1 | <!-- ******************************************************** --> |
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2 | <!-- --> |
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3 | <!-- [History] --> |
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4 | <!-- Converted to DocBook: Katsuya Amako, Aug-2006 --> |
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5 | <!-- Changed by: Dennis Wright, 25-Jun-2002 --> |
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6 | <!-- --> |
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7 | <!-- ******************************************************** --> |
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8 | |
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9 | |
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10 | <!-- ******************* Section (Level#1) ****************** --> |
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11 | <sect1 id="sect.EvtBias"> |
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12 | <title> |
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13 | Event Biasing Techniques |
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14 | </title> |
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15 | |
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16 | <!-- ******************* Section (Level#2) ****************** --> |
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17 | <sect2 id="sect.EvtBias.ScorImpRoul"> |
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18 | <title> |
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19 | Scoring, Geometrical Importance Sampling and Weight Roulette |
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20 | </title> |
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21 | |
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22 | <para> |
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23 | Geant4 provides event biasing techniques which may be used to save |
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24 | computing time in such applications as the simulation of radiation |
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25 | shielding. These are <emphasis>geometrical splitting |
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26 | </emphasis> and <emphasis>Russian roulette</emphasis> |
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27 | (also called geometrical importance sampling), and |
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28 | <emphasis>weight roulette</emphasis>. Scoring is carried out by <emphasis>G4MultiFunctionalDetector</emphasis> (see <xref |
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29 | linkend="sect.Hits.G4Multi" /> and <xref linkend="sect.Hits.G4VPrim" />) using the standard Geant4 scoring technique. |
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30 | Biasing specific scorers have been implemented and are described within |
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31 | <emphasis>G4MultiFunctionDetector</emphasis> documentation. In this chapter, it is assumed that |
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32 | the reader is familiar with both the usage of Geant4 and the |
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33 | concepts of importance sampling. More detailed documentation may be |
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34 | found in the documents |
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35 | <ulink url="http://geant4.cern.ch/collaboration/working_groups/geometry/biasing/Sampling.html"> |
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36 | 'Scoring, geometrical importance sampling and weight roulette' |
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37 | </ulink>. |
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38 | A detailed description of different use-cases which employ the sampling |
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39 | and scoring techniques can be found in the document |
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40 | <ulink url="http://geant4.cern.ch/collaboration/working_groups/geometry/biasing/BiasScoreUseCases.html"> |
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41 | 'Use cases of importance sampling and scoring in Geant4' |
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42 | </ulink>. |
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43 | </para> |
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44 | |
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45 | <para> |
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46 | The purpose of importance sampling is to save computing time by |
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47 | sampling less often the particle histories entering "less |
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48 | important" geometry regions, and more often in more "important" |
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49 | regions. Given the same amount of computing time, an |
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50 | importance-sampled and an analogue-sampled simulation must show |
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51 | equal mean values, while the importance-sampled simulation will |
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52 | have a decreased variance. |
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53 | </para> |
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54 | |
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55 | <para> |
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56 | The implementation of scoring is independent of the implementation |
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57 | of importance sampling. However both share common concepts. |
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58 | <emphasis>Scoring and importance sampling apply to particle types chosen |
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59 | by the user</emphasis>, which should be borne in mind when interpreting the |
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60 | output of any biased simulation. |
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61 | </para> |
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62 | |
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63 | <para> |
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64 | Examples on how to use scoring and importance sampling may be found |
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65 | in <literal>examples/extended/biasing</literal>. |
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66 | </para> |
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67 | |
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68 | |
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69 | <!-- ******************* Section (Level#3) ****************** --> |
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70 | <sect3 id="sect.EvtBias.ScorImpRoul.Geom"> |
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71 | <title> |
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72 | Geometries |
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73 | </title> |
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74 | |
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75 | <para> |
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76 | The kind of scoring referred to in this note and the importance |
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77 | sampling apply to spatial cells provided by the user. |
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78 | </para> |
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79 | |
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80 | <para> |
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81 | A <emphasis role="bold">cell</emphasis> is a physical volume (further specified |
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82 | by it's replica number, if the volume is a replica). Cells may be defined |
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83 | in two kinds of geometries: |
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84 | |
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85 | <orderedlist spacing="compact"> |
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86 | <listitem><para> |
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87 | <emphasis role="bold">mass geometry</emphasis>: the geometry setup of the |
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88 | experiment to be simulated. Physics processes apply to this geometry. |
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89 | </para></listitem> |
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90 | <listitem><para> |
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91 | <emphasis role="bold">parallel-geometry</emphasis>: a geometry constructed |
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92 | to define the physical volumes according to which scoring and/or importance |
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93 | sampling is applied. |
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94 | </para></listitem> |
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95 | </orderedlist> |
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96 | </para> |
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97 | |
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98 | <para> |
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99 | The user has the choice to score and/or sample by importance the |
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100 | particles of the chosen type, according to mass geometry or to |
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101 | parallel geometry. It is possible to utilize several parallel |
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102 | geometries in addition to the mass geometry. This provides the user |
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103 | with a lot of flexibility to define separate geometries for |
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104 | different particle types in order to apply scoring or/and |
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105 | importance sampling. |
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106 | </para> |
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107 | |
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108 | <para> |
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109 | <note> |
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110 | Parallel geometries should be constructed using the implementation as |
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111 | described in <xref linkend="sect.ParaGeom"/>. |
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112 | |
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113 | There are a few conditions for parallel geometries: |
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114 | |
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115 | <itemizedlist spacing="compact"> |
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116 | <listitem><para> |
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117 | The world volume for parallel and mass geometries must be identical copies. |
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118 | </para></listitem> |
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119 | <listitem><para> |
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120 | Scoring and importance cells must not share boundaries with the world volume. |
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121 | </para></listitem> |
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122 | </itemizedlist> |
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123 | </note> |
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124 | </para> |
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125 | |
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126 | </sect3> |
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127 | |
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128 | <!-- ******************* Section (Level#3) ****************** --> |
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129 | <sect3 id="sect.EvtBias.ScorImpRoul.ChgSamp"> |
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130 | <title> |
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131 | Changing the Sampling |
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132 | </title> |
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133 | |
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134 | <para> |
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135 | Samplers are higher level tools which perform the necessary |
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136 | changes of the Geant4 sampling in order to apply importance |
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137 | sampling and weight roulette. |
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138 | </para> |
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139 | |
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140 | <para> |
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141 | Variance reduction (and scoring through the |
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142 | <emphasis>G4MultiFunctionalDetector</emphasis>) |
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143 | may be combined arbitrarily for chosen particle types and may be applied to the |
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144 | mass or to parallel geometries. |
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145 | </para> |
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146 | |
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147 | <para> |
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148 | The <literal>G4GeometrySampler</literal> can be applied equally to mass or |
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149 | parallel geometries with an abstract interface supplied by <literal>G4VSampler</literal>. |
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150 | <literal>G4VSampler</literal> provides |
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151 | <literal>Prepare...</literal> methods and a <literal>Configure</literal> |
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152 | method: |
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153 | |
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154 | <anchor id="anchor_EvtBias_G4VSampler" /> |
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155 | <informalexample> |
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156 | <programlisting> |
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157 | class G4VSampler |
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158 | { |
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159 | public: |
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160 | G4VSampler(); |
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161 | virtual ~G4VSampler(); |
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162 | virtual void PrepareImportanceSampling(G4VIStore *istore, |
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163 | const G4VImportanceAlgorithm |
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164 | *ialg = 0) = 0; |
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165 | virtual void PrepareWeightRoulett(G4double wsurvive = 0.5, |
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166 | G4double wlimit = 0.25, |
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167 | G4double isource = 1) = 0; |
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168 | virtual void PrepareWeightWindow(G4VWeightWindowStore *wwstore, |
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169 | G4VWeightWindowAlgorithm *wwAlg = 0, |
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170 | G4PlaceOfAction placeOfAction = |
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171 | onBoundary) = 0; |
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172 | virtual void Configure() = 0; |
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173 | virtual void ClearSampling() = 0; |
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174 | virtual G4bool IsConfigured() const = 0; |
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175 | }; |
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176 | </programlisting> |
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177 | </informalexample> |
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178 | </para> |
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179 | |
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180 | <para> |
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181 | The methods for setting up the desired combination need specific |
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182 | information: |
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183 | |
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184 | <itemizedlist spacing="compact"> |
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185 | <listitem><para> |
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186 | Importance sampling: message <literal>PrepareImportanceSampling</literal> |
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187 | with a |
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188 | <link linkend="anchor_EvtBias_G4VIStore"> |
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189 | <literal>G4VIStore</literal></link> |
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190 | and optionally a <literal>G4VImportanceAlgorithm</literal> |
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191 | </para></listitem> |
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192 | <listitem><para> |
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193 | Weight window: message <literal>PrepareWeightWindow</literal> with the |
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194 | arguments: |
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195 | <itemizedlist spacing="compact"> |
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196 | <listitem><para> |
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197 | <emphasis>*wwstore</emphasis>: a |
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198 | <literal>G4VWeightWindowStore</literal> for retrieving the lower |
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199 | weight bounds for the energy-space cells |
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200 | </para></listitem> |
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201 | <listitem><para> |
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202 | <emphasis>*wwAlg</emphasis>: a |
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203 | <literal>G4VWeightWindowAlgorithm</literal> if a customized algorithm |
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204 | should be used |
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205 | </para></listitem> |
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206 | <listitem><para> |
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207 | <emphasis>placeOfAction</emphasis>: a |
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208 | <literal>G4PlaceOfAction</literal> specifying where to perform the |
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209 | biasing |
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210 | </para></listitem> |
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211 | </itemizedlist> |
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212 | </para></listitem> |
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213 | <listitem><para> |
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214 | Weight roulette: message <literal>PrepareWeightRoulett</literal> with the |
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215 | optional parameters: |
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216 | <itemizedlist spacing="compact"> |
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217 | <listitem><para> |
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218 | <emphasis>wsurvive</emphasis>: survival weight |
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219 | </para></listitem> |
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220 | <listitem><para> |
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221 | <emphasis>wlimit</emphasis>: minimal allowed |
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222 | value of weight * source importance / cell importance |
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223 | </para></listitem> |
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224 | <listitem><para> |
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225 | <emphasis>isource</emphasis>: importance of the source cell |
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226 | </para></listitem> |
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227 | </itemizedlist> |
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228 | </para></listitem> |
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229 | </itemizedlist> |
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230 | </para> |
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231 | |
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232 | <para> |
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233 | Each object of a sampler class is responsible for one particle |
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234 | type. The particle type is given to the constructor of the sampler |
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235 | classes via the particle type name, e.g. "neutron". Depending on |
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236 | the specific purpose, the <literal>Configure()</literal> of a sampler will |
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237 | set up specialized processes (derived from <literal>G4VProcess</literal>) for |
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238 | transportation in the parallel geometry, importance |
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239 | sampling and weight roulette for the given particle type. When |
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240 | <literal>Configure()</literal> is invoked the sampler places the processes in |
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241 | the correct order independent of the order in which user invoked |
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242 | the <literal>Prepare...</literal> methods. |
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243 | </para> |
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244 | |
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245 | <para> |
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246 | <note> |
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247 | <para> |
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248 | <itemizedlist spacing="compact"> |
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249 | <listitem><para> |
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250 | The <literal>Prepare...()</literal> functions may each only be invoked |
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251 | once. |
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252 | </para></listitem> |
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253 | <listitem><para> |
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254 | To configure the sampling the function <literal>Configure()</literal> |
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255 | must be called <emphasis>after</emphasis> the |
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256 | <literal>G4RunManager</literal> has been initialized and the PhysicsList has |
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257 | been instantiated. |
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258 | </para></listitem> |
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259 | </itemizedlist> |
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260 | </para> |
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261 | </note> |
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262 | </para> |
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263 | |
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264 | |
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265 | |
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266 | |
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267 | <para> |
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268 | The interface and framework are demonstrated in the |
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269 | <literal>examples/extended/biasing</literal> directory, with the main changes being to the |
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270 | G4GeometrySampler class and the fact that in the parallel case the WorldVolume |
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271 | is a copy of the Mass World. |
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272 | |
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273 | The parallel geometry now has to inherit from |
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274 | <emphasis>G4VUserParallelWorld</emphasis> which also has the <emphasis>GetWorld()</emphasis> method |
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275 | in order to retrieve a copy of the mass geometry WorldVolume. |
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276 | |
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277 | <informalexample> |
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278 | <programlisting> |
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279 | class B02ImportanceDetectorConstruction : public G4VUserParallelWorld |
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280 | ghostWorld = GetWorld(); |
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281 | </programlisting> |
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282 | </informalexample> |
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283 | |
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284 | </para> |
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285 | |
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286 | <para> |
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287 | The constructor for <emphasis>G4GeometrySampler</emphasis> takes a pointer to |
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288 | the physical world volume and the particle type name (e.g. "neutron") as arguments. |
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289 | In a single mass geometry the sampler is created as follows: |
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290 | |
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291 | <informalexample> |
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292 | <programlisting> |
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293 | G4GeometrySampler mgs(detector->GetWorldVolume(),"neutron"); |
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294 | mgs.SetParallel(false); |
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295 | </programlisting> |
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296 | </informalexample> |
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297 | |
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298 | |
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299 | Whilst the following lines of code are required in order to set up the sampler for the |
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300 | parallel geometry case: |
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301 | |
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302 | <informalexample> |
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303 | <programlisting> |
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304 | G4VPhysicalVolume* ghostWorld = pdet->GetWorldVolume(); |
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305 | |
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306 | G4GeometrySampler pgs(ghostWorld,"neutron"); |
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307 | |
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308 | pgs.SetParallel(true); |
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309 | </programlisting> |
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310 | </informalexample> |
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311 | |
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312 | |
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313 | Also note that the preparation and configuration of the samplers has to be |
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314 | carried out <emphasis>after</emphasis> the instantiation of the UserPhysicsList |
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315 | and after the initialisation of the <emphasis>G4RunManager</emphasis>: |
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316 | |
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317 | <informalexample> |
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318 | <programlisting> |
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319 | pgs.PrepareImportanceSampling(&aIstore, 0); |
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320 | pgs.Configure(); |
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321 | </programlisting> |
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322 | </informalexample> |
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323 | |
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324 | Due to the fact that biasing is a process and has to be inserted after all the |
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325 | other processes have been created. |
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326 | </para> |
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327 | |
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328 | |
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329 | </sect3> |
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330 | |
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331 | <!-- ******************* Section (Level#3) ****************** --> |
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332 | <sect3 id="sect.EvtBias.ScorImpRoul.ImpSamp"> |
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333 | <title> |
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334 | Importance Sampling |
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335 | </title> |
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336 | |
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337 | <para> |
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338 | Importance sampling acts on particles crossing boundaries |
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339 | between "importance cells". The action taken depends on the |
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340 | importance values assigned to the cells. In general a particle |
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341 | history is either split or Russian roulette is played if the |
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342 | importance increases or decreases, respectively. A weight assigned |
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343 | to the history is changed according to the action taken. |
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344 | </para> |
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345 | |
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346 | <para> |
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347 | The tools provided for importance sampling require the user to |
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348 | have a good understanding of the physics in the problem. This is |
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349 | because the user has to decide which particle types require |
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350 | importance sampled, define the cells, and assign importance values |
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351 | to the cells. If this is not done properly the results cannot be |
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352 | expected to describe a real experiment. |
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353 | </para> |
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354 | |
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355 | <para> |
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356 | The assignment of importance values to a cell is done using an |
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357 | importance store described below. |
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358 | </para> |
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359 | |
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360 | <para> |
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361 | An "importance store" with the interface <literal>G4VIStore</literal> is |
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362 | used to store importance values related to cells. In order to do |
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363 | importance sampling the user has to create an object (e.g. of class |
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364 | <literal>G4IStore</literal>) of type <literal>G4VIStore</literal>. The samplers may be |
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365 | given a <literal>G4VIStore</literal>. The user fills the store with cells and |
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366 | their importance values. |
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367 | </para> |
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368 | |
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369 | <para> |
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370 | An importance store has to be constructed with a reference to |
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371 | the world volume of the geometry used for importance sampling. This |
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372 | may be the world volume of the mass or of a parallel geometry. |
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373 | Importance stores derive from the interface <literal>G4VIStore</literal>: |
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374 | |
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375 | <anchor id="anchor_EvtBias_G4VIStore" /> |
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376 | <informalexample> |
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377 | <programlisting> |
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378 | class G4VIStore |
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379 | { |
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380 | public: |
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381 | G4VIStore(); |
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382 | virtual ~G4VIStore(); |
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383 | virtual G4double GetImportance(const G4GeometryCell &gCell) const = 0; |
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384 | virtual G4bool IsKnown(const G4GeometryCell &gCell) const = 0; |
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385 | virtual const G4VPhysicalVolume &GetWorldVolume() const = 0; |
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386 | }; |
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387 | </programlisting> |
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388 | </informalexample> |
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389 | </para> |
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390 | |
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391 | <para> |
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392 | A concrete implementation of an importance store is provided by |
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393 | the class <literal>G4VStore</literal>. The <emphasis>public</emphasis> |
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394 | part of the class is: |
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395 | |
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396 | <informalexample> |
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397 | <programlisting> |
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398 | class G4IStore : public G4VIStore |
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399 | { |
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400 | public: |
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401 | explicit G4IStore(const G4VPhysicalVolume &worldvolume); |
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402 | virtual ~G4IStore(); |
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403 | virtual G4double GetImportance(const G4GeometryCell &gCell) const; |
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404 | virtual G4bool IsKnown(const G4GeometryCell &gCell) const; |
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405 | virtual const G4VPhysicalVolume &GetWorldVolume() const; |
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406 | void AddImportanceGeometryCell(G4double importance, |
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407 | const G4GeometryCell &gCell); |
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408 | void AddImportanceGeometryCell(G4double importance, |
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409 | const G4VPhysicalVolume &, |
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410 | G4int aRepNum = 0); |
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411 | void ChangeImportance(G4double importance, |
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412 | const G4GeometryCell &gCell); |
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413 | void ChangeImportance(G4double importance, |
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414 | const G4VPhysicalVolume &, |
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415 | G4int aRepNum = 0); |
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416 | G4double GetImportance(const G4VPhysicalVolume &, |
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417 | G4int aRepNum = 0) const ; |
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418 | private: ..... |
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419 | }; |
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420 | </programlisting> |
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421 | </informalexample> |
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422 | </para> |
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423 | |
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424 | <para> |
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425 | The member function <literal>AddImportanceGeometryCell()</literal> enters |
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426 | a cell and an importance value into the importance store. The |
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427 | importance values may be returned either according to a physical |
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428 | volume and a replica number or according to a |
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429 | <literal>G4GeometryCell</literal>. The user must be aware of the |
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430 | interpretation of assigning importance values to a cell. |
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431 | If scoring is also implemented then this is attached to logical volumes, in |
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432 | which case the physical volume and replica number method should be used for |
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433 | assigning importance values. See <literal>examples/extended/biasing |
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434 | B01</literal> and <literal>B02</literal> for examples of this. |
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435 | </para> |
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436 | |
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437 | <para> |
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438 | <note> |
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439 | <para> |
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440 | <itemizedlist spacing="compact"> |
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441 | <listitem><para> |
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442 | An importance value must be assigned to every cell. |
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443 | </para></listitem> |
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444 | </itemizedlist> |
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445 | </para> |
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446 | </note> |
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447 | </para> |
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448 | |
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449 | <para> |
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450 | The different cases: |
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451 | |
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452 | <itemizedlist spacing="compact"> |
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453 | <listitem><para> |
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454 | <emphasis>Cell is not in store</emphasis> |
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455 | <para> |
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456 | Not filling a certain cell in the store will cause an |
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457 | exception. |
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458 | </para> |
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459 | </para></listitem> |
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460 | <listitem><para> |
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461 | <emphasis>Importance value = zero</emphasis> |
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462 | <para> |
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463 | Tracks of the chosen particle type will be killed. |
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464 | </para> |
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465 | </para></listitem> |
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466 | <listitem><para> |
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467 | <emphasis>importance values > 0</emphasis> |
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468 | <para> |
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469 | Normal allowed values |
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470 | </para> |
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471 | </para></listitem> |
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472 | <listitem><para> |
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473 | <emphasis>Importance value smaller zero</emphasis> |
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474 | <para> |
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475 | Not allowed! |
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476 | </para> |
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477 | </para></listitem> |
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478 | </itemizedlist> |
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479 | </para> |
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480 | |
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481 | </sect3> |
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482 | |
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483 | <!-- ******************* Section (Level#3) ****************** --> |
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484 | <sect3 id="sect.EvtBias.ScorImpRoul.SampAlgor"> |
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485 | <title> |
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486 | The Importance Sampling Algorithm |
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487 | </title> |
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488 | |
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489 | <para> |
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490 | Importance sampling supports using a customized importance |
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491 | sampling algorithm. To this end, the sampler interface |
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492 | <link linkend="anchor_EvtBias_G4VSampler"> |
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493 | <literal>G4VSampler</literal></link> |
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494 | may be given a pointer to the interface |
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495 | <literal>G4VImportanceAlgorithm</literal>: |
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496 | |
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497 | <informalexample> |
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498 | <programlisting> |
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499 | class G4VImportanceAlgorithm |
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500 | { |
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501 | public: |
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502 | G4VImportanceAlgorithm(); |
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503 | virtual ~G4VImportanceAlgorithm(); |
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504 | virtual G4Nsplit_Weight Calculate(G4double ipre, |
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505 | G4double ipost, |
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506 | G4double init_w) const = 0; |
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507 | }; |
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508 | </programlisting> |
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509 | </informalexample> |
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510 | </para> |
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511 | |
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512 | <para> |
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513 | The method <literal>Calculate()</literal> takes the arguments: |
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514 | |
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515 | <itemizedlist spacing="compact"> |
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516 | <listitem><para> |
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517 | <emphasis>ipre, ipost</emphasis>: importance |
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518 | of the previous cell and the importance of the current cell, |
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519 | respectively. |
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520 | </para></listitem> |
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521 | <listitem><para> |
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522 | <emphasis>init_w</emphasis>: the particles weight |
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523 | </para></listitem> |
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524 | </itemizedlist> |
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525 | </para> |
---|
526 | |
---|
527 | <para> |
---|
528 | It returns the struct: |
---|
529 | |
---|
530 | <informalexample> |
---|
531 | <programlisting> |
---|
532 | class G4Nsplit_Weight |
---|
533 | { |
---|
534 | public: |
---|
535 | |
---|
536 | G4int fN; |
---|
537 | G4double fW; |
---|
538 | }; |
---|
539 | </programlisting> |
---|
540 | </informalexample> |
---|
541 | |
---|
542 | <itemizedlist spacing="compact"> |
---|
543 | <listitem><para> |
---|
544 | <emphasis>fN</emphasis>: the calculated |
---|
545 | number of particles to exit the importance sampling |
---|
546 | </para></listitem> |
---|
547 | <listitem><para> |
---|
548 | <emphasis>fW</emphasis>: the weight of the particles |
---|
549 | </para></listitem> |
---|
550 | </itemizedlist> |
---|
551 | </para> |
---|
552 | |
---|
553 | <para> |
---|
554 | The user may have a customized algorithm used by providing a |
---|
555 | class inheriting from <literal>G4VImportanceAlgorithm</literal>. |
---|
556 | </para> |
---|
557 | |
---|
558 | <para> |
---|
559 | If no customized algorithm is given to the sampler the default |
---|
560 | importance sampling algorithm is used. This algorithm is |
---|
561 | implemented in <literal>G4ImportanceAlgorithm</literal>. |
---|
562 | </para> |
---|
563 | |
---|
564 | </sect3> |
---|
565 | |
---|
566 | <!-- ******************* Section (Level#3) ****************** --> |
---|
567 | <sect3 id="sect.EvtBias.ScorImpRoul.WeightWin"> |
---|
568 | <title> |
---|
569 | The Weight Window Technique |
---|
570 | </title> |
---|
571 | |
---|
572 | <para> |
---|
573 | The weight window technique is a weight-based alternative to |
---|
574 | importance sampling: |
---|
575 | |
---|
576 | <itemizedlist spacing="compact"> |
---|
577 | <listitem><para> |
---|
578 | applies splitting and Russian roulette depending on space |
---|
579 | (cells) and energy |
---|
580 | </para></listitem> |
---|
581 | <listitem><para> |
---|
582 | user defines weight windows in contrast to defining importance |
---|
583 | values as in importance sampling |
---|
584 | </para></listitem> |
---|
585 | </itemizedlist> |
---|
586 | </para> |
---|
587 | |
---|
588 | <para> |
---|
589 | In contrast to importance sampling this technique is not weight |
---|
590 | blind. Instead the technique is applied according to the particle |
---|
591 | weight with respect to the current energy-space cell. |
---|
592 | </para> |
---|
593 | |
---|
594 | <para> |
---|
595 | Therefore the technique is convenient to apply in combination |
---|
596 | with other variance reduction techniques such as cross-section |
---|
597 | biasing and implicit capture. |
---|
598 | </para> |
---|
599 | |
---|
600 | <para> |
---|
601 | A weight window may be specified for every cell and for several |
---|
602 | energy regions: <emphasis>space-energy cell</emphasis>. |
---|
603 | |
---|
604 | <figure id="fig.EvtBias.WeightWindow"> |
---|
605 | <title> |
---|
606 | Weight window concept |
---|
607 | </title> |
---|
608 | |
---|
609 | <mediaobject> |
---|
610 | <imageobject role="fo"> |
---|
611 | <imagedata fileref="./AllResources/Fundamentals/wwconcept.jpg" |
---|
612 | format="JPG" contentwidth="9.0cm" align="center" /> |
---|
613 | </imageobject> |
---|
614 | <imageobject role="html"> |
---|
615 | <imagedata fileref="./AllResources/Fundamentals/wwconcept.jpg" |
---|
616 | format="JPG" align="center" /> |
---|
617 | </imageobject> |
---|
618 | <textobject> |
---|
619 | <phrase>Weight window concept</phrase> |
---|
620 | </textobject> |
---|
621 | </mediaobject> |
---|
622 | </figure> |
---|
623 | </para> |
---|
624 | |
---|
625 | <!-- ******* Bridgehead ******* --> |
---|
626 | <bridgehead renderas='sect4'> |
---|
627 | Weight window concept |
---|
628 | </bridgehead> |
---|
629 | |
---|
630 | <para> |
---|
631 | The user specifies a <emphasis>lower weight bound W_L</emphasis> |
---|
632 | for every space-energy cell. |
---|
633 | |
---|
634 | <itemizedlist spacing="compact"> |
---|
635 | <listitem><para> |
---|
636 | The upper weight bound W_U and the survival weight W_S are |
---|
637 | calculated as: |
---|
638 | <para> |
---|
639 | W_U = C_U <emphasis>W_L</emphasis> and |
---|
640 | </para> |
---|
641 | <para> |
---|
642 | W_S = C_S <emphasis>W_L</emphasis>. |
---|
643 | </para> |
---|
644 | </para></listitem> |
---|
645 | <listitem><para> |
---|
646 | The user specifies C_S and C_U once for the whole problem. |
---|
647 | </para></listitem> |
---|
648 | <listitem><para> |
---|
649 | The user may give different sets of energy bounds for every cell |
---|
650 | or one set for all geometrical cells |
---|
651 | </para></listitem> |
---|
652 | <listitem><para> |
---|
653 | Special case: if C_S = C_U = 1 for all energies then weight |
---|
654 | window is equivalent to importance sampling |
---|
655 | </para></listitem> |
---|
656 | <listitem><para> |
---|
657 | The user can choose to apply the technique: at boundaries, on |
---|
658 | collisions or on boundaries and collisions |
---|
659 | </para></listitem> |
---|
660 | </itemizedlist> |
---|
661 | </para> |
---|
662 | |
---|
663 | <para> |
---|
664 | The energy-space cells are realized by <literal>G4GeometryCell</literal> |
---|
665 | as in importance sampling. The cells are stored in a weight window |
---|
666 | store defined by <literal>G4VWeightWindowStore</literal>: |
---|
667 | |
---|
668 | <informalexample> |
---|
669 | <programlisting> |
---|
670 | class G4VWeightWindowStore { |
---|
671 | public: |
---|
672 | G4VWeightWindowStore(); |
---|
673 | virtual ~G4VWeightWindowStore(); |
---|
674 | virtual G4double GetLowerWeitgh(const G4GeometryCell &gCell, |
---|
675 | G4double partEnergy) const = 0; |
---|
676 | virtual G4bool IsKnown(const G4GeometryCell &gCell) const = 0; |
---|
677 | virtual const G4VPhysicalVolume &GetWorldVolume() const = 0; |
---|
678 | }; |
---|
679 | </programlisting> |
---|
680 | </informalexample> |
---|
681 | </para> |
---|
682 | |
---|
683 | <para> |
---|
684 | A concrete implementation is provided: |
---|
685 | |
---|
686 | <informalexample> |
---|
687 | <programlisting> |
---|
688 | class G4WeightWindowStore: public G4VWeightWindowStore { |
---|
689 | public: |
---|
690 | explicit G4WeightWindowStore(const G4VPhysicalVolume &worldvolume); |
---|
691 | virtual ~G4WeightWindowStore(); |
---|
692 | virtual G4double GetLowerWeitgh(const G4GeometryCell &gCell, |
---|
693 | G4double partEnergy) const; |
---|
694 | virtual G4bool IsKnown(const G4GeometryCell &gCell) const; |
---|
695 | virtual const G4VPhysicalVolume &GetWorldVolume() const; |
---|
696 | void AddLowerWeights(const G4GeometryCell &gCell, |
---|
697 | const std::vector<G4double> &lowerWeights); |
---|
698 | void AddUpperEboundLowerWeightPairs(const G4GeometryCell &gCell, |
---|
699 | const G4UpperEnergyToLowerWeightMap& |
---|
700 | enWeMap); |
---|
701 | void SetGeneralUpperEnergyBounds(const |
---|
702 | std::set<G4double, std::less<G4double> > & enBounds); |
---|
703 | |
---|
704 | private:: |
---|
705 | ... |
---|
706 | }; |
---|
707 | </programlisting> |
---|
708 | </informalexample> |
---|
709 | </para> |
---|
710 | |
---|
711 | <para> |
---|
712 | The user may choose equal energy bounds for all cells. In this |
---|
713 | case a set of upper energy bounds must be given to the store using |
---|
714 | the method <literal>SetGeneralUpperEnergyBounds</literal>. If a general set |
---|
715 | of energy bounds have been set <literal>AddLowerWeights</literal> can be used |
---|
716 | to add the cells. |
---|
717 | </para> |
---|
718 | |
---|
719 | <para> |
---|
720 | Alternatively, the user may chose different energy regions for |
---|
721 | different cells. In this case the user must provide a mapping of |
---|
722 | upper energy bounds to lower weight bounds for every cell using the |
---|
723 | method <literal>AddUpperEboundLowerWeightPairs</literal>. |
---|
724 | </para> |
---|
725 | |
---|
726 | <para> |
---|
727 | Weight window algorithms implementing the interface class |
---|
728 | <literal>G4VWeightWindowAlgorithm</literal> can be used to define a |
---|
729 | customized algorithm: |
---|
730 | |
---|
731 | <informalexample> |
---|
732 | <programlisting> |
---|
733 | class G4VWeightWindowAlgorithm { |
---|
734 | public: |
---|
735 | G4VWeightWindowAlgorithm(); |
---|
736 | virtual ~G4VWeightWindowAlgorithm(); |
---|
737 | virtual G4Nsplit_Weight Calculate(G4double init_w, |
---|
738 | G4double lowerWeightBound) const = 0; |
---|
739 | }; |
---|
740 | </programlisting> |
---|
741 | </informalexample> |
---|
742 | </para> |
---|
743 | |
---|
744 | <para> |
---|
745 | A concrete implementation is provided and used as a default: |
---|
746 | |
---|
747 | <informalexample> |
---|
748 | <programlisting> |
---|
749 | class G4WeightWindowAlgorithm : public G4VWeightWindowAlgorithm { |
---|
750 | public: |
---|
751 | G4WeightWindowAlgorithm(G4double upperLimitFaktor = 5, |
---|
752 | G4double survivalFaktor = 3, |
---|
753 | G4int maxNumberOfSplits = 5); |
---|
754 | virtual ~G4WeightWindowAlgorithm(); |
---|
755 | virtual G4Nsplit_Weight Calculate(G4double init_w, |
---|
756 | G4double lowerWeightBound) const; |
---|
757 | private: |
---|
758 | ... |
---|
759 | }; |
---|
760 | </programlisting> |
---|
761 | </informalexample> |
---|
762 | </para> |
---|
763 | |
---|
764 | <para> |
---|
765 | The constructor takes three parameters which are used to: |
---|
766 | calculate the upper weight bound (upperLimitFaktor), calculate the |
---|
767 | survival weight (survivalFaktor), and introduce a maximal number |
---|
768 | (maxNumberOfSplits) of copies to be created in one go. |
---|
769 | </para> |
---|
770 | |
---|
771 | <para> |
---|
772 | In addition, the inverse of the maxNumberOfSplits is used to |
---|
773 | specify the minimum survival probability in case of Russian |
---|
774 | roulette. |
---|
775 | </para> |
---|
776 | |
---|
777 | </sect3> |
---|
778 | |
---|
779 | <!-- ******************* Section (Level#3) ****************** --> |
---|
780 | <sect3 id="sect.EvtBias.ScorImpRoul.WeigRoul"> |
---|
781 | <title> |
---|
782 | The Weight Roulette Technique |
---|
783 | </title> |
---|
784 | |
---|
785 | <para> |
---|
786 | Weight roulette (also called weight cutoff) is usually applied |
---|
787 | if importance sampling and implicit capture are used together. |
---|
788 | Implicit capture is not described here but it is useful to note |
---|
789 | that this procedure reduces a particle weight in every collision |
---|
790 | instead of killing the particle with some probability. |
---|
791 | </para> |
---|
792 | |
---|
793 | <para> |
---|
794 | Together with importance sampling the weight of a particle may |
---|
795 | become so low that it does not change any result significantly. |
---|
796 | Hence tracking a very low weight particle is a waste of computing |
---|
797 | time. Weight roulette is applied in order to solve this |
---|
798 | problem. |
---|
799 | </para> |
---|
800 | |
---|
801 | <!-- ******* Bridgehead ******* --> |
---|
802 | <bridgehead renderas='sect4'> |
---|
803 | The weight roulette concept |
---|
804 | </bridgehead> |
---|
805 | |
---|
806 | <para> |
---|
807 | Weight roulette takes into account the importance "Ic" of the |
---|
808 | current cell and the importance "Is" of the cell in which the |
---|
809 | source is located, by using the ratio "R=Is/Ic". |
---|
810 | </para> |
---|
811 | |
---|
812 | <para> |
---|
813 | Weight roulette uses a relative minimal weight limit and a |
---|
814 | relative survival weight. When a particle falls below the weight |
---|
815 | limit Russian roulette is applied. If the particle survives, |
---|
816 | tracking will be continued and the particle weight will be set to |
---|
817 | the survival weight. |
---|
818 | </para> |
---|
819 | |
---|
820 | <para> |
---|
821 | The weight roulette uses the following parameters with their |
---|
822 | default values: |
---|
823 | |
---|
824 | <itemizedlist spacing="compact"> |
---|
825 | <listitem><para> |
---|
826 | <emphasis>wsurvival</emphasis>: 0.5 |
---|
827 | </para></listitem> |
---|
828 | <listitem><para> |
---|
829 | <emphasis>wlimit</emphasis>: 0.25 |
---|
830 | </para></listitem> |
---|
831 | <listitem><para> |
---|
832 | <emphasis>isource</emphasis>: 1 |
---|
833 | </para></listitem> |
---|
834 | </itemizedlist> |
---|
835 | </para> |
---|
836 | |
---|
837 | <para> |
---|
838 | The following algorithm is applied: |
---|
839 | </para> |
---|
840 | |
---|
841 | <para> |
---|
842 | If a particle weight "w" is lower than R*wlimit: |
---|
843 | |
---|
844 | <itemizedlist spacing="compact"> |
---|
845 | <listitem><para> |
---|
846 | the weight of the particle will be changed to "ws = wsurvival*R" |
---|
847 | </para></listitem> |
---|
848 | <listitem><para> |
---|
849 | the probability for the particle to survive is "p = w/ws" |
---|
850 | </para></listitem> |
---|
851 | </itemizedlist> |
---|
852 | </para> |
---|
853 | |
---|
854 | </sect3> |
---|
855 | </sect2> |
---|
856 | |
---|
857 | |
---|
858 | <!-- ******************* Section (Level#2) ****************** --> |
---|
859 | <sect2 id="sect.EvtBias.PhysBias"> |
---|
860 | <title> |
---|
861 | Physics Based Biasing |
---|
862 | </title> |
---|
863 | |
---|
864 | <para> |
---|
865 | Geant4 supports physics based biasing through a number of general |
---|
866 | use, built in biasing techniques. A utility class, |
---|
867 | G4WrapperProcess, is also available to support user defined |
---|
868 | biasing. |
---|
869 | </para> |
---|
870 | |
---|
871 | <!-- ******************* Section (Level#3) ****************** --> |
---|
872 | <sect3 id="sect.EvtBias.PhysBias.BuiltInOpt"> |
---|
873 | <title> |
---|
874 | Built in Biasing Options |
---|
875 | </title> |
---|
876 | |
---|
877 | <!-- ******************* Section (Level#4) ****************** --> |
---|
878 | <sect4 id="sect.EvtBias.PhysBias.BuiltInOpt.PrimPart"> |
---|
879 | <title> |
---|
880 | Primary Particle Biasing |
---|
881 | </title> |
---|
882 | |
---|
883 | <para> |
---|
884 | Primary particle biasing can be used to increase the number of |
---|
885 | primary particles generated in a particular phase space region of |
---|
886 | interest. The weight of the primary particle is modified as |
---|
887 | appropriate. A general implementation is provided through the |
---|
888 | G4GeneralParticleSource class. It is possible to bias position, |
---|
889 | angular and energy distributions. |
---|
890 | </para> |
---|
891 | |
---|
892 | <para> |
---|
893 | G4GeneralParticleSource is a concrete implementation of |
---|
894 | G4VPrimaryGenerator. To use, instantiate G4GeneralParticleSource in |
---|
895 | the G4VUserPrimaryGeneratorAction class, as demonstrated below. |
---|
896 | |
---|
897 | <informalexample> |
---|
898 | <programlisting> |
---|
899 | MyPrimaryGeneratorAction::MyPrimaryGeneratorAction() { |
---|
900 | generator = new G4GeneralParticleSource; |
---|
901 | } |
---|
902 | |
---|
903 | void |
---|
904 | MyPrimaryGeneratorAction::GeneratePrimaries(G4Event*anEvent){ |
---|
905 | generator->GeneratePrimaryVertex(anEvent); |
---|
906 | } |
---|
907 | </programlisting> |
---|
908 | </informalexample> |
---|
909 | </para> |
---|
910 | |
---|
911 | <para> |
---|
912 | The biasing can be configured through interactive commands. |
---|
913 | Extensive documentation can be found in |
---|
914 | <ulink url="http://reat.space.qinetiq.com/gps/"> |
---|
915 | Primary particle biasing</ulink>. Examples are also distributed |
---|
916 | with the Geant4 distribution in |
---|
917 | <emphasis role="bold">examples/extended/eventgenerator/exgps</emphasis>. |
---|
918 | </para> |
---|
919 | |
---|
920 | </sect4> |
---|
921 | |
---|
922 | <!-- ******************* Section (Level#4) ****************** --> |
---|
923 | <sect4 id="sect.EvtBias.PhysBias.BuiltInOpt.RadDcy"> |
---|
924 | <title> |
---|
925 | Radioactive Decay Biasing |
---|
926 | </title> |
---|
927 | |
---|
928 | <para> |
---|
929 | The G4RadioactiveDecay class simulates the decay of radioactive |
---|
930 | nuclei and implements the following biasing options: |
---|
931 | |
---|
932 | <itemizedlist spacing="compact"> |
---|
933 | <listitem><para> |
---|
934 | Increase the sampling rate of radionuclides within observation |
---|
935 | times through a user defined probability distribution function |
---|
936 | </para></listitem> |
---|
937 | <listitem><para> |
---|
938 | Nuclear splitting, where the parent nuclide is split into a |
---|
939 | user defined number of nuclides |
---|
940 | </para></listitem> |
---|
941 | <listitem><para> |
---|
942 | Branching ratio biasing where branching ratios are sampled with |
---|
943 | equal probability |
---|
944 | </para></listitem> |
---|
945 | </itemizedlist> |
---|
946 | </para> |
---|
947 | |
---|
948 | <para> |
---|
949 | G4RadioactiveDecay is a process which must be registered with a |
---|
950 | process manager, as demonstrated below. |
---|
951 | |
---|
952 | <informalexample> |
---|
953 | <programlisting> |
---|
954 | void MyPhysicsList::ConstructProcess() |
---|
955 | { |
---|
956 | ... |
---|
957 | G4RadioactiveDecay* theRadioactiveDecay = |
---|
958 | new G4RadioactiveDecay(); |
---|
959 | |
---|
960 | G4ProcessManager* pmanager = ... |
---|
961 | pmanager ->AddProcess(theRadioactiveDecay); |
---|
962 | ... |
---|
963 | } |
---|
964 | </programlisting> |
---|
965 | </informalexample> |
---|
966 | </para> |
---|
967 | |
---|
968 | <para> |
---|
969 | The biasing can be controlled either in compiled code or through |
---|
970 | interactive commands. Extensive documentation can be found in |
---|
971 | |
---|
972 | <ulink url="http://reat.space.qinetiq.com/septimess/exrdm/"> |
---|
973 | Radioactive decay biasing example |
---|
974 | </ulink> |
---|
975 | and |
---|
976 | <ulink url="http://www.space.qinetiq.com/geant4/rdm.html"> |
---|
977 | Radioactive decay biasing |
---|
978 | </ulink>. |
---|
979 | </para> |
---|
980 | |
---|
981 | <para> |
---|
982 | Radioactive decay biasing examples are also distributed with the Geant4 |
---|
983 | distribution in |
---|
984 | <emphasis role="bold">examples/extended/radioactivedecay/exrdm</emphasis>. |
---|
985 | </para> |
---|
986 | |
---|
987 | </sect4> |
---|
988 | |
---|
989 | <!-- ******************* Section (Level#4) ****************** --> |
---|
990 | <sect4 id="sect.EvtBias.PhysBias.BuiltInOpt.HadLeadPar"> |
---|
991 | <title> |
---|
992 | Hadronic Leading Particle Biasing |
---|
993 | </title> |
---|
994 | |
---|
995 | <para> |
---|
996 | One hadronic leading particle biasing technique is |
---|
997 | implemented in the G4HadLeadBias utility. This method keeps only |
---|
998 | the most important part of the event, as well as representative |
---|
999 | tracks of each given particle type. So the track with the highest |
---|
1000 | energy as well as one of each of Baryon, pi0, mesons and leptons. |
---|
1001 | As usual, appropriate weights are assigned to the particles. |
---|
1002 | Setting the <emphasis role="bold">SwitchLeadBiasOn</emphasis> |
---|
1003 | environmental variable will activate this utility. |
---|
1004 | </para> |
---|
1005 | |
---|
1006 | </sect4> |
---|
1007 | |
---|
1008 | <!-- ******************* Section (Level#4) ****************** --> |
---|
1009 | <sect4 id="sect.EvtBias.PhysBias.BuiltInOpt.HadCrsSect"> |
---|
1010 | <title> |
---|
1011 | Hadronic Cross Section Biasing |
---|
1012 | </title> |
---|
1013 | |
---|
1014 | <para> |
---|
1015 | Cross section biasing artificially enhances/reduces the cross |
---|
1016 | section of a process. This may be useful for studying thin layer |
---|
1017 | interactions or thick layer shielding. The built in hadronic cross |
---|
1018 | section biasing applies to photon inelastic, electron nuclear and |
---|
1019 | positron nuclear processes. |
---|
1020 | </para> |
---|
1021 | |
---|
1022 | <para> |
---|
1023 | The biasing is controlled through the |
---|
1024 | <emphasis role="bold">BiasCrossSectionByFactor</emphasis> method |
---|
1025 | in G4HadronicProcess, as demonstrated below. |
---|
1026 | |
---|
1027 | <informalexample> |
---|
1028 | <programlisting> |
---|
1029 | void MyPhysicsList::ConstructProcess() |
---|
1030 | { |
---|
1031 | ... |
---|
1032 | G4ElectroNuclearReaction * theElectroReaction = |
---|
1033 | new G4ElectroNuclearReaction; |
---|
1034 | |
---|
1035 | G4ElectronNuclearProcess theElectronNuclearProcess; |
---|
1036 | theElectronNuclearProcess.RegisterMe(theElectroReaction); |
---|
1037 | theElectronNuclearProcess.BiasCrossSectionByFactor(100); |
---|
1038 | |
---|
1039 | pManager->AddDiscreteProcess(&theElectronNuclearProcess); |
---|
1040 | ... |
---|
1041 | } |
---|
1042 | </programlisting> |
---|
1043 | </informalexample> |
---|
1044 | </para> |
---|
1045 | |
---|
1046 | </sect4> |
---|
1047 | </sect3> |
---|
1048 | |
---|
1049 | |
---|
1050 | <!-- ******************* Section (Level#3) ****************** --> |
---|
1051 | <sect3 id="sect.EvtBias.PhysBias."> |
---|
1052 | <title> |
---|
1053 | G4WrapperProcess |
---|
1054 | </title> |
---|
1055 | |
---|
1056 | <para> |
---|
1057 | G4WrapperProcess can be used to implement user defined event |
---|
1058 | biasing. G4WrapperProcess, which is a process itself, wraps an |
---|
1059 | existing process. By default, all function calls are forwared to |
---|
1060 | the wrapped process. It is a non-invasive way to modify the |
---|
1061 | behaviour of an existing process. |
---|
1062 | </para> |
---|
1063 | |
---|
1064 | <para> |
---|
1065 | To use this utility, first create a derived class inheriting |
---|
1066 | from G4WrapperProcess. Override the methods whose behaviour you |
---|
1067 | would like to modify, for example, PostStepDoIt, and register the |
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1068 | derived class in place of the process to be wrapped. Finally, |
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1069 | register the wrapped process with G4WrapperProcess. The code |
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1070 | snippets below demonstrate its use. |
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1071 | |
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1072 | <informalexample> |
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1073 | <programlisting> |
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1074 | class MyWrapperProcess : public G4WrapperProcess { |
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1075 | ... |
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1076 | G4VParticleChange* PostStepDoIt(const G4Track& track, |
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1077 | const G4Step& step) { |
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1078 | // Do something interesting |
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1079 | } |
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1080 | }; |
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1081 | |
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1082 | |
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1083 | void MyPhysicsList::ConstructProcess() |
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1084 | { |
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1085 | ... |
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1086 | G4LowEnergyBremsstrahlung* bremProcess = |
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1087 | new G4LowEnergyBremsstrahlung(); |
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1088 | |
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1089 | MyWrapperProcess* wrapper = new MyWrapperProcess(); |
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1090 | wrapper->RegisterProcess(bremProcess); |
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1091 | |
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1092 | processManager->AddProcess(wrapper, -1, -1, 3); |
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1093 | } |
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1094 | </programlisting> |
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1095 | </informalexample> |
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1096 | </para> |
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1097 | |
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1098 | </sect3> |
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1099 | |
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1100 | </sect2> |
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1101 | </sect1> |
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