source: trunk/documents/UserDoc/DocBookUsersGuides/ForApplicationDeveloper/xml/GettingStarted/geometryDef.xml @ 1358

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<!--  [History]                                               -->
<!--    Changed by: Katsuya Amako,  6-Aug-1998                -->
<!--    Changed by: Dennis Wright, 28-Nov-2001                -->
<!--    Proof read by: Joe Chuma,  14-Jun-1999                -->
<!--    Converted to DocBook: Katsuya Amako, Aug-2006         -->
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<!-- ******************* Section (Level#1) ****************** -->
<sect1 id="sect.HowToDefDetectorGeom">
<title>
How to Define a Detector Geometry
</title>

<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.HowToDefDetectorGeom.BasicConcepts">
<title>
Basic Concepts
</title>

<para>
A detector geometry in Geant4 is made of a number of volumes. The
largest volume is called the <emphasis role="bold">World</emphasis> 
volume. It must contain, with some margin, all other volumes 
in the detector geometry. The other volumes are created and placed 
inside previous volumes, included in the World volume. 
The most simple (and efficient) shape to describe the World is a box.
</para>

<para>
Each volume is created by describing its shape and its physical
characteristics, and then placing it inside a containing
volume.
</para>

<para>
When a volume is placed within another volume, we call the
former volume the daughter volume and the latter the mother volume.
The coordinate system used to specify where the daughter volume is
placed, is the coordinate system of the mother volume.
</para>

<para>
To describe a volume's shape, we use the concept of a solid. A
solid is a geometrical object that has a shape and specific values
for each of that shape's dimensions. A cube with a side of 10
centimeters and a cylinder of radius 30 cm and length 75 cm are
examples of solids.
</para>

<para>
To describe a volume's full properties, we use a logical volume.
It includes the geometrical properties of the solid, and adds
physical characteristics: the material of the volume; whether it
contains any sensitive detector elements; the magnetic field;
etc.
</para>

<para>
We have yet to describe how to position the volume. To do this
you create a physical volume, which places a copy of the logical
volume inside a larger, containing, volume.
</para>

</sect2>

<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.HowToDefDetectorGeom.CreateSimpleVol">
<title>
Create a Simple Volume
</title>

<para>
What do you need to do to create a volume?
<itemizedlist spacing="compact">
  <listitem><para>
  Create a solid.
  </para></listitem>
  <listitem><para>
  Create a logical volume, using this solid, and adding other
  attributes.
  </para></listitem>
</itemizedlist>
</para>

</sect2>

<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.HowToDefDetectorGeom.ChooseSolid">
<title>Choose a Solid
</title>

<para>
To create a simple box, you only need to define its name and its
extent along each of the Cartesian axes. You can find an example
how to do this in Novice Example N01.
</para>

<para>
In the detector description in the source file
<literal>ExN01DetectorConstruction.cc</literal>, you will find the following
box definition:

<example id="programlist_HowToDefDetectorGeom_1">
<title>
Creating a box.
</title>
<programlisting>
  G4double expHall_x = 3.0*m;
  G4double expHall_y = 1.0*m;
  G4double expHall_z = 1.0*m;

  G4Box* experimentalHall_box
     = new G4Box("expHall_box",expHall_x,expHall_y,expHall_z);
</programlisting>
</example>

This creates a box named "expHall_box" with extent from -3.0
meters to +3.0 meters along the X axis, from -1.0 to 1.0 meters in
Y, and from -1.0 to 1.0 meters in Z.
</para>

<para>
It is also very simple to create a cylinder. To do this, you can
use the <emphasis>G4Tubs</emphasis> class.

<example id="programlist_HowToDefDetectorGeom_2">
<title>
Creating a cylinder.
</title>
<programlisting>
  G4double innerRadiusOfTheTube = 0.*cm;
  G4double outerRadiusOfTheTube = 60.*cm;
  G4double hightOfTheTube = 25.*cm;
  G4double startAngleOfTheTube = 0.*deg;
  G4double spanningAngleOfTheTube = 360.*deg;

  G4Tubs* tracker_tube
    = new G4Tubs("tracker_tube",
                 innerRadiusOfTheTube, 
                 outerRadiusOfTheTube,
                 hightOfTheTube,
                 startAngleOfTheTube, 
                 spanningAngleOfTheTube);
</programlisting>
</example>

This creates a full cylinder, named "tracker_tube", of radius 60
centimeters and length 50 cm.
</para>

</sect2>

<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.HowToDefDetectorGeom.CreateLogicalVol">
<title>
Create a Logical Volume
</title>

<para>
To create a logical volume, you must start with a solid and a
material. So, using the box created above, you can create a simple
logical volume filled with argon gas (see materials) by entering:

<informalexample>
<programlisting>
 G4LogicalVolume* experimentalHall_log
    = new G4LogicalVolume(experimentalHall_box,Ar,"expHall_log");
</programlisting>
</informalexample>

This logical volume is named "expHall_log".
</para>

<para>
Similarly we create a logical volume with the cylindrical solid
filled with aluminium

<informalexample>
<programlisting>
  G4LogicalVolume* tracker_log
    = new G4LogicalVolume(tracker_tube,Al,"tracker_log");
</programlisting>
</informalexample>


and named "tracker_log"
</para>

</sect2>

<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.HowToDefDetectorGeom.PlaceVolume">
<title>
Place a Volume
</title>

<para>
How do you place a volume? You start with a logical volume, and
then you decide the already existing volume inside of which to
place it. Then you decide where to place its center within that
volume, and how to rotate it. Once you have made these decisions,
you can create a physical volume, which is the placed instance of
the volume, and embodies all of these atributes.
</para>

</sect2>

<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.HowToDefDetectorGeom.CreatePhysicalVol">
<title>Create a Physical Volume</title>

<para>
You create a physical volume starting with your logical volume.
A physical volume is simply a placed instance of the logical
volume. This instance must be placed inside a mother logical
volume. For simplicity it is unrotated:
</para>

<example id="programlist_HowToDefDetectorGeom_3">
<title>
A simple physical volume.
</title>
<programlisting>
  G4double trackerPos_x = -1.0*meter;
  G4double trackerPos_y =  0.0*meter;
  G4double trackerPos_z =  0.0*meter;

  G4VPhysicalVolume* tracker_phys
    = new G4PVPlacement(0,                       // no rotation
                        G4ThreeVector(trackerPos_x,trackerPos_y,trackerPos_z),
                                                 // translation position
                        tracker_log,             // its logical volume
                        "tracker",               // its name
                        experimentalHall_log,    // its mother (logical) volume
                        false,                   // no boolean operations
                        0);                      // its copy number
</programlisting>
</example>

<para>
This places the logical volume <literal>tracker_log</literal> at the
origin of the mother volume <literal>experimentalHall_log</literal>, shifted
by one meter along X and unrotated. The resulting physical volume
is named "tracker" and has a copy number of 0.
</para>

<para>
An exception exists to the rule that a physical volume must be
placed inside a mother volume. That exception is for the World
volume, which is the largest volume created, and which contains all
other volumes. This volume obviously cannot be contained in any
other. Instead, it must be created as a <emphasis>G4PVPlacement</emphasis> 
with a null mother pointer. It also must be unrotated, and it must be
placed at the origin of the global coordinate system.
</para>

<para>
Generally, it is best to choose a simple solid as the World
volume, and in Example N01, we use the experimental hall:

<example id="programlist_HowToDefDetectorGeom_4">
<title>
The World volume from Example N01.
</title>
<programlisting>
  G4VPhysicalVolume* experimentalHall_phys
      = new G4PVPlacement(0,                       // no rotation
                          G4ThreeVector(0.,0.,0.), // translation position
                          experimentalHall_log,    // its logical volume
                          "expHall",               // its name
                          0,                       // its mother volume
                          false,                   // no boolean operations
                          0);                      // its copy number
</programlisting>
</example>
</para>

</sect2>


<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.HowToDefDetectorGeom.CoordinateRotations">
<title>
Coordinate Systems and Rotations
</title>

<para>
In Geant4, the rotation matrix associated to a placed physical
volume represents the rotation of the reference system of this
volume with respect to its mother.
</para>

<para>
A rotation matrix is normally constructed as in CLHEP, by
instantiating the identity matrix and then applying a rotation to
it. This is also demonstrated in Example N04.
</para>

</sect2>
</sect1>