[904] | 1 | <!-- ******************************************************** --> |
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| 2 | <!-- --> |
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| 3 | <!-- [History] --> |
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[1222] | 4 | <!-- New section on Reverse MC: L. Desorgher, Dec-2009 --> |
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[904] | 5 | <!-- Converted to DocBook: Katsuya Amako, Aug-2006 --> |
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| 6 | <!-- Changed by: Dennis Wright, 25-Jun-2002 --> |
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| 7 | <!-- --> |
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| 8 | <!-- ******************************************************** --> |
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| 9 | |
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| 10 | |
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| 11 | <!-- ******************* Section (Level#1) ****************** --> |
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| 12 | <sect1 id="sect.EvtBias"> |
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| 13 | <title> |
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| 14 | Event Biasing Techniques |
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| 15 | </title> |
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| 16 | |
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| 17 | <!-- ******************* Section (Level#2) ****************** --> |
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| 18 | <sect2 id="sect.EvtBias.ScorImpRoul"> |
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| 19 | <title> |
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| 20 | Scoring, Geometrical Importance Sampling and Weight Roulette |
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| 21 | </title> |
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| 22 | |
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| 23 | <para> |
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| 24 | Geant4 provides event biasing techniques which may be used to save |
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| 25 | computing time in such applications as the simulation of radiation |
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| 26 | shielding. These are <emphasis>geometrical splitting |
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| 27 | </emphasis> and <emphasis>Russian roulette</emphasis> |
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| 28 | (also called geometrical importance sampling), and |
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| 29 | <emphasis>weight roulette</emphasis>. Scoring is carried out by <emphasis>G4MultiFunctionalDetector</emphasis> (see <xref |
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| 30 | linkend="sect.Hits.G4Multi" /> and <xref linkend="sect.Hits.G4VPrim" />) using the standard Geant4 scoring technique. |
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| 31 | Biasing specific scorers have been implemented and are described within |
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| 32 | <emphasis>G4MultiFunctionDetector</emphasis> documentation. In this chapter, it is assumed that |
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| 33 | the reader is familiar with both the usage of Geant4 and the |
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| 34 | concepts of importance sampling. More detailed documentation may be |
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| 35 | found in the documents |
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[1211] | 36 | <ulink url="http://geant4.cern.ch/collaboration/working_groups/geometry/biasing/Sampling.html"> |
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| 37 | 'Scoring, geometrical importance sampling and weight roulette' |
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[904] | 38 | </ulink>. |
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| 39 | A detailed description of different use-cases which employ the sampling |
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| 40 | and scoring techniques can be found in the document |
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[1211] | 41 | <ulink url="http://geant4.cern.ch/collaboration/working_groups/geometry/biasing/BiasScoreUseCases.html"> |
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| 42 | 'Use cases of importance sampling and scoring in Geant4' |
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[904] | 43 | </ulink>. |
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| 44 | </para> |
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| 45 | |
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| 46 | <para> |
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| 47 | The purpose of importance sampling is to save computing time by |
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| 48 | sampling less often the particle histories entering "less |
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| 49 | important" geometry regions, and more often in more "important" |
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| 50 | regions. Given the same amount of computing time, an |
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| 51 | importance-sampled and an analogue-sampled simulation must show |
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| 52 | equal mean values, while the importance-sampled simulation will |
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| 53 | have a decreased variance. |
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| 54 | </para> |
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| 55 | |
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| 56 | <para> |
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| 57 | The implementation of scoring is independent of the implementation |
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| 58 | of importance sampling. However both share common concepts. |
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| 59 | <emphasis>Scoring and importance sampling apply to particle types chosen |
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| 60 | by the user</emphasis>, which should be borne in mind when interpreting the |
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| 61 | output of any biased simulation. |
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| 62 | </para> |
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| 63 | |
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| 64 | <para> |
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| 65 | Examples on how to use scoring and importance sampling may be found |
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[1211] | 66 | in <literal>examples/extended/biasing</literal>. |
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[904] | 67 | </para> |
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| 68 | |
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| 69 | |
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| 70 | <!-- ******************* Section (Level#3) ****************** --> |
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| 71 | <sect3 id="sect.EvtBias.ScorImpRoul.Geom"> |
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| 72 | <title> |
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| 73 | Geometries |
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| 74 | </title> |
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| 75 | |
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| 76 | <para> |
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| 77 | The kind of scoring referred to in this note and the importance |
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| 78 | sampling apply to spatial cells provided by the user. |
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| 79 | </para> |
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| 80 | |
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| 81 | <para> |
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| 82 | A <emphasis role="bold">cell</emphasis> is a physical volume (further specified |
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| 83 | by it's replica number, if the volume is a replica). Cells may be defined |
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| 84 | in two kinds of geometries: |
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| 85 | |
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| 86 | <orderedlist spacing="compact"> |
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| 87 | <listitem><para> |
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| 88 | <emphasis role="bold">mass geometry</emphasis>: the geometry setup of the |
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| 89 | experiment to be simulated. Physics processes apply to this geometry. |
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| 90 | </para></listitem> |
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| 91 | <listitem><para> |
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| 92 | <emphasis role="bold">parallel-geometry</emphasis>: a geometry constructed |
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| 93 | to define the physical volumes according to which scoring and/or importance |
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| 94 | sampling is applied. |
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| 95 | </para></listitem> |
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| 96 | </orderedlist> |
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| 97 | </para> |
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| 98 | |
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| 99 | <para> |
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| 100 | The user has the choice to score and/or sample by importance the |
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| 101 | particles of the chosen type, according to mass geometry or to |
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| 102 | parallel geometry. It is possible to utilize several parallel |
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| 103 | geometries in addition to the mass geometry. This provides the user |
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| 104 | with a lot of flexibility to define separate geometries for |
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| 105 | different particle types in order to apply scoring or/and |
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| 106 | importance sampling. |
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| 107 | </para> |
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| 108 | |
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| 109 | <para> |
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| 110 | <note> |
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| 111 | Parallel geometries should be constructed using the implementation as |
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| 112 | described in <xref linkend="sect.ParaGeom"/>. |
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| 113 | |
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| 114 | There are a few conditions for parallel geometries: |
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| 115 | |
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| 116 | <itemizedlist spacing="compact"> |
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| 117 | <listitem><para> |
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| 118 | The world volume for parallel and mass geometries must be identical copies. |
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| 119 | </para></listitem> |
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| 120 | <listitem><para> |
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| 121 | Scoring and importance cells must not share boundaries with the world volume. |
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| 122 | </para></listitem> |
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| 123 | </itemizedlist> |
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| 124 | </note> |
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| 125 | </para> |
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| 126 | |
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| 127 | </sect3> |
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| 128 | |
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| 129 | <!-- ******************* Section (Level#3) ****************** --> |
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| 130 | <sect3 id="sect.EvtBias.ScorImpRoul.ChgSamp"> |
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| 131 | <title> |
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| 132 | Changing the Sampling |
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| 133 | </title> |
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| 134 | |
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| 135 | <para> |
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| 136 | Samplers are higher level tools which perform the necessary |
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| 137 | changes of the Geant4 sampling in order to apply importance |
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| 138 | sampling and weight roulette. |
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| 139 | </para> |
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| 140 | |
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| 141 | <para> |
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| 142 | Variance reduction (and scoring through the |
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| 143 | <emphasis>G4MultiFunctionalDetector</emphasis>) |
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| 144 | may be combined arbitrarily for chosen particle types and may be applied to the |
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| 145 | mass or to parallel geometries. |
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| 146 | </para> |
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| 147 | |
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| 148 | <para> |
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| 149 | The <literal>G4GeometrySampler</literal> can be applied equally to mass or |
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| 150 | parallel geometries with an abstract interface supplied by <literal>G4VSampler</literal>. |
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| 151 | <literal>G4VSampler</literal> provides |
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| 152 | <literal>Prepare...</literal> methods and a <literal>Configure</literal> |
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| 153 | method: |
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| 154 | |
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| 155 | <anchor id="anchor_EvtBias_G4VSampler" /> |
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| 156 | <informalexample> |
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| 157 | <programlisting> |
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| 158 | class G4VSampler |
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| 159 | { |
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| 160 | public: |
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| 161 | G4VSampler(); |
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| 162 | virtual ~G4VSampler(); |
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| 163 | virtual void PrepareImportanceSampling(G4VIStore *istore, |
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| 164 | const G4VImportanceAlgorithm |
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| 165 | *ialg = 0) = 0; |
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| 166 | virtual void PrepareWeightRoulett(G4double wsurvive = 0.5, |
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| 167 | G4double wlimit = 0.25, |
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| 168 | G4double isource = 1) = 0; |
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| 169 | virtual void PrepareWeightWindow(G4VWeightWindowStore *wwstore, |
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| 170 | G4VWeightWindowAlgorithm *wwAlg = 0, |
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| 171 | G4PlaceOfAction placeOfAction = |
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| 172 | onBoundary) = 0; |
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| 173 | virtual void Configure() = 0; |
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| 174 | virtual void ClearSampling() = 0; |
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| 175 | virtual G4bool IsConfigured() const = 0; |
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| 176 | }; |
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| 177 | </programlisting> |
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| 178 | </informalexample> |
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| 179 | </para> |
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| 180 | |
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| 181 | <para> |
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| 182 | The methods for setting up the desired combination need specific |
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| 183 | information: |
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| 184 | |
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| 185 | <itemizedlist spacing="compact"> |
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| 186 | <listitem><para> |
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| 187 | Importance sampling: message <literal>PrepareImportanceSampling</literal> |
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| 188 | with a |
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| 189 | <link linkend="anchor_EvtBias_G4VIStore"> |
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| 190 | <literal>G4VIStore</literal></link> |
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| 191 | and optionally a <literal>G4VImportanceAlgorithm</literal> |
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| 192 | </para></listitem> |
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| 193 | <listitem><para> |
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| 194 | Weight window: message <literal>PrepareWeightWindow</literal> with the |
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| 195 | arguments: |
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| 196 | <itemizedlist spacing="compact"> |
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| 197 | <listitem><para> |
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| 198 | <emphasis>*wwstore</emphasis>: a |
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| 199 | <literal>G4VWeightWindowStore</literal> for retrieving the lower |
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| 200 | weight bounds for the energy-space cells |
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| 201 | </para></listitem> |
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| 202 | <listitem><para> |
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| 203 | <emphasis>*wwAlg</emphasis>: a |
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| 204 | <literal>G4VWeightWindowAlgorithm</literal> if a customized algorithm |
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| 205 | should be used |
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| 206 | </para></listitem> |
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| 207 | <listitem><para> |
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| 208 | <emphasis>placeOfAction</emphasis>: a |
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| 209 | <literal>G4PlaceOfAction</literal> specifying where to perform the |
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| 210 | biasing |
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| 211 | </para></listitem> |
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| 212 | </itemizedlist> |
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| 213 | </para></listitem> |
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| 214 | <listitem><para> |
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| 215 | Weight roulette: message <literal>PrepareWeightRoulett</literal> with the |
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| 216 | optional parameters: |
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| 217 | <itemizedlist spacing="compact"> |
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| 218 | <listitem><para> |
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| 219 | <emphasis>wsurvive</emphasis>: survival weight |
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| 220 | </para></listitem> |
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| 221 | <listitem><para> |
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| 222 | <emphasis>wlimit</emphasis>: minimal allowed |
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| 223 | value of weight * source importance / cell importance |
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| 224 | </para></listitem> |
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| 225 | <listitem><para> |
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| 226 | <emphasis>isource</emphasis>: importance of the source cell |
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| 227 | </para></listitem> |
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| 228 | </itemizedlist> |
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| 229 | </para></listitem> |
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| 230 | </itemizedlist> |
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| 231 | </para> |
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| 232 | |
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| 233 | <para> |
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| 234 | Each object of a sampler class is responsible for one particle |
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| 235 | type. The particle type is given to the constructor of the sampler |
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| 236 | classes via the particle type name, e.g. "neutron". Depending on |
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| 237 | the specific purpose, the <literal>Configure()</literal> of a sampler will |
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| 238 | set up specialized processes (derived from <literal>G4VProcess</literal>) for |
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| 239 | transportation in the parallel geometry, importance |
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| 240 | sampling and weight roulette for the given particle type. When |
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| 241 | <literal>Configure()</literal> is invoked the sampler places the processes in |
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| 242 | the correct order independent of the order in which user invoked |
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| 243 | the <literal>Prepare...</literal> methods. |
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| 244 | </para> |
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| 245 | |
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| 246 | <para> |
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| 247 | <note> |
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| 248 | <para> |
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| 249 | <itemizedlist spacing="compact"> |
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| 250 | <listitem><para> |
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| 251 | The <literal>Prepare...()</literal> functions may each only be invoked |
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| 252 | once. |
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| 253 | </para></listitem> |
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| 254 | <listitem><para> |
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| 255 | To configure the sampling the function <literal>Configure()</literal> |
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| 256 | must be called <emphasis>after</emphasis> the |
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| 257 | <literal>G4RunManager</literal> has been initialized and the PhysicsList has |
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| 258 | been instantiated. |
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| 259 | </para></listitem> |
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| 260 | </itemizedlist> |
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| 261 | </para> |
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| 262 | </note> |
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| 263 | </para> |
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| 264 | |
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| 265 | |
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| 266 | |
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| 267 | |
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| 268 | <para> |
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| 269 | The interface and framework are demonstrated in the |
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| 270 | <literal>examples/extended/biasing</literal> directory, with the main changes being to the |
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| 271 | G4GeometrySampler class and the fact that in the parallel case the WorldVolume |
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| 272 | is a copy of the Mass World. |
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| 273 | |
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| 274 | The parallel geometry now has to inherit from |
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| 275 | <emphasis>G4VUserParallelWorld</emphasis> which also has the <emphasis>GetWorld()</emphasis> method |
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| 276 | in order to retrieve a copy of the mass geometry WorldVolume. |
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| 277 | |
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| 278 | <informalexample> |
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| 279 | <programlisting> |
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| 280 | class B02ImportanceDetectorConstruction : public G4VUserParallelWorld |
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| 281 | ghostWorld = GetWorld(); |
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| 282 | </programlisting> |
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| 283 | </informalexample> |
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| 284 | |
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| 285 | </para> |
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| 286 | |
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| 287 | <para> |
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| 288 | The constructor for <emphasis>G4GeometrySampler</emphasis> takes a pointer to |
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| 289 | the physical world volume and the particle type name (e.g. "neutron") as arguments. |
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| 290 | In a single mass geometry the sampler is created as follows: |
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| 291 | |
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| 292 | <informalexample> |
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| 293 | <programlisting> |
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| 294 | G4GeometrySampler mgs(detector->GetWorldVolume(),"neutron"); |
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| 295 | mgs.SetParallel(false); |
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| 296 | </programlisting> |
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| 297 | </informalexample> |
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| 298 | |
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| 299 | |
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| 300 | Whilst the following lines of code are required in order to set up the sampler for the |
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| 301 | parallel geometry case: |
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| 302 | |
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| 303 | <informalexample> |
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| 304 | <programlisting> |
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| 305 | G4VPhysicalVolume* ghostWorld = pdet->GetWorldVolume(); |
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| 306 | |
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| 307 | G4GeometrySampler pgs(ghostWorld,"neutron"); |
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| 308 | |
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| 309 | pgs.SetParallel(true); |
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| 310 | </programlisting> |
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| 311 | </informalexample> |
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| 312 | |
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| 313 | |
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| 314 | Also note that the preparation and configuration of the samplers has to be |
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| 315 | carried out <emphasis>after</emphasis> the instantiation of the UserPhysicsList |
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| 316 | and after the initialisation of the <emphasis>G4RunManager</emphasis>: |
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| 317 | |
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| 318 | <informalexample> |
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| 319 | <programlisting> |
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| 320 | pgs.PrepareImportanceSampling(&aIstore, 0); |
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| 321 | pgs.Configure(); |
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| 322 | </programlisting> |
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| 323 | </informalexample> |
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| 324 | |
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| 325 | Due to the fact that biasing is a process and has to be inserted after all the |
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| 326 | other processes have been created. |
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| 327 | </para> |
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| 328 | |
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| 329 | |
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| 330 | </sect3> |
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| 331 | |
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| 332 | <!-- ******************* Section (Level#3) ****************** --> |
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| 333 | <sect3 id="sect.EvtBias.ScorImpRoul.ImpSamp"> |
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| 334 | <title> |
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| 335 | Importance Sampling |
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| 336 | </title> |
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| 337 | |
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| 338 | <para> |
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| 339 | Importance sampling acts on particles crossing boundaries |
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| 340 | between "importance cells". The action taken depends on the |
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| 341 | importance values assigned to the cells. In general a particle |
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| 342 | history is either split or Russian roulette is played if the |
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| 343 | importance increases or decreases, respectively. A weight assigned |
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| 344 | to the history is changed according to the action taken. |
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| 345 | </para> |
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| 346 | |
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| 347 | <para> |
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| 348 | The tools provided for importance sampling require the user to |
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| 349 | have a good understanding of the physics in the problem. This is |
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| 350 | because the user has to decide which particle types require |
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| 351 | importance sampled, define the cells, and assign importance values |
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| 352 | to the cells. If this is not done properly the results cannot be |
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| 353 | expected to describe a real experiment. |
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| 354 | </para> |
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| 355 | |
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| 356 | <para> |
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| 357 | The assignment of importance values to a cell is done using an |
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| 358 | importance store described below. |
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| 359 | </para> |
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| 360 | |
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| 361 | <para> |
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| 362 | An "importance store" with the interface <literal>G4VIStore</literal> is |
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| 363 | used to store importance values related to cells. In order to do |
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| 364 | importance sampling the user has to create an object (e.g. of class |
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| 365 | <literal>G4IStore</literal>) of type <literal>G4VIStore</literal>. The samplers may be |
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| 366 | given a <literal>G4VIStore</literal>. The user fills the store with cells and |
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| 367 | their importance values. |
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| 368 | </para> |
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| 369 | |
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| 370 | <para> |
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| 371 | An importance store has to be constructed with a reference to |
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| 372 | the world volume of the geometry used for importance sampling. This |
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| 373 | may be the world volume of the mass or of a parallel geometry. |
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| 374 | Importance stores derive from the interface <literal>G4VIStore</literal>: |
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| 375 | |
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| 376 | <anchor id="anchor_EvtBias_G4VIStore" /> |
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| 377 | <informalexample> |
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| 378 | <programlisting> |
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| 379 | class G4VIStore |
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| 380 | { |
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| 381 | public: |
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| 382 | G4VIStore(); |
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| 383 | virtual ~G4VIStore(); |
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| 384 | virtual G4double GetImportance(const G4GeometryCell &gCell) const = 0; |
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| 385 | virtual G4bool IsKnown(const G4GeometryCell &gCell) const = 0; |
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| 386 | virtual const G4VPhysicalVolume &GetWorldVolume() const = 0; |
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| 387 | }; |
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| 388 | </programlisting> |
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| 389 | </informalexample> |
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| 390 | </para> |
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| 391 | |
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| 392 | <para> |
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| 393 | A concrete implementation of an importance store is provided by |
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| 394 | the class <literal>G4VStore</literal>. The <emphasis>public</emphasis> |
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| 395 | part of the class is: |
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| 396 | |
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| 397 | <informalexample> |
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| 398 | <programlisting> |
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| 399 | class G4IStore : public G4VIStore |
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| 400 | { |
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| 401 | public: |
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| 402 | explicit G4IStore(const G4VPhysicalVolume &worldvolume); |
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| 403 | virtual ~G4IStore(); |
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| 404 | virtual G4double GetImportance(const G4GeometryCell &gCell) const; |
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| 405 | virtual G4bool IsKnown(const G4GeometryCell &gCell) const; |
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| 406 | virtual const G4VPhysicalVolume &GetWorldVolume() const; |
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| 407 | void AddImportanceGeometryCell(G4double importance, |
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| 408 | const G4GeometryCell &gCell); |
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| 409 | void AddImportanceGeometryCell(G4double importance, |
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| 410 | const G4VPhysicalVolume &, |
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| 411 | G4int aRepNum = 0); |
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| 412 | void ChangeImportance(G4double importance, |
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| 413 | const G4GeometryCell &gCell); |
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| 414 | void ChangeImportance(G4double importance, |
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| 415 | const G4VPhysicalVolume &, |
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| 416 | G4int aRepNum = 0); |
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| 417 | G4double GetImportance(const G4VPhysicalVolume &, |
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| 418 | G4int aRepNum = 0) const ; |
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| 419 | private: ..... |
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| 420 | }; |
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| 421 | </programlisting> |
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| 422 | </informalexample> |
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| 423 | </para> |
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| 424 | |
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| 425 | <para> |
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| 426 | The member function <literal>AddImportanceGeometryCell()</literal> enters |
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| 427 | a cell and an importance value into the importance store. The |
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| 428 | importance values may be returned either according to a physical |
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| 429 | volume and a replica number or according to a |
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| 430 | <literal>G4GeometryCell</literal>. The user must be aware of the |
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| 431 | interpretation of assigning importance values to a cell. |
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| 432 | If scoring is also implemented then this is attached to logical volumes, in |
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| 433 | which case the physical volume and replica number method should be used for |
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| 434 | assigning importance values. See <literal>examples/extended/biasing |
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| 435 | B01</literal> and <literal>B02</literal> for examples of this. |
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| 436 | </para> |
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| 437 | |
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| 438 | <para> |
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| 439 | <note> |
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| 440 | <para> |
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| 441 | <itemizedlist spacing="compact"> |
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| 442 | <listitem><para> |
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| 443 | An importance value must be assigned to every cell. |
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| 444 | </para></listitem> |
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| 445 | </itemizedlist> |
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| 446 | </para> |
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| 447 | </note> |
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| 448 | </para> |
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| 449 | |
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| 450 | <para> |
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| 451 | The different cases: |
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| 452 | |
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| 453 | <itemizedlist spacing="compact"> |
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| 454 | <listitem><para> |
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| 455 | <emphasis>Cell is not in store</emphasis> |
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| 456 | <para> |
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| 457 | Not filling a certain cell in the store will cause an |
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| 458 | exception. |
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| 459 | </para> |
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| 460 | </para></listitem> |
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| 461 | <listitem><para> |
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| 462 | <emphasis>Importance value = zero</emphasis> |
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| 463 | <para> |
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| 464 | Tracks of the chosen particle type will be killed. |
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| 465 | </para> |
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| 466 | </para></listitem> |
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| 467 | <listitem><para> |
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| 468 | <emphasis>importance values > 0</emphasis> |
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| 469 | <para> |
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| 470 | Normal allowed values |
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| 471 | </para> |
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| 472 | </para></listitem> |
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| 473 | <listitem><para> |
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| 474 | <emphasis>Importance value smaller zero</emphasis> |
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| 475 | <para> |
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| 476 | Not allowed! |
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| 477 | </para> |
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| 478 | </para></listitem> |
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| 479 | </itemizedlist> |
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| 480 | </para> |
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| 481 | |
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| 482 | </sect3> |
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| 483 | |
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| 484 | <!-- ******************* Section (Level#3) ****************** --> |
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| 485 | <sect3 id="sect.EvtBias.ScorImpRoul.SampAlgor"> |
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| 486 | <title> |
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| 487 | The Importance Sampling Algorithm |
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| 488 | </title> |
---|
| 489 | |
---|
| 490 | <para> |
---|
| 491 | Importance sampling supports using a customized importance |
---|
| 492 | sampling algorithm. To this end, the sampler interface |
---|
| 493 | <link linkend="anchor_EvtBias_G4VSampler"> |
---|
| 494 | <literal>G4VSampler</literal></link> |
---|
| 495 | may be given a pointer to the interface |
---|
| 496 | <literal>G4VImportanceAlgorithm</literal>: |
---|
| 497 | |
---|
| 498 | <informalexample> |
---|
| 499 | <programlisting> |
---|
| 500 | class G4VImportanceAlgorithm |
---|
| 501 | { |
---|
| 502 | public: |
---|
| 503 | G4VImportanceAlgorithm(); |
---|
| 504 | virtual ~G4VImportanceAlgorithm(); |
---|
| 505 | virtual G4Nsplit_Weight Calculate(G4double ipre, |
---|
| 506 | G4double ipost, |
---|
| 507 | G4double init_w) const = 0; |
---|
| 508 | }; |
---|
| 509 | </programlisting> |
---|
| 510 | </informalexample> |
---|
| 511 | </para> |
---|
| 512 | |
---|
| 513 | <para> |
---|
| 514 | The method <literal>Calculate()</literal> takes the arguments: |
---|
| 515 | |
---|
| 516 | <itemizedlist spacing="compact"> |
---|
| 517 | <listitem><para> |
---|
| 518 | <emphasis>ipre, ipost</emphasis>: importance |
---|
| 519 | of the previous cell and the importance of the current cell, |
---|
| 520 | respectively. |
---|
| 521 | </para></listitem> |
---|
| 522 | <listitem><para> |
---|
| 523 | <emphasis>init_w</emphasis>: the particles weight |
---|
| 524 | </para></listitem> |
---|
| 525 | </itemizedlist> |
---|
| 526 | </para> |
---|
| 527 | |
---|
| 528 | <para> |
---|
| 529 | It returns the struct: |
---|
| 530 | |
---|
| 531 | <informalexample> |
---|
| 532 | <programlisting> |
---|
| 533 | class G4Nsplit_Weight |
---|
| 534 | { |
---|
| 535 | public: |
---|
| 536 | |
---|
| 537 | G4int fN; |
---|
| 538 | G4double fW; |
---|
| 539 | }; |
---|
| 540 | </programlisting> |
---|
| 541 | </informalexample> |
---|
| 542 | |
---|
| 543 | <itemizedlist spacing="compact"> |
---|
| 544 | <listitem><para> |
---|
| 545 | <emphasis>fN</emphasis>: the calculated |
---|
| 546 | number of particles to exit the importance sampling |
---|
| 547 | </para></listitem> |
---|
| 548 | <listitem><para> |
---|
| 549 | <emphasis>fW</emphasis>: the weight of the particles |
---|
| 550 | </para></listitem> |
---|
| 551 | </itemizedlist> |
---|
| 552 | </para> |
---|
| 553 | |
---|
| 554 | <para> |
---|
| 555 | The user may have a customized algorithm used by providing a |
---|
| 556 | class inheriting from <literal>G4VImportanceAlgorithm</literal>. |
---|
| 557 | </para> |
---|
| 558 | |
---|
| 559 | <para> |
---|
| 560 | If no customized algorithm is given to the sampler the default |
---|
| 561 | importance sampling algorithm is used. This algorithm is |
---|
| 562 | implemented in <literal>G4ImportanceAlgorithm</literal>. |
---|
| 563 | </para> |
---|
| 564 | |
---|
| 565 | </sect3> |
---|
| 566 | |
---|
| 567 | <!-- ******************* Section (Level#3) ****************** --> |
---|
| 568 | <sect3 id="sect.EvtBias.ScorImpRoul.WeightWin"> |
---|
| 569 | <title> |
---|
| 570 | The Weight Window Technique |
---|
| 571 | </title> |
---|
| 572 | |
---|
| 573 | <para> |
---|
| 574 | The weight window technique is a weight-based alternative to |
---|
| 575 | importance sampling: |
---|
| 576 | |
---|
| 577 | <itemizedlist spacing="compact"> |
---|
| 578 | <listitem><para> |
---|
| 579 | applies splitting and Russian roulette depending on space |
---|
| 580 | (cells) and energy |
---|
| 581 | </para></listitem> |
---|
| 582 | <listitem><para> |
---|
| 583 | user defines weight windows in contrast to defining importance |
---|
| 584 | values as in importance sampling |
---|
| 585 | </para></listitem> |
---|
| 586 | </itemizedlist> |
---|
| 587 | </para> |
---|
| 588 | |
---|
| 589 | <para> |
---|
| 590 | In contrast to importance sampling this technique is not weight |
---|
| 591 | blind. Instead the technique is applied according to the particle |
---|
| 592 | weight with respect to the current energy-space cell. |
---|
| 593 | </para> |
---|
| 594 | |
---|
| 595 | <para> |
---|
| 596 | Therefore the technique is convenient to apply in combination |
---|
| 597 | with other variance reduction techniques such as cross-section |
---|
| 598 | biasing and implicit capture. |
---|
| 599 | </para> |
---|
| 600 | |
---|
| 601 | <para> |
---|
| 602 | A weight window may be specified for every cell and for several |
---|
| 603 | energy regions: <emphasis>space-energy cell</emphasis>. |
---|
| 604 | |
---|
| 605 | <figure id="fig.EvtBias.WeightWindow"> |
---|
| 606 | <title> |
---|
| 607 | Weight window concept |
---|
| 608 | </title> |
---|
| 609 | |
---|
| 610 | <mediaobject> |
---|
| 611 | <imageobject role="fo"> |
---|
[921] | 612 | <imagedata fileref="./AllResources/Fundamentals/wwconcept.jpg" |
---|
| 613 | format="JPG" contentwidth="9.0cm" align="center" /> |
---|
[904] | 614 | </imageobject> |
---|
| 615 | <imageobject role="html"> |
---|
[921] | 616 | <imagedata fileref="./AllResources/Fundamentals/wwconcept.jpg" |
---|
| 617 | format="JPG" align="center" /> |
---|
[904] | 618 | </imageobject> |
---|
| 619 | <textobject> |
---|
| 620 | <phrase>Weight window concept</phrase> |
---|
| 621 | </textobject> |
---|
| 622 | </mediaobject> |
---|
| 623 | </figure> |
---|
| 624 | </para> |
---|
| 625 | |
---|
| 626 | <!-- ******* Bridgehead ******* --> |
---|
| 627 | <bridgehead renderas='sect4'> |
---|
| 628 | Weight window concept |
---|
| 629 | </bridgehead> |
---|
| 630 | |
---|
| 631 | <para> |
---|
| 632 | The user specifies a <emphasis>lower weight bound W_L</emphasis> |
---|
| 633 | for every space-energy cell. |
---|
| 634 | |
---|
| 635 | <itemizedlist spacing="compact"> |
---|
| 636 | <listitem><para> |
---|
| 637 | The upper weight bound W_U and the survival weight W_S are |
---|
| 638 | calculated as: |
---|
| 639 | <para> |
---|
| 640 | W_U = C_U <emphasis>W_L</emphasis> and |
---|
| 641 | </para> |
---|
| 642 | <para> |
---|
| 643 | W_S = C_S <emphasis>W_L</emphasis>. |
---|
| 644 | </para> |
---|
| 645 | </para></listitem> |
---|
| 646 | <listitem><para> |
---|
| 647 | The user specifies C_S and C_U once for the whole problem. |
---|
| 648 | </para></listitem> |
---|
| 649 | <listitem><para> |
---|
| 650 | The user may give different sets of energy bounds for every cell |
---|
| 651 | or one set for all geometrical cells |
---|
| 652 | </para></listitem> |
---|
| 653 | <listitem><para> |
---|
| 654 | Special case: if C_S = C_U = 1 for all energies then weight |
---|
| 655 | window is equivalent to importance sampling |
---|
| 656 | </para></listitem> |
---|
| 657 | <listitem><para> |
---|
| 658 | The user can choose to apply the technique: at boundaries, on |
---|
| 659 | collisions or on boundaries and collisions |
---|
| 660 | </para></listitem> |
---|
| 661 | </itemizedlist> |
---|
| 662 | </para> |
---|
| 663 | |
---|
| 664 | <para> |
---|
| 665 | The energy-space cells are realized by <literal>G4GeometryCell</literal> |
---|
| 666 | as in importance sampling. The cells are stored in a weight window |
---|
| 667 | store defined by <literal>G4VWeightWindowStore</literal>: |
---|
| 668 | |
---|
| 669 | <informalexample> |
---|
| 670 | <programlisting> |
---|
| 671 | class G4VWeightWindowStore { |
---|
| 672 | public: |
---|
| 673 | G4VWeightWindowStore(); |
---|
| 674 | virtual ~G4VWeightWindowStore(); |
---|
| 675 | virtual G4double GetLowerWeitgh(const G4GeometryCell &gCell, |
---|
| 676 | G4double partEnergy) const = 0; |
---|
| 677 | virtual G4bool IsKnown(const G4GeometryCell &gCell) const = 0; |
---|
| 678 | virtual const G4VPhysicalVolume &GetWorldVolume() const = 0; |
---|
| 679 | }; |
---|
| 680 | </programlisting> |
---|
| 681 | </informalexample> |
---|
| 682 | </para> |
---|
| 683 | |
---|
| 684 | <para> |
---|
| 685 | A concrete implementation is provided: |
---|
| 686 | |
---|
| 687 | <informalexample> |
---|
| 688 | <programlisting> |
---|
| 689 | class G4WeightWindowStore: public G4VWeightWindowStore { |
---|
| 690 | public: |
---|
| 691 | explicit G4WeightWindowStore(const G4VPhysicalVolume &worldvolume); |
---|
| 692 | virtual ~G4WeightWindowStore(); |
---|
| 693 | virtual G4double GetLowerWeitgh(const G4GeometryCell &gCell, |
---|
| 694 | G4double partEnergy) const; |
---|
| 695 | virtual G4bool IsKnown(const G4GeometryCell &gCell) const; |
---|
| 696 | virtual const G4VPhysicalVolume &GetWorldVolume() const; |
---|
| 697 | void AddLowerWeights(const G4GeometryCell &gCell, |
---|
| 698 | const std::vector<G4double> &lowerWeights); |
---|
| 699 | void AddUpperEboundLowerWeightPairs(const G4GeometryCell &gCell, |
---|
| 700 | const G4UpperEnergyToLowerWeightMap& |
---|
| 701 | enWeMap); |
---|
| 702 | void SetGeneralUpperEnergyBounds(const |
---|
| 703 | std::set<G4double, std::less<G4double> > & enBounds); |
---|
| 704 | |
---|
| 705 | private:: |
---|
| 706 | ... |
---|
| 707 | }; |
---|
| 708 | </programlisting> |
---|
| 709 | </informalexample> |
---|
| 710 | </para> |
---|
| 711 | |
---|
| 712 | <para> |
---|
| 713 | The user may choose equal energy bounds for all cells. In this |
---|
| 714 | case a set of upper energy bounds must be given to the store using |
---|
| 715 | the method <literal>SetGeneralUpperEnergyBounds</literal>. If a general set |
---|
| 716 | of energy bounds have been set <literal>AddLowerWeights</literal> can be used |
---|
| 717 | to add the cells. |
---|
| 718 | </para> |
---|
| 719 | |
---|
| 720 | <para> |
---|
| 721 | Alternatively, the user may chose different energy regions for |
---|
| 722 | different cells. In this case the user must provide a mapping of |
---|
| 723 | upper energy bounds to lower weight bounds for every cell using the |
---|
| 724 | method <literal>AddUpperEboundLowerWeightPairs</literal>. |
---|
| 725 | </para> |
---|
| 726 | |
---|
| 727 | <para> |
---|
| 728 | Weight window algorithms implementing the interface class |
---|
| 729 | <literal>G4VWeightWindowAlgorithm</literal> can be used to define a |
---|
| 730 | customized algorithm: |
---|
| 731 | |
---|
| 732 | <informalexample> |
---|
| 733 | <programlisting> |
---|
| 734 | class G4VWeightWindowAlgorithm { |
---|
| 735 | public: |
---|
| 736 | G4VWeightWindowAlgorithm(); |
---|
| 737 | virtual ~G4VWeightWindowAlgorithm(); |
---|
| 738 | virtual G4Nsplit_Weight Calculate(G4double init_w, |
---|
| 739 | G4double lowerWeightBound) const = 0; |
---|
| 740 | }; |
---|
| 741 | </programlisting> |
---|
| 742 | </informalexample> |
---|
| 743 | </para> |
---|
| 744 | |
---|
| 745 | <para> |
---|
| 746 | A concrete implementation is provided and used as a default: |
---|
| 747 | |
---|
| 748 | <informalexample> |
---|
| 749 | <programlisting> |
---|
| 750 | class G4WeightWindowAlgorithm : public G4VWeightWindowAlgorithm { |
---|
| 751 | public: |
---|
| 752 | G4WeightWindowAlgorithm(G4double upperLimitFaktor = 5, |
---|
| 753 | G4double survivalFaktor = 3, |
---|
| 754 | G4int maxNumberOfSplits = 5); |
---|
| 755 | virtual ~G4WeightWindowAlgorithm(); |
---|
| 756 | virtual G4Nsplit_Weight Calculate(G4double init_w, |
---|
| 757 | G4double lowerWeightBound) const; |
---|
| 758 | private: |
---|
| 759 | ... |
---|
| 760 | }; |
---|
| 761 | </programlisting> |
---|
| 762 | </informalexample> |
---|
| 763 | </para> |
---|
| 764 | |
---|
| 765 | <para> |
---|
| 766 | The constructor takes three parameters which are used to: |
---|
| 767 | calculate the upper weight bound (upperLimitFaktor), calculate the |
---|
| 768 | survival weight (survivalFaktor), and introduce a maximal number |
---|
| 769 | (maxNumberOfSplits) of copies to be created in one go. |
---|
| 770 | </para> |
---|
| 771 | |
---|
| 772 | <para> |
---|
| 773 | In addition, the inverse of the maxNumberOfSplits is used to |
---|
| 774 | specify the minimum survival probability in case of Russian |
---|
| 775 | roulette. |
---|
| 776 | </para> |
---|
| 777 | |
---|
| 778 | </sect3> |
---|
| 779 | |
---|
| 780 | <!-- ******************* Section (Level#3) ****************** --> |
---|
| 781 | <sect3 id="sect.EvtBias.ScorImpRoul.WeigRoul"> |
---|
| 782 | <title> |
---|
| 783 | The Weight Roulette Technique |
---|
| 784 | </title> |
---|
| 785 | |
---|
| 786 | <para> |
---|
| 787 | Weight roulette (also called weight cutoff) is usually applied |
---|
| 788 | if importance sampling and implicit capture are used together. |
---|
| 789 | Implicit capture is not described here but it is useful to note |
---|
| 790 | that this procedure reduces a particle weight in every collision |
---|
| 791 | instead of killing the particle with some probability. |
---|
| 792 | </para> |
---|
| 793 | |
---|
| 794 | <para> |
---|
| 795 | Together with importance sampling the weight of a particle may |
---|
| 796 | become so low that it does not change any result significantly. |
---|
| 797 | Hence tracking a very low weight particle is a waste of computing |
---|
| 798 | time. Weight roulette is applied in order to solve this |
---|
| 799 | problem. |
---|
| 800 | </para> |
---|
| 801 | |
---|
| 802 | <!-- ******* Bridgehead ******* --> |
---|
| 803 | <bridgehead renderas='sect4'> |
---|
| 804 | The weight roulette concept |
---|
| 805 | </bridgehead> |
---|
| 806 | |
---|
| 807 | <para> |
---|
| 808 | Weight roulette takes into account the importance "Ic" of the |
---|
| 809 | current cell and the importance "Is" of the cell in which the |
---|
| 810 | source is located, by using the ratio "R=Is/Ic". |
---|
| 811 | </para> |
---|
| 812 | |
---|
| 813 | <para> |
---|
| 814 | Weight roulette uses a relative minimal weight limit and a |
---|
| 815 | relative survival weight. When a particle falls below the weight |
---|
| 816 | limit Russian roulette is applied. If the particle survives, |
---|
| 817 | tracking will be continued and the particle weight will be set to |
---|
| 818 | the survival weight. |
---|
| 819 | </para> |
---|
| 820 | |
---|
| 821 | <para> |
---|
| 822 | The weight roulette uses the following parameters with their |
---|
| 823 | default values: |
---|
| 824 | |
---|
| 825 | <itemizedlist spacing="compact"> |
---|
| 826 | <listitem><para> |
---|
| 827 | <emphasis>wsurvival</emphasis>: 0.5 |
---|
| 828 | </para></listitem> |
---|
| 829 | <listitem><para> |
---|
| 830 | <emphasis>wlimit</emphasis>: 0.25 |
---|
| 831 | </para></listitem> |
---|
| 832 | <listitem><para> |
---|
| 833 | <emphasis>isource</emphasis>: 1 |
---|
| 834 | </para></listitem> |
---|
| 835 | </itemizedlist> |
---|
| 836 | </para> |
---|
| 837 | |
---|
| 838 | <para> |
---|
| 839 | The following algorithm is applied: |
---|
| 840 | </para> |
---|
| 841 | |
---|
| 842 | <para> |
---|
| 843 | If a particle weight "w" is lower than R*wlimit: |
---|
| 844 | |
---|
| 845 | <itemizedlist spacing="compact"> |
---|
| 846 | <listitem><para> |
---|
| 847 | the weight of the particle will be changed to "ws = wsurvival*R" |
---|
| 848 | </para></listitem> |
---|
| 849 | <listitem><para> |
---|
| 850 | the probability for the particle to survive is "p = w/ws" |
---|
| 851 | </para></listitem> |
---|
| 852 | </itemizedlist> |
---|
| 853 | </para> |
---|
| 854 | |
---|
| 855 | </sect3> |
---|
| 856 | </sect2> |
---|
| 857 | |
---|
| 858 | |
---|
| 859 | <!-- ******************* Section (Level#2) ****************** --> |
---|
| 860 | <sect2 id="sect.EvtBias.PhysBias"> |
---|
| 861 | <title> |
---|
| 862 | Physics Based Biasing |
---|
| 863 | </title> |
---|
| 864 | |
---|
| 865 | <para> |
---|
| 866 | Geant4 supports physics based biasing through a number of general |
---|
| 867 | use, built in biasing techniques. A utility class, |
---|
| 868 | G4WrapperProcess, is also available to support user defined |
---|
| 869 | biasing. |
---|
| 870 | </para> |
---|
| 871 | |
---|
| 872 | <!-- ******************* Section (Level#3) ****************** --> |
---|
| 873 | <sect3 id="sect.EvtBias.PhysBias.BuiltInOpt"> |
---|
| 874 | <title> |
---|
| 875 | Built in Biasing Options |
---|
| 876 | </title> |
---|
| 877 | |
---|
| 878 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 879 | <sect4 id="sect.EvtBias.PhysBias.BuiltInOpt.PrimPart"> |
---|
| 880 | <title> |
---|
| 881 | Primary Particle Biasing |
---|
| 882 | </title> |
---|
| 883 | |
---|
| 884 | <para> |
---|
| 885 | Primary particle biasing can be used to increase the number of |
---|
| 886 | primary particles generated in a particular phase space region of |
---|
| 887 | interest. The weight of the primary particle is modified as |
---|
| 888 | appropriate. A general implementation is provided through the |
---|
| 889 | G4GeneralParticleSource class. It is possible to bias position, |
---|
| 890 | angular and energy distributions. |
---|
| 891 | </para> |
---|
| 892 | |
---|
| 893 | <para> |
---|
| 894 | G4GeneralParticleSource is a concrete implementation of |
---|
| 895 | G4VPrimaryGenerator. To use, instantiate G4GeneralParticleSource in |
---|
| 896 | the G4VUserPrimaryGeneratorAction class, as demonstrated below. |
---|
| 897 | |
---|
| 898 | <informalexample> |
---|
| 899 | <programlisting> |
---|
| 900 | MyPrimaryGeneratorAction::MyPrimaryGeneratorAction() { |
---|
| 901 | generator = new G4GeneralParticleSource; |
---|
| 902 | } |
---|
| 903 | |
---|
| 904 | void |
---|
| 905 | MyPrimaryGeneratorAction::GeneratePrimaries(G4Event*anEvent){ |
---|
| 906 | generator->GeneratePrimaryVertex(anEvent); |
---|
| 907 | } |
---|
| 908 | </programlisting> |
---|
| 909 | </informalexample> |
---|
| 910 | </para> |
---|
| 911 | |
---|
| 912 | <para> |
---|
| 913 | The biasing can be configured through interactive commands. |
---|
| 914 | Extensive documentation can be found in |
---|
| 915 | <ulink url="http://reat.space.qinetiq.com/gps/"> |
---|
| 916 | Primary particle biasing</ulink>. Examples are also distributed |
---|
| 917 | with the Geant4 distribution in |
---|
| 918 | <emphasis role="bold">examples/extended/eventgenerator/exgps</emphasis>. |
---|
| 919 | </para> |
---|
| 920 | |
---|
| 921 | </sect4> |
---|
| 922 | |
---|
| 923 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 924 | <sect4 id="sect.EvtBias.PhysBias.BuiltInOpt.RadDcy"> |
---|
| 925 | <title> |
---|
| 926 | Radioactive Decay Biasing |
---|
| 927 | </title> |
---|
| 928 | |
---|
| 929 | <para> |
---|
| 930 | The G4RadioactiveDecay class simulates the decay of radioactive |
---|
| 931 | nuclei and implements the following biasing options: |
---|
| 932 | |
---|
| 933 | <itemizedlist spacing="compact"> |
---|
| 934 | <listitem><para> |
---|
| 935 | Increase the sampling rate of radionuclides within observation |
---|
| 936 | times through a user defined probability distribution function |
---|
| 937 | </para></listitem> |
---|
| 938 | <listitem><para> |
---|
| 939 | Nuclear splitting, where the parent nuclide is split into a |
---|
| 940 | user defined number of nuclides |
---|
| 941 | </para></listitem> |
---|
| 942 | <listitem><para> |
---|
| 943 | Branching ratio biasing where branching ratios are sampled with |
---|
| 944 | equal probability |
---|
| 945 | </para></listitem> |
---|
| 946 | </itemizedlist> |
---|
| 947 | </para> |
---|
| 948 | |
---|
| 949 | <para> |
---|
| 950 | G4RadioactiveDecay is a process which must be registered with a |
---|
| 951 | process manager, as demonstrated below. |
---|
| 952 | |
---|
| 953 | <informalexample> |
---|
| 954 | <programlisting> |
---|
| 955 | void MyPhysicsList::ConstructProcess() |
---|
| 956 | { |
---|
| 957 | ... |
---|
| 958 | G4RadioactiveDecay* theRadioactiveDecay = |
---|
| 959 | new G4RadioactiveDecay(); |
---|
| 960 | |
---|
| 961 | G4ProcessManager* pmanager = ... |
---|
| 962 | pmanager ->AddProcess(theRadioactiveDecay); |
---|
| 963 | ... |
---|
| 964 | } |
---|
| 965 | </programlisting> |
---|
| 966 | </informalexample> |
---|
| 967 | </para> |
---|
| 968 | |
---|
| 969 | <para> |
---|
| 970 | The biasing can be controlled either in compiled code or through |
---|
| 971 | interactive commands. Extensive documentation can be found in |
---|
| 972 | |
---|
| 973 | <ulink url="http://reat.space.qinetiq.com/septimess/exrdm/"> |
---|
| 974 | Radioactive decay biasing example |
---|
| 975 | </ulink> |
---|
| 976 | and |
---|
| 977 | <ulink url="http://www.space.qinetiq.com/geant4/rdm.html"> |
---|
| 978 | Radioactive decay biasing |
---|
| 979 | </ulink>. |
---|
| 980 | </para> |
---|
| 981 | |
---|
| 982 | <para> |
---|
| 983 | Radioactive decay biasing examples are also distributed with the Geant4 |
---|
| 984 | distribution in |
---|
| 985 | <emphasis role="bold">examples/extended/radioactivedecay/exrdm</emphasis>. |
---|
| 986 | </para> |
---|
| 987 | |
---|
| 988 | </sect4> |
---|
| 989 | |
---|
| 990 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 991 | <sect4 id="sect.EvtBias.PhysBias.BuiltInOpt.HadLeadPar"> |
---|
| 992 | <title> |
---|
| 993 | Hadronic Leading Particle Biasing |
---|
| 994 | </title> |
---|
| 995 | |
---|
| 996 | <para> |
---|
| 997 | One hadronic leading particle biasing technique is |
---|
| 998 | implemented in the G4HadLeadBias utility. This method keeps only |
---|
| 999 | the most important part of the event, as well as representative |
---|
| 1000 | tracks of each given particle type. So the track with the highest |
---|
| 1001 | energy as well as one of each of Baryon, pi0, mesons and leptons. |
---|
| 1002 | As usual, appropriate weights are assigned to the particles. |
---|
| 1003 | Setting the <emphasis role="bold">SwitchLeadBiasOn</emphasis> |
---|
| 1004 | environmental variable will activate this utility. |
---|
| 1005 | </para> |
---|
| 1006 | |
---|
| 1007 | </sect4> |
---|
| 1008 | |
---|
| 1009 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 1010 | <sect4 id="sect.EvtBias.PhysBias.BuiltInOpt.HadCrsSect"> |
---|
| 1011 | <title> |
---|
| 1012 | Hadronic Cross Section Biasing |
---|
| 1013 | </title> |
---|
| 1014 | |
---|
| 1015 | <para> |
---|
| 1016 | Cross section biasing artificially enhances/reduces the cross |
---|
| 1017 | section of a process. This may be useful for studying thin layer |
---|
| 1018 | interactions or thick layer shielding. The built in hadronic cross |
---|
| 1019 | section biasing applies to photon inelastic, electron nuclear and |
---|
| 1020 | positron nuclear processes. |
---|
| 1021 | </para> |
---|
| 1022 | |
---|
| 1023 | <para> |
---|
| 1024 | The biasing is controlled through the |
---|
| 1025 | <emphasis role="bold">BiasCrossSectionByFactor</emphasis> method |
---|
| 1026 | in G4HadronicProcess, as demonstrated below. |
---|
| 1027 | |
---|
| 1028 | <informalexample> |
---|
| 1029 | <programlisting> |
---|
| 1030 | void MyPhysicsList::ConstructProcess() |
---|
| 1031 | { |
---|
| 1032 | ... |
---|
| 1033 | G4ElectroNuclearReaction * theElectroReaction = |
---|
| 1034 | new G4ElectroNuclearReaction; |
---|
| 1035 | |
---|
| 1036 | G4ElectronNuclearProcess theElectronNuclearProcess; |
---|
| 1037 | theElectronNuclearProcess.RegisterMe(theElectroReaction); |
---|
| 1038 | theElectronNuclearProcess.BiasCrossSectionByFactor(100); |
---|
| 1039 | |
---|
| 1040 | pManager->AddDiscreteProcess(&theElectronNuclearProcess); |
---|
| 1041 | ... |
---|
| 1042 | } |
---|
| 1043 | </programlisting> |
---|
| 1044 | </informalexample> |
---|
| 1045 | </para> |
---|
| 1046 | |
---|
| 1047 | </sect4> |
---|
| 1048 | </sect3> |
---|
| 1049 | |
---|
| 1050 | |
---|
| 1051 | <!-- ******************* Section (Level#3) ****************** --> |
---|
| 1052 | <sect3 id="sect.EvtBias.PhysBias."> |
---|
| 1053 | <title> |
---|
| 1054 | G4WrapperProcess |
---|
| 1055 | </title> |
---|
| 1056 | |
---|
| 1057 | <para> |
---|
| 1058 | G4WrapperProcess can be used to implement user defined event |
---|
| 1059 | biasing. G4WrapperProcess, which is a process itself, wraps an |
---|
| 1060 | existing process. By default, all function calls are forwared to |
---|
| 1061 | the wrapped process. It is a non-invasive way to modify the |
---|
| 1062 | behaviour of an existing process. |
---|
| 1063 | </para> |
---|
| 1064 | |
---|
| 1065 | <para> |
---|
| 1066 | To use this utility, first create a derived class inheriting |
---|
| 1067 | from G4WrapperProcess. Override the methods whose behaviour you |
---|
| 1068 | would like to modify, for example, PostStepDoIt, and register the |
---|
| 1069 | derived class in place of the process to be wrapped. Finally, |
---|
| 1070 | register the wrapped process with G4WrapperProcess. The code |
---|
| 1071 | snippets below demonstrate its use. |
---|
| 1072 | |
---|
| 1073 | <informalexample> |
---|
| 1074 | <programlisting> |
---|
| 1075 | class MyWrapperProcess : public G4WrapperProcess { |
---|
| 1076 | ... |
---|
| 1077 | G4VParticleChange* PostStepDoIt(const G4Track& track, |
---|
| 1078 | const G4Step& step) { |
---|
| 1079 | // Do something interesting |
---|
| 1080 | } |
---|
| 1081 | }; |
---|
| 1082 | |
---|
| 1083 | |
---|
| 1084 | void MyPhysicsList::ConstructProcess() |
---|
| 1085 | { |
---|
| 1086 | ... |
---|
| 1087 | G4LowEnergyBremsstrahlung* bremProcess = |
---|
| 1088 | new G4LowEnergyBremsstrahlung(); |
---|
| 1089 | |
---|
| 1090 | MyWrapperProcess* wrapper = new MyWrapperProcess(); |
---|
| 1091 | wrapper->RegisterProcess(bremProcess); |
---|
| 1092 | |
---|
| 1093 | processManager->AddProcess(wrapper, -1, -1, 3); |
---|
| 1094 | } |
---|
| 1095 | </programlisting> |
---|
| 1096 | </informalexample> |
---|
| 1097 | </para> |
---|
| 1098 | |
---|
| 1099 | </sect3> |
---|
| 1100 | |
---|
| 1101 | </sect2> |
---|
[1222] | 1102 | |
---|
| 1103 | <!-- ******************* Section (Level#2) ****************** --> |
---|
| 1104 | <sect2 id="sect.EvtBias.ReverseMC"> |
---|
| 1105 | <title> |
---|
| 1106 | Adjoint/Reverse Monte Carlo |
---|
| 1107 | </title> |
---|
| 1108 | |
---|
| 1109 | <para> |
---|
| 1110 | Another powerful biasing technique available in Geant4 is the Reverse Monte Carlo (RMC) method, |
---|
| 1111 | also known as the Adjoint Monte Carlo method. In this method particles are generated on the external |
---|
| 1112 | boundary of the sensitive part of the geometry and then are tracked backward in the geometry till they |
---|
| 1113 | reach the external source surface, |
---|
| 1114 | or exceed an energy threshold. By this way the computing time is focused only on particle tracks |
---|
| 1115 | that are contributing to the tallies. |
---|
| 1116 | The RMC method is much rapid than the Forward MC method when the sensitive part of the geometry is |
---|
| 1117 | small compared to the rest of the geometry and to the external source, that has to be extensive and |
---|
| 1118 | not beam like. At the moment the RMC method is implemented in Geant4 only for some electromagnetic |
---|
| 1119 | processes (see <xref linkend="sect.EvtBias.ReverseMC.InG4.Process" />). An example illustrating the use |
---|
| 1120 | of the Reverse MC method in Geant4 is distributed within the Geant4 |
---|
| 1121 | toolkit in <emphasis role="bold">examples/extended/biasing/ReverseMC01</emphasis>. |
---|
| 1122 | </para> |
---|
| 1123 | |
---|
| 1124 | <!-- ******************* Section (Level#3) ****************** --> |
---|
| 1125 | <sect3 id="sect.EvtBias.ReverseMC.InG4"> |
---|
| 1126 | <title> |
---|
| 1127 | Treatment of the Reverse MC method in Geant4 |
---|
| 1128 | </title> |
---|
| 1129 | |
---|
| 1130 | <para> |
---|
| 1131 | Different G4Adjoint classes have been implemented into the Geant4 |
---|
| 1132 | toolkit in order to run an adjoint/reverse simulation in a Geant4 application. |
---|
| 1133 | This implementation is illustrated in <xref linkend="fig.EvtBias.ReverseMC.InG4_1" />. |
---|
| 1134 | An adjoint run is divided in a serie of alternative adjoint and forward tracking of adjoint and normal particles. |
---|
| 1135 | One Geant4 event treats one of this tracking phase. |
---|
| 1136 | </para> |
---|
| 1137 | |
---|
| 1138 | <figure id="fig.EvtBias.ReverseMC.InG4_1"> |
---|
| 1139 | <title> |
---|
| 1140 | Schematic view of an adjoint/reverse simulation in Geant4 |
---|
| 1141 | </title> |
---|
| 1142 | <mediaobject> |
---|
| 1143 | <imageobject role="fo"> |
---|
| 1144 | <imagedata fileref="./AllResources/Fundamentals/ReverseMC_tracking.png" |
---|
| 1145 | format="png" align="center" /> |
---|
| 1146 | </imageobject> |
---|
| 1147 | <imageobject role="html"> |
---|
| 1148 | <imagedata fileref="./AllResources/Fundamentals/ReverseMC_tracking.png" |
---|
| 1149 | format="png" align="center" /> |
---|
| 1150 | </imageobject> |
---|
| 1151 | </mediaobject> |
---|
| 1152 | </figure> |
---|
| 1153 | |
---|
| 1154 | |
---|
| 1155 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 1156 | <sect4 id="sect.EvtBias.ReverseMC.InG4.AdjointTracking"> |
---|
| 1157 | <title> |
---|
| 1158 | Adjoint tracking phase |
---|
| 1159 | </title> |
---|
| 1160 | |
---|
| 1161 | <para> |
---|
| 1162 | Adjoint particles (adjoint_e-, adjoint_gamma,...) are generated one by one on the so called adjoint source |
---|
| 1163 | with random position, energy (1/E distribution) and direction. The adjoint source is the |
---|
| 1164 | external surface of a user defined volume or of a user defined sphere. The adjoint |
---|
| 1165 | source should contain one or several sensitive volumes and should be small compared to the entire geometry. |
---|
| 1166 | The user can set the minimum and maximum energy of the adjoint source. After its |
---|
| 1167 | generation the adjoint primary particle is tracked backward in the geometry till a user defined |
---|
| 1168 | external surface |
---|
| 1169 | (spherical or boundary of a volume) or is killed before if it reaches a user defined upper energy limit that |
---|
| 1170 | represents the maximum energy of the external source. During the reverse tracking, reverse |
---|
| 1171 | processes take place where the adjoint particle being tracked can be either scattered |
---|
| 1172 | or transformed in another type of adjoint particle. During the reverse tracking the |
---|
| 1173 | G4AdjointSimulationManager replaces the user defined primary, run, stepping, ... actions, by its own actions. |
---|
| 1174 | A reverse tracking phase corresponds to one Geant4 event. |
---|
| 1175 | </para> |
---|
| 1176 | |
---|
| 1177 | </sect4> |
---|
| 1178 | |
---|
| 1179 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 1180 | <sect4 id="sect.EvtBias.ReverseMC.InG4.ForwardTracking"> |
---|
| 1181 | <title> |
---|
| 1182 | Forward tracking phase |
---|
| 1183 | </title> |
---|
| 1184 | |
---|
| 1185 | <para> |
---|
| 1186 | When an adjoint particle reaches the external surface its weight, type, position, |
---|
| 1187 | and direction are registered and a normal primary particle, with a type equivalent to the last generated primary |
---|
| 1188 | adjoint, is generated with the same energy, position but opposite direction and is tracked in the forward direction |
---|
| 1189 | in the sensitive region as in a forward MC simulation. |
---|
| 1190 | During this forward tracking phase the |
---|
| 1191 | event, stacking, stepping, tracking actions defined by the user for his forward simulation are used. |
---|
| 1192 | By this clear separation between |
---|
| 1193 | adjoint and forward tracking phases, the code of the user developed for a forward simulation |
---|
| 1194 | should be only slightly modified to adapt it for an adjoint simulation (see <xref linkend="sect.EvtBias.ReverseMC.UserCoding" />). |
---|
| 1195 | Indeed the computation of the signals is done by the same actions |
---|
| 1196 | or classes that the one used in the forward simulation mode. |
---|
| 1197 | A forward tracking phase corresponds to one G4 event. |
---|
| 1198 | </para> |
---|
| 1199 | |
---|
| 1200 | </sect4> |
---|
| 1201 | |
---|
| 1202 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 1203 | <sect4 id="sect.EvtBias.ReverseMC.InG4.Process"> |
---|
| 1204 | <title> |
---|
| 1205 | Reverse processes |
---|
| 1206 | </title> |
---|
| 1207 | |
---|
| 1208 | <para> |
---|
| 1209 | During the reverse tracking, reverse processes act on the adjoint particles. |
---|
| 1210 | The reverse processes that are at the moment available in Geant4 are the: |
---|
| 1211 | <itemizedlist spacing="compact"> |
---|
| 1212 | <listitem><para> |
---|
| 1213 | Reverse discrete ionization for e-, proton and ions |
---|
| 1214 | </para></listitem> |
---|
| 1215 | <listitem><para> |
---|
| 1216 | Continuous gain of energy by ionization and bremsstrahlung for e- and by ionization for protons and ions |
---|
| 1217 | </para></listitem> |
---|
| 1218 | <listitem><para> |
---|
| 1219 | Reverse discrete e- bremsstrahlung |
---|
| 1220 | </para></listitem> |
---|
| 1221 | <listitem><para> |
---|
| 1222 | Reverse photo-electric effect |
---|
| 1223 | </para></listitem> |
---|
| 1224 | <listitem><para> |
---|
| 1225 | Reverse Compton scattering |
---|
| 1226 | </para></listitem> |
---|
| 1227 | <listitem><para> |
---|
| 1228 | Approximated multiple scattering (see comment in <xref linkend="sect.EvtBias.ReverseMC.Limitation.ReverseMS"/>) |
---|
| 1229 | </para></listitem> |
---|
| 1230 | </itemizedlist> |
---|
| 1231 | It is important to note that the electromagnetic reverse processes are cut dependent |
---|
| 1232 | as their equivalent forward processes. The implementation of the reverse processes is based on |
---|
| 1233 | the forward processes implemented in the G4 standard electromagnetic package. |
---|
| 1234 | </para> |
---|
| 1235 | |
---|
| 1236 | </sect4> |
---|
| 1237 | |
---|
| 1238 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 1239 | <sect4 id="sect.EvtBias.ReverseMC.InG4.RemarkOnNbEvents"> |
---|
| 1240 | <title> |
---|
| 1241 | Nb of adjoint particle types and nb of G4 events of an adjoint simulation |
---|
| 1242 | </title> |
---|
| 1243 | |
---|
| 1244 | <para> |
---|
| 1245 | The list of type of adjoint and forward particles that are generated on the adjoint source |
---|
| 1246 | and considered in the simulation is a function of the adjoint processes declared in the physics list. |
---|
| 1247 | For example if only the e- and gamma electromagnetic processes are considered, |
---|
| 1248 | only adjoint e- and adjoint gamma will be considered as primaries. |
---|
| 1249 | In this case an adjoint event will be divided in four G4 event consisting in the reverse tracking of an adjoint e-, |
---|
| 1250 | the forward tracking of its equivalent forward e-, the reverse tracking of |
---|
| 1251 | an adjoint gamma, and the forward tracking of its equivalent forward gamma. |
---|
| 1252 | In this case a run of 100 adjoint events will consist into 400 Geant4 events. |
---|
| 1253 | If the proton ionization is also considered adjoint and forward protons are also generated as primaries |
---|
| 1254 | and 600 Geant4 events are processed for 100 adjoint events. |
---|
| 1255 | </para> |
---|
| 1256 | |
---|
| 1257 | </sect4> |
---|
| 1258 | |
---|
| 1259 | </sect3> |
---|
| 1260 | |
---|
| 1261 | <!-- ******************* Section (Level#3) ****************** --> |
---|
| 1262 | <sect3 id="sect.EvtBias.ReverseMC.UserCoding"> |
---|
| 1263 | <title> |
---|
| 1264 | How to update a G4 application to use the reverse Monte Carlo mode |
---|
| 1265 | </title> |
---|
| 1266 | |
---|
| 1267 | <para> |
---|
| 1268 | Some modifications are needed to an existing Geant4 application in order to adapt |
---|
| 1269 | it for the use of the reverse simulation mode |
---|
| 1270 | (see also the G4 example <emphasis role="bold">examples/extended/biasing/ReverseMC01</emphasis>). |
---|
| 1271 | It consists into the: |
---|
| 1272 | |
---|
| 1273 | <itemizedlist spacing="compact"> |
---|
| 1274 | <listitem><para> |
---|
| 1275 | Creation of the adjoint simulation manager in the main code |
---|
| 1276 | </para></listitem> |
---|
| 1277 | <listitem><para> |
---|
| 1278 | Optional declaration of user actions that will be used during the adjoint tracking phase |
---|
| 1279 | </para></listitem> |
---|
| 1280 | <listitem><para> |
---|
| 1281 | Use of a special physics lists that combine the adjoint and forward processes |
---|
| 1282 | </para></listitem> |
---|
| 1283 | <listitem><para> |
---|
| 1284 | Modification of the user analysis part of the code |
---|
| 1285 | </para></listitem> |
---|
| 1286 | </itemizedlist> |
---|
| 1287 | </para> |
---|
| 1288 | |
---|
| 1289 | |
---|
| 1290 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 1291 | <sect4 id="sect.EvtBias.ReverseMC.UserCoding.Main"> |
---|
| 1292 | <title> |
---|
| 1293 | Creation of G4AdjointSimManager in the main |
---|
| 1294 | </title> |
---|
| 1295 | <para> |
---|
| 1296 | The class G4AdjointSimManager represents the manager of an adjoint simulation. |
---|
| 1297 | This static class should be created somewhere in the main code. The way to do that is illustrated below |
---|
| 1298 | <informalexample> |
---|
| 1299 | <programlisting> |
---|
| 1300 | int main(int argc,char** argv) { |
---|
| 1301 | ... |
---|
| 1302 | G4AdjointSimManager* theAdjointSimManager = G4AdjointSimManager::GetInstance(); |
---|
| 1303 | ... |
---|
| 1304 | } |
---|
| 1305 | </programlisting> |
---|
| 1306 | </informalexample> |
---|
| 1307 | By doing this the G4 application can be run in the reverse MC mode as well |
---|
| 1308 | as in the forward MC mode. |
---|
| 1309 | It is important to note that G4AdjointSimManager |
---|
| 1310 | is not a new G4RunManager and that the creation of G4RunManager in the main and the declaration of the geometry, |
---|
| 1311 | physics list, and user actions to G4RunManager is still needed. |
---|
| 1312 | The definition of the adjoint and external sources and the start of an adjoint simulation can be controlled |
---|
| 1313 | by G4UI commands in the directory <emphasis role="bold">/adjoint</emphasis>. |
---|
| 1314 | </para> |
---|
| 1315 | </sect4> |
---|
| 1316 | |
---|
| 1317 | |
---|
| 1318 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 1319 | <sect4 id="sect.EvtBias.ReverseMC.UserCoding.AdjointActions"> |
---|
| 1320 | <title> |
---|
| 1321 | Optional declaration of adjoint user actions |
---|
| 1322 | </title> |
---|
| 1323 | <para> |
---|
| 1324 | During an adjoint simulation the user stepping, tracking, stacking and event actions declared to G4RunManager |
---|
| 1325 | are used only during the G4 events dedicated to the forward tracking of normal particles in the sensitive region, |
---|
| 1326 | while during the events where adjoint particles are tracked backward the following happen concerning these actions: |
---|
| 1327 | |
---|
| 1328 | <itemizedlist spacing="compact"> |
---|
| 1329 | <listitem><para> |
---|
| 1330 | The user stepping action is replaced by G4AdjointSteppingAction that is reponsible to stop an adjoint track when |
---|
| 1331 | it reaches the external source, exceed the maximum energy of the external source, or cross the adjoint source surface. |
---|
| 1332 | If needed the user can declare its own stepping action that will be called by G4AdjointSteppingAction after the |
---|
| 1333 | check of stopping track conditions. This stepping action can be different that the stepping action used for the |
---|
| 1334 | forward simulation. It is declared to G4AdjointSimManager by the following lines of code : |
---|
| 1335 | |
---|
| 1336 | <informalexample><programlisting> |
---|
| 1337 | G4AdjointSimManager* theAdjointSimManager = G4AdjointSimManager::GetInstance(); |
---|
| 1338 | theAdjointSimManager->SetAdjointSteppingAction(aUserDefinedSteppingAction); |
---|
| 1339 | </programlisting></informalexample> |
---|
| 1340 | |
---|
| 1341 | </para></listitem> |
---|
| 1342 | |
---|
| 1343 | <listitem><para> |
---|
| 1344 | No stacking, tracking and event actions are considered by default. If needed the user can declare to G4AdjointSimManager |
---|
| 1345 | stacking, tracking and event actions that will be used only during the adjoint tracking phase. |
---|
| 1346 | The following lines of code show how to declare these adjoint actions to G4AdjointSimManager: |
---|
| 1347 | |
---|
| 1348 | <informalexample><programlisting> |
---|
| 1349 | G4AdjointSimManager* theAdjointSimManager = G4AdjointSimManager::GetInstance(); |
---|
| 1350 | theAdjointSimManager->SetAdjointEventAction(aUserDefinedEventAction); |
---|
| 1351 | theAdjointSimManager->SetAdjointStackingAction(aUserDefinedStackingAction); |
---|
| 1352 | theAdjointSimManager->SetAdjointTrackingAction(aUserDefinedTrackingAction); |
---|
| 1353 | </programlisting></informalexample> |
---|
| 1354 | |
---|
| 1355 | </para></listitem> |
---|
| 1356 | |
---|
| 1357 | </itemizedlist> |
---|
| 1358 | By default no user run action is considered in an adjoint simulation but if needed such action can be declared to |
---|
| 1359 | G4AdjointSimManager as such: |
---|
| 1360 | <informalexample><programlisting> |
---|
| 1361 | G4AdjointSimManager* theAdjointSimManager = G4AdjointSimManager::GetInstance(); |
---|
| 1362 | theAdjointSimManager->SetAdjointRunAction(aUserDefinedRunAction); |
---|
| 1363 | </programlisting></informalexample> |
---|
| 1364 | |
---|
| 1365 | </para> |
---|
| 1366 | |
---|
| 1367 | </sect4> |
---|
| 1368 | |
---|
| 1369 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 1370 | <sect4 id="sect.EvtBias.ReverseMC.UserCoding.PhysicsList"> |
---|
| 1371 | <title> |
---|
| 1372 | Physics list for reverse and forward electromagnetic processes |
---|
| 1373 | </title> |
---|
| 1374 | |
---|
| 1375 | <para> |
---|
| 1376 | To run an adjoint simulation a specific physics list should be used where existing G4 adjoint electromagnetic processes |
---|
| 1377 | and their forward equivalent have to be declared. |
---|
| 1378 | An example of such physics list is provided by the class G4AdjointPhysicsLits in the G4 example |
---|
| 1379 | <emphasis role="bold">extended/biasing/ReverseMC01</emphasis>. |
---|
| 1380 | </para> |
---|
| 1381 | |
---|
| 1382 | </sect4> |
---|
| 1383 | |
---|
| 1384 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 1385 | <sect4 id="sect.EvtBias.ReverseMC.UserCoding.Analysis"> |
---|
| 1386 | <title> |
---|
| 1387 | Modification in the analysis part of the code |
---|
| 1388 | </title> |
---|
| 1389 | |
---|
| 1390 | <para> |
---|
| 1391 | The user code should be modified to normalize the signals computed during the forward tracking phase to the weight of the last adjoint particle |
---|
| 1392 | that reaches the external surface. |
---|
| 1393 | This weight represents the statistical weight that the last full adjoint tracks (from the adjoint source |
---|
| 1394 | to the external source) would have in a forward simulation. If multiplied by a signal and registered in function of energy |
---|
| 1395 | and/or direction the simulation results will give an answer matrix of this signal. |
---|
| 1396 | To normalize it to a given spectrum it has to be furthermore multiplied by a directional differential flux |
---|
| 1397 | corresponding to this spectrum |
---|
| 1398 | |
---|
| 1399 | The weight, direction, position , kinetic energy and type of the last adjoint particle that reaches the |
---|
| 1400 | external source, and that would represents the primary of a forward simulation, can be get from G4AdjointSimManager by using for example the following line of codes |
---|
| 1401 | <informalexample><programlisting> |
---|
| 1402 | |
---|
| 1403 | G4AdjointSimManager* theAdjointSimManager = G4AdjointSimManager::GetInstance(); |
---|
| 1404 | G4String particle_name = theAdjointSimManager->GetFwdParticleNameAtEndOfLastAdjointTrack(); |
---|
| 1405 | G4int PDGEncoding= theAdjointSimManager->GetFwdParticlePDGEncodingAtEndOfLastAdjointTrack(); |
---|
| 1406 | G4double weight = theAdjointSimManager->GetWeightAtEndOfLastAdjointTrack(); |
---|
| 1407 | G4double Ekin = theAdjointSimManager->GetEkinAtEndOfLastAdjointTrack(); |
---|
| 1408 | G4double Ekin_per_nuc=theAdjointSimManager->GetEkinNucAtEndOfLastAdjointTrack(); // in case of ions |
---|
| 1409 | G4ThreeVector dir = theAdjointSimManager->GetDirectionAtEndOfLastAdjointTrack(); |
---|
| 1410 | G4ThreeVector pos = theAdjointSimManager->GetPositionAtEndOfLastAdjointTrack(); |
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| 1411 | |
---|
| 1412 | </programlisting></informalexample> |
---|
| 1413 | |
---|
| 1414 | </para> |
---|
| 1415 | |
---|
| 1416 | <para> |
---|
| 1417 | In order to have a code working for both forward and adjoint simulation mode, the extra code needed in user actions or analysis |
---|
| 1418 | manager for the adjoint |
---|
| 1419 | simulation mode can be separated to the code needed only for the normal forward simulation by using the following public method |
---|
| 1420 | of G4AdjointSimManager: |
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| 1421 | <informalexample><programlisting> |
---|
| 1422 | G4bool GetAdjointSimMode(); |
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| 1423 | </programlisting></informalexample> |
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| 1424 | that returns true if an adjoint simulation is running and false if not. |
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| 1425 | |
---|
| 1426 | </para> |
---|
| 1427 | |
---|
| 1428 | <para> |
---|
| 1429 | The following code example shows how to normalize a detector signal and compute an answer matrix |
---|
| 1430 | in the case of an adjoint simulation. |
---|
| 1431 | <example id="sect.EvtBias.ReverseMC.UserCoding.Analysis_1"> |
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| 1432 | <title> |
---|
| 1433 | Normalization in the case of an adjoint simulation. The detector signal S computed during the forward tracking |
---|
| 1434 | phase is normalized to a primary source of e- with a differential directional flux given by the function F. |
---|
| 1435 | An answer matrix of the signal is also computed. |
---|
| 1436 | </title> |
---|
| 1437 | |
---|
| 1438 | <programlisting> |
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| 1439 | |
---|
| 1440 | G4double S = ...; // signal in the sensitive volume computed during a forward tracking phase |
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| 1441 | |
---|
| 1442 | //Normalization of the signal for an adjoint simulation |
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| 1443 | G4AdjointSimManager* theAdjSimManager = G4AdjointSimManager::GetInstance(); |
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| 1444 | if (theAdjSimManager->GetAdjointSimMode()) { |
---|
| 1445 | G4double normalized_S=0.; //normalized to a given e- primary spectrum |
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| 1446 | G4double S_for_answer_matrix=0.; //for e- answer matrix |
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| 1447 | |
---|
| 1448 | if (theAdjSimManager->GetFwdParticleNameAtEndOfLastAdjointTrack() == "e-"){ |
---|
| 1449 | G4double ekin_prim = theAdjSimManager->GetEkinAtEndOfLastAdjointTrack(); |
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| 1450 | G4ThreeVector dir_prim = theAdjointSimManager->GetDirectionAtEndOfLastAdjointTrack(); |
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| 1451 | G4double weight_prim = theAdjSimManager->GetWeightAtEndOfLastAdjointTrack(); |
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| 1452 | S_for_answer_matrix = S*weight_prim; |
---|
| 1453 | normalized_S = S_for_answer_matrix*F(ekin_prim,dir); //F(ekin_prim,dir_prim) gives the differential directional flux of primary e- |
---|
| 1454 | } |
---|
| 1455 | //follows the code where normalized_S and S_for_answer_matrix are registered or whatever |
---|
| 1456 | .... |
---|
| 1457 | } |
---|
| 1458 | |
---|
| 1459 | //analysis/normalization code for forward simulation |
---|
| 1460 | else { |
---|
| 1461 | ... |
---|
| 1462 | } |
---|
| 1463 | ... |
---|
| 1464 | </programlisting> |
---|
| 1465 | </example> |
---|
| 1466 | |
---|
| 1467 | </para> |
---|
| 1468 | |
---|
| 1469 | </sect4> |
---|
| 1470 | |
---|
| 1471 | </sect3> |
---|
| 1472 | |
---|
| 1473 | <!-- ******************* Section (Level#3) ****************** --> |
---|
| 1474 | <sect3 id="sect.EvtBias.ReverseMC.Control"> |
---|
| 1475 | <title> |
---|
| 1476 | Control of an adjoint simulation |
---|
| 1477 | </title> |
---|
| 1478 | |
---|
| 1479 | <para> |
---|
| 1480 | The G4UI commands in the directory |
---|
| 1481 | <ulink url="./AllResources/Control/UIcommands/_adjoint_.html">/adjoint</ulink>. |
---|
| 1482 | allow the user to : |
---|
| 1483 | <itemizedlist spacing="compact"> |
---|
| 1484 | <listitem><para> |
---|
| 1485 | Define the adjoint source where adjoint primaries are generated |
---|
| 1486 | </para></listitem> |
---|
| 1487 | <listitem><para> |
---|
| 1488 | Define the external source till which adjoint particles are tracked |
---|
| 1489 | </para></listitem> |
---|
| 1490 | <listitem><para> |
---|
| 1491 | Start an adjoint simulation |
---|
| 1492 | </para></listitem> |
---|
| 1493 | </itemizedlist> |
---|
| 1494 | </para> |
---|
| 1495 | </sect3> |
---|
| 1496 | |
---|
| 1497 | <!-- ******************* Section (Level#3) ****************** --> |
---|
| 1498 | <sect3 id="sect.EvtBias.ReverseMC.Limitation"> |
---|
| 1499 | <title> |
---|
| 1500 | Known issues in the Reverse MC mode |
---|
| 1501 | </title> |
---|
| 1502 | |
---|
| 1503 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 1504 | <sect4 id="sect.EvtBias.ReverseMC.Limitation.Weight"> |
---|
| 1505 | <title> |
---|
| 1506 | Occasional wrong high weight in the adjoint simulation |
---|
| 1507 | </title> |
---|
| 1508 | |
---|
| 1509 | <para> |
---|
| 1510 | In rare cases an adjoint track may get a wrong high weight when reaching the external source. |
---|
| 1511 | While this happens not often it may corrupt the simulation results significantly. This happens |
---|
| 1512 | in some tracks where both reverse photo-electric and bremsstrahlung processes take place at low energy. |
---|
| 1513 | We still need some investigations to remove this problem at the level of physical adjoint/reverse processes. |
---|
| 1514 | However this problem can be solved at the level of event actions or analysis in the user code by adding a test on the |
---|
| 1515 | normalized signal during an adjoint simulation. An example of such test has been implemented in the Geant4 example |
---|
| 1516 | <emphasis role="bold"> extended/biasing/ReverseMC01 </emphasis>. |
---|
| 1517 | In this implementation an event is rejected when the relative error of the computed normalized energy deposited |
---|
| 1518 | increases during one event by more than 50% while the computed precision is already below 10%. |
---|
| 1519 | </para> |
---|
| 1520 | |
---|
| 1521 | </sect4> |
---|
| 1522 | |
---|
| 1523 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 1524 | <sect4 id="sect.EvtBias.ReverseMC.Limitation.ReverseBrem"> |
---|
| 1525 | <title> |
---|
| 1526 | Reverse bremsstrahlung |
---|
| 1527 | </title> |
---|
| 1528 | |
---|
| 1529 | <para> |
---|
| 1530 | A difference between the differential cross sections used in the adjoint and forward bremsstrahlung |
---|
| 1531 | models is the source of a higher flux of >100 keV gamma in the reverse simulation compared to the forward simulation mode. |
---|
| 1532 | In principle the adjoint processes/models should make use of the direct differential cross section to sample |
---|
| 1533 | the adjoint secondaries and compute the adjoint cross section. However due to the way the effective differential cross section is |
---|
| 1534 | considered in the forward model G4eBremsstrahlungModel this was not possible to achieve for the reverse bremsstrahlung. |
---|
| 1535 | Indeed the differential cross section used in G4AdjointeBremstrahlungModel is obtained by the numerical derivation |
---|
| 1536 | over the cut energy of the direct cross section provided by G4eBremsstrahlungModel. |
---|
| 1537 | This would be a correct procedure if the distribution of secondary in G4eBremsstrahlungModel |
---|
| 1538 | would match this differential cross section. Unfortunately it is not the case as independent parameterization are used |
---|
| 1539 | in G4eBremsstrahlungModel for both the cross sections and the sampling of secondaries. (It means that in the forward case |
---|
| 1540 | if one would integrate the effective differential cross section considered in the simulation we would not find back |
---|
| 1541 | the used cross section). |
---|
| 1542 | In the future we plan to correct this problem by using an extra weight correction factor after the occurrence of a reverse |
---|
| 1543 | bremsstrahlung. This weight factor should be the ratio between the differential CS used in the adjoint simulation and the |
---|
| 1544 | one effectively used in the forward processes. As it is impossible to have a simple and direct access to the forward differential CS |
---|
| 1545 | in G4eBremsstrahlungModel we are investigating the feasibility to use the differential CS considered in |
---|
| 1546 | G4Penelope models. |
---|
| 1547 | </para> |
---|
| 1548 | |
---|
| 1549 | </sect4> |
---|
| 1550 | |
---|
| 1551 | <!-- ******************* Section (Level#4) ****************** --> |
---|
| 1552 | <sect4 id="sect.EvtBias.ReverseMC.Limitation.ReverseMS"> |
---|
| 1553 | <title> |
---|
| 1554 | Reverse multiple scattering |
---|
| 1555 | </title> |
---|
| 1556 | |
---|
| 1557 | <para> |
---|
| 1558 | For the reverse multiple scattering the same model is used than in the forward case. |
---|
| 1559 | This approximation makes that the discrepancy between the adjoint and forward |
---|
| 1560 | simulation cases can get to a level of ~ 10-15% relative differences in the test cases that we have considered. |
---|
| 1561 | In the future we plan to improve the adjoint multiple scattering models by forcing the computation of |
---|
| 1562 | multiple scattering effect at the end of an adjoint step. |
---|
| 1563 | </para> |
---|
| 1564 | |
---|
| 1565 | </sect4> |
---|
| 1566 | |
---|
| 1567 | </sect3> |
---|
| 1568 | </sect2> |
---|
[1211] | 1569 | </sect1> |
---|