[1208] | 1 | <HTML> |
---|
| 2 | <TITLE> |
---|
| 3 | </TITLE> |
---|
| 4 | <!-- Changed by: Katsuya Amako, 6-Aug-1998 --> |
---|
| 5 | <!-- Changed by: Dennis Wright, 28-Nov-2001 --> |
---|
| 6 | <!-- Proof read by: Joe Chuma, 14-Jun-1999 --> |
---|
| 7 | |
---|
| 8 | <BODY> |
---|
| 9 | <TABLE WIDTH="100%"><TR> |
---|
| 10 | <TD> |
---|
| 11 | |
---|
| 12 | |
---|
| 13 | <A HREF="index.html"> |
---|
| 14 | <IMG SRC="../../../../resources/html/IconsGIF/Contents.gif" ALT="Contents"></A> |
---|
| 15 | <A HREF="mainProgram.html"> |
---|
| 16 | <IMG SRC="../../../../resources/html/IconsGIF/Previous.gif" ALT="Previous"></A> |
---|
| 17 | <A HREF="materialDef.html"> |
---|
| 18 | <IMG SRC="../../../../resources/html/IconsGIF/Next.gif" ALT="Next"></A> |
---|
| 19 | </TD> |
---|
| 20 | <TD ALIGN="Right"> |
---|
| 21 | <FONT SIZE="-1" COLOR="#238E23"> |
---|
| 22 | <B>Geant4 User's Guide</B> |
---|
| 23 | <BR> |
---|
| 24 | <B>For Application Developers</B> |
---|
| 25 | <BR> |
---|
| 26 | <B>Getting Started with Geant4</B> |
---|
| 27 | </FONT> |
---|
| 28 | </TD> |
---|
| 29 | </TR></TABLE> |
---|
| 30 | <BR><BR> |
---|
| 31 | |
---|
| 32 | <P ALIGN="Center"> |
---|
| 33 | <FONT SIZE="+3" COLOR="#238E23"> |
---|
| 34 | <B>2.2 How to Define a Detector Geometry</B> |
---|
| 35 | </FONT> |
---|
| 36 | <P><BR> |
---|
| 37 | |
---|
| 38 | <HR ALIGN="Center" SIZE="7%"> |
---|
| 39 | <p> |
---|
| 40 | |
---|
| 41 | <A NAME="2.2.1"> |
---|
| 42 | <h2>2.2.1 Basic Concepts</H2></A> |
---|
| 43 | |
---|
| 44 | A detector geometry in Geant4 is made of a number of volumes. The largest |
---|
| 45 | volume is called the <b>World</b> volume. It must contain, with some margin, |
---|
| 46 | all other volumes in the detector geometry. The other volumes are created |
---|
| 47 | and placed inside previous volumes, included in the World volume. |
---|
| 48 | The most simple (and efficient) shape to describe the World is a box. |
---|
| 49 | <P> |
---|
| 50 | Each volume is created by describing its shape and its physical |
---|
| 51 | characteristics, and then placing it inside a containing volume. |
---|
| 52 | <P> |
---|
| 53 | When a volume is placed within another volume, we call the former volume the |
---|
| 54 | daughter volume and the latter the mother volume. The coordinate system used |
---|
| 55 | to specify where the daughter volume is placed, is the coordinate system of the |
---|
| 56 | mother volume. |
---|
| 57 | <P> |
---|
| 58 | To describe a volume's shape, we use the concept of a solid. A solid is |
---|
| 59 | a geometrical object that has a shape and specific values for each of |
---|
| 60 | that shape's dimensions. A cube with a side of 10 centimeters |
---|
| 61 | and a cylinder of radius 30 cm and length 75 cm are examples of solids. |
---|
| 62 | <P> |
---|
| 63 | To describe a volume's full properties, we use a logical volume. |
---|
| 64 | It includes the geometrical properties of the solid, and adds physical |
---|
| 65 | characteristics: the material of the volume; whether it contains any |
---|
| 66 | sensitive detector elements; the magnetic field; etc. |
---|
| 67 | <P> |
---|
| 68 | We have yet to describe how to position the volume. To do this you |
---|
| 69 | create a physical volume, which places a copy of the logical volume inside |
---|
| 70 | a larger, containing, volume. |
---|
| 71 | <P> |
---|
| 72 | |
---|
| 73 | <HR> |
---|
| 74 | <A NAME="2.2.2"> |
---|
| 75 | <H2>2.2.2 Create a Simple Volume</H2></A> |
---|
| 76 | |
---|
| 77 | What do you need to do to create a volume? |
---|
| 78 | <P> |
---|
| 79 | <UL> |
---|
| 80 | <LI> Create a solid. |
---|
| 81 | <LI> Create a logical volume, using this solid, and adding other attributes. |
---|
| 82 | </UL> |
---|
| 83 | <p> |
---|
| 84 | |
---|
| 85 | <HR> |
---|
| 86 | <A NAME="2.2.3"> |
---|
| 87 | <H2>2.2.3 Choose a Solid</H2></A> |
---|
| 88 | |
---|
| 89 | To create a simple box, you only need to define its name and its |
---|
| 90 | extent along each of the Cartesian axes. You can find an example how to do |
---|
| 91 | this in Novice Example N01. |
---|
| 92 | <P> |
---|
| 93 | In the detector description in the source file <tt>ExN01DetectorConstruction.cc</tt>, |
---|
| 94 | you will find the following box definition: |
---|
| 95 | <p> |
---|
| 96 | <center> |
---|
| 97 | <table border=2 cellpadding=10> |
---|
| 98 | <tr> |
---|
| 99 | <td> |
---|
| 100 | <PRE> |
---|
| 101 | G4double expHall_x = 3.0*m; |
---|
| 102 | G4double expHall_y = 1.0*m; |
---|
| 103 | G4double expHall_z = 1.0*m; |
---|
| 104 | |
---|
| 105 | G4Box* experimentalHall_box |
---|
| 106 | = new G4Box("expHall_box",expHall_x,expHall_y,expHall_z); |
---|
| 107 | </PRE> |
---|
| 108 | </td> |
---|
| 109 | </tr> |
---|
| 110 | <tr> |
---|
| 111 | <td align=center> |
---|
| 112 | Source listing 2.2.1<BR> |
---|
| 113 | Creating a box. |
---|
| 114 | </td> |
---|
| 115 | </tr> |
---|
| 116 | </table></center> |
---|
| 117 | <p> |
---|
| 118 | This creates a box named "expHall_box" |
---|
| 119 | with extent from -3.0 meters to +3.0 meters along the X axis, |
---|
| 120 | from -1.0 to 1.0 meters in Y, and from -1.0 to 1.0 meters in Z. |
---|
| 121 | <P> |
---|
| 122 | It is also very simple to create a cylinder. To do this, you can use the |
---|
| 123 | <i>G4Tubs</i> class. |
---|
| 124 | <p> |
---|
| 125 | <center> |
---|
| 126 | <table border=2 cellpadding=10> |
---|
| 127 | <tr> |
---|
| 128 | <td> |
---|
| 129 | <PRE> |
---|
| 130 | G4double innerRadiusOfTheTube = 0.*cm; |
---|
| 131 | G4double outerRadiusOfTheTube = 60.*cm; |
---|
| 132 | G4double hightOfTheTube = 25.*cm; |
---|
| 133 | G4double startAngleOfTheTube = 0.*deg; |
---|
| 134 | G4double spanningAngleOfTheTube = 360.*deg; |
---|
| 135 | |
---|
| 136 | G4Tubs* tracker_tube |
---|
| 137 | = new G4Tubs("tracker_tube", |
---|
| 138 | innerRadiusOfTheTube, |
---|
| 139 | outerRadiusOfTheTube, |
---|
| 140 | hightOfTheTube, |
---|
| 141 | startAngleOfTheTube, |
---|
| 142 | spanningAngleOfTheTube); |
---|
| 143 | </PRE> |
---|
| 144 | </td> |
---|
| 145 | </tr> |
---|
| 146 | <tr> |
---|
| 147 | <td align=center> |
---|
| 148 | Source listing 2.2.2<BR> |
---|
| 149 | Creating a cylinder. |
---|
| 150 | </td> |
---|
| 151 | </tr> |
---|
| 152 | </table></center> |
---|
| 153 | <p> |
---|
| 154 | This creates a full cylinder, named "tracker_tube", |
---|
| 155 | of radius 60 centimeters and length 50 cm. |
---|
| 156 | <P> |
---|
| 157 | |
---|
| 158 | <HR> |
---|
| 159 | <A NAME="2.2.4"> |
---|
| 160 | <H2>2.2.4 Create a Logical Volume</H2></A> |
---|
| 161 | |
---|
| 162 | To create a logical volume, you must start with a solid and a material. |
---|
| 163 | So, using the box created above, you can create a simple logical volume filled |
---|
| 164 | with argon gas (see materials) by entering: |
---|
| 165 | <PRE> |
---|
| 166 | G4LogicalVolume* experimentalHall_log |
---|
| 167 | = new G4LogicalVolume(experimentalHall_box,Ar,"expHall_log"); |
---|
| 168 | </PRE> |
---|
| 169 | This logical volume is named "expHall_log". |
---|
| 170 | <P> |
---|
| 171 | Similarly we create a logical volume with the cylindrical solid filled with |
---|
| 172 | aluminium |
---|
| 173 | <PRE> |
---|
| 174 | G4LogicalVolume* tracker_log |
---|
| 175 | = new G4LogicalVolume(tracker_tube,Al,"tracker_log"); |
---|
| 176 | </PRE> |
---|
| 177 | and named "tracker_log" |
---|
| 178 | <P> |
---|
| 179 | |
---|
| 180 | <HR> |
---|
| 181 | <A NAME="2.2.5"> |
---|
| 182 | <H2>2.2.5 Place a Volume</H2></A> |
---|
| 183 | |
---|
| 184 | How do you place a volume? You start with a logical volume, and then you decide |
---|
| 185 | the already existing volume inside of which to place it. Then you decide |
---|
| 186 | where to place its center within that volume, and how to rotate it. Once you have |
---|
| 187 | made these decisions, you can create a physical volume, which is the placed |
---|
| 188 | instance of the volume, and embodies all of these atributes. |
---|
| 189 | <P> |
---|
| 190 | |
---|
| 191 | <HR> |
---|
| 192 | <A NAME="2.2.6"> |
---|
| 193 | <H2>2.2.6 Create a Physical Volume</H2></A> |
---|
| 194 | <P> |
---|
| 195 | You create a physical volume starting with your logical volume. |
---|
| 196 | A physical volume is simply a placed instance of the logical volume. |
---|
| 197 | This instance must be placed inside a mother logical volume. |
---|
| 198 | For simplicity it is unrotated: |
---|
| 199 | <p> |
---|
| 200 | <center> |
---|
| 201 | <table border=2 cellpadding=10> |
---|
| 202 | <tr> |
---|
| 203 | <td> |
---|
| 204 | <PRE> |
---|
| 205 | G4double trackerPos_x = -1.0*meter; |
---|
| 206 | G4double trackerPos_y = 0.0*meter; |
---|
| 207 | G4double trackerPos_z = 0.0*meter; |
---|
| 208 | |
---|
| 209 | G4VPhysicalVolume* tracker_phys |
---|
| 210 | = new G4PVPlacement(0, // no rotation |
---|
| 211 | G4ThreeVector(trackerPos_x,trackerPos_y,trackerPos_z), |
---|
| 212 | // translation position |
---|
| 213 | tracker_log, // its logical volume |
---|
| 214 | "tracker", // its name |
---|
| 215 | experimentalHall_log, // its mother (logical) volume |
---|
| 216 | false, // no boolean operations |
---|
| 217 | 0); // its copy number |
---|
| 218 | </PRE> |
---|
| 219 | </td> |
---|
| 220 | </tr> |
---|
| 221 | <tr> |
---|
| 222 | <td align=center> |
---|
| 223 | Source listing 2.2.3<BR> |
---|
| 224 | A simple physical volume. |
---|
| 225 | </td> |
---|
| 226 | </tr> |
---|
| 227 | </table></center> |
---|
| 228 | <p> |
---|
| 229 | This places the logical volume <tt>tracker_log</tt> at the origin of the |
---|
| 230 | mother volume <tt>experimentalHall_log</tt>, shifted by one meter along X and |
---|
| 231 | unrotated. The resulting physical volume is named "tracker" and has a copy |
---|
| 232 | number of 0. |
---|
| 233 | <P> |
---|
| 234 | An exception exists to the rule that a physical volume must be placed inside |
---|
| 235 | a mother volume. That exception is for the World volume, which is the largest |
---|
| 236 | volume created, and which contains all other volumes. This volume obviously |
---|
| 237 | cannot be contained in any other. Instead, it must be created as a |
---|
| 238 | <i>G4PVPlacement</i> with a null mother pointer. It also must be |
---|
| 239 | unrotated, and it must be placed at the origin of the global coordinate system. |
---|
| 240 | <P> |
---|
| 241 | Generally, it is best to choose a simple solid as the World volume, and in |
---|
| 242 | Example N01, we use the experimental hall: |
---|
| 243 | <p> |
---|
| 244 | <center> |
---|
| 245 | <table border=2 cellpadding=10> |
---|
| 246 | <tr> |
---|
| 247 | <td> |
---|
| 248 | <PRE> |
---|
| 249 | G4VPhysicalVolume* experimentalHall_phys |
---|
| 250 | = new G4PVPlacement(0, // no rotation |
---|
| 251 | G4ThreeVector(0.,0.,0.), // translation position |
---|
| 252 | experimentalHall_log, // its logical volume |
---|
| 253 | "expHall", // its name |
---|
| 254 | 0, // its mother volume |
---|
| 255 | false, // no boolean operations |
---|
| 256 | 0); // its copy number |
---|
| 257 | </PRE> |
---|
| 258 | </td> |
---|
| 259 | </tr> |
---|
| 260 | <tr> |
---|
| 261 | <td align=center> |
---|
| 262 | Source listing 2.2.4<BR> |
---|
| 263 | The World volume from Example N01. |
---|
| 264 | </td> |
---|
| 265 | </tr> |
---|
| 266 | </table></center> |
---|
| 267 | <p> |
---|
| 268 | |
---|
| 269 | <HR> |
---|
| 270 | <A NAME="2.2.7"> |
---|
| 271 | <H2>2.2.7 Coordinate Systems and Rotations</H2></A> |
---|
| 272 | |
---|
| 273 | In Geant4, the rotation matrix associated to a placed physical volume represents |
---|
| 274 | the rotation of the reference system of this volume with respect to |
---|
| 275 | its mother. |
---|
| 276 | <p> |
---|
| 277 | A rotation matrix is normally constructed as in CLHEP, by instantiating the |
---|
| 278 | identity matrix and then applying a rotation to it. This is also demonstrated |
---|
| 279 | in Example N04. |
---|
| 280 | <P> |
---|
| 281 | |
---|
| 282 | <HR> |
---|
| 283 | <A HREF="../../../../Authors/html/subjectsToAuthors.html"> |
---|
| 284 | <I>About the authors</I></A> |
---|
| 285 | |
---|
| 286 | </BODY> |
---|
| 287 | </HTML> |
---|