| 1 | <HTML>
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| 2 | <TITLE>
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| 3 | </TITLE>
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| 4 | <!-- Changed by: Katsuya Amako, 4-Aug-1998 -->
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| 5 | <!-- Proof read by: Joe Chuma, 15-Jun-1999 -->
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| 6 | <!-- Changed by: Hisaya Kurashige, 28-Oct-2001 -->
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| 7 | <!-- Changed by: Dennis Wright, 29-Nov-2001 -->
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| 8 | <!-- Changed by: Hisaya Kurashige, 18-Jan-2007 -->
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| 9 |
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| 10 |
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| 11 | <BODY>
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| 12 | <TABLE WIDTH="100%"><TR>
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| 13 | <TD>
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| 14 |
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| 15 |
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| 16 | <A HREF="index.html">
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| 17 | <IMG SRC="../../../../resources/html/IconsGIF/Contents.gif" ALT="Contents"></A>
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| 18 | <A HREF="materialDef.html">
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| 19 | <IMG SRC="../../../../resources/html/IconsGIF/Previous.gif" ALT="Previous"></A>
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| 20 | <A HREF="physicsDef.html">
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| 21 | <IMG SRC="../../../../resources/html/IconsGIF/Next.gif" ALT="Next"></A>
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| 22 | </TD>
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| 23 | <TD ALIGN="Right">
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| 24 | <FONT SIZE="-1" COLOR="#238E23">
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| 25 | <B>Geant4 User's Guide</B>
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| 26 | <BR>
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| 27 | <B>For Application Developers</B>
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| 28 | <BR>
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| 29 | <B>Getting Started with Geant4</B>
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| 30 | </FONT>
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| 31 | </TD>
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| 32 | </TR></TABLE>
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| 33 | <BR>
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| 34 |
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| 35 | <P ALIGN="Center">
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| 36 | <FONT SIZE="+3" COLOR="#238E23">
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| 37 | <B>2.4 How to Specify Particles</B>
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| 38 | </FONT>
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| 39 | <P><BR>
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| 40 |
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| 41 | <HR ALIGN="Center" SIZE="7%">
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| 42 | <tt>G4VuserPhysicsList</tt> is one of the mandatory user base classes
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| 43 | described in <a href="mainProgram.html">Section 2.1</a> . Within this class
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| 44 | all particles and physics processes to be used in your simulation must be
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| 45 | defined. The range cut-off parameter should also be defined in this class.
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| 46 | <P>
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| 47 | The user must create a class derived from <tt>G4VuserPhysicsList</tt> and
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| 48 | implement the following pure virtual methods:
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| 49 | <table>
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| 50 | <tr>
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| 51 | <td><TT>ConstructParticle()</TT>:
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| 52 | <td>construction of particles
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| 53 | <tr>
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| 54 | <td><TT>ConstructProcess()</TT>:
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| 55 | <td>construct processes and register them to particles
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| 56 | <tr>
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| 57 | <td><TT>SetCuts()</TT>:
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| 58 | <td>setting a range cut value for all particles
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| 59 | </table>
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| 60 | <p>
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| 61 | This section provides some simple examples of the
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| 62 | <tt>ConstructParticle()</tt> and <tt>SetCuts()</tt> methods.
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| 63 | For infomation on <tt>ConstructProcess()</tt> methods, please see
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| 64 | <a href="physicsDef.html">Section 2.5</a>.
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| 65 | <P>
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| 66 | <HR>
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| 67 | <a name="2.4.1">
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| 68 | <H2>2.4.1 Particle Definition</H2></a>
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| 69 |
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| 70 | Geant4 provides various types of particles for use in simulations:
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| 71 | <UL>
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| 72 | <LI>ordinary particles, such as electrons, protons, and gammas
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| 73 | <LI>resonant particles with very short lifetimes,
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| 74 | such as vector mesons and delta baryons
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| 75 | <LI>nuclei, such as deuterons, alphas, and heavy ions
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| 76 | <LI>quarks, di-quarks, and gluons
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| 77 | </UL>
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| 78 | <p>
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| 79 | Each particle is represented by its own class, which is derived from
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| 80 | <tt>G4ParticleDefinition</tt>. Particles are organized into six major
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| 81 | categories:
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| 82 | <ul>
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| 83 | <li>lepton,
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| 84 | <li>meson,
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| 85 | <li>baryon,
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| 86 | <li>boson,
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| 87 | <li>shortlived and
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| 88 | <li>ion,
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| 89 | </ul>
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| 90 | each of which is defined in a corresponding sub-directory under
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| 91 | <TT>geant4/source/particles</TT>. There is also a corresponding granular
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| 92 | library for each particle category.
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| 93 | </p>
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| 94 |
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| 95 | <H4>2.4.1.1 The <tt>G4ParticleDefinition</tt> Class </H4>
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| 96 |
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| 97 | <tt>G4ParticleDefinition</tt> has properties which characterize individual
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| 98 | particles, such as, name, mass, charge, spin, and so on. Most of these
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| 99 | properties are "read-only" and can not be changed by users without rebuilding
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| 100 | the libraries.
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| 101 |
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| 102 | <H4>2.4.1.2 How to Access a Particle</H4>
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| 103 | <P>
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| 104 | Each particle class type represents an individual particle type, and each
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| 105 | class has a single static object. There are some exceptions to this rule;
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| 106 | please see <a href="../TrackingAndPhysics/particle.html">Section 5.3</a> for
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| 107 | details.
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| 108 | <P>
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| 109 | For example, the class <tt>G4Electron</tt> represents the electron and its
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| 110 | only object is <TT>G4Electron::theElectron</TT>. The object is therefore
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| 111 | referred to as a "singleton" of the <i>G4Electron</i> class. The pointer to
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| 112 | this object is available through the static method
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| 113 | <TT>G4Electron::ElectronDefinition()</TT>.
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| 114 | <P>
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| 115 | More than 100 types of particles are provided by default, to be used in
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| 116 | various physics processes. In normal applications, users will not need to
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| 117 | define their own particles.
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| 118 | <p>
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| 119 | Because particles are static objects of individual particle classes, these
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| 120 | objects are instantiated automatically before the <tt>main()</tt> routine is
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| 121 | executed. However, you must explicitly declare the particle classes required
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| 122 | by your program, otherwise the compiler can not recognize which classes you
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| 123 | need, and no particles will be instantiated.
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| 124 | <P>
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| 125 |
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| 126 | <H4>2.4.1.3 Dictionary of Particles</H4>
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| 127 | The <tt>G4ParticleTable</tt> class is provided as a dictionary of particles.
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| 128 | Various utility methods are provided, such as:
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| 129 | <center><table>
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| 130 | <tr>
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| 131 | <td><tt>FindParticle(G4String name)</tt>:
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| 132 | <td>find the particle by name
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| 133 | <tr>
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| 134 | <td><tt>FindParticle(G4int PDGencoding)</tt>:
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| 135 | <td>find the particle by PDG encoding .
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| 136 | </table></center>
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| 137 | <p>
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| 138 | <tt>G4ParticleTable</tt> is also defined as a singleton object, and the
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| 139 | static method <TT>G4ParticleTable::GetParticleTable()</TT> provides its
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| 140 | pointer.
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| 141 | <P>
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| 142 | Particles are registered automatically during construction. The user has no
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| 143 | control over particle registration.
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| 144 | <p>
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| 145 |
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| 146 | <H4>2.4.1.4 Constructing Particles</H4>
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| 147 | <P>
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| 148 | <TT>ConstructParticle()</TT> is a pure virtual method, in which the static
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| 149 | member functions for all the particles you require should be called. This
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| 150 | ensures that objects of these particles will be created.
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| 151 |
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| 152 | <p>
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| 153 | WARNING: You must define "All PARTICLE TYPES" which are used in your application, except for heavy ions.
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| 154 | "All PARTICLE TYPES" means not only primary particles, but also all other particles which may
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| 155 | appear as secondaries generated by physics processes you use.
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| 156 | Beginning with Geant4 version 8.0, you should keep this rule strictly because all particle definitions are revised to "non-static" objects.
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| 157 | <p>
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| 158 |
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| 159 | For example, suppose you need a proton and a geantino, which is a virtual
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| 160 | particle used for simulation and which does not interact with materials.
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| 161 | The <TT>ConstructParticle()</TT> method is implemented as below:
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| 162 |
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| 163 | <center>
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| 164 | <table border=2 cellpadding=10>
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| 165 | <tr>
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| 166 | <td>
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| 167 | <PRE>
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| 168 | void ExN01PhysicsList::ConstructParticle()
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| 169 | {
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| 170 | G4Proton::ProtonDefinition();
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| 171 | G4Geantino::GeantinoDefinition();
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| 172 | }
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| 173 | </PRE>
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| 174 | </td>
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| 175 | </tr>
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| 176 | <tr>
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| 177 | <td align=center>
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| 178 | Source listing 2.4.1<BR>
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| 179 | Construct a proton and a geantino.
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| 180 | </td>
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| 181 | </tr>
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| 182 | </table></center>
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| 183 | <P>
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| 184 | Due to the large number of pre-defined particles in Geant4, it is
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| 185 | cumbersome to list all the particles by this method. If you want all the
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| 186 | particles in a Geant4 particle category, there are six utility classes,
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| 187 | corresponding to each of the particle categories, which perform this function:
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| 188 | <UL>
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| 189 | <LI><tt>G4BosonConstructor</tt>
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| 190 | <LI><tt>G4LeptonConstructor</tt>
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| 191 | <LI><tt>G4MesonConstructor</tt>
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| 192 | <LI><tt>G4BarionConstructor</tt>
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| 193 | <LI><tt>G4IonConstructor</tt>
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| 194 | <LI><tt>G4ShortlivedConstructor</tt> .
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| 195 | </UL>
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| 196 | <P>
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| 197 | An example of this is shown in <tt>ExN05PhysicsList</tt>, listed below.
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| 198 | <P>
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| 199 | <center>
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| 200 | <table border=2 cellpadding=10>
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| 201 | <tr>
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| 202 | <td>
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| 203 | <PRE>
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| 204 | void ExN05PhysicsList::ConstructLeptons()
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| 205 | {
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| 206 | // Construct all leptons
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| 207 | G4LeptonConstructor pConstructor;
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| 208 | pConstructor.ConstructParticle();
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| 209 | }
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| 210 | </PRE>
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| 211 | </td>
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| 212 | </tr>
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| 213 | <tr>
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| 214 | <td align=center>
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| 215 | Source listing 2.4.2<BR>
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| 216 | Construct all leptons.
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| 217 | </td>
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| 218 | </tr>
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| 219 | </table></center>
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| 220 | <P>
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| 221 |
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| 222 | <HR>
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| 223 | <a name="2.4.2">
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| 224 | <H2>2.4.2 Range Cuts</H2></a>
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| 225 |
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| 226 | To avoid infrared divergence, some electromagnetic processes require a
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| 227 | threshold below which no secondary will be generated. Because of this
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| 228 | requirement, gammas, electrons and positrons require production thresholds
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| 229 | which the user should define. This threshold should be defined as a distance,
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| 230 | or range cut-off, which is internally converted to an energy for individual
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| 231 | materials. The range threshold should be defined in the initialization phase
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| 232 | using the <tt>SetCuts()</tt> method of <tt>G4VUserPhysicsList</tt>.
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| 233 | <a href="../TrackingAndPhysics/thresholdVScut.html">Section 5.4</a> discusses
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| 234 | threshold and tracking cuts in detail.
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| 235 | </P>
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| 236 |
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| 237 | <H4>2.4.2.1 Setting the cuts</H4>
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| 238 |
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| 239 | Production threshold values should be defined in <TT>SetCuts()</TT> which is a
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| 240 | pure virtual method of the <tt>G4VUserPhysicsList</tt> class.
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| 241 | Construction of particles, materials, and processes should precede the
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| 242 | invocation of <TT>SetCuts()</TT>. <tt>G4RunManager</tt> takes care of this
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| 243 | sequence in usual applications.
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| 244 |
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| 245 | <P>
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| 246 | The idea of a "unique cut value in range" is one of the important features of
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| 247 | Geant4 and is used to handle cut values in a coherent manner.
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| 248 | For most applications, users need to determine only one cut value in range,
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| 249 | and apply this value to gammas, electrons and positrons alike.
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| 250 | <P>
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| 251 | In such a case, the <TT>SetCutsWithDefault()</TT> method may be used. It
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| 252 | is provided by the <tt>G4VuserPhysicsList</tt> base class, which has a
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| 253 | <tt>defaultCutValue</tt> member as the default range cut-off value.
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| 254 | <TT>SetCutsWithDefault()</TT> uses this value.
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| 255 | <P>
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| 256 | It is possible to set different range cut values for gammas, electrons and
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| 257 | positrons, and also to set different range cut values for each geometrical
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| 258 | region. In such cases however, one must be careful with physics outputs
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| 259 | because Geant4 processes (especially energy loss) are designed to conform
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| 260 | to the "unique cut value in range" scheme.
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| 261 | <P>
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| 262 | <center>
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| 263 | <table border=2 cellpadding=10>
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| 264 | <tr>
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| 265 | <td>
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| 266 | <PRE>
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| 267 | void ExN04PhysicsList::SetCuts()
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| 268 | {
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| 269 | // the G4VUserPhysicsList::SetCutsWithDefault() method sets
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| 270 | // the default cut value for all particle types
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| 271 | SetCutsWithDefault();
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| 272 | }
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| 273 | </PRE>
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| 274 | </td>
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| 275 | </tr>
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| 276 | <tr>
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| 277 | <td align=center>
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| 278 | Source listing 2.4.3<BR>
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| 279 | Set cut values by using the default cut value.
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| 280 | </td>
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| 281 | </tr>
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| 282 | </table></center>
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| 283 | <P>
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| 284 | The <tt>defaultCutValue</tt> is set to 1.0 mm by default. Of
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| 285 | course, you can set the new default cut value in the constructor
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| 286 | of your physics list class as shown below.
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| 287 | <P>
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| 288 | <center>
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| 289 | <table border=2 cellpadding=10>
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| 290 | <tr>
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| 291 | <td>
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| 292 | <PRE>
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| 293 | ExN04PhysicsList::ExN04PhysicsList(): G4VUserPhysicsList()
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| 294 | {
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| 295 | // default cut value (1.0mm)
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| 296 | defaultCutValue = 1.0*mm;
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| 297 | }
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| 298 | </PRE>
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| 299 | </td>
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| 300 | </tr>
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| 301 | <tr>
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| 302 | <td align=center>
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| 303 | Source listing 2.4.4<BR>
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| 304 | Set the default cut value.
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| 305 | </td>
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| 306 | </tr>
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| 307 | </table></center>
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| 308 | <P>
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| 309 | The <TT>SetDefaultCutValue() </TT> method in <tt>G4VUserPhysicsList</tt> may
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| 310 | also be used, and the "/run/setCut" command may be used to change
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| 311 | the default cut value interactively.
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| 312 | <P>
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| 313 | WARNING: DO NOT change cut values inside the event loop. Cut values may
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| 314 | however be changed between runs.
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| 315 | <P>
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| 316 | An example implementation of <TT>SetCuts()</TT> is shown below:
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| 317 | <p>
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| 318 | <center>
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| 319 | <table border=2 cellpadding=10>
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| 320 | <tr>
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| 321 | <td>
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| 322 | <PRE>
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| 323 | void ExN03PhysicsList::SetCuts()
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| 324 | {
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| 325 | // set cut values for gamma at first and for e- second and next for e+,
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| 326 | // because some processes for e+/e- need cut values for gamma
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| 327 | SetCutValue(cutForGamma, "gamma");
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| 328 | SetCutValue(cutForElectron, "e-");
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| 329 | SetCutValue(cutForElectron, "e+");
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| 330 | }
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| 331 | </PRE>
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| 332 | </td>
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| 333 | </tr>
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| 334 | <tr>
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| 335 | <td align=center>
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| 336 | Source listing 2.4.5<BR>
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| 337 | Example implementation of the <tt>SetCuts()</tt> method.
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| 338 | </td>
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| 339 | </tr>
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| 340 | </table></center>
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| 341 | <P>
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| 342 | Beginning with Geant4 version 5.1, it is now possible to set production
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| 343 | thresholds for each geometrical region. This new functionality is described
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| 344 | in <a href="../TrackingAndPhysics/cutsPerRegion.html">Section 5.5</a>.
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| 345 | <P>
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| 346 |
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| 347 | <HR>
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| 348 | <A HREF="../../../../Authors/html/subjectsToAuthors.html">
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| 349 | <I>About the authors</I></A>
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| 350 |
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| 351 | </BODY>
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| 352 | </HTML>
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| 353 |
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