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1 Example1 for Reverse Monte Carlo
2 --------------------------------
3
4
5 Author
6 ------
7 This example code and the adjoint classes in the G4 toolkit have been developed by L.Desorgher (SpaceIT GmbH)
8 under the ESA contract 21435/08/NL/AT. For any (reasonable) question you may contact the author
9 at the following email address : desorgher@spaceit.ch
10
11
12 Abstract
13 --------
14 This is the README file for the first G4 example illustrating the use of the Reverse Monte Carlo (RMC) mode in a Geant4
15 application. The Reverse Monte Carlo method is also known as the Adjoint Monte Carlo (AMC) method and
16 in this document we will alternate both Reverse and Adjoint terms.
17
18 Other documentation
19 -------------------
20 See also the section 3.7.3 Adjoint/Reverse Monte carlo in the
21 Geant4 User guide for application developers.
22
23
24 Table of Contents:
25 -----------------
26
27 1.Definition of Reverse/Adjoint Monte Carlo
28
29 2.The Reverse Monte Carlo mode in Geant4 (since G4.9.3 release)
30 2.1. Reverse tracking phase
31 2.2. Forward tracking phase
32 2.3. Reverse processes
33 2.4. Remark on Nb of adjoint particle types and G4 events considered in an adjoint simulation
34 2.5. Modifications to bring in a existing G4 application to use the Reverse MC method
35
36 3.exampleRMC01
37 3.1. Geometry
38 3.2. Physics
39 3.3. Analysis and output of the code
40 3.4. Run macrofiles
41 3.5. Comparison of adjoint and forward simulation results. Normalization!
42
43 4.Control of the adjoint simulation and the RMC01 code by G4 macro UI commands
44 4.1. G4UI commands in the directory /adjoint
45 4.2. G4UI commands in the directory /adjoint_physics
46 4.3. G4UI commands in the directory /RMC01
47
48 5. Known issues
49 5.1. Rare too high weight in the adjoint simulation
50 5.2. Limitation of the reverse bremsstrahlung
51 5.3.Limitation of the reverse multiple scattering
52
53
54
55 1. Definition of Reverse/Adjoint Monte Carlo
56 -----------------------------------------
57 -----------------------------------------
58 When the sensitive part of a detector is small compared to its entire size and to the size of the
59 external extended primary particle source, a lot of computing time is spent during a normal Monte Carlo run
60 in the simulation of particle showers that are not contributing to the detector signal.
61 In such particular case the Reverse Monte Carlo (RMC) method, also known as the
62 Adjoint Monte Carlo method, can be used.
63 In this method particles are generated in or on the external surface of the sensitive volume
64 of the instrument and then are tracked backward in the geometry till they reach the source surface,
65 or exceed an energy threshold. During the reverse tracking reverse reactions are applied to the particles.
66
67
68
69 2. The Reverse Monte Carlo mode in Geant4 (since G4.9.3 release)
70 ----------------------------------------------------------------
71 ----------------------------------------------------------------
72 (See also the section 3.7.3 Adjoint/Reverse Monte carlo in the
73 Geant4 User guide for application developers.)
74
75 Different G4Adjoint classes have been implemented into the Geant4
76 toolkit to run an adjoint/reverse simulation in a Geant4 application.
77 In this implementation an adjoint run is divided in a succession
78 of alternative adjoint and forward tracking of adjoint and normal particles.
79 One Geant4 event treats the reverse tracking of an adjoint primary particle
80 and its secondaries, and the forward tracking of a primary particle euqivalent
81 to the adjoint primary as well as its secondaries.
82
83
84 2.1. Reverse tracking phase:
85 -------------------------
86
87 Adjoint particles (adjoint_e-, adjoint_gamma,...) are generated one by one on the so called
88 adjoint source with random position, energy (1/E distribution) and direction. The adjoint
89 source is the external surface of a user defined volume or of a user defined sphere. The
90 adjoint source should contain one or several sensitive volumes and should be small
91 compared to the entire geometry. The user can set the minimum and maximum energy of the
92 adjoint source. After its generation the adjoint primary particle is tracked backward in
93 the geometry till a user defined external surface (spherical or boundary of a volume)
94 or is killed before if it reaches a user defined upper energy limit that represents the
95 maximum energy of the external source. During the reverse tracking, reverse processes take
96 place where the adjoint particle being tracked can be either scattered or transformed in
97 another type of adjoint particle. During the reverse tracking the
98 G4AdjointSimulationManager replaces the user defined primary, run, stepping, ... actions,
99 by its own actions.
100
101 2.2. Forward tracking phase:
102 --------------------------
103
104 When an adjoint particle reaches the external surface its weight, type, position,
105 and direction are registered and a normal primary particle with a type equivalent
106 to the last generated adjoint primary is generated with the same energy,
107 position but opposite direction and is tracked in the forward direction
108 in the sensitive region as in a forward MC simulation.
109 During this forward tracking phase the event, stacking, stepping, tracking actions defined
110 by the user for its general forward application are used.
111 By this clear separation between adjoint and forward tracking phases, the code of the
112 user developed for a forward simulation should be only slightly
113 modified to adapt it for an adjoint simulation. Indeed the computation of the signal
114 is done by the same user actions or analysis classes that the one used in the forward
115 simulation mode. Before the G4.10.0 release the reverse and forward tracking mode
116 took place in separated events. Since the G4.10.0 release,
117 in order to prepare to the migration of the
118 ReverseMC to the G4 Multiple Threading mode, the reverse and forward tracking
119 phase of corresponding adjoint and forward primaries have been merged in the same
120 event.
121
122
123 2.3. Reverse Processes:
124 ---------------------
125
126 During the reverse tracking phase reverse processes act on the adjoint particles.
127 The Reverse processes that are available at the moment in Geant4 are the:
128 - Reverse discrete Ionization for e-, proton and ions
129 - Continuous gain of energy by ionization and bremsstrahlung for e- and by ionization for protons and ions
130 - Reverse discrete e- bremsstrahlung
131 - Reverse photoelectric effect
132 - Reverse Compton scattering
133 - Approximated multiple scattering (MS) (see section 5.3)
134
135 For the gamma reverse physics an adjoint gamma reverse forced interaction process has been implemented
136 since GEANT4.10.3. THis process splits a new created gamma in two tracks.
137 The first tracks is used to force a free flight of the adjoint gamma through the geometry.
138 The second track is used to force a reverse bremsstrahlung or a reverse compton at some random
139 position along the free flight track.
140
141 It is important to note that the electromagnetic reverse processes are cut dependent
142 as their equivalent forward processes. The implementation of the reverse processes is
143 based on the forward processes
144 implemented in the G4 standard electromagnetic package.
145
146
147 2.4. Remark on Nb of adjoint particle types and Nb of G4 events considered in an adjoint simulation:
148 ---------------------------------------------------------------------------------
149
150 The list of type of adjoint and forward particles that are generated on the adjoint source
151 and considered in the simulation is a function of the adjoint processes declared in the
152 physics list. For example if only the e- and gamma electromagnetic processes are considered
153 , only adjoint e- and adjoint gamma will be considered as primaries. In this case an
154 adjoint event will be divided in two G4 events. The first event will consist
155 into the coupled reverse and forward tracking of an adjoint e- and its equivalent
156 forward e-, while the second events will process the reverse and forward trackings
157 of corresponsing adjoint and forward primary gammas. In this case a
158 run of 100 adjoint events will consist into 200 Geant4 events. If the proton ionization is
159 also considered adjoint and forward protons are also generated as primaries
160 and 300 Geant4 events are processed for 100 adjoint events.
161
162 2.5. Modifications to bring in a existing G4 application to use the Reverse MC method
163 --------------------------------------------------------------------------------
164 (for more details see also the section 3.7.3 Adjoint/Reverse Monte carlo in the
165 Geant4 User guide for application developers.)
166
167 Due the clear separation between the reverse and forward tracking phase only few modifications are needed
168 to an existing Geant4 application in order to adapt it for the use of the reverse simulation mode.
169 Except in the physics list where all the reverse processes and their forward equivalent
170 have to be declared, the principal code modifications are needed only in the analysis phase at the end
171 of the forward tracking where computed signals have to be multiplied by the weight
172 of the reverse tracks that have reached the external surface of the simulatrion
173 and then normalized to different user defined spectra and angular distribution representing
174 the external source.
175 The weight of the adjoint tracks is computed by the G4Adjoint classes and the user needs
176 only to multiply them to the primary differential, directional spectrum of its choice.
177 The adjoint weight a the end of tracks can be also registered if needed in answer matrices.
178
179 More precisely, in order to be able to use the Reverse MC method in his simulation, the user should modify
180 its code as such:
181
182 - Adapt its physics list to use Reverse Processes for adjoint particles. An example of such physics list is provided in an extended
183 example.
184 - Create an instance of G4AdjointSimManager somewhere in the main code.
185
186 - Modify the analysis part of the code to normalize the signal computed during the forward phase to the weight
187 of adjoint particle that reached the external surface during the last tracking phase.
188 This is done by using the following method of G4AdjointSimManager.
189 size_t GetNbOfAdointTracksReachingTheExternalSurface()
190 G4int GetIDOfLastAdjParticleReachingExtSource(size_t i)
191 G4ThreeVector GetPositionAtEndOfLastAdjointTrack(size_t i)
192 G4ThreeVector GetDirectionAtEndOfLastAdjointTrack(size_t i)
193 G4double GetEkinAtEndOfLastAdjointTrack(size_t i)
194 G4double GetEkinNucAtEndOfLastAdjointTrack(size_t i)
195 G4double GetWeightAtEndOfLastAdjointTrack(size_t i)
196 G4double GetCosthAtEndOfLastAdjointTrack(size_t i)
197 G4String GetFwdParticleNameAtEndOfLastAdjointTrack(size_t i)
198 G4int GetFwdParticlePDGEncodingAtEndOfLastAdjointTrack(size_t i)
199 G4int GetFwdParticleIndexAtEndOfLastAdjointTrack(size_t i).
200 Since the version Geant4.10.3 several adjoint tracks can arrive on the external surface during the same events.
201 It is therefore important to loop over alll these tracks when normalizing the weights at the end of the event.
202 The method GetNbOfAdointTracksReachingTheExternalSurface() returns the number of adjoint tracks that reached the
203 external surface. Ine the other methods the input parameter i allows to get the information of the ith track.
204
205 In order to have a code working for both forward and adjoint simulation mode, the extra code needed in user actions for the adjoint
206 simulation mode can be separated to the code needed only for the normal forward simulation by using the following method
207
208 G4bool GetAdjointSimMode() that return true if an adjoint simulation is running and false if not!
209
210
211
212 3. exampleRMC01
213 ---------------
214 ---------------
215 The example RMC01 illustrates how to modify a G4 application in order to use
216 both forward and reverse MC modes in the same code.
217
218
219 3.1. Geometry:
220 --------------
221
222 The following simple geometry is considered:
223 - sensitive Silicon cylinder at the center of an Aluminum spherical shielding with 10 cm Radius.
224 - two 0.5mm thick Tantalum plates set horizontally above and below the Sensitive Cylinder
225
226 The free parameters of the geometry that can bes set by the user are:
227 - the thickness of the Aluminum shielding
228 - the height of the sensitive Si cylinder
229 - the radius of the sensitive Si cylinder
230
231
232
233 3.2. Physics:
234 -------------
235
236 The physical processes considered are:
237 - Reverse and forward discrete Ionization for e- and proton
238 - Continuous gain and loss of energy by ionization and bremsstrahlung for e- and by ionization for protons
239 - Reverse and forward discrete e- bremsstrahlung
240 - Reverse and forward photoelectric effect
241 - Reverse and forward Compton scattering
242 - Reverse and forward Multiple scattering
243
244 These processes are implemented in the class G4AdjointPhysicsList distributed with the example. The G4AdjointPhysicsMessenger allows the user
245 to switch on/off some processes for testing purpose. By default all processes cited above are considered except the proton ionization that
246 has to be specifically switch on in the macro file by the user.
247
248
249
250 3.3. Analysis and output of the code:
251 ----------------------------------
252
253 The example computes the energy deposited in the sensitive Si cylinder and the current of e-, protons, and gamma
254 entering this cylinder.
255 The Hits are registered in the sensitive detector class RMC01SD that is a typical G4 sensitive detector class
256 used in a forward simulation and is not modified at all
257 for the adjoint simulation mode.
258 The analysis of the registered hits during forward events is done by the RMCO1AnalysisManager.
259 That is the class that illustrates how to adapt an analysis code of a fwd simulation in order to use it also for
260 an adjoint simulation.
261 In this class during a forward simulation the method EndOfEventForForwardSimulation is used at the end of an event
262 while during an adjoint simulation at the end of fwd tracking event the method EndOfEventForAdjointSimulation is called.
263 By looking at the source of RMCO1AnalysisManager and more particularly to its method EndOfEventForAdjointSimulation the user will
264 learn how to adapt its G4 analysis code for an adjoint simulation.
265
266 The outputs of an adjoint simulation are:
267
268 -The total energy deposited and particle current entering the sensitive cylinder normalized
269 automatically to a user defined primary spectrum(exponential or power law) .
270 These results are stored in the files:
271 -Adj_Edep_vs_EkinPrim.txt
272 -Adj_ElectronCurrent.txt
273 -Adj_GammaCurrent.txt
274 -Adj_ProtonCurrent.txt
275 -ConvergenceOfAdjointSimulationResults.txt:
276 The total normalized edep and its relative error registered every 5000 adjoint events
277
278
279 -The answer matrix of the energy deposited and particles current on the sensitive cylinder in function of primary energy of e-, gamma and
280 protons. These results are stored in the files Adj********_Answer.txt
281
282
283
284 The outputs of a forward simulation are:
285 -The mean energy deposited and particle current entering the sensitive cylinder per event.
286 These results are stored in the files:
287 -Fwd_Edep_vs_EkinPrim.txt
288 -Fwd_ElectronCurrent.txt
289 -Fwd_GammaCurrent.txt
290 -Fwd_ProtonCurrent.txt
291 -ConvergenceOfAdjointSimulationResults.txt: The total normalized edep and its relative error registered every 5000 adjoint events
292
293
294
295 3.4. Run macrofiles:
296 ------------------
297 The following example run macro files are distributed with the code:
298
299 -run_adjoint_simulation_electron.mac and run_adjoint_simulation_proton.mac for adjoint simulations
300
301 -run_forward_simulation_electron.mac and run_forward_simulation_proton.mac for forward simulations
302
303
304 3.5. Comparison of adjoint and forward simulation results:
305 ----------------------------------------------------------
306 It is the responsibility of the user to select in the macro file the same external spectrum
307 for both the forward and adjoint simulations and to normalize the per event results of the forward simulation
308 to the fluence considered in the adjoint simulation.
309
310 For the macro files that are provided with the examples it consists into multiplying the forward results by pi*100.
311 This normalization factor is explained by the following:
312
313 -For the forward simulation the results are given per number of events. It corresponds
314 to a normalization to a fluence of 1 particle emanating from the external source.
315
316 -In run_fwd_simulation.mac the source is set on a sphere of 10 cm radius (see /gps commands in
317 macrofile).Therefore the omnidirectional fluence for the fwd simulation is 1./(pi*R^2) with R=10cm.
318
319 -The adjoint results are normalized to a fluence of 1/cm2.
320 (See command /RMC01/analysis/SetExponentialSpectrumForAdjointSim in macrofile)
321
322 -In conclusion to compare the adjoint and forward results, the forward results should
323 be multiplied by pi*R^2/cm2= pi*100.
324
325
326
327 4. Control of the adjoint simulation and the RMC01 code by G4 macro UI commands:
328 -------------------------------------------------------------------------
329 Different G4 macro UI commands are provided to control the RMC01 example and the adjoint simulation.
330 Some macro commands are provided within the geant4 toolkit and appears in a G4 application when the singleton
331 class G4AdjointSimManager is called somewhere in the code, the other macro commands are
332 declared in the code distributed within the example.
333
334
335 4.1. G4UI commands in the directory /adjoint
336 -----------------------------------------------
337 The macro commands in the directory /adjoint appears in a user application when the singleton
338 class G4AdjointSimManager is called somewhere in the code.
339 It allows to control the adjoint source, the external source and start an adjoint simulation.
340
341 The command to start an adjoint run is:
342
343 -/adjoint/start_run nb
344 Start an adjoint simulation with a number of events given by nb. It is important to note that the total number of events in the sense of G4
345 will be nb*2*nb_primary_considered (see 3.4.)
346
347
348 The commands to control the adjoint source are:
349
350 -/adjoint/DefineSphericalAdjSource R X Y Z unit_length
351 The adjoint source is set on a sphere with radius R and centered on position (X,Y,Z)
352
353 -/adjoint/DefineSphericalAdjSourceCenteredOnAVolume phys_vol_name R unit_length
354 The external source is set on a sphere with radius R and with its center position located at the center of the
355 the physical volume specified by the name phys_vol_name.
356 -/adjoint/DefineAdjSourceOnExtSurfaceOfAVolume phys_vol_name
357 The external surface is set as the external boundary of a the physical volume with name phys_vol_name
358
359 -/adjoint/SetAdjSourceEmin Emin energy_unit
360 Set the minimum energy of the external source
361
362 -/adjoint/SetAdjSourceEmax Emax energy_unit
363 Set the maximum energy of the external source
364
365 -/adjoint/ConsiderAsPrimary particle_name
366 The type of particle specified by "particle_name" will be added in the list of primary adjoint particles.
367 The list of candidates depends on the reverse physics processes considered in the simulation. At the most the
368 potential candidates are (e-, gamma, proton , ion). For this example only e-, gamma, proton
369 can be chosen. As the proton ionization is not considered by default, the default list of particles is
370 [e-,gamma]. To have also the proton as candidate the proton ionization should
371 be switch on (/adjoint_physics/UseProtonIonisation true).
372
373 -/adjoint/NeglectAsPrimary particle_name
374 The type of particle specified by "particle_name" will be removed from the list of primary adjoint particles.
375 The list of candidates depends on the reverse physics processes considered in the simulation. At the most the
376 potential candidates are (e-, gamma, proton , ion). For this example only e-, gamma, proton
377 can be chosen. As the proton ionization is not considered by default, the default list of particles is
378 [e-,gamma].To have also the proton as candidate the proton ionization should
379 be switch on (/adjoint_physics/UseProtonIonisation true).
380
381
382 The commands to control the external source are:
383
384 -/adjoint/DefineSphericalExtSource R X Y Z unit_length:
385 The external source is set on a sphere with radius R and centered on position (X,Y,Z)
386
387 -/adjoint/DefineSphericalExtSourceCenteredOnAVolume phys_vol_name R unit_length
388 The external source is set on a sphere with radius R and with its center position located at the center of the
389 the physical volume specified by the name phys_vol_name.
390
391 -/adjoint/DefineExtSourceOnExtSurfaceOfAVolume phys_vol_name
392 The external surface is set as the external boundary of a the physical volume with name phys_vol_name
393
394 -/adjoint/SetExtSourceEmax Emax energy_unit
395 Set the maximum energy of the external source. An adjoint track will be stop when a an adjoint particle get an energy higher than this maximum energy.
396
397
398
399 4.2. G4UI commands in the directory /adjoint_physics
400 ------------------------------------------------------
401 These commands allow to control the electromagnetic processes that will be considered in the simulation.
402
403 The processes that can be used are:
404 -Reverse and forward e- continuous and discrete Ionization. Always switch on
405 -Reverse and forward e- Bremsstrahlung. Switch on by default
406 -Reverse and forward Compton scattering. Switch on by default
407 -Reverse and forward photo electric effect. Switch on by default
408 -Reverse and forward photo electric effect. Switch on by default
409 -Reverse and forward multiple scattering. Switch on by default
410 -Reverse and forward proton continuous and discrete Ionization. Switch off by default
411 -Forward e-e+ pair production. Switch off by default.
412 If switch all the e+ electromagnetic physics is considered.
413
414
415 The commands that can be used to switch on of these processes are:
416
417 /adjoint_physics/UseProtonIonisation true/false
418 -Switch on/off the reverse and forward proton ionization. Off by default.
419
420 /adjoint_physics/UseBremsstrahlung true/false
421 -Switch on/off the reverse and forward e- bremsstrahlung. On by default.
422
423 /adjoint_physics/UseCompton true/false
424 -Switch on/off the Compton scattering. On by default.
425
426
427 /adjoint_physics/UseMS true/false
428 -Switch on/off the multiple scattering. On by default.
429
430
431 /adjoint_physics/UseEgainElossFluctuation true/false
432 -Switch on/off the fluctuation in the continuous energy loss/gain. On by default. Only for test purpose.
433
434 /adjoint_physics/UsePEEffect true/false
435 -Switch on/off the photo electric effect. On by default.
436
437
438 /adjoint_physics/UseGammaConversion true/false
439 -Switch on/off the forward e-e+ pair production from gamma. Off by default. When On all the e+
440 electromagnetic physics is considered.
441
442
443 The user can also fix the maximum energy Emax and minimum energy Emin of the adjoint physical processes used
444 in the simulation. The adjoint process will be applied to particles within the energy range [Emin, Emax]
445 and will produce adjoint secondary only in this energy range. It is recommended to fix Emin to the minimum
446 energy of the adjoint source and fix Emax to the maximum energy of the external source.
447 The commands controlling Emin and Emax are:
448
449 /adjoint_physics/SetEminForAdjointModels Emin Energy_unit
450 -Set the minimum energy of the adjoint processes/models.
451
452 /adjoint_physics/SetEmaxForAdjointModels Emin Energy_unit
453 -Set the maximum energy of the adjoint processes/models.
454
455
456 4.3. G4UI commands in the directory /RMC01
457 ----------------------------------------------
458
459 Commands/RMC01/geometry/ to control the geometry:
460
461 /RMC01/geometry/SetSensitiveVolumeHeight H length_unit
462 Set the height H of the Si sensitive cylinder.
463
464
465 /RMC01/geometry/SetSensitiveVolumeRadius R length_unit
466 Set the radius R of the Si sensitive cylinder.
467
468 /RMC01/geometry/SetShieldingThickness D length_unit
469 Set the thickness D of the aluminum shielding.
470
471 Commands /RMC01/analysis/ to control the primary spectrum used for the normalization of the
472 adjoint simulation results and fix the expected precision of the computed Edep:
473
474 /RMC01/analysis/SetPowerLawPrimSpectrumForAdjointSim particle_name F F_unit alpha Emin Emax E_unit
475 Set the primary spectrum to which the adjoint simulation results will be normalised to a power law
476 spectrum E^(-alpha) of particle defined by particle_name, with an omnidirectional fluence F, and
477 energy range [Emin,Emax]. The fluence unit candidates for F_unit are [1/cm2, 1/m2, cm-2, m-2].
478
479
480 /RMC01/analysis/SetExponentialSpectrumForAdjointSim particle_name F F_unit E0 Emin Emax E_unit
481 Set the primary spectrum to which the adjoint simulation results will be normalised to an exponential
482 spectrum exp(-E/E0) of particle defined by particle_name, with an omnidirectional fluence F, and
483 energy range [Emin,Emax]. The fluence unit candidates for F_unit are [1/cm2, 1/m2, cm-2, m-2].
484
485
486
487 /RMC01/analysis/SetExpectedPrecisionOfResults precision
488 Set the expected precision in % for the computed energy deposited in the sensitive volume
489 for both the forward and adjoint simulation case. When the relative statistical error
490 of the computed energy deposited reach this precision the run is aborted and the results are registered.
491 Otherwise the run continue till the nb of events specified by the user are processed. By default the precision is set
492 to 0. meaning that the run will not be aborted in this case.
493
494
495
496
497
498
499 5. Known issues
500 --------------------------------
501 --------------------------------
502
503 5.1 Rare too high weight in the adjoint simulation
504 ---------------------------------------------------
505
506 In rare cases an adjoint track may get a much too high weight when reaching the external source.
507 While this happen not often it may corrupt the simulation results significantly. The reason of this high weight is
508 the joint use at low e- and gamma energy of both the photoelectric and bremsstrahlung processes.
509 Unfortunately we still need some investigations to remove this problem at the level of physical processes.
510 However this problem can be solved at the level of event action in the user code by adding a test on the adjoint
511 weight. Such test has been implemented in the example RMC01.
512 In this implementation an event is rejected when the relative error of the computed normalised edep
513 increase during one event by more than 50% when the precision is already below 10%.
514
515
516 5.2 Limitation of the reverse bremsstrahlung
517 -------------------------------------------
518 The difference between the differential cross sections used in the adjoint and forward bremsstrahlung
519 models is the source of a higher flux of >100 keV gamma in the reverse simulation compared to the forward simulation.
520 The adjoint processes/models should make use of the direct differential cross section to sample
521 the adjoint secondaries and compute the adjoint cross section.
522 The differential cross section used in G4AdjointeBremstrahlungModel is obtained by the numerical derivation
523 over the cut energy of the direct cross section provided by G4eBremsstrahlungModel.
524 This would be a correct procedure if the distribution of secondary in G4eBremsstrahlungModel
525 would match this differential cross section. Unfortunately it is not the case as independent parameterization are used
526 in G4eBremsstrahlungModel for both the cross sections and the sample of secondary. (It means that in the forward case
527 if one would integrate the effective differential cross section considered in the simulation we would not find back
528 the used cross section).
529 In the future we plan to correct this problem by using an extra weight correction factor after the occurrence of a reverse
530 bremsstrahlung. This weight factor should be the ratio between the differential CS used in the adjoint simulation and the
531 one effectively used in the forward processes. As it is impossible to have access to the forward differential CS
532 in G4eBremsstrahlungModel we are investigating the feasibility to use the differential CS considered in
533 G4Penelope models.
534
535
536 5.3 Limitation of the reverse multiple scattering
537 -------------------------------------------------
538 For the reverse multiple scattering we are using the same models than for the forward case.
539 This approximation makes that the discrepancy between the adjoint and forward
540 simulation cases can get to a level of ~ 10-15% relative differences in the test cases that we have considered.
541 In the future we plan to improve the adjoint multiple scattering models by forcing the computation of
542 multiple scattering effect at the end of an adjoint step.
