Gabriele Cosmo
101 lines
5.0 KiB
Plaintext
101 lines
5.0 KiB
Plaintext
Example GB06: parallel geometries with generic biasing
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This example demonstrates the use of parallel geometries in generic biasing,
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on a classical shield problem, using geometry-based importance biasing.
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1) Geometry and activation of navigation in parallel world:
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--------------------------------------------------------
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The geometry is made of two parts:
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- the mass (standard) geometry, which is made of a single block of
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concrete ; this is implemented in GB06DetectorConstuction ;
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- a parallel geometry, in which a series of slices is defined, these
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slices being created using a replica volume ; this is implemeted in
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GB06ParallelGeometryForSlices, which derives from the base class
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G4VUserParallelWorld .
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The navigation in the parallel geometry is activated for neutrons. This is
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done in the main program exampleGB06.cc. The activation is made using the
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facilities of the G4GenericBiasingPhysics class, as:
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biasingPhysics->AddParallelGeometry("neutron",
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"parallelWorldForSlices");
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where the first name is for the particle type to be aware of the parallel word,
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the second argument is the name of the parallel world.
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When checking the process list of neutrons (/particle/select neutron and
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then /particle/process dump ) a new process, `biasingLimiter', is visible. This
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process handles the step limitation in the parallel geometry. This process can
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handle several parallel geometries, these being passed to the process as
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biasingPhysics->AddParallelGeometry("neutron", "parallelWorld1") ,
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biasingPhysics->AddParallelGeometry("neutron", "parallelWorld2") , etc.
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The geometry-based importance technique utilizes only splitting and killing,
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hence techniques which are "non-physics biasing" techniques, in the sense they
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don't modify the behavior of physics processes. For this reason, only a process
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making the interface between the tracking and the biaising is inserted in the
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physics list, the physics processes themselves being untouched, this is made as:
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biasingPhysics->NonPhysicsBias("neutron");
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Finally, the volume (ie the slice) importances are defined in a simple
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"importance map" that is created in the GB06ParallelGeometryForSlices class, this
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map associating a replica number to a volume importance. The map is hold by the
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biasing operator.
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2) Biasing classes:
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----------------
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As usual, with the generic biasing scheme, a biasing operator and a biasing
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operation are defined, these are, respectively the
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GB06BOptrSplitAndKillByImportance and
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GB06BOptnSplitAndKillByImportance
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classes. The operator here only handles one particle type. In the StartRun()
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method, it configures the biasing operation GB06BOptnSplitAndKillByImportance
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passing it the information related to the parallel geometry, and passing it the
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importance map.
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The biasing operation GB06BOptnSplitAndKillByImportance applies a classical
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importance-based geometry technique, with spliting / killing at the slice
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bondaries. Splitting is made if the track goes from a smaller importance to a
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volume of larger importance, and killing (Russian roulette) is applied in the
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other case.
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The particularity of this biasing operation is its handling of the parallel
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geometry information. It has to get by itself geometry information that, in the
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case of information of the mass geometry, are provided in the G4StepPoint objects
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(pre step point, post step point) of the G4Step. Here, in the
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DistanceToApplyOperation(...), which is called at the beginning of the step, it
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gets a "snapshot" of the geometry state keeping a G4TouchableHistoryHandle. Then
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in the GenerateBiasingFinalState, which is called at the end of the step, it gets
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the new geometry state, with an other G4TouchableHistoryHandle. For a step that
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ends on the boundary, this last touchable history will logically point to the
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next volume. In this case, the biasing is applied, and the importances are
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obtained from the replica numbers taken from the two touchable histories, and
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then from the importance map.
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3) Output:
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-------
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A simple sensitive detector is defined (GB06SD) and is attached to a thin
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volume ("meas.logical") placed after the concrete shield. This sensitive
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detector simply prints the information (particle type, kinetic energy, etc,
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and weight) leaving the shield.
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4) Known problems:
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---------------
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In exampleGB06.in the neutron killer process, nKiller, is de-activated
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(process that kills neutrons after some time), for two reasons. First, killing
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neutrons in a shield problem is not desirable because neutrons may fly for some
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time before leaving the shield, and hence must be accounted for. Second, if
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nKiller is left active, an exception message about a spurious displacement by
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1e-7mm will appear sometimes : this happens when a neutron is killed on a volume
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boundary, and the navigation "sees" a (tiny) displacement, that should not exist.
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