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