[1208] | 1 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"> |
---|
| 2 | <HTML> |
---|
| 3 | <HEAD> |
---|
| 4 | <TITLE></TITLE> |
---|
| 5 | <!-- Changed by: Katsuya Amako, 14-Jul-1998 --> |
---|
| 6 | <!-- Changed by: Katsuya Amako, 30-Nov-1998 --> |
---|
| 7 | <!-- Changed by: Dennis Wright, 20-Nov-2001 --> |
---|
| 8 | <!-- Proof read by: Joe Chuma, 28-Jun-1999 --> |
---|
| 9 | |
---|
| 10 | <META NAME="GENERATOR" CONTENT="Mozilla/3.0Gold (X11; I; OSF1 V4.0 alpha) [Netscape]"> |
---|
| 11 | </HEAD> |
---|
| 12 | <BODY> |
---|
| 13 | <TABLE WIDTH="100%" > |
---|
| 14 | <TR> |
---|
| 15 | <TD> |
---|
| 16 | |
---|
| 17 | </A> |
---|
| 18 | <A HREF="index.html"> |
---|
| 19 | <IMG SRC="../../../../resources/html/IconsGIF/Contents.gif" ALT="Contents" HEIGHT=16 WIDTH=59></A> |
---|
| 20 | <A HREF="global.html"> |
---|
| 21 | <IMG SRC="../../../../resources/html/IconsGIF/Previous.gif" ALT="Previous" HEIGHT=16 WIDTH=59> |
---|
| 22 | <A HREF="run.html"> |
---|
| 23 | <IMG SRC="../../../../resources/html/IconsGIF/Next.gif" ALT="Next" HEIGHT=16 WIDTH=59></A> |
---|
| 24 | </TD> |
---|
| 25 | |
---|
| 26 | <TD ALIGN="Right"><FONT COLOR="#238E23"><FONT SIZE=-1> |
---|
| 27 | <B>Geant4 User's Guide</B> <BR> |
---|
| 28 | <B>For Application Developers</B> <BR> |
---|
| 29 | <B>Toolkit Fundamentals</B> </FONT></FONT> |
---|
| 30 | </TD> |
---|
| 31 | </TR> |
---|
| 32 | </TABLE> |
---|
| 33 | |
---|
| 34 | <P><BR> |
---|
| 35 | <CENTER><FONT COLOR="#238E23"><FONT SIZE=+3> |
---|
| 36 | <B>3.3 System of units </B> |
---|
| 37 | </FONT></FONT> <BR> |
---|
| 38 | </CENTER> |
---|
| 39 | <BR> |
---|
| 40 | |
---|
| 41 | <HR ALIGN="Center" SIZE="7%"> |
---|
| 42 | <P> |
---|
| 43 | |
---|
| 44 | <a name="3.3.1"> |
---|
| 45 | <H2>3.3.1 Basic units</h2></a> |
---|
| 46 | |
---|
| 47 | <P> |
---|
| 48 | Geant4 offers the user the possibility to choose and use the units |
---|
| 49 | he prefers for any quantity. In fact, the Geant4 kernel takes care of |
---|
| 50 | the units. Internally it uses a consistent set on units based on the |
---|
| 51 | <tt>HepSystemOfUnits</tt>: |
---|
| 52 | <P> |
---|
| 53 | <PRE> |
---|
| 54 | millimeter (mm) |
---|
| 55 | nanosecond (ns) |
---|
| 56 | Mega electron Volt (MeV) |
---|
| 57 | positron charge (eplus) |
---|
| 58 | degree Kelvin (kelvin) |
---|
| 59 | the amount of substance (mole) |
---|
| 60 | luminous intensity (candela) |
---|
| 61 | radian (radian) |
---|
| 62 | steradian (steradian) |
---|
| 63 | </PRE> |
---|
| 64 | <P> |
---|
| 65 | All other units are defined from the basic ones. |
---|
| 66 | <P> |
---|
| 67 | For instance: |
---|
| 68 | <PRE> |
---|
| 69 | millimeter = mm = 1; |
---|
| 70 | meter = m = 1000*mm; |
---|
| 71 | ... |
---|
| 72 | m3 = m*m*m; |
---|
| 73 | ... |
---|
| 74 | </PRE> |
---|
| 75 | <P> |
---|
| 76 | In the file <TT>source/global/management/include/SystemOfUnits.h</TT> |
---|
| 77 | you will find all of these definitions. That file is part of CLHEP. |
---|
| 78 | <P> |
---|
| 79 | Moreover, the user is free to change the system of units to be used by the |
---|
| 80 | kernel. |
---|
| 81 | <p> |
---|
| 82 | |
---|
| 83 | <HR> |
---|
| 84 | <a name="3.3.2"> |
---|
| 85 | <H2>3.3.2 Input your data</H2></a> |
---|
| 86 | |
---|
| 87 | <b>Avoid 'hard coded' data</b> |
---|
| 88 | <P> |
---|
| 89 | You <B>must</B> give the units for the data you are going to introduce: |
---|
| 90 | <PRE> |
---|
| 91 | G4double Size = 15*km, KineticEnergy = 90.3*GeV, density = 11*mg/cm3; |
---|
| 92 | </PRE> |
---|
| 93 | <P> |
---|
| 94 | Indeed, the full Geant4 code is written respecting these specifications, |
---|
| 95 | and this makes it independent of the units chosen by the user. |
---|
| 96 | <P> |
---|
| 97 | If the units are not specified, it is understood that the data is implicitly |
---|
| 98 | in the internal G4 system, but this is strongly discouraged. |
---|
| 99 | <P> |
---|
| 100 | If the data set comes from an array or from an external file, it is strongly |
---|
| 101 | recommended to set the units as soon as the data are read, before any |
---|
| 102 | treatment. For instance: |
---|
| 103 | <P> |
---|
| 104 | <PRE> |
---|
| 105 | for (int j=0, j<jmax, j++) CrossSection[j] *= millibarn; |
---|
| 106 | ... |
---|
| 107 | my calculations |
---|
| 108 | ... |
---|
| 109 | </PRE> |
---|
| 110 | <P><BR> |
---|
| 111 | |
---|
| 112 | <b>Interactive commands</b> |
---|
| 113 | |
---|
| 114 | <P> |
---|
| 115 | Some built-in commands from the User Interface (UI) also require the units |
---|
| 116 | to be specified. |
---|
| 117 | <P> |
---|
| 118 | For instance: |
---|
| 119 | <PRE> |
---|
| 120 | /gun/energy 15.2 keV |
---|
| 121 | /gun/position 3 2 -7 meter |
---|
| 122 | </PRE> |
---|
| 123 | <P> |
---|
| 124 | If the units are not specified, or are not valid, the command is refused. |
---|
| 125 | <P> |
---|
| 126 | |
---|
| 127 | <HR> |
---|
| 128 | <a name="3.3.3"> |
---|
| 129 | <H2>3.3.3 Output your data</H2></a> |
---|
| 130 | |
---|
| 131 | You can output your data with the units you wish. To do so, it |
---|
| 132 | is sufficient to <b>divide</b> the data by the corresponding unit: |
---|
| 133 | <PRE> |
---|
| 134 | G4cout << KineticEnergy/keV << " keV"; |
---|
| 135 | G4cout << density/(g/cm3) << " g/cm3"; |
---|
| 136 | </PRE> |
---|
| 137 | <P> |
---|
| 138 | Of course, <tt>G4cout << KineticEnergy</tt> will print the energy in the |
---|
| 139 | internal units system. |
---|
| 140 | <P> |
---|
| 141 | There is another way to output your data. Let Geant4 choose the most |
---|
| 142 | appropriate units for the actual numerical value of your data. It is |
---|
| 143 | sufficient to specify to which category your data belong (Length, Time, |
---|
| 144 | Energy, etc.). For example |
---|
| 145 | <PRE> |
---|
| 146 | G4cout << G4BestUnit(StepSize, "Length"); |
---|
| 147 | </PRE> |
---|
| 148 | <P> |
---|
| 149 | <tt>StepSize</tt> will be printed in km, m, mm, fermi, etc. depending of |
---|
| 150 | its actual value. |
---|
| 151 | <P> |
---|
| 152 | |
---|
| 153 | <HR> |
---|
| 154 | <a name="3.3.4"> |
---|
| 155 | <H2>3.3.4 Introduce new units</H2></a> |
---|
| 156 | |
---|
| 157 | <P> |
---|
| 158 | If you wish to introduce new units, there are two methods: |
---|
| 159 | |
---|
| 160 | <UL> |
---|
| 161 | <P> |
---|
| 162 | <LI> You can complete the file <tt>SystemOfUnits.h</tt> </LI> |
---|
| 163 | |
---|
| 164 | <PRE> |
---|
| 165 | #include "SystemOfUnits.h" |
---|
| 166 | |
---|
| 167 | static const G4double inch = 2.54*cm; |
---|
| 168 | </PRE> |
---|
| 169 | <P> |
---|
| 170 | Using this method, it is not easy to define composed units. It is better |
---|
| 171 | to do the following: |
---|
| 172 | <P> |
---|
| 173 | <LI> You can instantiate an object of the class <i>G4UnitDefinition</i> </LI> |
---|
| 174 | |
---|
| 175 | <PRE> |
---|
| 176 | G4UnitDefinition ( name, symbol, category, value ) |
---|
| 177 | </PRE> |
---|
| 178 | <P> |
---|
| 179 | For example: define a few units for speed |
---|
| 180 | |
---|
| 181 | <PRE> |
---|
| 182 | G4UnitDefinition ( "km/hour" , "km/h", "Speed", km/(3600*s) ); |
---|
| 183 | G4UnitDefinition ( "meter/ns", "m/ns", "Speed", m/ns ); |
---|
| 184 | </PRE> |
---|
| 185 | <P> |
---|
| 186 | The category "Speed" does not exist by default in <i>G4UnitsTable</i>, |
---|
| 187 | but it will be created automatically. |
---|
| 188 | <P> |
---|
| 189 | The class <i>G4UnitDefinition</i> is located in <tt>source/global/management</tt>. |
---|
| 190 | </UL> |
---|
| 191 | <P> |
---|
| 192 | |
---|
| 193 | <HR> |
---|
| 194 | <a name="3.3.5"> |
---|
| 195 | <H2>3.3.5 Print the list of units</H2></a> |
---|
| 196 | |
---|
| 197 | <P> |
---|
| 198 | You can print the list of units with the static function: |
---|
| 199 | <tt>G4UnitDefinition::PrintUnitsTable();</tt> |
---|
| 200 | <P> |
---|
| 201 | or with the interactive command: <TT>/units/list</TT> |
---|
| 202 | |
---|
| 203 | <P><BR><BR> |
---|
| 204 | |
---|
| 205 | <HR> |
---|
| 206 | <A HREF="../../../../Authors/html/subjectsToAuthors.html"> |
---|
| 207 | <I>About the authors</A></I> |
---|
| 208 | |
---|
| 209 | </BODY> |
---|
| 210 | </HTML> |
---|