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29<TD ALIGN=RIGHT><FONT COLOR="#238E23"><FONT SIZE=-1>
30<B>Geant4 User's Guide</B></FONT></FONT>
31<BR><FONT COLOR="#238E23"><FONT SIZE=-1>
32<B>For Application Developers</B></FONT></FONT>
33<BR><FONT COLOR="#238E23"><FONT SIZE=-1>
34<B>Tracking and Physics</B></FONT></FONT></TD>
35</TR>
36</TABLE>
37<P><BR>
38
39<CENTER><FONT COLOR="#238E23"><FONT SIZE=+3>
40<B>5.4 Production Threshold versus Tracking Cut</B></FONT></FONT>
41</CENTER>
42<P><BR>
43
44<HR ALIGN="Center" SIZE="7%">
45<P>
46
47<a name="5.4.1">
48<H2>5.4.1 General considerations</H2></a>
49 
50 We have to fulfill two contradictory requirements. It is the responsibility
51 of each individual <b>process</b> to produce secondary particles according to its
52 own capabilities. On the other hand, it is only the Geant4 kernel (i.e., tracking)
53 which can ensure an overall coherence of the simulation.
54<P>
55 The general principles in Geant4 are the following:
56<p>
57<OL>
58  <LI>Each <b>process</b> has its intrinsic limit(s) to produce secondary
59      particles.</LI>
60  <LI>All particles produced (and accepted) will be tracked up to <b>zero
61      range</b>.</LI>
62  <LI>Each <b>particle</b> has a suggested cut in range (which is converted
63      to energy for all materials), and defined via a <tt>SetCut()</tt> 
64      method (see
65      <a href="../GettingStarted/particleDef.html#2.4.2">Section 2.4.2</a>).</LI>
66</OL>
67<p>
68Points 1 and 2 imply that the cut associated with the <b>particle</b> is a
69(recommended) <b>production</b> threshold of secondary particles.
70<P>
71
72<HR>
73<a name="5.4.2">
74<H2>5.4.2 Set production threshold (<tt>SetCut</tt> methods)</H2></a>
75
76As already mentioned, each kind of particle has a suggested production
77threshold.  Some of the processes will not use this threshold (e.g., decay),
78while other processes will use it as a default value for their intrinsic
79limits (e.g., ionisation and bremsstrahlung).
80<P>
81See <a href="../GettingStarted/particleDef.html#2.4.2">Section 2.4.2</a> to
82see how to set the production threshold.
83<p>
84
85<HR>
86<a name="5.4.3">
87<H2>5.4.3 Apply cut</H2></a>
88
89 The <tt>DoIt</tt> methods of each process can produce secondary particles. Two cases
90 can happen:
91<p>
92 <UL>
93  <LI>a process sets its intrinsic limit greater than or equal to the recommended
94      production threshold. OK. Nothing has to be done (nothing can be done !).</LI>
95  <LI>a process sets its intrinsic limit smaller than the production threshold
96      (for instance 0).</LI>
97      <P>
98       The list of secondaries is sent to the <i>SteppingManager</i> via a <i>ParticleChange</i>
99        object.
100 </UL>
101<p>
102 BEFORE being recopied to the temporary stack for later tracking, the particles
103 below the production threshold will be kept or deleted according to the
104 safe mechanism explained hereafter.
105<p>
106 <UL>
107  <LI>The <i>ParticleDefinition</i> (or <i>ParticleWithCuts</i>) has a boolean data member:
108      <tt>ApplyCut</tt>.</LI>
109  <LI><tt>ApplyCut</tt> is OFF: do nothing. All the secondaries are stacked (and then
110      tracked later on), regardless of their initial energy. The Geant4 kernel respects
111      the best that the physics can do, but neglects the overall coherence and
112      the efficiency. Energy conservation is respected as far as the processes
113      know how to handle correctly the particles they produced!</LI>
114  <LI><tt>ApplyCut</tt> in ON: the <i>TrackingManager</i> checks the range of each secondary
115      against the production threshold and against the safety. The particle is
116      stacked if <TT>range > min(cut,safety).</TT></LI>
117    <UL>
118     <LI>If not, check if the process has nevertheless set the flag ``good for tracking''
119        and then stack it (see Section 5.4.4 below for the explanation of the
120        <tt>GoodForTracking</tt> flag).</LI>
121     <LI>If not, recuperate its kinetic energy in the <TT>localEnergyDeposit</TT>,
122         and set <TT>tkin=0</TT>.</LI>
123     <LI>Then check in the <i>ProcessManager</i> if the vector of <i>ProcessAtRest</i> is not
124         empty. If yes, stack the particle for performing the ``Action At Rest'' later.
125         If not, and only in this case, abandon this secondary.</LI>
126    </UL>
127      With this sophisticated mechanism we have the global cut that we wanted,
128      but with energy conservation, and we respect boundary constraint (safety)
129      and the wishes of the processes (via ``good for tracking'').
130 </UL>
131<P>
132
133<HR>
134<a name="5.4.4">
135<H2>5.4.4 Why produce secondaries below threshold?</H2></a>
136 
137 A process may have good reasons to produce particles below the recommended
138 threshold:
139<p>
140 <UL>
141  <LI>checking the range of the secondary versus geometrical quantities like
142      safety may allow one to realize the possibility that the produced particle,
143      even below threshold, will reach a sensitive part of the detector;</LI>
144  <LI>another example is the gamma conversion: the positron is always produced,
145      even at zero energy, for further annihilation.</LI>
146 </UL>
147<p>
148 These secondary particles are sent to the ``Stepping Manager'' with a flag
149 <tt>GoodForTracking</tt> to pass the filter explained in the previous section
150 (even when <tt>ApplyCut</tt> is ON).
151<P>
152
153<HR>
154<a name="5.4.5">
155<H2>5.4.5 Cuts in stopping range or in energy?</H2></a>
156 
157 The cuts in stopping range allow one to say that the energy has been released
158 at the correct space position, limiting the approximation within a given
159 distance. On the contrary, cuts in energy imply accuracies of the energy
160 depositions which depend on the material.
161<P>
162
163<HR>
164<a name="5.4.6">
165<H2>5.4.6 Summary</H2></a>
166
167 In summary, we do not have tracking cuts; we only have production thresholds
168 in range. All particles produced and accepted are tracked up to zero range.
169<P>
170 It must be clear that the overall coherency that we provide cannot go
171 beyond the capability of processes to produce particles down to the recommended
172 threshold.
173<P>
174 In other words a process can produce the secondaries down to the recommended
175 threshold, and by interrogating the geometry, or by realizing when mass-to-energy
176 conversion can occur, recognize when particles below the threshold have
177 to be produced.
178<P>
179
180<HR>
181<a name="5.4.7">
182<H2>5.4.7 Special tracking cuts</H2></a>
183
184 One may need to cut given particle types in given volumes for optimisation
185 reasons. This decision is under user control, and can happen for particles
186 during tracking as well.
187<P>
188 The user must be able to apply these special cuts only for the desired
189 particles and in the desired volumes, without introducing an overhead for
190 all the rest.
191<P>
192 The approach is as follows:
193<p>
194 <UL>
195  <LI>special user cuts are registered in the <i>UserLimits</i> class (or its descendant),
196      which is associated with the logical volume class.</LI>
197      <P>
198      The current default list is:
199      <UL>
200       <LI>max allowed step size</LI>
201       <LI>max total track length</LI>
202       <LI>max total time of flight</LI>
203       <LI>min kinetic energy</LI>
204       <LI>min remaining range</LI>
205      </UL>
206      The user can instantiate a <i>UserLimits</i> object only for the desired logical
207      volumes and do the association.<p>
208      The first item (max step size) is automatically taken into account by the G4 kernel
209      while the others items must be managed by the user, as explained below.
210      <P>
211      <b>Example</b>(see novice/N02): in the Tracker region, in order to force the step size not
212      to exceed 1/10 of the Tracker thickness, it is enough to put the following code in
213      <TT>DetectorConstruction::Construct()</TT>:
214      <PRE>
215       G4double maxStep = 0.1*TrackerLength;
216       logicTracker->SetUserLimits(new G4UserLimits(maxStep));
217      </PRE>
218      The <i>G4UserLimits</i> class is in <TT>source/global/management</TT>.<p>
219  <LI>Concerning the others cuts, the user must define dedicaced process(es).
220      He registers this process (or its descendant) only for the desired particles
221      in their process manager.
222      He can apply his cuts in the <TT>DoIt</TT> of this process,
223      since, via <i>G4Track</i>, he can access the logical volume and <i>UserLimits</i>.
224      <p>
225      An example of such process (called <i>UserSpecialCuts</i>) is provided in the repository,
226      but not inserted in any process manager of any particle.</LI>
227      <P>
228      <b>Example: neutrons. </b>
229      One may need to abandon the tracking of neutrons after a given time
230      of flight (or a charged particle in a magnetic field after a given total
231      track length ... etc ...).
232      <P>
233      Example(see novice/N02): in the Tracker region, in order to force the total time of
234      flight of the neutrons not to exceed 10 milliseconds, put the following
235      code in <TT>DetectorConstruction::Construct()</TT>:
236      <PRE>
237        G4double maxTime = 10*ms;
238        logicTracker->SetUserLimits(new G4UserLimits(DBL_MAX,DBL_MAX,maxTime));
239      </PRE>
240      and put the following code in <TT>N02PhysicsList</TT>:
241      <PRE>
242        G4ProcessManager* pmanager = G4Neutron::Neutron->GetProcessManager();
243        pmanager->AddProcess(new G4UserSpecialCuts(),-1,-1,1);
244      </PRE>
245      (The default <i>G4UserSpecialCuts</i> class is in
246      <TT>source/processes/transportation</TT>.)
247 </UL>
248<P>
249
250<BR><BR>
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