1 | <!-- ******************************************************** --> |
---|
2 | <!-- --> |
---|
3 | <!-- [History] --> |
---|
4 | <!-- Changed by: Katsuya Amako, 4-Aug-1998 --> |
---|
5 | <!-- Changed by: Katsuya Amako, 9-Jul-1998 --> |
---|
6 | <!-- Proof read by: Joe Chuma, 2-Jul-1999 --> |
---|
7 | <!-- few corrections in 3.18.7: mma, 11-Jan-2001 --> |
---|
8 | <!-- Converted to DocBook: Katsuya Amako, Aug-2006 --> |
---|
9 | <!-- --> |
---|
10 | <!-- ******************************************************** --> |
---|
11 | |
---|
12 | |
---|
13 | <!-- ******************* Section (Level#1) ****************** --> |
---|
14 | <sect1 id="sect.ProThres"> |
---|
15 | <title> |
---|
16 | Production Threshold versus Tracking Cut |
---|
17 | </title> |
---|
18 | |
---|
19 | |
---|
20 | <!-- ******************* Section (Level#2) ****************** --> |
---|
21 | <sect2 id="sect.ProThres.Gen"> |
---|
22 | <title> |
---|
23 | General considerations |
---|
24 | </title> |
---|
25 | |
---|
26 | <para> |
---|
27 | We have to fulfill two contradictory requirements. It is the |
---|
28 | responsibility of each individual <emphasis role="bold">process</emphasis> |
---|
29 | to produce secondary particles according to its own capabilities. On |
---|
30 | the other hand, it is only the Geant4 kernel (i.e., tracking) which can |
---|
31 | ensure an overall coherence of the simulation. |
---|
32 | </para> |
---|
33 | |
---|
34 | <para> |
---|
35 | The general principles in Geant4 are the following: |
---|
36 | |
---|
37 | <orderedlist spacing="compact"> |
---|
38 | <listitem><para> |
---|
39 | Each <emphasis role="bold">process</emphasis> has its intrinsic limit(s) |
---|
40 | to produce secondary particles. |
---|
41 | </para></listitem> |
---|
42 | <listitem><para> |
---|
43 | All particles produced (and accepted) will be tracked up to |
---|
44 | <emphasis role="bold">zero range</emphasis>. |
---|
45 | </para></listitem> |
---|
46 | <listitem><para> |
---|
47 | Each <emphasis role="bold">particle</emphasis> has a suggested cut in range |
---|
48 | (which is converted to energy for all materials), and defined via a |
---|
49 | <literal>SetCut()</literal> method (see |
---|
50 | <xref linkend="sect.HowToSpecParti.RangeCuts" />). |
---|
51 | </para></listitem> |
---|
52 | </orderedlist> |
---|
53 | </para> |
---|
54 | |
---|
55 | <para> |
---|
56 | Points 1 and 2 imply that the cut associated with the |
---|
57 | <emphasis role="bold">particle</emphasis> is a (recommended) |
---|
58 | <emphasis role="bold">production</emphasis> threshold of secondary particles. |
---|
59 | </para> |
---|
60 | |
---|
61 | </sect2> |
---|
62 | |
---|
63 | <!-- ******************* Section (Level#2) ****************** --> |
---|
64 | <sect2 id="sect.ProThres.Set"> |
---|
65 | <title> |
---|
66 | Set production threshold (<literal>SetCut</literal> methods) |
---|
67 | </title> |
---|
68 | |
---|
69 | <para> |
---|
70 | As already mentioned, each kind of particle has a suggested |
---|
71 | production threshold. Some of the processes will not use this |
---|
72 | threshold (e.g., decay), while other processes will use it as a |
---|
73 | default value for their intrinsic limits (e.g., ionisation and |
---|
74 | bremsstrahlung). |
---|
75 | </para> |
---|
76 | |
---|
77 | <para>See |
---|
78 | <xref linkend="sect.HowToSpecParti.RangeCuts" /> to see how to set |
---|
79 | the production threshold. |
---|
80 | </para> |
---|
81 | |
---|
82 | </sect2> |
---|
83 | |
---|
84 | |
---|
85 | <!-- ******************* Section (Level#2) ****************** --> |
---|
86 | <sect2 id="sect.ProThres.Apply"> |
---|
87 | <title> |
---|
88 | Apply cut |
---|
89 | </title> |
---|
90 | |
---|
91 | <para> |
---|
92 | The <literal>DoIt</literal> methods of each process can produce secondary |
---|
93 | particles. Two cases can happen: |
---|
94 | |
---|
95 | <itemizedlist spacing="compact"> |
---|
96 | <listitem><para> |
---|
97 | a process sets its intrinsic limit greater than or equal to the |
---|
98 | recommended production threshold. OK. Nothing has to be done |
---|
99 | (nothing can be done !). |
---|
100 | </para></listitem> |
---|
101 | <listitem><para> |
---|
102 | a process sets its intrinsic limit smaller than the production |
---|
103 | threshold (for instance 0). |
---|
104 | </para></listitem> |
---|
105 | </itemizedlist> |
---|
106 | </para> |
---|
107 | |
---|
108 | <para> |
---|
109 | The list of secondaries is sent to the <emphasis>SteppingManager</emphasis> |
---|
110 | via a <emphasis>ParticleChange</emphasis> object. |
---|
111 | </para> |
---|
112 | |
---|
113 | <para> |
---|
114 | <emphasis>Before</emphasis> being recopied to the temporary stack for later |
---|
115 | tracking, the particles below the production threshold will be kept or |
---|
116 | deleted according to the safe mechanism explained hereafter. |
---|
117 | |
---|
118 | <itemizedlist spacing="compact"> |
---|
119 | <listitem><para> |
---|
120 | The <emphasis>ParticleDefinition</emphasis> |
---|
121 | (or <emphasis>ParticleWithCuts</emphasis>) has a boolean data member: |
---|
122 | <literal>ApplyCut</literal>.</para></listitem> |
---|
123 | <listitem><para><literal>ApplyCut</literal> is OFF: do nothing. |
---|
124 | All the secondaries are stacked (and then tracked later on), regardless |
---|
125 | of their initial energy. The Geant4 kernel respects the best that the |
---|
126 | physics can do, but neglects the overall coherence and the efficiency. |
---|
127 | Energy conservation is respected as far as the processes know how to |
---|
128 | handle correctly the particles they produced! |
---|
129 | </para></listitem> |
---|
130 | <listitem><para> |
---|
131 | <literal>ApplyCut</literal> in ON: the <emphasis>TrackingManager</emphasis> |
---|
132 | checks the range of each secondary against the production threshold and |
---|
133 | against the safety. The particle is stacked if <literal>range > |
---|
134 | min(cut,safety).</literal> |
---|
135 | <itemizedlist spacing="compact"> |
---|
136 | <listitem><para> |
---|
137 | If not, check if the process has nevertheless set the flag |
---|
138 | ``good for tracking'' and then stack it (see |
---|
139 | <xref linkend="sect.ProThres.WhyProd" /> |
---|
140 | below for the explanation of the <literal>GoodForTracking</literal> flag). |
---|
141 | </para></listitem> |
---|
142 | <listitem><para> |
---|
143 | If not, recuperate its kinetic energy in the |
---|
144 | <literal>localEnergyDeposit</literal>, and set |
---|
145 | <literal>tkin=0</literal>. |
---|
146 | </para></listitem> |
---|
147 | <listitem><para> |
---|
148 | Then check in the <emphasis>ProcessManager</emphasis> if the vector of |
---|
149 | <emphasis>ProcessAtRest</emphasis> is not empty. If yes, stack the |
---|
150 | particle for performing the ``Action At Rest'' later. If not, and only |
---|
151 | in this case, abandon this secondary. |
---|
152 | </para></listitem> |
---|
153 | </itemizedlist> |
---|
154 | </para></listitem> |
---|
155 | </itemizedlist> |
---|
156 | </para> |
---|
157 | |
---|
158 | <para> |
---|
159 | With this sophisticated mechanism we have the global cut that we |
---|
160 | wanted, but with energy conservation, and we respect boundary |
---|
161 | constraint (safety) and the wishes of the processes (via ``good for |
---|
162 | tracking''). |
---|
163 | </para> |
---|
164 | |
---|
165 | </sect2> |
---|
166 | |
---|
167 | |
---|
168 | <!-- ******************* Section (Level#2) ****************** --> |
---|
169 | <sect2 id="sect.ProThres.WhyProd"> |
---|
170 | <title> |
---|
171 | Why produce secondaries below threshold? |
---|
172 | </title> |
---|
173 | |
---|
174 | <para> |
---|
175 | A process may have good reasons to produce particles below the |
---|
176 | recommended threshold: |
---|
177 | |
---|
178 | <itemizedlist spacing="compact"> |
---|
179 | <listitem><para> |
---|
180 | checking the range of the secondary versus geometrical |
---|
181 | quantities like safety may allow one to realize the possibility |
---|
182 | that the produced particle, even below threshold, will reach a |
---|
183 | sensitive part of the detector; |
---|
184 | </para></listitem> |
---|
185 | <listitem><para> |
---|
186 | another example is the gamma conversion: the positron is always |
---|
187 | produced, even at zero energy, for further annihilation. |
---|
188 | </para></listitem> |
---|
189 | </itemizedlist> |
---|
190 | </para> |
---|
191 | |
---|
192 | <para> |
---|
193 | These secondary particles are sent to the ``Stepping Manager'' |
---|
194 | with a flag <literal>GoodForTracking</literal> to pass the filter explained |
---|
195 | in the previous section (even when <literal>ApplyCut</literal> is ON). |
---|
196 | </para> |
---|
197 | |
---|
198 | </sect2> |
---|
199 | |
---|
200 | |
---|
201 | <!-- ******************* Section (Level#2) ****************** --> |
---|
202 | <sect2 id="sect.ProThres.RangOrEner"> |
---|
203 | <title> |
---|
204 | Cuts in stopping range or in energy? |
---|
205 | </title> |
---|
206 | |
---|
207 | <para> |
---|
208 | The cuts in stopping range allow one to say that the energy has |
---|
209 | been released at the correct space position, limiting the |
---|
210 | approximation within a given distance. On the contrary, cuts in |
---|
211 | energy imply accuracies of the energy depositions which depend on |
---|
212 | the material. |
---|
213 | </para> |
---|
214 | |
---|
215 | </sect2> |
---|
216 | |
---|
217 | |
---|
218 | <!-- ******************* Section (Level#2) ****************** --> |
---|
219 | <sect2 id="sect.ProThres.Sum"> |
---|
220 | <title> |
---|
221 | Summary |
---|
222 | </title> |
---|
223 | |
---|
224 | <para> |
---|
225 | In summary, we do not have tracking cuts; we only have production |
---|
226 | thresholds in range. All particles produced and accepted are |
---|
227 | tracked up to zero range. |
---|
228 | </para> |
---|
229 | |
---|
230 | <para> |
---|
231 | It must be clear that the overall coherency that we provide |
---|
232 | cannot go beyond the capability of processes to produce particles |
---|
233 | down to the recommended threshold. |
---|
234 | </para> |
---|
235 | |
---|
236 | <para> |
---|
237 | In other words a process can produce the secondaries down to the |
---|
238 | recommended threshold, and by interrogating the geometry, or by |
---|
239 | realizing when mass-to-energy conversion can occur, recognize when |
---|
240 | particles below the threshold have to be produced. |
---|
241 | </para> |
---|
242 | |
---|
243 | </sect2> |
---|
244 | |
---|
245 | |
---|
246 | <!-- ******************* Section (Level#2) ****************** --> |
---|
247 | <sect2 id="sect.ProThres.Spe"> |
---|
248 | <title> |
---|
249 | Special tracking cuts |
---|
250 | </title> |
---|
251 | |
---|
252 | <para> |
---|
253 | One may need to cut given particle types in given volumes for |
---|
254 | optimisation reasons. This decision is under user control, and can |
---|
255 | happen for particles during tracking as well. |
---|
256 | </para> |
---|
257 | |
---|
258 | <para> |
---|
259 | The user must be able to apply these special cuts only for the |
---|
260 | desired particles and in the desired volumes, without introducing |
---|
261 | an overhead for all the rest. |
---|
262 | </para> |
---|
263 | |
---|
264 | <para> |
---|
265 | The approach is as follows: |
---|
266 | |
---|
267 | <itemizedlist spacing="compact"> |
---|
268 | <listitem><para> |
---|
269 | special user cuts are registered in the <emphasis>UserLimits</emphasis> |
---|
270 | class (or its descendant), which is associated with the logical volume |
---|
271 | class. |
---|
272 | <para> |
---|
273 | The current default list is: |
---|
274 | <itemizedlist spacing="compact"> |
---|
275 | <listitem><para> |
---|
276 | max allowed step size |
---|
277 | </para></listitem> |
---|
278 | <listitem><para> |
---|
279 | max total track length |
---|
280 | </para></listitem> |
---|
281 | <listitem><para> |
---|
282 | max total time of flight |
---|
283 | </para></listitem> |
---|
284 | <listitem><para> |
---|
285 | min kinetic energy |
---|
286 | </para></listitem> |
---|
287 | <listitem><para> |
---|
288 | min remaining range |
---|
289 | </para></listitem> |
---|
290 | </itemizedlist> |
---|
291 | </para> |
---|
292 | <para> |
---|
293 | The user can instantiate a <emphasis>UserLimits</emphasis> object only |
---|
294 | for the desired logical volumes and do the association. |
---|
295 | </para> |
---|
296 | <para> |
---|
297 | The first item (max step size) is automatically taken into |
---|
298 | account by the G4 kernel while the others items must be managed by |
---|
299 | the user, as explained below. |
---|
300 | </para> |
---|
301 | <para> |
---|
302 | <emphasis role="bold">Example</emphasis>(see novice/N02): |
---|
303 | in the Tracker region, in order to force the step size not to exceed |
---|
304 | 1/10 of the Tracker thickness, it is enough to put the following code in |
---|
305 | <literal>DetectorConstruction::Construct()</literal>: |
---|
306 | <informalexample> |
---|
307 | <programlisting> |
---|
308 | G4double maxStep = 0.1*TrackerLength; |
---|
309 | logicTracker->SetUserLimits(new G4UserLimits(maxStep)); |
---|
310 | </programlisting> |
---|
311 | </informalexample> |
---|
312 | and in <literal>PhysicsList</literal>, the process |
---|
313 | <literal>G4StepLimiter</literal> needs to be attached to each |
---|
314 | particle's process manager where step limitation in the Tracker region |
---|
315 | is required: |
---|
316 | <informalexample> |
---|
317 | <programlisting> |
---|
318 | // Step limitation seen as a process |
---|
319 | G4StepLimiter* stepLimiter = new G4StepLimiter(); |
---|
320 | pmanager->AddDiscreteProcess(StepLimiter); |
---|
321 | </programlisting> |
---|
322 | </informalexample> |
---|
323 | </para> |
---|
324 | <para> |
---|
325 | The <emphasis>G4UserLimits</emphasis> class is in |
---|
326 | <literal>source/global/management</literal>. |
---|
327 | </para> |
---|
328 | </para></listitem> |
---|
329 | <listitem><para> |
---|
330 | Concerning the others cuts, the user must define |
---|
331 | dedicaced process(es). He registers this process (or its descendant) |
---|
332 | only for the desired particles in their process manager. He can apply |
---|
333 | his cuts in the <literal>DoIt</literal> of this process, since, via |
---|
334 | <emphasis>G4Track</emphasis>, he can access the logical volume and |
---|
335 | <emphasis>UserLimits</emphasis>. |
---|
336 | |
---|
337 | <para> |
---|
338 | An example of such process (called <emphasis>UserSpecialCuts</emphasis>) is |
---|
339 | provided in the repository, but not inserted in any process manager |
---|
340 | of any particle. |
---|
341 | </para> |
---|
342 | <para> |
---|
343 | <emphasis role="bold">Example: neutrons.</emphasis> One may need to abandon |
---|
344 | the tracking of neutrons after a given time of flight (or a charged |
---|
345 | particle in a magnetic field after a given total track length ... etc ...). |
---|
346 | </para> |
---|
347 | <para> |
---|
348 | Example(see novice/N02): in the Tracker region, in order to |
---|
349 | force the total time of flight of the neutrons not to exceed 10 |
---|
350 | milliseconds, put the following code in |
---|
351 | <literal>DetectorConstruction::Construct()</literal>: |
---|
352 | |
---|
353 | <informalexample> |
---|
354 | <programlisting> |
---|
355 | G4double maxTime = 10*ms; |
---|
356 | logicTracker->SetUserLimits(new G4UserLimits(DBL_MAX,DBL_MAX,maxTime)); |
---|
357 | </programlisting> |
---|
358 | </informalexample> |
---|
359 | |
---|
360 | and put the following code in <literal>N02PhysicsList</literal>: |
---|
361 | |
---|
362 | <informalexample> |
---|
363 | <programlisting> |
---|
364 | G4ProcessManager* pmanager = G4Neutron::Neutron->GetProcessManager(); |
---|
365 | pmanager->AddProcess(new G4UserSpecialCuts(),-1,-1,1); |
---|
366 | </programlisting> |
---|
367 | </informalexample> |
---|
368 | </para> |
---|
369 | <para> |
---|
370 | (The default <emphasis>G4UserSpecialCuts</emphasis> class is in |
---|
371 | <literal>source/processes/transportation</literal>.) |
---|
372 | </para> |
---|
373 | </para></listitem> |
---|
374 | </itemizedlist> |
---|
375 | </para> |
---|
376 | |
---|
377 | </sect2> |
---|
378 | </sect1> |
---|