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4 | <!-- Changed by: Katsuya Amako, 4-Aug-1998 --> |
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8 | <!-- Changed by: Hisaya Kurashige, 18-Jan-2007 --> |
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11 | <BODY> |
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12 | <TABLE WIDTH="100%"><TR> |
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13 | <TD> |
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14 | |
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15 | |
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16 | <A HREF="index.html"> |
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17 | <IMG SRC="../../../../resources/html/IconsGIF/Contents.gif" ALT="Contents"></A> |
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18 | <A HREF="materialDef.html"> |
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19 | <IMG SRC="../../../../resources/html/IconsGIF/Previous.gif" ALT="Previous"></A> |
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20 | <A HREF="physicsDef.html"> |
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22 | </TD> |
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23 | <TD ALIGN="Right"> |
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24 | <FONT SIZE="-1" COLOR="#238E23"> |
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25 | <B>Geant4 User's Guide</B> |
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26 | <BR> |
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27 | <B>For Application Developers</B> |
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28 | <BR> |
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29 | <B>Getting Started with Geant4</B> |
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30 | </FONT> |
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31 | </TD> |
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32 | </TR></TABLE> |
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33 | <BR> |
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34 | |
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35 | <P ALIGN="Center"> |
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36 | <FONT SIZE="+3" COLOR="#238E23"> |
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37 | <B>2.4 How to Specify Particles</B> |
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38 | </FONT> |
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39 | <P><BR> |
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40 | |
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41 | <HR ALIGN="Center" SIZE="7%"> |
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42 | <tt>G4VuserPhysicsList</tt> is one of the mandatory user base classes |
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43 | described in <a href="mainProgram.html">Section 2.1</a> . Within this class |
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44 | all particles and physics processes to be used in your simulation must be |
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45 | defined. The range cut-off parameter should also be defined in this class. |
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46 | <P> |
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47 | The user must create a class derived from <tt>G4VuserPhysicsList</tt> and |
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48 | implement the following pure virtual methods: |
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49 | <table> |
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50 | <tr> |
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51 | <td><TT>ConstructParticle()</TT>: |
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52 | <td>construction of particles |
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53 | <tr> |
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54 | <td><TT>ConstructProcess()</TT>: |
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55 | <td>construct processes and register them to particles |
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56 | <tr> |
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57 | <td><TT>SetCuts()</TT>: |
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58 | <td>setting a range cut value for all particles |
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59 | </table> |
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60 | <p> |
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61 | This section provides some simple examples of the |
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62 | <tt>ConstructParticle()</tt> and <tt>SetCuts()</tt> methods. |
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63 | For infomation on <tt>ConstructProcess()</tt> methods, please see |
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64 | <a href="physicsDef.html">Section 2.5</a>. |
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65 | <P> |
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66 | <HR> |
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67 | <a name="2.4.1"> |
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68 | <H2>2.4.1 Particle Definition</H2></a> |
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69 | |
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70 | Geant4 provides various types of particles for use in simulations: |
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71 | <UL> |
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72 | <LI>ordinary particles, such as electrons, protons, and gammas |
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73 | <LI>resonant particles with very short lifetimes, |
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74 | such as vector mesons and delta baryons |
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75 | <LI>nuclei, such as deuterons, alphas, and heavy ions |
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76 | <LI>quarks, di-quarks, and gluons |
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77 | </UL> |
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78 | <p> |
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79 | Each particle is represented by its own class, which is derived from |
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80 | <tt>G4ParticleDefinition</tt>. Particles are organized into six major |
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81 | categories: |
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82 | <ul> |
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83 | <li>lepton, |
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84 | <li>meson, |
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85 | <li>baryon, |
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86 | <li>boson, |
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87 | <li>shortlived and |
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88 | <li>ion, |
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89 | </ul> |
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90 | each of which is defined in a corresponding sub-directory under |
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91 | <TT>geant4/source/particles</TT>. There is also a corresponding granular |
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92 | library for each particle category. |
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93 | </p> |
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94 | |
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95 | <H4>2.4.1.1 The <tt>G4ParticleDefinition</tt> Class </H4> |
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96 | |
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97 | <tt>G4ParticleDefinition</tt> has properties which characterize individual |
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98 | particles, such as, name, mass, charge, spin, and so on. Most of these |
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99 | properties are "read-only" and can not be changed by users without rebuilding |
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100 | the libraries. |
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101 | |
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102 | <H4>2.4.1.2 How to Access a Particle</H4> |
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103 | <P> |
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104 | Each particle class type represents an individual particle type, and each |
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105 | class has a single static object. There are some exceptions to this rule; |
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106 | please see <a href="../TrackingAndPhysics/particle.html">Section 5.3</a> for |
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107 | details. |
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108 | <P> |
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109 | For example, the class <tt>G4Electron</tt> represents the electron and its |
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110 | only object is <TT>G4Electron::theElectron</TT>. The object is therefore |
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111 | referred to as a "singleton" of the <i>G4Electron</i> class. The pointer to |
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112 | this object is available through the static method |
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113 | <TT>G4Electron::ElectronDefinition()</TT>. |
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114 | <P> |
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115 | More than 100 types of particles are provided by default, to be used in |
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116 | various physics processes. In normal applications, users will not need to |
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117 | define their own particles. |
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118 | <p> |
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119 | Because particles are static objects of individual particle classes, these |
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120 | objects are instantiated automatically before the <tt>main()</tt> routine is |
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121 | executed. However, you must explicitly declare the particle classes required |
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122 | by your program, otherwise the compiler can not recognize which classes you |
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123 | need, and no particles will be instantiated. |
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124 | <P> |
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125 | |
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126 | <H4>2.4.1.3 Dictionary of Particles</H4> |
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127 | The <tt>G4ParticleTable</tt> class is provided as a dictionary of particles. |
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128 | Various utility methods are provided, such as: |
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129 | <center><table> |
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130 | <tr> |
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131 | <td><tt>FindParticle(G4String name)</tt>: |
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132 | <td>find the particle by name |
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133 | <tr> |
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134 | <td><tt>FindParticle(G4int PDGencoding)</tt>: |
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135 | <td>find the particle by PDG encoding . |
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136 | </table></center> |
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137 | <p> |
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138 | <tt>G4ParticleTable</tt> is also defined as a singleton object, and the |
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139 | static method <TT>G4ParticleTable::GetParticleTable()</TT> provides its |
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140 | pointer. |
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141 | <P> |
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142 | Particles are registered automatically during construction. The user has no |
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143 | control over particle registration. |
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144 | <p> |
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145 | |
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146 | <H4>2.4.1.4 Constructing Particles</H4> |
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147 | <P> |
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148 | <TT>ConstructParticle()</TT> is a pure virtual method, in which the static |
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149 | member functions for all the particles you require should be called. This |
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150 | ensures that objects of these particles will be created. |
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151 | |
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152 | <p> |
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153 | WARNING: You must define "All PARTICLE TYPES" which are used in your application, except for heavy ions. |
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154 | "All PARTICLE TYPES" means not only primary particles, but also all other particles which may |
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155 | appear as secondaries generated by physics processes you use. |
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156 | Beginning with Geant4 version 8.0, you should keep this rule strictly because all particle definitions are revised to "non-static" objects. |
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157 | <p> |
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158 | |
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159 | For example, suppose you need a proton and a geantino, which is a virtual |
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160 | particle used for simulation and which does not interact with materials. |
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161 | The <TT>ConstructParticle()</TT> method is implemented as below: |
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162 | |
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163 | <center> |
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164 | <table border=2 cellpadding=10> |
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165 | <tr> |
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166 | <td> |
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167 | <PRE> |
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168 | void ExN01PhysicsList::ConstructParticle() |
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169 | { |
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170 | G4Proton::ProtonDefinition(); |
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171 | G4Geantino::GeantinoDefinition(); |
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172 | } |
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173 | </PRE> |
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174 | </td> |
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175 | </tr> |
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176 | <tr> |
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177 | <td align=center> |
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178 | Source listing 2.4.1<BR> |
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179 | Construct a proton and a geantino. |
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180 | </td> |
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181 | </tr> |
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182 | </table></center> |
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183 | <P> |
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184 | Due to the large number of pre-defined particles in Geant4, it is |
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185 | cumbersome to list all the particles by this method. If you want all the |
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186 | particles in a Geant4 particle category, there are six utility classes, |
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187 | corresponding to each of the particle categories, which perform this function: |
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188 | <UL> |
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189 | <LI><tt>G4BosonConstructor</tt> |
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190 | <LI><tt>G4LeptonConstructor</tt> |
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191 | <LI><tt>G4MesonConstructor</tt> |
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192 | <LI><tt>G4BarionConstructor</tt> |
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193 | <LI><tt>G4IonConstructor</tt> |
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194 | <LI><tt>G4ShortlivedConstructor</tt> . |
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195 | </UL> |
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196 | <P> |
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197 | An example of this is shown in <tt>ExN05PhysicsList</tt>, listed below. |
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198 | <P> |
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199 | <center> |
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200 | <table border=2 cellpadding=10> |
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201 | <tr> |
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202 | <td> |
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203 | <PRE> |
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204 | void ExN05PhysicsList::ConstructLeptons() |
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205 | { |
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206 | // Construct all leptons |
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207 | G4LeptonConstructor pConstructor; |
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208 | pConstructor.ConstructParticle(); |
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209 | } |
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210 | </PRE> |
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211 | </td> |
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212 | </tr> |
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213 | <tr> |
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214 | <td align=center> |
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215 | Source listing 2.4.2<BR> |
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216 | Construct all leptons. |
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217 | </td> |
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218 | </tr> |
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219 | </table></center> |
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220 | <P> |
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221 | |
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222 | <HR> |
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223 | <a name="2.4.2"> |
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224 | <H2>2.4.2 Range Cuts</H2></a> |
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225 | |
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226 | To avoid infrared divergence, some electromagnetic processes require a |
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227 | threshold below which no secondary will be generated. Because of this |
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228 | requirement, gammas, electrons and positrons require production thresholds |
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229 | which the user should define. This threshold should be defined as a distance, |
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230 | or range cut-off, which is internally converted to an energy for individual |
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231 | materials. The range threshold should be defined in the initialization phase |
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232 | using the <tt>SetCuts()</tt> method of <tt>G4VUserPhysicsList</tt>. |
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233 | <a href="../TrackingAndPhysics/thresholdVScut.html">Section 5.4</a> discusses |
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234 | threshold and tracking cuts in detail. |
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235 | </P> |
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236 | |
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237 | <H4>2.4.2.1 Setting the cuts</H4> |
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238 | |
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239 | Production threshold values should be defined in <TT>SetCuts()</TT> which is a |
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240 | pure virtual method of the <tt>G4VUserPhysicsList</tt> class. |
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241 | Construction of particles, materials, and processes should precede the |
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242 | invocation of <TT>SetCuts()</TT>. <tt>G4RunManager</tt> takes care of this |
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243 | sequence in usual applications. |
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244 | |
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245 | <P> |
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246 | The idea of a "unique cut value in range" is one of the important features of |
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247 | Geant4 and is used to handle cut values in a coherent manner. |
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248 | For most applications, users need to determine only one cut value in range, |
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249 | and apply this value to gammas, electrons and positrons alike. |
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250 | <P> |
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251 | In such a case, the <TT>SetCutsWithDefault()</TT> method may be used. It |
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252 | is provided by the <tt>G4VuserPhysicsList</tt> base class, which has a |
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253 | <tt>defaultCutValue</tt> member as the default range cut-off value. |
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254 | <TT>SetCutsWithDefault()</TT> uses this value. |
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255 | <P> |
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256 | It is possible to set different range cut values for gammas, electrons and |
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257 | positrons, and also to set different range cut values for each geometrical |
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258 | region. In such cases however, one must be careful with physics outputs |
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259 | because Geant4 processes (especially energy loss) are designed to conform |
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260 | to the "unique cut value in range" scheme. |
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261 | <P> |
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262 | <center> |
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263 | <table border=2 cellpadding=10> |
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264 | <tr> |
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265 | <td> |
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266 | <PRE> |
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267 | void ExN04PhysicsList::SetCuts() |
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268 | { |
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269 | // the G4VUserPhysicsList::SetCutsWithDefault() method sets |
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270 | // the default cut value for all particle types |
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271 | SetCutsWithDefault(); |
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272 | } |
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273 | </PRE> |
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274 | </td> |
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275 | </tr> |
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276 | <tr> |
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277 | <td align=center> |
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278 | Source listing 2.4.3<BR> |
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279 | Set cut values by using the default cut value. |
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280 | </td> |
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281 | </tr> |
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282 | </table></center> |
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283 | <P> |
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284 | The <tt>defaultCutValue</tt> is set to 1.0 mm by default. Of |
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285 | course, you can set the new default cut value in the constructor |
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286 | of your physics list class as shown below. |
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287 | <P> |
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288 | <center> |
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289 | <table border=2 cellpadding=10> |
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290 | <tr> |
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291 | <td> |
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292 | <PRE> |
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293 | ExN04PhysicsList::ExN04PhysicsList(): G4VUserPhysicsList() |
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294 | { |
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295 | // default cut value (1.0mm) |
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296 | defaultCutValue = 1.0*mm; |
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297 | } |
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298 | </PRE> |
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299 | </td> |
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300 | </tr> |
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301 | <tr> |
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302 | <td align=center> |
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303 | Source listing 2.4.4<BR> |
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304 | Set the default cut value. |
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305 | </td> |
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306 | </tr> |
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307 | </table></center> |
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308 | <P> |
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309 | The <TT>SetDefaultCutValue() </TT> method in <tt>G4VUserPhysicsList</tt> may |
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310 | also be used, and the "/run/setCut" command may be used to change |
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311 | the default cut value interactively. |
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312 | <P> |
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313 | WARNING: DO NOT change cut values inside the event loop. Cut values may |
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314 | however be changed between runs. |
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315 | <P> |
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316 | An example implementation of <TT>SetCuts()</TT> is shown below: |
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317 | <p> |
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318 | <center> |
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319 | <table border=2 cellpadding=10> |
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320 | <tr> |
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321 | <td> |
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322 | <PRE> |
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323 | void ExN03PhysicsList::SetCuts() |
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324 | { |
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325 | // set cut values for gamma at first and for e- second and next for e+, |
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326 | // because some processes for e+/e- need cut values for gamma |
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327 | SetCutValue(cutForGamma, "gamma"); |
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328 | SetCutValue(cutForElectron, "e-"); |
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329 | SetCutValue(cutForElectron, "e+"); |
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330 | } |
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331 | </PRE> |
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332 | </td> |
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333 | </tr> |
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334 | <tr> |
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335 | <td align=center> |
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336 | Source listing 2.4.5<BR> |
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337 | Example implementation of the <tt>SetCuts()</tt> method. |
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338 | </td> |
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339 | </tr> |
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340 | </table></center> |
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341 | <P> |
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342 | Beginning with Geant4 version 5.1, it is now possible to set production |
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343 | thresholds for each geometrical region. This new functionality is described |
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344 | in <a href="../TrackingAndPhysics/cutsPerRegion.html">Section 5.5</a>. |
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345 | <P> |
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346 | |
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347 | <HR> |
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348 | <A HREF="../../../../Authors/html/subjectsToAuthors.html"> |
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349 | <I>About the authors</I></A> |
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350 | |
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351 | </BODY> |
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352 | </HTML> |
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353 | |
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