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| 3 | <TITLE>Introduction to Geant4</TITLE>
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| 11 | <TABLE WIDTH="100%" >
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| 14 | <A HREF="../../../Overview/html/index.html">
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| 15 | <IMG SRC="../../../resources/html/IconsGIF/Overview.gif" ALT="Overview"></A>
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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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| 20 | </TD>
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| 21 |
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| 22 | <TD ALIGN=RIGHT><FONT COLOR="#238E23"><FONT SIZE=-1>
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| 23 | <B>Geant4 User's Documents</B></FONT></FONT>
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| 24 | </TD>
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| 25 | </TR>
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| 26 | </TABLE>
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| 27 | <P>
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| 28 | <BR>
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| 29 |
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| 30 | <CENTER><B><FONT COLOR="#238E23"><FONT SIZE=+4>
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| 31 | Introduction to Geant4
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| 32 | </FONT></FONT></B></CENTER>
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| 33 | <P>
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| 34 | <BR>
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| 35 |
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| 36 | <HR ALIGN="Center" SIZE="7%">
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| 37 |
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| 38 | <!-- ============================================== Section -->
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| 39 | <A NAME="1."></A>
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| 40 | <H2>1. Geant4 Scope of Application</H2>
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| 41 |
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| 42 | Geant4 is a free software package composed of tools which can be used to
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| 43 | accurately simulate the passage of particles through matter. All aspects
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| 44 | of the simulation process have been included in the toolkit:
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| 45 | <UL>
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| 46 | <LI>the geometry of the system,</LI>
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| 47 | <LI>the materials involved,</LI>
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| 48 | <LI>the fundamental particles of interest,</LI>
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| 49 | <LI>the generation of primary events,
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| 50 | <LI>the tracking of particles through materials and electromagnetic fields,
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| 51 | <LI>the physics processes governing particle interactions,
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| 52 | <LI>the response of sensitive detector components,
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| 53 | <LI>the generation of event data,
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| 54 | <LI>the storage of events and tracks,
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| 55 | <LI>the visualization of the detector and particle trajectories, and
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| 56 | <LI>the capture and analysis of simulation data at different levels of
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| 57 | detail and refinement.</LI>
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| 58 | </UL>
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| 59 |
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| 60 | Users may construct stand-alone applications or applications built upon
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| 61 | another object-oriented framework. In either case the toolkit will support
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| 62 | them from the initial problem definition to the production of results and
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| 63 | graphics for publication. To this end, the toolkit includes:
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| 64 | <UL>
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| 65 | <LI>user interfaces,
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| 66 | <LI>built-in steering routines, and
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| 67 | <LI>command interpreters
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| 68 | </UL>
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| 69 | which operate at every level of the simulation.
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| 70 |
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| 71 | <P>
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| 72 | At the heart of Geant4 is an abundant set of physics models to handle the
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| 73 | interactions of particles with matter across a very wide energy range. Data
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| 74 | and expertise have been drawn from many sources around the world and in this
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| 75 | respect, Geant4 acts as a repository which incorporates a large part of all
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| 76 | that is known about particle interactions.
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| 77 |
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| 78 | <P>
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| 79 | Geant4 is written in C++ and exploits advanced software-engineering
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| 80 | techniques and object-oriented technology to achieve transparency. For
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| 81 | example, the way in which cross sections are input or computed is separated
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| 82 | from the way in which they are used or accessed. The user can overload both
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| 83 | of these features. Similarly, the computation of the final state can be
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| 84 | divided into alternative or complementary models, according to the energy
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| 85 | range, the particle type, and the material. To build a specific application
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| 86 | the user-physicist chooses from among these options and implements code in
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| 87 | user action classes supplied by the toolkit. A serious problem with
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| 88 | previous simulation codes was the difficulty of adding new or variant
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| 89 | physics models; development was difficult due to the increased size,
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| 90 | complexity and interdependency of the procedure-based code. In contrast,
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| 91 | object-oriented methods help manage complexity and limit dependencies by
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| 92 | defining a uniform interface and common organizational principles for all
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| 93 | physics models. Within this framework the functionality of models can be
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| 94 | more easily recognized and understood, and the creation and addition of new
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| 95 | models is a well-defined procedure that entails little or no modification to
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| 96 | the existing code.
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| 97 |
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| 98 | <A NAME="2."></A>
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| 99 | <H2>2. History of Geant4</H2>
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| 100 |
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| 101 | These ideas first appeared in two studies done independently at CERN and KEK
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| 102 | in 1993. Both groups sought to investigate how modern computing techniques
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| 103 | could be applied to improve the existing FORTRAN based Geant3 simulation
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| 104 | program. Activities were merged in the fall of 1994 and a formal proposal,
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| 105 | RD44, to construct an entirely new program based on object-oriented
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| 106 | technology was submitted to CERN's Detector Research and Development
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| 107 | Committee. The initiative grew to become a large international
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| 108 | collaboration of physicist programmers and software engineers from a number
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| 109 | of institutes and universities participating in a range of high-energy
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| 110 | physics experiments in Europe, Japan, Canada and the United States. The
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| 111 | objective was to write a detector simulation program which had the
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| 112 | functionality and flexibility necessary to meet the requirements of the next
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| 113 | generation of subatomic physics experiments. The initial scope quickly
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| 114 | widened when it became apparent that such a tool would also benefit the
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| 115 | nuclear, accelerator, space and medical physics community, with more
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| 116 | individuals joining from these fields of science.
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| 117 |
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| 118 | <P>
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| 119 | The RD44 project represented a pioneering effort in redesigning a major CERN
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| 120 | software package for a modern object-oriented (OO) environment based on C++.
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| 121 | The R&D phase was completed in December 1998 with the delivery of the first
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| 122 | production release. The collaboration was subsequently renamed Geant4 and
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| 123 | reinstated on the basis of a formal Memorandum of Understanding (MoU) signed
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| 124 | by many of the same national institutes, laboratories and large HEP
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| 125 | experiments who participated in RD44. The agreement addresses the program
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| 126 | management, maintenance and user support during the production phase and the
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| 127 | continued development and refinement of the toolkit. It is subject to tacit
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| 128 | renewal every two years and sets out a collaboration structure defined by a
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| 129 | Collaboration Board (CB), a Technical Steering Board (TSB) and several
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| 130 | working groups.
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| 131 |
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| 132 | <P>
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| 133 | The collaboration now profits from the accumulated experience of many
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| 134 | contributors to the field of Monte Carlo simulation of physics detectors and
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| 135 | physical processes. While geographically distributed software development
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| 136 | and large-scale object-oriented systems are no longer a novelty, Geant4, in
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| 137 | terms of the size and scope of the code and the number of contributors, may
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| 138 | well represent the largest and most ambitious project of its kind outside
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| 139 | the corporate world. A clean overall problem decomposition has led to a
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| 140 | clear hierarchical structure of domains. Every section of the Geant4
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| 141 | software, which corresponds to a releasable component (library), is
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| 142 | individually managed by a working group of experts. In addition, there is a
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| 143 | working group for each of the activities: testing and quality assurance,
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| 144 | software management and documentation management. A release coordinator
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| 145 | heads each group. This consequent distribution of responsibility among a
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| 146 | relative large number of people permits a support structure whereby
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| 147 | outside users can address questions directly to the appropriate expert.
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| 148 |
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| 149 | <A NAME="3."></A>
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| 150 | <H2>3. Overview of Geant4 Functionality</H2>
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| 151 |
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| 152 | The Geant4 class category diagram is shown in Fig. 1.
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| 153 | <p>
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| 154 | Categories at the bottom of the diagram are used by virtually all higher
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| 155 | categories and provide the foundation of the toolkit.</p>
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| 156 |
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| 157 | The
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| 158 | <UL>
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| 159 | <LI><i>global</i>
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| 160 | </UL>
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| 161 | category covers the system of units, constants, numerics and random number
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| 162 | handling.
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| 163 | <p>
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| 164 | The two categories:
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| 165 |
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| 166 | <UL>
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| 167 | <LI><i>materials</i>
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| 168 | <LI><i>particles</i>
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| 169 | </UL>
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| 170 |
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| 171 | implement facilities necessary to describe the physical properties of
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| 172 | particles and materials for the simulation of particle-matter interactions.
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| 173 | <P>
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| 174 | The
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| 175 | <UL>
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| 176 | <LI><i>geometry</i>
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| 177 | </UL>
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| 178 |
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| 179 | module offers the ability to describe a geometrical structure and propagate
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| 180 | particles efficiently through it.
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| 181 | <p>
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| 182 | <center>
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| 183 | <table>
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| 184 | <tr>
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| 185 | <td>
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| 186 | <IMG SRC="introductionToGeant4.src/classCategory.gif"></a>
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| 187 | </td>
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| 188 | </tr>
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| 189 | <tr>
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| 190 | <td align=center>
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| 191 | Fig. 1 Geant4 class categories
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| 192 | </td>
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| 193 | </tr>
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| 194 | </table>
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| 195 | </center>
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| 196 |
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| 197 |
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| 198 | <P>
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| 199 | Above these reside categories required to describe the tracking of particles
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| 200 | and the physical processes they undergo. The
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| 201 |
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| 202 | <UL>
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| 203 | <LI><i>track</i>
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| 204 | </UL>
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| 205 |
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| 206 | category contains classes for tracks and steps, used by the
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| 207 | <UL>
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| 208 | <LI><i>processes</i>
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| 209 | </UL>
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| 210 |
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| 211 | category, which contains implementations of models of physical interactions: electromagnetic interactions of leptons, photons,
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| 212 | hadrons and ions, and hadronic interactions.
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| 213 | <p>
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| 214 | All processes are invoked by the
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| 215 | <UL>
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| 216 | <LI><i>tracking</i>
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| 217 | </UL>
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| 218 |
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| 219 | category, which manages their contribution to the evolution of a track's state
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| 220 | and provides information in sensitive volumes for hits and digitization.
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| 221 |
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| 222 | <P>
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| 223 | Above these the
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| 224 | <UL>
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| 225 | <LI><i>event</i>
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| 226 | </UL>
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| 227 |
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| 228 | category manages events in terms of their tracks and the
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| 229 | <UL>
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| 230 | <LI><i>run</i>
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| 231 | </UL>
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| 232 |
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| 233 | category manages collections of events that share a common beam and detector
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| 234 | implementation. A
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| 235 |
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| 236 | <UL>
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| 237 | <LI><i>readout</i>
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| 238 | </UL>
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| 239 |
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| 240 | category allows the handling of pile-up.
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| 241 |
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| 242 | <P>
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| 243 | Finally capabilities that use all of these categories and connect to
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| 244 | facilities outside the toolkit through abstract interfaces, provide
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| 245 | <i>visualization</i>, <i>persistency</i> and user <i>interface</i>
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| 246 | capabilities.
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| 247 |
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| 248 | <BR>
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| 249 |
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| 250 | <A NAME="4."></A>
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| 251 | <H2>4. Geant4 User Support</H2>
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| 252 |
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| 253 | The collaboration offers support for Geant4, providing
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| 254 | <UL>
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| 255 | <LI>assistance with problems relating to the code,
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| 256 | <LI>consultation on using the toolkit, and
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| 257 | <LI>response to enhancement requests.
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| 258 | </UL>
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| 259 |
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| 260 | A user can also expect assistance in
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| 261 | <UL>
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| 262 | <LI>investigating aberrant results.
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| 263 | </UL>
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| 264 |
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| 265 | Users of the software who encounter a problem in running the code can use an
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| 266 | <UL>
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| 267 | <LI>Internet-based <a href="http://cern.ch/geant4/problemreport">problem reporting system</a>.
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| 268 | </UL>
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| 269 |
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| 270 | The system is open to all users. It is set up automatically to assign problem
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| 271 | reports to the responsible person according to the category affected. The
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| 272 | contact person may then respond directly or forward it to a colleague. This
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| 273 | system is a customized version of the open source reporting tool
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| 274 | <a target="_ext" href="http://bugzilla.mozilla.org">Bugzilla</a>. Besides routing the
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| 275 | problem to specialists, it tracks and documents the responses until the
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| 276 | problem is resolved.
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| 277 |
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| 278 | <P>
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| 279 | New requirements, such as requests for new functionality, are presented to
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| 280 | and decided by the Technical Steering Board (TSB). The TSB sets the
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| 281 | priorities and agrees on time-scales for the fulfillment of new
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| 282 | requirements. Such support is guaranteed to collaboration members, while
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| 283 | requests from non-members are handled on a <i>best effort</i> basis.
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| 284 |
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| 285 | <P>
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| 286 | For each member organisation a contact person
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| 287 | <a href="http://cern.ch/geant4/collaboration/steering_board/members.shtml">(TSB member)</a> has been designated who acts as a first reference for Geant4
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| 288 | users in that locality, which may include affiliated institutions, user
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| 289 | groups, and others in the same geographic area. The contact person will
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| 290 | respond to enquiries, help resolve simple problems, and forward more
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| 291 | specialized queries to the relevant expert(s).
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| 292 |
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| 293 | <P>
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| 294 | Beyond that, a list of frequently asked questions
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| 295 | <a href="FAQ.html">(FAQs)</a>, and an internet-based
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| 296 | <a targer="_ext" href="http://geant4-hn.slac.stanford.edu:5090/Geant4-HyperNews/index">user forum</a> complete the available Geant4 user support.
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| 297 | <BR>
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| 298 |
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| 299 | <A NAME="5."></A>
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| 300 | <H2>5. Software Knowledge Required to Use the Geant4 Toolkit</H2>
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| 301 |
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| 302 | In general, there are three types of users:
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| 303 | <UL>
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| 304 | <LI>the <b>end user</b>,
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| 305 | <LI>the <b>application programmer</b>,
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| 306 | </UL>
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| 307 |
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| 308 | and for large simulation tasks:
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| 309 | <UL>
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| 310 | <LI>the <b>framework provider</b>.
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| 311 | </UL>
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| 312 |
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| 313 | The <b>end user</b> runs the simulation program by controlling run time
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| 314 | parameters. The interface with the program may be a graphical user
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| 315 | interface, an interactive command line interface, or the macro-based system
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| 316 | for batch. The end user needs a basic knowledge of how to control the
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| 317 | program flow but does not necessarily have to know object-oriented
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| 318 | programming or C++.
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| 319 |
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| 320 | <P>
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| 321 | The <b>application programmer</b> is central to any simulation task. A firm
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| 322 | knowledge of C++ is required to implement code in user action classes to
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| 323 | specify, at a minimum, the detector description, the relevant particles and
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| 324 | physics processes, and the initial event kinematics. A manual for the
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| 325 | application programmer is found in the
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| 326 | <a href="../../../UsersGuides/ForApplicationDeveloper/html/index.html">User's Guide: For Application Developers</a>.
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| 327 |
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| 328 | <P>
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| 329 | Using standard components of Geant4, a <b>framework provider</b> would add
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| 330 | interfaces to external tools, such as for example, to Computer Aided Design
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| 331 | (CAD) programs, Object-Oriented Data Base Management Systems (ODBMS) and
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| 332 | graphics systems. This requires the development of new classes overloading
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| 333 | standard Geant4 functionality and hence a solid understanding of
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| 334 | object-oriented Programming. A manual for the framework provider is found in the
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| 335 | <a href="../../../UsersGuides/ForToolkitDeveloper/html/index.html">User's Guide: For Toolkit Developers</a>.
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| 336 | <p>
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| 337 |
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| 338 |
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| 339 | <B>References</B>
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| 340 | <P>
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| 341 | All user documentation can be found on the Geant4 homepage
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| 342 | <a href="http://cern.ch/geant4">http://cern.ch/geant4</a>.
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| 343 | <P>
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| 344 | References for Object-Oriented Technology:
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| 345 | <UL>
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| 346 | <LI>Grady Booch, Object-Oriented Analysis and Design with Applications The Benjamin/Cummings Publishing Co. Inc, 1994,
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| 347 | ISBN 0-8053-5340-2
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| 348 | <LI>R.C.Martin, Designing Object-Oriented C++ Applications Using The Booch Method, Prentice Hall 1995, ISBN
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| 349 | 0-13-203837-4;
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| 350 | <LI>E. Gamma, et al., Design Patterns - Elements of Reusable Object-Oriented Software, Addison Wesley 1995, ISBN
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| 351 | 0-201-63361-2;
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| 352 | </UL>
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| 353 |
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| 354 | References for C++:
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| 355 | <UL>
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| 356 | <LI>B.Stroustrup, C++ Programming Language 3rd Edition, Addison Wesley,
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| 357 | ISBN: 0-201-88954-4</LI>
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| 358 | <LI>I.Pohl, Object-Oriented Programming Using C++, 2nd Edition, Addison
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| 359 | Wesley, ISBN: 0-201-89550-1.</LI>
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| 360 | </UL>
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| 361 | <BR>
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| 362 |
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| 363 | <A NAME="6."></A>
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| 364 | <H2>6. Computing Environment Required by the Geant4 Toolkit</H2>
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| 365 |
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| 366 | The Geant4 toolkit is available for a variety of operating systems:
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| 367 | <UL>
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| 368 | <LI>flavors of UNIX,
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| 369 | <LI>Linux,
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| 370 | <LI>and Windows systems.
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| 371 | </UL>
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| 372 |
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| 373 | In order to link and build the program only two underlying software packages are mandatory:
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| 374 | <UL>
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| 375 | <LI>CLHEP (Class Library of High Energy Physics) and the
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| 376 | <LI>STL (Standard Template Library for fundamental classes like C++ containers and strings).
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| 377 | </UL>
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| 378 |
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| 379 | The Geant4 source code is available from the
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| 380 | <a href="http://cern.ch/geant4">Geant4 web pages</a>
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| 381 | while CLHEP is available from the
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| 382 | <a target="_ext" href="http://cern.ch/clhep">CLHEP Home Page</a>.
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| 383 | For details on setting up the computing environment see the
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| 384 | <a href="../../../UsersGuides/InstallationGuide/html/index.html">Installation Guide</a>.
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| 385 |
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| 386 | </BODY>
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| 387 | </HTML>
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| 388 |
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