| 1 | <html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>2.4. How to Specify Particles</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="ch02s03.html" title="2.3. How to Specify Materials in the Detector"><link rel="next" href="ch02s05.html" title="2.5. How to Specify Physics Processes"><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">2.4.
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| 9 | How to Specify Particles
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| 10 | </th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch02s03.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="ch02s05.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.HowToSpecParti"></a>2.4.
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| 13 | How to Specify Particles
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| 14 | </h2></div></div></div><p>
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| 15 | <code class="literal">G4VuserPhysicsList</code> is one of the mandatory user base
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| 16 | classes described in
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| 17 | <a href="ch02.html#sect.HowToDefMain" title="2.1.
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| 18 | How to Define the main() Program
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| 19 | ">Section 2.1</a>.
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| 20 | Within this class all particles and physics processes to be used in
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| 21 | your simulation must be defined. The range cut-off parameter should
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| 22 | also be defined in this class.
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| 23 | </p><p>
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| 24 | The user must create a class derived from
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| 25 | <code class="literal">G4VuserPhysicsList</code> and implement the following pure
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| 26 | virtual methods:
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| 27 |
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| 28 | </p><div class="informalexample"><pre class="programlisting">
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| 29 | ConstructParticle(); // construction of particles
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| 30 | ConstructProcess(); // construct processes and register them to particles
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| 31 | SetCuts(); // setting a range cut value for all particles
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| 32 | </pre></div><p>
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| 33 | </p><p>
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| 34 | This section provides some simple examples of the
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| 35 | <code class="literal">ConstructParticle()</code> and <code class="literal">SetCuts()</code>
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| 36 | methods.
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| 37 | For information on <code class="literal">ConstructProcess()</code> methods, please see
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| 38 | <a href="ch02s05.html" title="2.5.
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| 39 | How to Specify Physics Processes
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| 40 | ">Section 2.5</a>.
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| 41 |
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| 42 | </p><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="sect.HowToSpecParti.PartiDef"></a>2.4.1.
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| 43 | Particle Definition
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| 44 | </h3></div></div></div><p>
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| 45 | Geant4 provides various types of particles for use in simulations:
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| 46 |
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| 47 | </p><div class="itemizedlist"><ul type="disc" compact><li><p>
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| 48 | ordinary particles, such as electrons, protons, and gammas
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| 49 | </p></li><li><p>
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| 50 | resonant particles with very short lifetimes, such as vector
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| 51 | mesons and delta baryons
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| 52 | </p></li><li><p>
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| 53 | nuclei, such as deuteron, alpha, and heavy ions (including hyper-nuclei)
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| 54 | </p></li><li><p>
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| 55 | quarks, di-quarks, and gluon
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| 56 | </p></li></ul></div><p>
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| 57 | </p><p>
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| 58 | Each particle is represented by its own class, which is derived
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| 59 | from <code class="literal">G4ParticleDefinition</code>.
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| 60 | (Exception: G4Ions represents all heavy nuclei. Please see
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| 61 | <a href="ch05s03.html" title="5.3.
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| 62 | Particles
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| 63 | ">Section 5.3</a>.)
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| 64 | Particles are organized into six major categories:
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| 65 |
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| 66 | </p><div class="itemizedlist"><ul type="disc" compact><li><p>
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| 67 | lepton,
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| 68 | </p></li><li><p>
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| 69 | meson,
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| 70 | </p></li><li><p>
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| 71 | baryon,
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| 72 | </p></li><li><p>
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| 73 | boson,
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| 74 | </p></li><li><p>
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| 75 | shortlived and
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| 76 | </p></li><li><p>
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| 77 | ion,
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| 78 | </p></li></ul></div><p>
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| 79 |
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| 80 | each of which is defined in a corresponding sub-directory under
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| 81 | <code class="literal">geant4/source/particles</code>. There is also a corresponding
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| 82 | granular library for each particle category.
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| 83 | </p><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="sect.HowToSpecParti.PartiDef.G4ParticleDefinition"></a>2.4.1.1.
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| 84 | The <code class="literal">G4ParticleDefinition</code> Class
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| 85 | </h4></div></div></div><p>
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| 86 | <code class="literal">G4ParticleDefinition</code> has properties which characterize
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| 87 | individual particles, such as, name, mass, charge, spin, and so on.
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| 88 | Most of these properties are "read-only" and can not be changed by
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| 89 | users without rebuilding the libraries.
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| 90 | </p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="sect.HowToSpecParti.PartiDef.HowToAccessParti"></a>2.4.1.2.
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| 91 | How to Access a Particle
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| 92 | </h4></div></div></div><p>
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| 93 | Each particle class type represents an individual particle type,
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| 94 | and each class has a single object. This object can be accessed by
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| 95 | using the static method of each class.
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| 96 | There are some exceptions to this rule; please see
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| 97 | <a href="ch05s03.html" title="5.3.
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| 98 | Particles
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| 99 | ">Section 5.3</a> for details.
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| 100 | </p><p>
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| 101 | For example, the class <code class="literal">G4Electron</code> represents the
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| 102 | electron and the member <code class="literal">G4Electron::theInstance</code>
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| 103 | points its only object.
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| 104 | The pointer to this object is available
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| 105 | through the static methods
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| 106 | <code class="literal">G4Electron::ElectronDefinition()</code>.
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| 107 | <code class="literal">G4Electron::Definition()</code>.
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| 108 | </p><p>
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| 109 | More than 100 types of particles are provided by default, to be
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| 110 | used in various physics processes. In normal applications, users
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| 111 | will not need to define their own particles.
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| 112 | </p><p>
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| 113 | Because particles are static objects of individual particle
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| 114 | classes, these objects are instantiated
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| 115 | when the static method getting the pointer is invoked at the first time
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| 116 | Therefore, you must explicitly declare the particle classes
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| 117 | required by your program at the initialization step,
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| 118 | otherwise no particles will be instantiated.
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| 119 | </p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="sect.HowToSpecParti.PartiDef.DictOfParti"></a>2.4.1.3. Dictionary of Particles
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| 120 | </h4></div></div></div><p>
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| 121 | The <code class="literal">G4ParticleTable</code> class is provided as a dictionary of
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| 122 | particles. Various utility methods are provided, such as:
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| 123 |
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| 124 | </p><div class="informalexample"><pre class="programlisting">
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| 125 | FindParticle(G4String name); // find the particle by name
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| 126 | FindParticle(G4int PDGencoding) // find the particle by PDG encoding .
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| 127 | </pre></div><p>
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| 128 | </p><p>
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| 129 | <code class="literal">G4ParticleTable</code> is defined as a singleton object,
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| 130 | and the static method <code class="literal">G4ParticleTable::GetParticleTable()</code>
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| 131 | provides its pointer.
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| 132 | </p><p>
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| 133 | As for heavy ions (including hyper-nuclei), objects are created
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| 134 | dynamically by requests from users and processes.
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| 135 | The <code class="literal">G4ParticleTable</code> class provides
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| 136 | methods to create ions, such as:
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| 137 | </p><div class="informalexample"><pre class="programlisting">
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| 138 | G4ParticleDefinition* GetIon( G4int atomicNumber,
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| 139 | G4int atomicMass,
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| 140 | G4double excitationEnergy);
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| 141 | </pre></div><p>
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| 142 | </p><p>
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| 143 | Particles are registered automatically during construction. The
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| 144 | user has no control over particle registration.
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| 145 | </p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="sect.HowToSpecParti.PartiDef.ConstruParti"></a>2.4.1.4.
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| 146 | Constructing Particles
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| 147 | </h4></div></div></div><p>
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| 148 | <code class="literal">ConstructParticle()</code> is a pure virtual method, in which
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| 149 | the static member functions for all the particles you require should be called.
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| 150 | This ensures that objects of these particles are created.
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| 151 | </p><p>
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| 152 | WARNING: You must define "All PARTICLE TYPES" which are used in your
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| 153 | application, except for heavy ions.
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| 154 | "All PARTICLE TYPES" means not only primary
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| 155 | particles, but also all other particles which may appear as secondaries
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| 156 | generated by physics processes you use. Beginning with Geant4 version 8.0,
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| 157 | you should keep this rule strictly because all particle definitions are
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| 158 | revised to "non-static" objects.
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| 159 | </p><p>
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| 160 | For example, suppose you need a proton and a geantino, which is
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| 161 | a virtual particle used for simulation and which does not interact
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| 162 | with materials. The <code class="literal">ConstructParticle()</code> method is
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| 163 | implemented as below:
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| 164 |
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| 165 | </p><div class="example"><a name="programlist_HowToSpecParti_1"></a><p class="title"><b>Example 2.12.
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| 166 | Construct a proton and a geantino.
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| 167 | </b></p><div class="example-contents"><pre class="programlisting">
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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></div></div><p><br class="example-break">
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| 174 | </p><p>
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| 175 | Due to the large number of pre-defined particles in Geant4, it
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| 176 | is cumbersome to list all the particles by this method. If you want
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| 177 | all the particles in a Geant4 particle category, there are six
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| 178 | utility classes, corresponding to each of the particle categories,
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| 179 | which perform this function:
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| 180 |
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| 181 | </p><div class="itemizedlist"><ul type="disc" compact><li><p>
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| 182 | <code class="literal">G4BosonConstructor</code>
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| 183 | </p></li><li><p>
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| 184 | <code class="literal">G4LeptonConstructor</code>
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| 185 | </p></li><li><p>
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| 186 | <code class="literal">G4MesonConstructor</code>
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| 187 | </p></li><li><p>
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| 188 | <code class="literal">G4BarionConstructor</code>
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| 189 | </p></li><li><p>
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| 190 | <code class="literal">G4IonConstructor</code>
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| 191 | </p></li><li><p>
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| 192 | <code class="literal">G4ShortlivedConstructor</code>.
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| 193 | </p></li></ul></div><p>
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| 194 | </p><p>
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| 195 | An example of this is shown in <code class="literal">ExN05PhysicsList</code>,
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| 196 | listed below.
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| 197 |
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| 198 | </p><div class="example"><a name="programlist_HowToSpecParti_2"></a><p class="title"><b>Example 2.13.
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| 199 | Construct all leptons.
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| 200 | </b></p><div class="example-contents"><pre class="programlisting">
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| 201 | void ExN05PhysicsList::ConstructLeptons()
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| 202 | {
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| 203 | // Construct all leptons
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| 204 | G4LeptonConstructor pConstructor;
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| 205 | pConstructor.ConstructParticle();
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| 206 | }
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| 207 | </pre></div></div><p><br class="example-break">
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| 208 | </p></div></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="sect.HowToSpecParti.RangeCuts"></a>2.4.2.
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| 209 | Range Cuts
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| 210 | </h3></div></div></div><p>
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| 211 | To avoid infrared divergence, some electromagnetic processes
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| 212 | require a threshold below which no secondary will be generated.
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| 213 | Because of this requirement, gammas, electrons and positrons
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| 214 | require production thresholds which the user should define. This
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| 215 | threshold should be defined as a distance, or range cut-off, which
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| 216 | is internally converted to an energy for individual materials. The
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| 217 | range threshold should be defined in the initialization phase using
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| 218 | the <code class="literal">SetCuts()</code> method of <code class="literal">G4VUserPhysicsList</code>.
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| 219 | <a href="ch05s05.html" title="5.5.
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| 220 | Cuts per Region
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| 221 | ">Section 5.5</a> discusses threshold and tracking
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| 222 | cuts in detail.
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| 223 | </p><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="sect.HowToSpecParti.RangeCuts.SetCuts"></a>2.4.2.1.
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| 224 | Setting the cuts
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| 225 | </h4></div></div></div><p>
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| 226 | Production threshold values should be defined in <code class="literal">SetCuts()</code>
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| 227 | which is a pure virtual method of the <code class="literal">G4VUserPhysicsList</code>
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| 228 | class. Construction of particles, materials, and processes should
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| 229 | precede the invocation of <code class="literal">SetCuts()</code>. <code class="literal">G4RunManager</code>
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| 230 | takes care of this sequence in usual applications.
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| 231 | </p><p>
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| 232 | The idea of a "unique cut value in range" is one of the
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| 233 | important features of Geant4 and is used to handle cut values in a
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| 234 | coherent manner. For most applications, users need to determine
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| 235 | only one cut value in range, and apply this value to gammas,
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| 236 | electrons and positrons alike.
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| 237 | </p><p>
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| 238 | In such a case, the <code class="literal">SetCutsWithDefault()</code> method may be
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| 239 | used. It is provided by the <code class="literal">G4VuserPhysicsList</code> base class,
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| 240 | which has a <code class="literal">defaultCutValue</code> member as the default range
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| 241 | cut-off value. <code class="literal">SetCutsWithDefault()</code> uses this value.
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| 242 | </p><p>
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| 243 | It is possible to set different range cut values for gammas,
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| 244 | electrons and positrons, and also to set different range cut values
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| 245 | for each geometrical region. In such cases however, one must be
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| 246 | careful with physics outputs because Geant4 processes (especially
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| 247 | energy loss) are designed to conform to the "unique cut value in
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| 248 | range" scheme.
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| 249 |
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| 250 | </p><div class="example"><a name="programlist_HowToSpecParti_3"></a><p class="title"><b>Example 2.14.
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| 251 | Set cut values by using the default cut value.
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| 252 | </b></p><div class="example-contents"><pre class="programlisting">
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| 253 | void ExN04PhysicsList::SetCuts()
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| 254 | {
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| 255 | // the G4VUserPhysicsList::SetCutsWithDefault() method sets
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| 256 | // the default cut value for all particle types
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| 257 | SetCutsWithDefault();
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| 258 | }
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| 259 | </pre></div></div><p><br class="example-break">
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| 260 | </p><p>
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| 261 | The <code class="literal">defaultCutValue</code> is set to 1.0 mm by default. Of
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| 262 | course, you can set the new default cut value in the constructor of
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| 263 | your physics list class as shown below.
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| 264 |
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| 265 | </p><div class="example"><a name="programlist_HowToSpecParti_4"></a><p class="title"><b>Example 2.15.
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| 266 | Set the default cut value.
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| 267 | </b></p><div class="example-contents"><pre class="programlisting">
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| 268 | ExN04PhysicsList::ExN04PhysicsList(): G4VUserPhysicsList()
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| 269 | {
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| 270 | // default cut value (1.0mm)
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| 271 | defaultCutValue = 1.0*mm;
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| 272 | }
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| 273 | </pre></div></div><p><br class="example-break">
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| 274 | </p><p>
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| 275 | The <code class="literal">SetDefaultCutValue()</code> method in
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| 276 | <code class="literal">G4VUserPhysicsList</code> may also be used, and the "/run/setCut"
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| 277 | command may be used to change the default cut value
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| 278 | interactively.
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| 279 | </p><p>
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| 280 | WARNING: DO NOT change cut values inside the event loop. Cut
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| 281 | values may however be changed between runs.
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| 282 | </p><p>
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| 283 | An example implementation of <code class="literal">SetCuts()</code> is shown
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| 284 | below:
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| 285 |
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| 286 | </p><div class="example"><a name="programlist_HowToSpecParti_5"></a><p class="title"><b>Example 2.16.
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| 287 | Example implementation of the <code class="literal">SetCuts()</code> method.
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| 288 | </b></p><div class="example-contents"><pre class="programlisting">
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| 289 | void ExN03PhysicsList::SetCuts()
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| 290 | {
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| 291 | // set cut values for gamma at first and for e- second and next for e+,
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| 292 | // because some processes for e+/e- need cut values for gamma
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| 293 | SetCutValue(cutForGamma, "gamma");
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| 294 | SetCutValue(cutForElectron, "e-");
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| 295 | SetCutValue(cutForElectron, "e+");
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| 296 | }
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| 297 | </pre></div></div><p><br class="example-break">
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| 298 | </p><p>
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| 299 | Beginning with Geant4 version 5.1, it is now possible to set
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| 300 | production thresholds for each geometrical region. This new
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| 301 | functionality is described in <a href="ch05s05.html" title="5.5.
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| 302 | Cuts per Region
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| 303 | ">Section 5.5</a>.
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| 304 | </p></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch02s03.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="ch02s05.html"><img src="AllResources/IconsGIF/next.gif" alt="Next"></a></td></tr><tr><td width="40%" align="left" valign="top">2.3.
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| 305 | How to Specify Materials in the Detector
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| 306 | </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"> 2.5.
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| 307 | How to Specify Physics Processes
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| 308 | </td></tr></table></div></body></html>
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