| Geant4 Cross Reference |
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25 //
26
27 #include "G4AdjointForcedInteractionForGamma.hh"
28
29 #include "G4AdjointCSManager.hh"
30 #include "G4AdjointGamma.hh"
31 #include "G4MaterialCutsCouple.hh"
32 #include "G4ParticleChange.hh"
33 #include "G4SystemOfUnits.hh"
34 #include "G4VEmAdjointModel.hh"
35
36 G4AdjointForcedInteractionForGamma::G4AdjointForcedInteractionForGamma(
37 const G4String& process_name)
38 : G4VContinuousDiscreteProcess(process_name)
39 , fAdjointComptonModel(nullptr)
40 , fAdjointBremModel(nullptr)
41 {
42 fCSManager = G4AdjointCSManager::GetAdjointCSManager();
43 fParticleChange = new G4ParticleChange();
44 }
45
46 //////////////////////////////////////////////////////////////////////////////
47 G4AdjointForcedInteractionForGamma::~G4AdjointForcedInteractionForGamma()
48 {
49 if(fParticleChange)
50 delete fParticleChange;
51 }
52
53 //////////////////////////////////////////////////////////////////////////////
54 void G4AdjointForcedInteractionForGamma::ProcessDescription(
55 std::ostream& out) const
56 {
57 out << "Forced interaction for gamma.\n";
58 }
59
60 //////////////////////////////////////////////////////////////////////////////
61 void G4AdjointForcedInteractionForGamma::BuildPhysicsTable(
62 const G4ParticleDefinition&)
63 {
64 fCSManager->BuildCrossSectionMatrices(); // it will be done just once
65 fCSManager->BuildTotalSigmaTables();
66 }
67
68 // Note on weight correction for forced interaction.
69 // For the forced interaction applied here we use a truncated exponential law
70 // for the probability of survival over a fixed total length. This is done by
71 // using a linear transformation of the non-biased probability survival. In
72 // math this is written P'(x)=C1P(x)+C2 , with P(x)=exp(-sum(sigma_ixi)) . x and
73 // L can cross different volumes with different cross section sigma. For forced
74 // interaction, we get the limit conditions:
75 // P'(L)=0 and P'(0)=1 (L can be used over different volumes)
76 // From simple solving of linear equations we
77 // get C1=1/(1-P(L)) and C2=-P(L)/(1-P(L))
78 // P'(x)=(P(x)-P(L))/(1-P(L))
79 // For the probability over a step x1 to x2, P'(x1->x2)=P'(x2)/P'(x1).
80 // The effective cross
81 // section is defined -d(P'(x))/dx/P'(x).
82 // We get therefore
83 // sigma_eff = C1sigmaP(x)/(C1P(x)+C2) = sigmaP(x)/(P(x)+C2/C1)
84 // = sigmaP(x)/(P(x)-P(L)) = sigma/(1-P(L)/P(x))
85 //////////////////////////////////////////////////////////////////////////////
86 G4VParticleChange* G4AdjointForcedInteractionForGamma::PostStepDoIt(
87 const G4Track& track, const G4Step&)
88 {
89 fParticleChange->Initialize(track);
90 // For the free flight gamma no interaction occurs but a gamma with same
91 // properties is produced for further forced interaction. It is done at the
92 // very beginning of the track so that the weight can be the same
93 if(fCopyGammaForForced)
94 {
95 G4ThreeVector theGammaMomentum = track.GetMomentum();
96 fParticleChange->AddSecondary(
97 new G4DynamicParticle(G4AdjointGamma::AdjointGamma(), theGammaMomentum));
98 fParticleChange->SetParentWeightByProcess(false);
99 fParticleChange->SetSecondaryWeightByProcess(false);
100 }
101 else
102 { // Occurrence of forced interaction
103 // Selection of the model to be called
104 G4VEmAdjointModel* theSelectedModel = nullptr;
105 G4bool is_scat_proj_to_proj_case = false;
106 G4double factor=1.;
107 if(!fAdjointComptonModel && !fAdjointBremModel)
108 return fParticleChange;
109 if(!fAdjointComptonModel)
110 {
111 theSelectedModel = fAdjointBremModel;
112 is_scat_proj_to_proj_case = false;
113 // This is needed because the results of it will be used in the post step
114 // do it weight correction inside the model
115 fAdjointBremModel->AdjointCrossSection(track.GetMaterialCutsCouple(),
116 track.GetKineticEnergy(), false);
117 }
118 else if(!fAdjointBremModel)
119 {
120 theSelectedModel = fAdjointComptonModel;
121 is_scat_proj_to_proj_case = true;
122 }
123 else
124 { // Choose the model according to a 50-50 % probability
125 G4double bremAdjCS = fAdjointBremModel->AdjointCrossSection(
126 track.GetMaterialCutsCouple(), track.GetKineticEnergy(), false);
127 if(G4UniformRand() < 0.5)
128 {
129 theSelectedModel = fAdjointBremModel;
130 is_scat_proj_to_proj_case = false;
131 factor=bremAdjCS/fLastAdjCS/0.5;
132 }
133 else
134 {
135 theSelectedModel = fAdjointComptonModel;
136 is_scat_proj_to_proj_case = true;
137 factor=(fLastAdjCS-bremAdjCS)/fLastAdjCS/0.5;
138 }
139 }
140
141 // Compute the weight correction factor
142 G4double invEffectiveAdjointCS =
143 (1. - std::exp(fNbAdjIntLength - fTotNbAdjIntLength)) / fLastAdjCS/fCSBias;
144
145 // Call the selected model without correction of the weight in the model
146 theSelectedModel->SetCorrectWeightForPostStepInModel(false);
147 theSelectedModel
148 ->SetAdditionalWeightCorrectionFactorForPostStepOutsideModel(
149 factor*fLastAdjCS * invEffectiveAdjointCS);
150 theSelectedModel->SampleSecondaries(track, is_scat_proj_to_proj_case,
151 fParticleChange);
152 theSelectedModel->SetCorrectWeightForPostStepInModel(true);
153
154 fContinueGammaAsNewFreeFlight = true;
155 }
156 return fParticleChange;
157 }
158
159 //////////////////////////////////////////////////////////////////////////////
160 G4VParticleChange* G4AdjointForcedInteractionForGamma::AlongStepDoIt(
161 const G4Track& track, const G4Step&)
162 {
163 fParticleChange->Initialize(track);
164 // Compute nb of interactions length over step length
165 G4ThreeVector position = track.GetPosition();
166 G4double stepLength = track.GetStep()->GetStepLength();
167 G4double ekin = track.GetKineticEnergy();
168 fLastAdjCS = fCSManager->GetTotalAdjointCS(track.GetDefinition(), ekin,
169 track.GetMaterialCutsCouple());
170 G4double nb_fwd_interaction_length_over_step =
171 stepLength * fCSManager->GetTotalForwardCS(G4AdjointGamma::AdjointGamma(),
172 ekin,
173 track.GetMaterialCutsCouple());
174
175 G4double nb_adj_interaction_length_over_step = stepLength * fLastAdjCS;
176 G4double fwd_survival_probability =
177 std::exp(-nb_fwd_interaction_length_over_step);
178 G4double mc_induced_survival_probability = 1.;
179
180 if(fFreeFlightGamma)
181 { // for free_flight survival probability stays 1
182 // Accumulate the number of interaction lengths during free flight of gamma
183 fTotNbAdjIntLength += nb_adj_interaction_length_over_step;
184 fAccTrackLength += stepLength;
185 }
186 else
187 {
188 G4double previous_acc_nb_adj_interaction_length = fNbAdjIntLength;
189 fNbAdjIntLength += fCSBias*nb_adj_interaction_length_over_step;
190 theNumberOfInteractionLengthLeft -= fCSBias*nb_adj_interaction_length_over_step;
191
192 // protection against rare race condition
193 if(std::abs(fTotNbAdjIntLength - previous_acc_nb_adj_interaction_length) <=
194 1.e-15)
195 {
196 mc_induced_survival_probability = 1.e50;
197 }
198 else
199 {
200 mc_induced_survival_probability =
201 std::exp(-fNbAdjIntLength) - std::exp(-fTotNbAdjIntLength);
202 mc_induced_survival_probability /=
203 (std::exp(-previous_acc_nb_adj_interaction_length) -
204 std::exp(-fTotNbAdjIntLength));
205 }
206 }
207 G4double weight_correction =
208 fwd_survival_probability / mc_induced_survival_probability;
209
210 // Caution!!!
211 // It is important to select the weight of the post_step_point as the
212 // current weight and not the weight of the track, as the weight of the track
213 // is changed after having applied all the along_step_do_it.
214 G4double new_weight =
215 weight_correction * track.GetStep()->GetPostStepPoint()->GetWeight();
216
217 fParticleChange->SetParentWeightByProcess(false);
218 fParticleChange->SetSecondaryWeightByProcess(false);
219 fParticleChange->ProposeParentWeight(new_weight);
220
221 return fParticleChange;
222 }
223
224 //////////////////////////////////////////////////////////////////////////////
225 G4double
226 G4AdjointForcedInteractionForGamma::PostStepGetPhysicalInteractionLength(
227 const G4Track& track, G4double, G4ForceCondition* condition)
228 {
229 G4int step_id = track.GetCurrentStepNumber();
230 *condition = NotForced;
231 fCopyGammaForForced = false;
232 G4int track_id = track.GetTrackID();
233 fFreeFlightGamma =
234 (track_id != fLastFreeFlightTrackId + 1 || fContinueGammaAsNewFreeFlight);
235 if(fFreeFlightGamma)
236 {
237 if(step_id == 1 || fContinueGammaAsNewFreeFlight)
238 {
239 *condition = Forced;
240 // A gamma with same conditions will be generate at next post_step do it
241 // for the forced interaction
242 fCopyGammaForForced = true;
243 fLastFreeFlightTrackId = track_id;
244 fAccTrackLength = 0.;
245 fTotNbAdjIntLength = 0.;
246 fContinueGammaAsNewFreeFlight = false;
247 return 1.e-90;
248 }
249 else
250 {
251 return DBL_MAX;
252 }
253 }
254 else
255 { // compute the interaction length for forced interaction
256 if(step_id == 1)
257 {
258 fCSBias=0.000001/fTotNbAdjIntLength;
259 fTotNbAdjIntLength*=fCSBias;
260 G4double min_val = std::exp(-fTotNbAdjIntLength);
261 theNumberOfInteractionLengthLeft =
262 -std::log(min_val + G4UniformRand() * (1. - min_val));
263 theInitialNumberOfInteractionLength = theNumberOfInteractionLengthLeft;
264 fNbAdjIntLength = 0.;
265 }
266 G4VPhysicalVolume* thePostPhysVolume =
267 track.GetStep()->GetPreStepPoint()->GetPhysicalVolume();
268 G4double ekin = track.GetKineticEnergy();
269 G4double postCS = 0.;
270 if(thePostPhysVolume)
271 {
272 postCS = fCSManager->GetTotalAdjointCS(
273 G4AdjointGamma::AdjointGamma(), ekin,
274 thePostPhysVolume->GetLogicalVolume()->GetMaterialCutsCouple());
275 }
276 if(postCS > 0.)
277 return theNumberOfInteractionLengthLeft / postCS /fCSBias;
278 else
279 return DBL_MAX;
280 }
281 }
282
283 ////////////////////////////////////////////////////////////////////////////////
284 G4double G4AdjointForcedInteractionForGamma::GetContinuousStepLimit(
285 const G4Track&, G4double, G4double, G4double&)
286 {
287 return DBL_MAX;
288 }
289
290 ////////////////////////////////////////////////////////////////////////////////
291 // Not used in this process but should be implemented as virtual method
292 G4double G4AdjointForcedInteractionForGamma::GetMeanFreePath(const G4Track&,
293 G4double,
294 G4ForceCondition*)
295 {
296 return 0.;
297 }
298