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25 //
26 // --------------------------------------------------------------------
27 ///
28 //
29 //
30 // --------------------------------------------------------------
31 //
32 // Author: A. Forti
33 // Maria Grazia Pia (Maria.Grazia.Pia@cern.ch)
34 //
35 // History:
36 // --------
37 // 02/03/1999 A. Forti 1st implementation
38 // 14.03.2000 Veronique Lefebure;
39 // Change initialisation of lowestEnergyLimit from 1.22 to 1.022.
40 // Note that the hard coded value 1.022 should be used instead of
41 // 2*electron_mass_c2 in order to agree with the value of the data bank EPDL97
42 // 24.04.01 V.Ivanchenko remove RogueWave
43 // 27.07.01 F.Longo correct bug in energy distribution
44 // 21.01.03 V.Ivanchenko Cut per region
45 // 25.03.03 F.Longo fix in angular distribution of e+/e-
46 // 24.04.03 V.Ivanchenko - Cut per region mfpt
47 //
48 // --------------------------------------------------------------
49
50 #include "G4LowEnergyGammaConversion.hh"
51
52 #include "Randomize.hh"
53 #include "G4PhysicalConstants.hh"
54 #include "G4SystemOfUnits.hh"
55 #include "G4ParticleDefinition.hh"
56 #include "G4Track.hh"
57 #include "G4Step.hh"
58 #include "G4ForceCondition.hh"
59 #include "G4Gamma.hh"
60 #include "G4Electron.hh"
61 #include "G4DynamicParticle.hh"
62 #include "G4VParticleChange.hh"
63 #include "G4ThreeVector.hh"
64 #include "G4Positron.hh"
65 #include "G4IonisParamElm.hh"
66 #include "G4Material.hh"
67 #include "G4RDVCrossSectionHandler.hh"
68 #include "G4RDCrossSectionHandler.hh"
69 #include "G4RDVEMDataSet.hh"
70 #include "G4RDVDataSetAlgorithm.hh"
71 #include "G4RDLogLogInterpolation.hh"
72 #include "G4RDVRangeTest.hh"
73 #include "G4RDRangeTest.hh"
74 #include "G4MaterialCutsCouple.hh"
75
76 G4LowEnergyGammaConversion::G4LowEnergyGammaConversion(const G4String& processName)
77 : G4VDiscreteProcess(processName),
78 lowEnergyLimit(1.022000*MeV),
79 highEnergyLimit(100*GeV),
80 intrinsicLowEnergyLimit(1.022000*MeV),
81 intrinsicHighEnergyLimit(100*GeV),
82 smallEnergy(2.*MeV)
83
84 {
85 if (lowEnergyLimit < intrinsicLowEnergyLimit ||
86 highEnergyLimit > intrinsicHighEnergyLimit)
87 {
88 G4Exception("G4LowEnergyGammaConversion::G4LowEnergyGammaConversion()",
89 "OutOfRange", FatalException,
90 "Energy limit outside intrinsic process validity range!");
91 }
92
93 // The following pointer is owned by G4DataHandler
94
95 crossSectionHandler = new G4RDCrossSectionHandler();
96 crossSectionHandler->Initialise(0,1.0220*MeV,100.*GeV,400);
97 meanFreePathTable = 0;
98 rangeTest = new G4RDRangeTest;
99
100 if (verboseLevel > 0)
101 {
102 G4cout << GetProcessName() << " is created " << G4endl
103 << "Energy range: "
104 << lowEnergyLimit / MeV << " MeV - "
105 << highEnergyLimit / GeV << " GeV"
106 << G4endl;
107 }
108 }
109
110 G4LowEnergyGammaConversion::~G4LowEnergyGammaConversion()
111 {
112 delete meanFreePathTable;
113 delete crossSectionHandler;
114 delete rangeTest;
115 }
116
117 void G4LowEnergyGammaConversion::BuildPhysicsTable(const G4ParticleDefinition& )
118 {
119
120 crossSectionHandler->Clear();
121 G4String crossSectionFile = "pair/pp-cs-";
122 crossSectionHandler->LoadData(crossSectionFile);
123
124 delete meanFreePathTable;
125 meanFreePathTable = crossSectionHandler->BuildMeanFreePathForMaterials();
126 }
127
128 G4VParticleChange* G4LowEnergyGammaConversion::PostStepDoIt(const G4Track& aTrack,
129 const G4Step& aStep)
130 {
131 // The energies of the e+ e- secondaries are sampled using the Bethe - Heitler
132 // cross sections with Coulomb correction. A modified version of the random
133 // number techniques of Butcher & Messel is used (Nuc Phys 20(1960),15).
134
135 // Note 1 : Effects due to the breakdown of the Born approximation at low
136 // energy are ignored.
137 // Note 2 : The differential cross section implicitly takes account of
138 // pair creation in both nuclear and atomic electron fields. However triplet
139 // prodution is not generated.
140
141 aParticleChange.Initialize(aTrack);
142
143 const G4MaterialCutsCouple* couple = aTrack.GetMaterialCutsCouple();
144
145 const G4DynamicParticle* incidentPhoton = aTrack.GetDynamicParticle();
146 G4double photonEnergy = incidentPhoton->GetKineticEnergy();
147 G4ParticleMomentum photonDirection = incidentPhoton->GetMomentumDirection();
148
149 G4double epsilon ;
150 G4double epsilon0 = electron_mass_c2 / photonEnergy ;
151
152 // Do it fast if photon energy < 2. MeV
153 if (photonEnergy < smallEnergy )
154 {
155 epsilon = epsilon0 + (0.5 - epsilon0) * G4UniformRand();
156 }
157 else
158 {
159 // Select randomly one element in the current material
160 const G4Element* element = crossSectionHandler->SelectRandomElement(couple,photonEnergy);
161
162 if (element == 0)
163 {
164 G4cout << "G4LowEnergyGammaConversion::PostStepDoIt - element = 0" << G4endl;
165 }
166 G4IonisParamElm* ionisation = element->GetIonisation();
167 if (ionisation == 0)
168 {
169 G4cout << "G4LowEnergyGammaConversion::PostStepDoIt - ionisation = 0" << G4endl;
170 }
171
172 // Extract Coulomb factor for this Element
173 G4double fZ = 8. * (ionisation->GetlogZ3());
174 if (photonEnergy > 50. * MeV) fZ += 8. * (element->GetfCoulomb());
175
176 // Limits of the screening variable
177 G4double screenFactor = 136. * epsilon0 / (element->GetIonisation()->GetZ3()) ;
178 G4double screenMax = std::exp ((42.24 - fZ)/8.368) - 0.952 ;
179 G4double screenMin = std::min(4.*screenFactor,screenMax) ;
180
181 // Limits of the energy sampling
182 G4double epsilon1 = 0.5 - 0.5 * std::sqrt(1. - screenMin / screenMax) ;
183 G4double epsilonMin = std::max(epsilon0,epsilon1);
184 G4double epsilonRange = 0.5 - epsilonMin ;
185
186 // Sample the energy rate of the created electron (or positron)
187 G4double screen;
188 G4double gReject ;
189
190 G4double f10 = ScreenFunction1(screenMin) - fZ;
191 G4double f20 = ScreenFunction2(screenMin) - fZ;
192 G4double normF1 = std::max(f10 * epsilonRange * epsilonRange,0.);
193 G4double normF2 = std::max(1.5 * f20,0.);
194
195 do {
196 if (normF1 / (normF1 + normF2) > G4UniformRand() )
197 {
198 epsilon = 0.5 - epsilonRange * std::pow(G4UniformRand(), 0.3333) ;
199 screen = screenFactor / (epsilon * (1. - epsilon));
200 gReject = (ScreenFunction1(screen) - fZ) / f10 ;
201 }
202 else
203 {
204 epsilon = epsilonMin + epsilonRange * G4UniformRand();
205 screen = screenFactor / (epsilon * (1 - epsilon));
206 gReject = (ScreenFunction2(screen) - fZ) / f20 ;
207 }
208 } while ( gReject < G4UniformRand() );
209
210 } // End of epsilon sampling
211
212 // Fix charges randomly
213
214 G4double electronTotEnergy;
215 G4double positronTotEnergy;
216
217 if (CLHEP::RandBit::shootBit())
218 {
219 electronTotEnergy = (1. - epsilon) * photonEnergy;
220 positronTotEnergy = epsilon * photonEnergy;
221 }
222 else
223 {
224 positronTotEnergy = (1. - epsilon) * photonEnergy;
225 electronTotEnergy = epsilon * photonEnergy;
226 }
227
228 // Scattered electron (positron) angles. ( Z - axis along the parent photon)
229 // Universal distribution suggested by L. Urban (Geant3 manual (1993) Phys211),
230 // derived from Tsai distribution (Rev. Mod. Phys. 49, 421 (1977)
231
232 G4double u;
233 const G4double a1 = 0.625;
234 G4double a2 = 3. * a1;
235 // G4double d = 27. ;
236
237 // if (9. / (9. + d) > G4UniformRand())
238 if (0.25 > G4UniformRand())
239 {
240 u = - std::log(G4UniformRand() * G4UniformRand()) / a1 ;
241 }
242 else
243 {
244 u = - std::log(G4UniformRand() * G4UniformRand()) / a2 ;
245 }
246
247 G4double thetaEle = u*electron_mass_c2/electronTotEnergy;
248 G4double thetaPos = u*electron_mass_c2/positronTotEnergy;
249 G4double phi = twopi * G4UniformRand();
250
251 G4double dxEle= std::sin(thetaEle)*std::cos(phi),dyEle= std::sin(thetaEle)*std::sin(phi),dzEle=std::cos(thetaEle);
252 G4double dxPos=-std::sin(thetaPos)*std::cos(phi),dyPos=-std::sin(thetaPos)*std::sin(phi),dzPos=std::cos(thetaPos);
253
254
255 // Kinematics of the created pair:
256 // the electron and positron are assumed to have a symetric angular
257 // distribution with respect to the Z axis along the parent photon
258
259 G4double localEnergyDeposit = 0. ;
260
261 aParticleChange.SetNumberOfSecondaries(2) ;
262 G4double electronKineEnergy = std::max(0.,electronTotEnergy - electron_mass_c2) ;
263
264 // Generate the electron only if with large enough range w.r.t. cuts and safety
265
266 G4double safety = aStep.GetPostStepPoint()->GetSafety();
267
268 if (rangeTest->Escape(G4Electron::Electron(),couple,electronKineEnergy,safety))
269 {
270 G4ThreeVector electronDirection (dxEle, dyEle, dzEle);
271 electronDirection.rotateUz(photonDirection);
272
273 G4DynamicParticle* particle1 = new G4DynamicParticle (G4Electron::Electron(),
274 electronDirection,
275 electronKineEnergy);
276 aParticleChange.AddSecondary(particle1) ;
277 }
278 else
279 {
280 localEnergyDeposit += electronKineEnergy ;
281 }
282
283 // The e+ is always created (even with kinetic energy = 0) for further annihilation
284 G4double positronKineEnergy = std::max(0.,positronTotEnergy - electron_mass_c2) ;
285
286 // Is the local energy deposit correct, if the positron is always created?
287 if (! (rangeTest->Escape(G4Positron::Positron(),couple,positronKineEnergy,safety)))
288 {
289 localEnergyDeposit += positronKineEnergy ;
290 positronKineEnergy = 0. ;
291 }
292
293 G4ThreeVector positronDirection (dxPos, dyPos, dzPos);
294 positronDirection.rotateUz(photonDirection);
295
296 // Create G4DynamicParticle object for the particle2
297 G4DynamicParticle* particle2 = new G4DynamicParticle(G4Positron::Positron(),
298 positronDirection, positronKineEnergy);
299 aParticleChange.AddSecondary(particle2) ;
300
301 aParticleChange.ProposeLocalEnergyDeposit(localEnergyDeposit) ;
302
303 // Kill the incident photon
304 aParticleChange.ProposeMomentumDirection(0.,0.,0.) ;
305 aParticleChange.ProposeEnergy(0.) ;
306 aParticleChange.ProposeTrackStatus(fStopAndKill) ;
307
308 // Reset NbOfInteractionLengthLeft and return aParticleChange
309 return G4VDiscreteProcess::PostStepDoIt(aTrack,aStep);
310 }
311
312 G4bool G4LowEnergyGammaConversion::IsApplicable(const G4ParticleDefinition& particle)
313 {
314 return ( &particle == G4Gamma::Gamma() );
315 }
316
317 G4double G4LowEnergyGammaConversion::GetMeanFreePath(const G4Track& track,
318 G4double, // previousStepSize
319 G4ForceCondition*)
320 {
321 const G4DynamicParticle* photon = track.GetDynamicParticle();
322 G4double energy = photon->GetKineticEnergy();
323 const G4MaterialCutsCouple* couple = track.GetMaterialCutsCouple();
324 size_t materialIndex = couple->GetIndex();
325
326 G4double meanFreePath;
327 if (energy > highEnergyLimit) meanFreePath = meanFreePathTable->FindValue(highEnergyLimit,materialIndex);
328 else if (energy < lowEnergyLimit) meanFreePath = DBL_MAX;
329 else meanFreePath = meanFreePathTable->FindValue(energy,materialIndex);
330 return meanFreePath;
331 }
332
333 G4double G4LowEnergyGammaConversion::ScreenFunction1(G4double screenVariable)
334 {
335 // Compute the value of the screening function 3*phi1 - phi2
336
337 G4double value;
338
339 if (screenVariable > 1.)
340 value = 42.24 - 8.368 * std::log(screenVariable + 0.952);
341 else
342 value = 42.392 - screenVariable * (7.796 - 1.961 * screenVariable);
343
344 return value;
345 }
346
347 G4double G4LowEnergyGammaConversion::ScreenFunction2(G4double screenVariable)
348 {
349 // Compute the value of the screening function 1.5*phi1 - 0.5*phi2
350
351 G4double value;
352
353 if (screenVariable > 1.)
354 value = 42.24 - 8.368 * std::log(screenVariable + 0.952);
355 else
356 value = 41.405 - screenVariable * (5.828 - 0.8945 * screenVariable);
357
358 return value;
359 }
360