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16 | |
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17 | </A> |
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18 | <A HREF="index.html"> |
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24 | </TD> |
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25 | |
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26 | <TD ALIGN="Right"><FONT COLOR="#238E23"><FONT SIZE=-1> |
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27 | <B>Geant4 User's Guide</B> <BR> |
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28 | <B>For Application Developers</B> <BR> |
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29 | <B>Toolkit Fundamentals</B> </FONT></FONT> |
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30 | </TD> |
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31 | </TR> |
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32 | </TABLE> |
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33 | |
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34 | <P><BR> |
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35 | <CENTER><FONT COLOR="#238E23"><FONT SIZE=+3> |
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36 | <B>3.3 System of units </B> |
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37 | </FONT></FONT> <BR> |
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38 | </CENTER> |
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39 | <BR> |
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40 | |
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41 | <HR ALIGN="Center" SIZE="7%"> |
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42 | <P> |
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43 | |
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44 | <a name="3.3.1"> |
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45 | <H2>3.3.1 Basic units</h2></a> |
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46 | |
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47 | <P> |
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48 | Geant4 offers the user the possibility to choose and use the units |
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49 | he prefers for any quantity. In fact, the Geant4 kernel takes care of |
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50 | the units. Internally it uses a consistent set on units based on the |
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51 | <tt>HepSystemOfUnits</tt>: |
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52 | <P> |
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53 | <PRE> |
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54 | millimeter (mm) |
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55 | nanosecond (ns) |
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56 | Mega electron Volt (MeV) |
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57 | positron charge (eplus) |
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58 | degree Kelvin (kelvin) |
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59 | the amount of substance (mole) |
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60 | luminous intensity (candela) |
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61 | radian (radian) |
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62 | steradian (steradian) |
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63 | </PRE> |
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64 | <P> |
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65 | All other units are defined from the basic ones. |
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66 | <P> |
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67 | For instance: |
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68 | <PRE> |
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69 | millimeter = mm = 1; |
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70 | meter = m = 1000*mm; |
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71 | ... |
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72 | m3 = m*m*m; |
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73 | ... |
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74 | </PRE> |
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75 | <P> |
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76 | In the file <TT>source/global/management/include/SystemOfUnits.h</TT> |
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77 | you will find all of these definitions. That file is part of CLHEP. |
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78 | <P> |
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79 | Moreover, the user is free to change the system of units to be used by the |
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80 | kernel. |
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81 | <p> |
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82 | |
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83 | <HR> |
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84 | <a name="3.3.2"> |
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85 | <H2>3.3.2 Input your data</H2></a> |
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86 | |
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87 | <b>Avoid 'hard coded' data</b> |
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88 | <P> |
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89 | You <B>must</B> give the units for the data you are going to introduce: |
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90 | <PRE> |
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91 | G4double Size = 15*km, KineticEnergy = 90.3*GeV, density = 11*mg/cm3; |
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92 | </PRE> |
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93 | <P> |
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94 | Indeed, the full Geant4 code is written respecting these specifications, |
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95 | and this makes it independent of the units chosen by the user. |
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96 | <P> |
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97 | If the units are not specified, it is understood that the data is implicitly |
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98 | in the internal G4 system, but this is strongly discouraged. |
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99 | <P> |
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100 | If the data set comes from an array or from an external file, it is strongly |
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101 | recommended to set the units as soon as the data are read, before any |
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102 | treatment. For instance: |
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103 | <P> |
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104 | <PRE> |
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105 | for (int j=0, j<jmax, j++) CrossSection[j] *= millibarn; |
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106 | ... |
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107 | my calculations |
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108 | ... |
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109 | </PRE> |
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110 | <P><BR> |
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111 | |
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112 | <b>Interactive commands</b> |
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113 | |
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114 | <P> |
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115 | Some built-in commands from the User Interface (UI) also require the units |
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116 | to be specified. |
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117 | <P> |
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118 | For instance: |
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119 | <PRE> |
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120 | /gun/energy 15.2 keV |
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121 | /gun/position 3 2 -7 meter |
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122 | </PRE> |
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123 | <P> |
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124 | If the units are not specified, or are not valid, the command is refused. |
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125 | <P> |
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126 | |
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127 | <HR> |
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128 | <a name="3.3.3"> |
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129 | <H2>3.3.3 Output your data</H2></a> |
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130 | |
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131 | You can output your data with the units you wish. To do so, it |
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132 | is sufficient to <b>divide</b> the data by the corresponding unit: |
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133 | <PRE> |
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134 | G4cout << KineticEnergy/keV << " keV"; |
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135 | G4cout << density/(g/cm3) << " g/cm3"; |
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136 | </PRE> |
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137 | <P> |
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138 | Of course, <tt>G4cout << KineticEnergy</tt> will print the energy in the |
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139 | internal units system. |
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140 | <P> |
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141 | There is another way to output your data. Let Geant4 choose the most |
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142 | appropriate units for the actual numerical value of your data. It is |
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143 | sufficient to specify to which category your data belong (Length, Time, |
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144 | Energy, etc.). For example |
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145 | <PRE> |
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146 | G4cout << G4BestUnit(StepSize, "Length"); |
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147 | </PRE> |
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148 | <P> |
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149 | <tt>StepSize</tt> will be printed in km, m, mm, fermi, etc. depending of |
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150 | its actual value. |
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151 | <P> |
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152 | |
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153 | <HR> |
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154 | <a name="3.3.4"> |
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155 | <H2>3.3.4 Introduce new units</H2></a> |
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156 | |
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157 | <P> |
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158 | If you wish to introduce new units, there are two methods: |
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159 | |
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160 | <UL> |
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161 | <P> |
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162 | <LI> You can complete the file <tt>SystemOfUnits.h</tt> </LI> |
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163 | |
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164 | <PRE> |
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165 | #include "SystemOfUnits.h" |
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166 | |
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167 | static const G4double inch = 2.54*cm; |
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168 | </PRE> |
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169 | <P> |
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170 | Using this method, it is not easy to define composed units. It is better |
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171 | to do the following: |
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172 | <P> |
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173 | <LI> You can instantiate an object of the class <i>G4UnitDefinition</i> </LI> |
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174 | |
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175 | <PRE> |
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176 | G4UnitDefinition ( name, symbol, category, value ) |
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177 | </PRE> |
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178 | <P> |
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179 | For example: define a few units for speed |
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180 | |
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181 | <PRE> |
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182 | G4UnitDefinition ( "km/hour" , "km/h", "Speed", km/(3600*s) ); |
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183 | G4UnitDefinition ( "meter/ns", "m/ns", "Speed", m/ns ); |
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184 | </PRE> |
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185 | <P> |
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186 | The category "Speed" does not exist by default in <i>G4UnitsTable</i>, |
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187 | but it will be created automatically. |
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188 | <P> |
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189 | The class <i>G4UnitDefinition</i> is located in <tt>source/global/management</tt>. |
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190 | </UL> |
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191 | <P> |
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192 | |
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193 | <HR> |
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194 | <a name="3.3.5"> |
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195 | <H2>3.3.5 Print the list of units</H2></a> |
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196 | |
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197 | <P> |
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198 | You can print the list of units with the static function: |
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199 | <tt>G4UnitDefinition::PrintUnitsTable();</tt> |
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200 | <P> |
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201 | or with the interactive command: <TT>/units/list</TT> |
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202 | |
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203 | <P><BR><BR> |
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204 | |
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205 | <HR> |
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206 | <A HREF="../../../../Authors/html/subjectsToAuthors.html"> |
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207 | <I>About the authors</A></I> |
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208 | |
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209 | </BODY> |
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210 | </HTML> |
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