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21<FONT SIZE="-1" COLOR="#238E23">
22<B>Geant4 User's Guide</B>
23<BR>
24<B>For Application Developers</B>
25<BR>
26<B>Getting Started with Geant4</B>
27</FONT>
28</TD>
29</TR></TABLE>
30<BR><BR>
31
32<P ALIGN="Center">
33<FONT SIZE="+3" COLOR="#238E23">
34<B>2.6 How to Generate a Primary Event</B>
35</FONT>
36<BR><BR>
37
38<HR ALIGN="Center" SIZE="7%">
39<P>
40
41<a name="2.6.1">
42<H2>2.6.1 Generating Primary Events</H2></a>
43
44 <I>G4VuserPrimaryGeneratorAction</I> is one of the mandatory classes
45 available for deriving your own concrete class. In your concrete class, you
46 have to specify how a primary event should be generated. Actual
47 generation of primary particles will be done by concrete classes of
48 <I>G4VPrimaryGenerator</I>, explained in the following sub-section.
49 Your <I>G4VUserPrimaryGeneratorAction</I> concrete class just arranges
50 the way primary particles are generated.
51<P>
52<center>
53<table border=2 cellpadding=10>
54<tr>
55<td>
56<PRE>
57<FONT FACE="Courier New">
58#ifndef ExN01PrimaryGeneratorAction_h
59#define ExN01PrimaryGeneratorAction_h 1
60
61#include "G4VUserPrimaryGeneratorAction.hh"
62
63class G4ParticleGun;
64class G4Event;
65
66class ExN01PrimaryGeneratorAction : public G4VUserPrimaryGeneratorAction
67{
68  public:
69    ExN01PrimaryGeneratorAction();
70    ~ExN01PrimaryGeneratorAction();
71
72  public:
73    void generatePrimaries(G4Event* anEvent);
74
75  private:
76    G4ParticleGun* particleGun;
77};
78
79#endif
80
81#include "ExN01PrimaryGeneratorAction.hh"
82#include "G4Event.hh"
83#include "G4ParticleGun.hh"
84#include "G4ThreeVector.hh"
85#include "G4Geantino.hh"
86#include "globals.hh"
87
88ExN01PrimaryGeneratorAction::ExN01PrimaryGeneratorAction()
89{
90  G4int n_particle = 1;
91  particleGun = new G4ParticleGun(n_particle);
92
93  particleGun->SetParticleDefinition(G4Geantino::GeantinoDefinition());
94  particleGun->SetParticleEnergy(1.0*GeV);
95  particleGun->SetParticlePosition(G4ThreeVector(-2.0*m,0.0*m,0.0*m));
96}
97
98ExN01PrimaryGeneratorAction::~ExN01PrimaryGeneratorAction()
99{
100  delete particleGun;
101}
102
103void ExN01PrimaryGeneratorAction::generatePrimaries(G4Event* anEvent)
104{
105  G4int i = anEvent->get_eventID() % 3;
106  switch(i)
107  {
108    case 0:
109      particleGun->SetParticleMomentumDirection(G4ThreeVector(1.0,0.0,0.0));
110      break;
111    case 1:
112      particleGun->SetParticleMomentumDirection(G4ThreeVector(1.0,0.1,0.0));
113      break;
114    case 2:
115      particleGun->SetParticleMomentumDirection(G4ThreeVector(1.0,0.0,0.1));
116      break;
117  }
118
119  particleGun->generatePrimaryVertex(anEvent);
120}
121</PRE>
122</FONT>
123</td>
124</tr>
125<tr>
126<td align=center>
127 Source listing 2.6.1<BR>
128 An example of a <I>G4VUserPrimaryGeneratorAction</I> 
129 concrete class using <I>G4ParticleGun</I>. For the usage of <I>G4ParticleGun</I>
130 refer to 2.6.2.
131</td>
132</tr>
133</table></center>
134<P>
135
136<h4>2.6.1.1 Selection of the generator</h4>
137
138 In the constructor of your <I>G4VUserPrimaryGeneratorAction</I>,
139 you should instantiate the primary generator(s). If necessary, you need to
140 set some initial conditions for the generator(s).
141<P>
142 In Source listing 2.6.1, <I>G4ParticleGun</I> is constructed to use
143 as the actual primary particle generator. Methods of <I>G4ParticleGun</I> 
144 are described in the following section. Please note that the primary
145 generator object(s) you construct in your <I>G4VUserPrimaryGeneratorAction</I>
146 concrete class must be deleted in your destructor.
147<P>
148
149<h4>2.6.1.2 Generation of an event</h4>
150
151 <I>G4VUserPrimaryGeneratorAction</I> has a pure virtual method
152 named <tt>generatePrimaries()</tt>. This method is invoked at the
153 beginning of each event. In this method, you have to invoke the
154 <I>G4VPrimaryGenerator</I> concrete class you instantiated via
155 the <tt>generatePrimaryVertex()</tt> method.
156<P>
157 You can invoke more than one generator and/or invoke one generator
158 more than once. Mixing up several generators can produce a more
159 complicated primary event.
160<P>
161
162<HR>
163<a name="2.6.2">
164<H2>2.6.2 <i>G4VPrimaryGenerator</i></H2></a>
165
166Geant4 provides two <I>G4VPrimaryGenerator</I> concrete classes.  One is
167<I>G4ParticleGun</I>, which will be discussed here, and the other is
168<I>G4HEPEvtInterface</I>, which will be discussed in
169 <a href="../Fundamentals/eventGenerator.html">Section 3.6</a>.
170<P>
171<h4>2.6.2.1 <i>G4ParticleGun</i></h4>
172
173<I>G4ParticleGun</I> is a generator provided by Geant4.  This class generates
174primary particle(s) with a given momentum and position.  It does not provide
175any sort of randomizing.  The constructor of <I>G4ParticleGun</I> takes an
176integer which causes the generation of one or more primaries of exactly same
177kinematics.  It is a rather frequent user requirement to generate a primary
178with randomized energy, momentum, and/or position.  Such randomization can
179be achieved by invoking various set methods provided by <I>G4ParticleGun</I>.
180The invocation of these methods should be implemented in the
181<tt>generatePrimaries()</tt> method of your concrete 
182<I>G4VUserPrimaryGeneratorAction</I> class before invoking
183<tt>generatePrimaryVertex()</tt> of <I>G4ParticleGun</I>.  Geant4 provides
184various random number generation methods with various distributions (see
185<a href="../Fundamentals/global.html">Section 3.2</a>).
186<P>
187
188<h4>2.6.2.2 Public methods of <I>G4ParticleGun</I></h4>
189
190 The following methods are provided by <I>G4ParticleGun</I>, and all of
191 them can be invoked from the <tt>generatePrimaries()</tt> method in your
192 concrete <I>G4VUserPrimaryGeneratorAction</I> class.
193<UL>
194 <li><tt>void SetParticleDefinition(G4ParticleDefinition*)</tt>
195 <li><tt>void SetParticleMomentum(G4ParticleMomentum)</tt>
196 <li><tt>void SetParticleMomentumDirection(G4ThreeVector)</tt>
197 <li><tt>void SetParticleEnergy(G4double)</tt>
198 <li><tt>void SetParticleTime(G4double)</tt>
199 <li><tt>void SetParticlePosition(G4ThreeVector)</tt>
200 <li><tt>void SetParticlePolarization(G4ThreeVector)</tt>
201 <li><tt>void SetNumberOfParticles(G4int)</tt>
202</UL>
203<p>
204
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