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9<TABLE WIDTH="100%"><TR>
10<TD>
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13<A HREF="index.html">
14<IMG SRC="../../../../resources/html/IconsGIF/Contents.gif" ALT="Contents"></A>
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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
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