| 1 | <html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>2.2. How to Define a Detector Geometry</title><link rel="stylesheet" href="../xml/XSLCustomizationLayer/G4HTMLStylesheet.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.71.1"><link rel="start" href="index.html" title="Geant4 User's Guide for Application Developers"><link rel="up" href="ch02.html" title="Chapter 2. Getting Started with Geant4 - Running a Simple Example"><link rel="prev" href="ch02.html" title="Chapter 2. Getting Started with Geant4 - Running a Simple Example"><link rel="next" href="ch02s03.html" title="2.3. How to Specify Materials in the Detector"><script language="JavaScript">
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| 8 | </script></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">2.2.
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| 9 | How to Define a Detector Geometry
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| 10 | </th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch02.html"><img src="AllResources/IconsGIF/prev.gif" alt="Prev"></a> </td><th width="60%" align="center">Chapter 2.
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| 11 | Getting Started with Geant4 - Running a Simple Example
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| 12 | </th><td width="20%" align="right"> <a accesskey="n" href="ch02s03.html"><img src="AllResources/IconsGIF/next.gif" alt="Next"></a></td></tr></table><hr></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="sect.HowToDefDetectorGeom"></a>2.2.
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| 13 | How to Define a Detector Geometry
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| 14 | </h2></div></div></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="sect.HowToDefDetectorGeom.BasicConcepts"></a>2.2.1.
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| 15 | Basic Concepts
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| 16 | </h3></div></div></div><p>
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| 17 | A detector geometry in Geant4 is made of a number of volumes. The
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| 18 | largest volume is called the <span class="bold"><strong>World</strong></span>
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| 19 | volume. It must contain, with some margin, all other volumes
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| 20 | in the detector geometry. The other volumes are created and placed
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| 21 | inside previous volumes, included in the World volume.
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| 22 | The most simple (and efficient) shape to describe the World is a box.
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| 23 | </p><p>
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| 24 | Each volume is created by describing its shape and its physical
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| 25 | characteristics, and then placing it inside a containing
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| 26 | volume.
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| 27 | </p><p>
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| 28 | When a volume is placed within another volume, we call the
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| 29 | former volume the daughter volume and the latter the mother volume.
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| 30 | The coordinate system used to specify where the daughter volume is
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| 31 | placed, is the coordinate system of the mother volume.
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| 32 | </p><p>
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| 33 | To describe a volume's shape, we use the concept of a solid. A
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| 34 | solid is a geometrical object that has a shape and specific values
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| 35 | for each of that shape's dimensions. A cube with a side of 10
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| 36 | centimeters and a cylinder of radius 30 cm and length 75 cm are
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| 37 | examples of solids.
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| 38 | </p><p>
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| 39 | To describe a volume's full properties, we use a logical volume.
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| 40 | It includes the geometrical properties of the solid, and adds
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| 41 | physical characteristics: the material of the volume; whether it
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| 42 | contains any sensitive detector elements; the magnetic field;
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| 43 | etc.
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| 44 | </p><p>
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| 45 | We have yet to describe how to position the volume. To do this
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| 46 | you create a physical volume, which places a copy of the logical
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| 47 | volume inside a larger, containing, volume.
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| 48 | </p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="sect.HowToDefDetectorGeom.CreateSimpleVol"></a>2.2.2.
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| 49 | Create a Simple Volume
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| 50 | </h3></div></div></div><p>
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| 51 | What do you need to do to create a volume?
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| 52 | </p><div class="itemizedlist"><ul type="disc" compact><li><p>
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| 53 | Create a solid.
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| 54 | </p></li><li><p>
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| 55 | Create a logical volume, using this solid, and adding other
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| 56 | attributes.
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| 57 | </p></li></ul></div><p>
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| 58 | </p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="sect.HowToDefDetectorGeom.ChooseSolid"></a>2.2.3. Choose a Solid
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| 59 | </h3></div></div></div><p>
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| 60 | To create a simple box, you only need to define its name and its
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| 61 | extent along each of the Cartesian axes. You can find an example
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| 62 | how to do this in Novice Example N01.
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| 63 | </p><p>
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| 64 | In the detector description in the source file
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| 65 | <code class="literal">ExN01DetectorConstruction.cc</code>, you will find the following
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| 66 | box definition:
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| 67 |
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| 68 | </p><div class="example"><a name="programlist_HowToDefDetectorGeom_1"></a><p class="title"><b>Example 2.3.
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| 69 | Creating a box.
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| 70 | </b></p><div class="example-contents"><pre class="programlisting">
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| 71 | G4double expHall_x = 3.0*m;
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| 72 | G4double expHall_y = 1.0*m;
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| 73 | G4double expHall_z = 1.0*m;
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| 74 |
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| 75 | G4Box* experimentalHall_box
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| 76 | = new G4Box("expHall_box",expHall_x,expHall_y,expHall_z);
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| 77 | </pre></div></div><p><br class="example-break">
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| 78 |
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| 79 | This creates a box named "expHall_box" with extent from -3.0
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| 80 | meters to +3.0 meters along the X axis, from -1.0 to 1.0 meters in
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| 81 | Y, and from -1.0 to 1.0 meters in Z.
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| 82 | </p><p>
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| 83 | It is also very simple to create a cylinder. To do this, you can
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| 84 | use the <span class="emphasis"><em>G4Tubs</em></span> class.
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| 85 |
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| 86 | </p><div class="example"><a name="programlist_HowToDefDetectorGeom_2"></a><p class="title"><b>Example 2.4.
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| 87 | Creating a cylinder.
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| 88 | </b></p><div class="example-contents"><pre class="programlisting">
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| 89 | G4double innerRadiusOfTheTube = 0.*cm;
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| 90 | G4double outerRadiusOfTheTube = 60.*cm;
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| 91 | G4double hightOfTheTube = 25.*cm;
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| 92 | G4double startAngleOfTheTube = 0.*deg;
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| 93 | G4double spanningAngleOfTheTube = 360.*deg;
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| 94 |
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| 95 | G4Tubs* tracker_tube
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| 96 | = new G4Tubs("tracker_tube",
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| 97 | innerRadiusOfTheTube,
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| 98 | outerRadiusOfTheTube,
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| 99 | hightOfTheTube,
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| 100 | startAngleOfTheTube,
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| 101 | spanningAngleOfTheTube);
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| 102 | </pre></div></div><p><br class="example-break">
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| 103 |
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| 104 | This creates a full cylinder, named "tracker_tube", of radius 60
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| 105 | centimeters and length 50 cm.
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| 106 | </p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="sect.HowToDefDetectorGeom.CreateLogicalVol"></a>2.2.4.
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| 107 | Create a Logical Volume
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| 108 | </h3></div></div></div><p>
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| 109 | To create a logical volume, you must start with a solid and a
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| 110 | material. So, using the box created above, you can create a simple
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| 111 | logical volume filled with argon gas (see materials) by entering:
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| 112 |
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| 113 | </p><div class="informalexample"><pre class="programlisting">
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| 114 | G4LogicalVolume* experimentalHall_log
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| 115 | = new G4LogicalVolume(experimentalHall_box,Ar,"expHall_log");
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| 116 | </pre></div><p>
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| 117 |
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| 118 | This logical volume is named "expHall_log".
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| 119 | </p><p>
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| 120 | Similarly we create a logical volume with the cylindrical solid
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| 121 | filled with aluminium
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| 122 |
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| 123 | </p><div class="informalexample"><pre class="programlisting">
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| 124 | G4LogicalVolume* tracker_log
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| 125 | = new G4LogicalVolume(tracker_tube,Al,"tracker_log");
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| 126 | </pre></div><p>
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| 127 |
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| 128 |
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| 129 | and named "tracker_log"
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| 130 | </p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="sect.HowToDefDetectorGeom.PlaceVolume"></a>2.2.5.
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| 131 | Place a Volume
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| 132 | </h3></div></div></div><p>
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| 133 | How do you place a volume? You start with a logical volume, and
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| 134 | then you decide the already existing volume inside of which to
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| 135 | place it. Then you decide where to place its center within that
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| 136 | volume, and how to rotate it. Once you have made these decisions,
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| 137 | you can create a physical volume, which is the placed instance of
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| 138 | the volume, and embodies all of these atributes.
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| 139 | </p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="sect.HowToDefDetectorGeom.CreatePhysicalVol"></a>2.2.6. Create a Physical Volume</h3></div></div></div><p>
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| 140 | You create a physical volume starting with your logical volume.
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| 141 | A physical volume is simply a placed instance of the logical
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| 142 | volume. This instance must be placed inside a mother logical
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| 143 | volume. For simplicity it is unrotated:
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| 144 | </p><div class="example"><a name="programlist_HowToDefDetectorGeom_3"></a><p class="title"><b>Example 2.5.
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| 145 | A simple physical volume.
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| 146 | </b></p><div class="example-contents"><pre class="programlisting">
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| 147 | G4double trackerPos_x = -1.0*meter;
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| 148 | G4double trackerPos_y = 0.0*meter;
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| 149 | G4double trackerPos_z = 0.0*meter;
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| 150 |
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| 151 | G4VPhysicalVolume* tracker_phys
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| 152 | = new G4PVPlacement(0, // no rotation
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| 153 | G4ThreeVector(trackerPos_x,trackerPos_y,trackerPos_z),
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| 154 | // translation position
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| 155 | tracker_log, // its logical volume
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| 156 | "tracker", // its name
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| 157 | experimentalHall_log, // its mother (logical) volume
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| 158 | false, // no boolean operations
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| 159 | 0); // its copy number
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| 160 | </pre></div></div><br class="example-break"><p>
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| 161 | This places the logical volume <code class="literal">tracker_log</code> at the
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| 162 | origin of the mother volume <code class="literal">experimentalHall_log</code>, shifted
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| 163 | by one meter along X and unrotated. The resulting physical volume
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| 164 | is named "tracker" and has a copy number of 0.
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| 165 | </p><p>
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| 166 | An exception exists to the rule that a physical volume must be
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| 167 | placed inside a mother volume. That exception is for the World
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| 168 | volume, which is the largest volume created, and which contains all
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| 169 | other volumes. This volume obviously cannot be contained in any
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| 170 | other. Instead, it must be created as a <span class="emphasis"><em>G4PVPlacement</em></span>
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| 171 | with a null mother pointer. It also must be unrotated, and it must be
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| 172 | placed at the origin of the global coordinate system.
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| 173 | </p><p>
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| 174 | Generally, it is best to choose a simple solid as the World
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| 175 | volume, and in Example N01, we use the experimental hall:
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| 176 |
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| 177 | </p><div class="example"><a name="programlist_HowToDefDetectorGeom_4"></a><p class="title"><b>Example 2.6.
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| 178 | The World volume from Example N01.
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| 179 | </b></p><div class="example-contents"><pre class="programlisting">
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| 180 | G4VPhysicalVolume* experimentalHall_phys
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| 181 | = new G4PVPlacement(0, // no rotation
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| 182 | G4ThreeVector(0.,0.,0.), // translation position
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| 183 | experimentalHall_log, // its logical volume
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| 184 | "expHall", // its name
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| 185 | 0, // its mother volume
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| 186 | false, // no boolean operations
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| 187 | 0); // its copy number
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| 188 | </pre></div></div><p><br class="example-break">
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| 189 | </p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="sect.HowToDefDetectorGeom.CoordinateRotations"></a>2.2.7.
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| 190 | Coordinate Systems and Rotations
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| 191 | </h3></div></div></div><p>
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| 192 | In Geant4, the rotation matrix associated to a placed physical
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| 193 | volume represents the rotation of the reference system of this
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| 194 | volume with respect to its mother.
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| 195 | </p><p>
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| 196 | A rotation matrix is normally constructed as in CLHEP, by
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| 197 | instantiating the identity matrix and then applying a rotation to
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| 198 | it. This is also demonstrated in Example N04.
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| 199 | </p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch02.html"><img src="AllResources/IconsGIF/prev.gif" alt="Prev"></a> </td><td width="20%" align="center"><a accesskey="u" href="ch02.html"><img src="AllResources/IconsGIF/up.gif" alt="Up"></a></td><td width="40%" align="right"> <a accesskey="n" href="ch02s03.html"><img src="AllResources/IconsGIF/next.gif" alt="Next"></a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 2.
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| 200 | Getting Started with Geant4 - Running a Simple Example
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| 201 | </td><td width="20%" align="center"><a accesskey="h" href="index.html"><img src="AllResources/IconsGIF/home.gif" alt="Home"></a></td><td width="40%" align="right" valign="top"> 2.3.
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| 202 | How to Specify Materials in the Detector
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| 203 | </td></tr></table></div></body></html>
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