adjoint tracks is computed by the G4Adjoint classes and the user needs
|
||||
only to multiply them to the primary differential, directional spectrum of its choice.
|
||||
The adjoint weight a the end of tracks can be also registered if needed in answer matrices.
|
||||
|
||||
More precisely, in order to be able to use the Reverse MC method in his simulation, the user should modify
|
||||
its code as such:
|
||||
|
||||
- Adapt its physics list to use Reverse Processes for adjoint particles. An example of such physics list is provided in an extended
|
||||
example.
|
||||
- Create an instance of G4AdjointSimManager somewhere in the main code.
|
||||
|
||||
- Modify the analysis part of the code to normalize the signal computed during the forward phase to the weight
|
||||
of adjoint particle that reached the external surface during the last tracking phase.
|
||||
This is done by using the following method of G4AdjointSimManager.
|
||||
size_t GetNbOfAdointTracksReachingTheExternalSurface()
|
||||
G4int GetIDOfLastAdjParticleReachingExtSource(size_t i)
|
||||
G4ThreeVector GetPositionAtEndOfLastAdjointTrack(size_t i)
|
||||
G4ThreeVector GetDirectionAtEndOfLastAdjointTrack(size_t i)
|
||||
G4double GetEkinAtEndOfLastAdjointTrack(size_t i)
|
||||
G4double GetEkinNucAtEndOfLastAdjointTrack(size_t i)
|
||||
G4double GetWeightAtEndOfLastAdjointTrack(size_t i)
|
||||
G4double GetCosthAtEndOfLastAdjointTrack(size_t i)
|
||||
G4String GetFwdParticleNameAtEndOfLastAdjointTrack(size_t i)
|
||||
G4int GetFwdParticlePDGEncodingAtEndOfLastAdjointTrack(size_t i)
|
||||
G4int GetFwdParticleIndexAtEndOfLastAdjointTrack(size_t i).
|
||||
Since the version Geant4.10.3 several adjoint tracks can arrive on the external surface during the same events.
|
||||
It is therefore important to loop over alll these tracks when normalizing the weights at the end of the event.
|
||||
The method GetNbOfAdointTracksReachingTheExternalSurface() returns the number of adjoint tracks that reached the
|
||||
external surface. Ine the other methods the input parameter i allows to get the information of the ith track.
|
||||
|
||||
In order to have a code working for both forward and adjoint simulation mode, the extra code needed in user actions for the adjoint
|
||||
simulation mode can be separated to the code needed only for the normal forward simulation by using the following method
|
||||
|
||||
G4bool GetAdjointSimMode() that return true if an adjoint simulation is running and false if not!
|
||||
|
||||
|
||||
|
||||
3. exampleRMC01
|
||||
---------------
|
||||
---------------
|
||||
The example RMC01 illustrates how to modify a G4 application in order to use
|
||||
both forward and reverse MC modes in the same code.
|
||||
|
||||
|
||||
3.1. Geometry:
|
||||
--------------
|
||||
|
||||
The following simple geometry is considered:
|
||||
- sensitive Silicon cylinder at the center of an Aluminum spherical shielding with 10 cm Radius.
|
||||
- two 0.5mm thick Tantalum plates set horizontally above and below the Sensitive Cylinder
|
||||
|
||||
The free parameters of the geometry that can bes set by the user are:
|
||||
- the thickness of the Aluminum shielding
|
||||
- the height of the sensitive Si cylinder
|
||||
- the radius of the sensitive Si cylinder
|
||||
|
||||
|
||||
|
||||
3.2. Physics:
|
||||
-------------
|
||||
|
||||
The physical processes considered are:
|
||||
- Reverse and forward discrete Ionization for e- and proton
|
||||
- Continuous gain and loss of energy by ionization and bremsstrahlung for e- and by ionization for protons
|
||||
- Reverse and forward discrete e- bremsstrahlung
|
||||
- Reverse and forward photoelectric effect
|
||||
- Reverse and forward Compton scattering
|
||||
- Reverse and forward Multiple scattering
|
||||
|
||||
These processes are implemented in the class G4AdjointPhysicsList distributed with the example. The G4AdjointPhysicsMessenger allows the user
|
||||
to switch on/off some processes for testing purpose. By default all processes cited above are considered except the proton ionization that
|
||||
has to be specifically switch on in the macro file by the user.
|
||||
|
||||
|
||||
|
||||
3.3. Analysis and output of the code:
|
||||
----------------------------------
|
||||
|
||||
The example computes the energy deposited in the sensitive Si cylinder and the current of e-, protons, and gamma
|
||||
entering this cylinder.
|
||||
The Hits are registered in the sensitive detector class RMC01SD that is a typical G4 sensitive detector class
|
||||
used in a forward simulation and is not modified at all
|
||||
for the adjoint simulation mode.
|
||||
The analysis of the registered hits during forward events is done by the RMCO1AnalysisManager.
|
||||
That is the class that illustrates how to adapt an analysis code of a fwd simulation in order to use it also for
|
||||
an adjoint simulation.
|
||||
In this class during a forward simulation the method EndOfEventForForwardSimulation is used at the end of an event
|
||||
while during an adjoint simulation at the end of fwd tracking event the method EndOfEventForAdjointSimulation is called.
|
||||
By looking at the source of RMCO1AnalysisManager and more particularly to its method EndOfEventForAdjointSimulation the user will
|
||||
learn how to adapt its G4 analysis code for an adjoint simulation.
|
||||
|
||||
The outputs of an adjoint simulation are:
|
||||
|
||||
-The total energy deposited and particle current entering the sensitive cylinder normalized
|
||||
automatically to a user defined primary spectrum(exponential or power law) .
|
||||
These results are stored in the files:
|
||||
-Adj_Edep_vs_EkinPrim.txt
|
||||
-Adj_ElectronCurrent.txt
|
||||
-Adj_GammaCurrent.txt
|
||||
-Adj_ProtonCurrent.txt
|
||||
-ConvergenceOfAdjointSimulationResults.txt:
|
||||
The total normalized edep and its relative error registered every 5000 adjoint events
|
||||
|
||||
|
||||
-The answer matrix of the energy deposited and particles current on the sensitive cylinder in function of primary energy of e-, gamma and
|
||||
protons. These results are stored in the files Adj********_Answer.txt
|
||||
|
||||
|
||||
|
||||
The outputs of a forward simulation are:
|
||||
-The mean energy deposited and particle current entering the sensitive cylinder per event.
|
||||
These results are stored in the files:
|
||||
-Fwd_Edep_vs_EkinPrim.txt
|
||||
-Fwd_ElectronCurrent.txt
|
||||
-Fwd_GammaCurrent.txt
|
||||
-Fwd_ProtonCurrent.txt
|
||||
-ConvergenceOfAdjointSimulationResults.txt: The total normalized edep and its relative error registered every 5000 adjoint events
|
||||
|
||||
|
||||
|
||||
3.4. Run macrofiles:
|
||||
------------------
|
||||
The following example run macro files are distributed with the code:
|
||||
|
||||
-run_adjoint_simulation_electron.mac and run_adjoint_simulation_proton.mac for adjoint simulations
|
||||
|
||||
-run_forward_simulation_electron.mac and run_forward_simulation_proton.mac for forward simulations
|
||||
|
||||
|
||||
3.5. Comparison of adjoint and forward simulation results:
|
||||
----------------------------------------------------------
|
||||
It is the responsibility of the user to select in the macro file the same external spectrum
|
||||
for both the forward and adjoint simulations and to normalize the per event results of the forward simulation
|
||||
to the fluence considered in the adjoint simulation.
|
||||
|
||||
For the macro files that are provided with the examples it consists into multiplying the forward results by pi*100.
|
||||
This normalization factor is explained by the following:
|
||||
|
||||
-For the forward simulation the results are given per number of events. It corresponds
|
||||
to a normalization to a fluence of 1 particle emanating from the external source.
|
||||
|
||||
-In run_fwd_simulation.mac the source is set on a sphere of 10 cm radius (see /gps commands in
|
||||
macrofile).Therefore the omnidirectional fluence for the fwd simulation is 1./(pi*R^2) with R=10cm.
|
||||
|
||||
-The adjoint results are normalized to a fluence of 1/cm2.
|
||||
(See command /RMC01/analysis/SetExponentialSpectrumForAdjointSim in macrofile)
|
||||
|
||||
-In conclusion to compare the adjoint and forward results, the forward results should
|
||||
be multiplied by pi*R^2/cm2= pi*100.
|
||||
|
||||
|
||||
|
||||
4. Control of the adjoint simulation and the RMC01 code by G4 macro UI commands:
|
||||
-------------------------------------------------------------------------
|
||||
Different G4 macro UI commands are provided to control the RMC01 example and the adjoint simulation.
|
||||
Some macro commands are provided within the geant4 toolkit and appears in a G4 application when the singleton
|
||||
class G4AdjointSimManager is called somewhere in the code, the other macro commands are
|
||||
declared in the code distributed within the example.
|
||||
|
||||
|
||||
4.1. G4UI commands in the directory /adjoint
|
||||
-----------------------------------------------
|
||||
The macro commands in the directory /adjoint appears in a user application when the singleton
|
||||
class G4AdjointSimManager is called somewhere in the code.
|
||||
It allows to control the adjoint source, the external source and start an adjoint simulation.
|
||||
|
||||
The command to start an adjoint run is:
|
||||
|
||||
-/adjoint/start_run nb
|
||||
Start an adjoint simulation with a number of events given by nb. It is important to note that the total number of events in the sense of G4
|
||||
will be nb*2*nb_primary_considered (see 3.4.)
|
||||
|
||||
|
||||
The commands to control the adjoint source are:
|
||||
|
||||
-/adjoint/DefineSphericalAdjSource R X Y Z unit_length
|
||||
The adjoint source is set on a sphere with radius R and centered on position (X,Y,Z)
|
||||
|
||||
-/adjoint/DefineSphericalAdjSourceCenteredOnAVolume phys_vol_name R unit_length
|
||||
The external source is set on a sphere with radius R and with its center position located at the center of the
|
||||
the physical volume specified by the name phys_vol_name.
|
||||
-/adjoint/DefineAdjSourceOnExtSurfaceOfAVolume phys_vol_name
|
||||
The external surface is set as the external boundary of a the physical volume with name phys_vol_name
|
||||
|
||||
-/adjoint/SetAdjSourceEmin Emin energy_unit
|
||||
Set the minimum energy of the external source
|
||||
|
||||
-/adjoint/SetAdjSourceEmax Emax energy_unit
|
||||
Set the maximum energy of the external source
|
||||
|
||||
-/adjoint/ConsiderAsPrimary particle_name
|
||||
The type of particle specified by "particle_name" will be added in the list of primary adjoint particles.
|
||||
The list of candidates depends on the reverse physics processes considered in the simulation. At the most the
|
||||
potential candidates are (e-, gamma, proton , ion). For this example only e-, gamma, proton
|
||||
can be chosen. As the proton ionization is not considered by default, the default list of particles is
|
||||
[e-,gamma]. To have also the proton as candidate the proton ionization should
|
||||
be switch on (/adjoint_physics/UseProtonIonisation true).
|
||||
|
||||
-/adjoint/NeglectAsPrimary particle_name
|
||||
The type of particle specified by "particle_name" will be removed from the list of primary adjoint particles.
|
||||
The list of candidates depends on the reverse physics processes considered in the simulation. At the most the
|
||||
potential candidates are (e-, gamma, proton , ion). For this example only e-, gamma, proton
|
||||
can be chosen. As the proton ionization is not considered by default, the default list of particles is
|
||||
[e-,gamma].To have also the proton as candidate the proton ionization should
|
||||
be switch on (/adjoint_physics/UseProtonIonisation true).
|
||||
|
||||
|
||||
The commands to control the external source are:
|
||||
|
||||
-/adjoint/DefineSphericalExtSource R X Y Z unit_length:
|
||||
The external source is set on a sphere with radius R and centered on position (X,Y,Z)
|
||||
|
||||
-/adjoint/DefineSphericalExtSourceCenteredOnAVolume phys_vol_name R unit_length
|
||||
The external source is set on a sphere with radius R and with its center position located at the center of the
|
||||
the physical volume specified by the name phys_vol_name.
|
||||
|
||||
-/adjoint/DefineExtSourceOnExtSurfaceOfAVolume phys_vol_name
|
||||
The external surface is set as the external boundary of a the physical volume with name phys_vol_name
|
||||
|
||||
-/adjoint/SetExtSourceEmax Emax energy_unit
|
||||
Set the maximum energy of the external source. An adjoint track will be stop when a an adjoint particle get an energy higher than this maximum energy.
|
||||
|
||||
|
||||
|
||||
4.2. G4UI commands in the directory /adjoint_physics
|
||||
------------------------------------------------------
|
||||
These commands allow to control the electromagnetic processes that will be considered in the simulation.
|
||||
|
||||
The processes that can be used are:
|
||||
-Reverse and forward e- continuous and discrete Ionization. Always switch on
|
||||
-Reverse and forward e- Bremsstrahlung. Switch on by default
|
||||
-Reverse and forward Compton scattering. Switch on by default
|
||||
-Reverse and forward photo electric effect. Switch on by default
|
||||
-Reverse and forward photo electric effect. Switch on by default
|
||||
-Reverse and forward multiple scattering. Switch on by default
|
||||
-Reverse and forward proton continuous and discrete Ionization. Switch off by default
|
||||
-Forward e-e+ pair production. Switch off by default.
|
||||
If switch all the e+ electromagnetic physics is considered.
|
||||
|
||||
|
||||
The commands that can be used to switch on of these processes are:
|
||||
|
||||
/adjoint_physics/UseProtonIonisation true/false
|
||||
-Switch on/off the reverse and forward proton ionization. Off by default.
|
||||
|
||||
/adjoint_physics/UseBremsstrahlung true/false
|
||||
-Switch on/off the reverse and forward e- bremsstrahlung. On by default.
|
||||
|
||||
/adjoint_physics/UseCompton true/false
|
||||
-Switch on/off the Compton scattering. On by default.
|
||||
|
||||
|
||||
/adjoint_physics/UseMS true/false
|
||||
-Switch on/off the multiple scattering. On by default.
|
||||
|
||||
|
||||
/adjoint_physics/UseEgainElossFluctuation true/false
|
||||
-Switch on/off the fluctuation in the continuous energy loss/gain. On by default. Only for test purpose.
|
||||
|
||||
/adjoint_physics/UsePEEffect true/false
|
||||
-Switch on/off the photo electric effect. On by default.
|
||||
|
||||
|
||||
/adjoint_physics/UseGammaConversion true/false
|
||||
-Switch on/off the forward e-e+ pair production from gamma. Off by default. When On all the e+
|
||||
electromagnetic physics is considered.
|
||||
|
||||
|
||||
The user can also fix the maximum energy Emax and minimum energy Emin of the adjoint physical processes used
|
||||
in the simulation. The adjoint process will be applied to particles within the energy range [Emin, Emax]
|
||||
and will produce adjoint secondary only in this energy range. It is recommended to fix Emin to the minimum
|
||||
energy of the adjoint source and fix Emax to the maximum energy of the external source.
|
||||
The commands controlling Emin and Emax are:
|
||||
|
||||
/adjoint_physics/SetEminForAdjointModels Emin Energy_unit
|
||||
-Set the minimum energy of the adjoint processes/models.
|
||||
|
||||
/adjoint_physics/SetEmaxForAdjointModels Emin Energy_unit
|
||||
-Set the maximum energy of the adjoint processes/models.
|
||||
|
||||
|
||||
4.3. G4UI commands in the directory /RMC01
|
||||
----------------------------------------------
|
||||
|
||||
Commands/RMC01/geometry/ to control the geometry:
|
||||
|
||||
/RMC01/geometry/SetSensitiveVolumeHeight H length_unit
|
||||
Set the height H of the Si sensitive cylinder.
|
||||
|
||||
|
||||
/RMC01/geometry/SetSensitiveVolumeRadius R length_unit
|
||||
Set the radius R of the Si sensitive cylinder.
|
||||
|
||||
/RMC01/geometry/SetShieldingThickness D length_unit
|
||||
Set the thickness D of the aluminum shielding.
|
||||
|
||||
Commands /RMC01/analysis/ to control the primary spectrum used for the normalization of the
|
||||
adjoint simulation results and fix the expected precision of the computed Edep:
|
||||
|
||||
/RMC01/analysis/SetPowerLawPrimSpectrumForAdjointSim particle_name F F_unit alpha Emin Emax E_unit
|
||||
Set the primary spectrum to which the adjoint simulation results will be normalised to a power law
|
||||
spectrum E^(-alpha) of particle defined by particle_name, with an omnidirectional fluence F, and
|
||||
energy range [Emin,Emax]. The fluence unit candidates for F_unit are [1/cm2, 1/m2, cm-2, m-2].
|
||||
|
||||
|
||||
/RMC01/analysis/SetExponentialSpectrumForAdjointSim particle_name F F_unit E0 Emin Emax E_unit
|
||||
Set the primary spectrum to which the adjoint simulation results will be normalised to an exponential
|
||||
spectrum exp(-E/E0) of particle defined by particle_name, with an omnidirectional fluence F, and
|
||||
energy range [Emin,Emax]. The fluence unit candidates for F_unit are [1/cm2, 1/m2, cm-2, m-2].
|
||||
|
||||
|
||||
|
||||
/RMC01/analysis/SetExpectedPrecisionOfResults precision
|
||||
Set the expected precision in % for the computed energy deposited in the sensitive volume
|
||||
for both the forward and adjoint simulation case. When the relative statistical error
|
||||
of the computed energy deposited reach this precision the run is aborted and the results are registered.
|
||||
Otherwise the run continue till the nb of events specified by the user are processed. By default the precision is set
|
||||
to 0. meaning that the run will not be aborted in this case.
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
5. Known issues
|
||||
--------------------------------
|
||||
--------------------------------
|
||||
|
||||
5.1 Rare too high weight in the adjoint simulation
|
||||
---------------------------------------------------
|
||||
|
||||
In rare cases an adjoint track may get a much too high weight when reaching the external source.
|
||||
While this happen not often it may corrupt the simulation results significantly. The reason of this high weight is
|
||||
the joint use at low e- and gamma energy of both the photoelectric and bremsstrahlung processes.
|
||||
Unfortunately we still need some investigations to remove this problem at the level of physical processes.
|
||||
However this problem can be solved at the level of event action in the user code by adding a test on the adjoint
|
||||
weight. Such test has been implemented in the example RMC01.
|
||||
In this implementation an event is rejected when the relative error of the computed normalised edep
|
||||
increase during one event by more than 50% when the precision is already below 10%.
|
||||
|
||||
|
||||
5.2 Limitation of the reverse bremsstrahlung
|
||||
-------------------------------------------
|
||||
The difference between the differential cross sections used in the adjoint and forward bremsstrahlung
|
||||
models is the source of a higher flux of >100 keV gamma in the reverse simulation compared to the forward simulation.
|
||||
The adjoint processes/models should make use of the direct differential cross section to sample
|
||||
the adjoint secondaries and compute the adjoint cross section.
|
||||
The differential cross section used in G4AdjointeBremstrahlungModel is obtained by the numerical derivation
|
||||
over the cut energy of the direct cross section provided by G4eBremsstrahlungModel.
|
||||
This would be a correct procedure if the distribution of secondary in G4eBremsstrahlungModel
|
||||
would match this differential cross section. Unfortunately it is not the case as independent parameterization are used
|
||||
in G4eBremsstrahlungModel for both the cross sections and the sample of secondary. (It means that in the forward case
|
||||
if one would integrate the effective differential cross section considered in the simulation we would not find back
|
||||
the used cross section).
|
||||
In the future we plan to correct this problem by using an extra weight correction factor after the occurrence of a reverse
|
||||
bremsstrahlung. This weight factor should be the ratio between the differential CS used in the adjoint simulation and the
|
||||
one effectively used in the forward processes. As it is impossible to have access to the forward differential CS
|
||||
in G4eBremsstrahlungModel we are investigating the feasibility to use the differential CS considered in
|
||||
G4Penelope models.
|
||||
|
||||
|
||||
5.3 Limitation of the reverse multiple scattering
|
||||
-------------------------------------------------
|
||||
For the reverse multiple scattering we are using the same models than for the forward case.
|
||||
This approximation makes that the discrepancy between the adjoint and forward
|
||||
simulation cases can get to a level of ~ 10-15% relative differences in the test cases that we have considered.
|
||||
In the future we plan to improve the adjoint multiple scattering models by forcing the computation of
|
||||
multiple scattering effect at the end of an adjoint step.
|
||||
Reference in New Issue
Block a user