source: trunk/source/processes/electromagnetic/standard/src/G4BetheHeitlerModel.cc @ 961

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25//
26// $Id: G4BetheHeitlerModel.cc,v 1.12 2008/10/15 15:54:57 vnivanch Exp $
27// GEANT4 tag $Name: geant4-09-02-ref-02 $
28//
29// -------------------------------------------------------------------
30//
31// GEANT4 Class file
32//
33//
34// File name:     G4BetheHeitlerModel
35//
36// Author:        Vladimir Ivanchenko on base of Michel Maire code
37//
38// Creation date: 15.03.2005
39//
40// Modifications:
41// 18-04-05 Use G4ParticleChangeForGamma (V.Ivantchenko)
42// 24-06-05 Increase number of bins to 200 (V.Ivantchenko)
43// 16-11-05 replace shootBit() by G4UniformRand()  mma
44// 04-12-05 SetProposedKineticEnergy(0.) for the killed photon (mma)
45// 20-02-20 SelectRandomElement is called for any initial gamma energy
46//          in order to have selected element for polarized model (VI)
47//
48// Class Description:
49//
50// -------------------------------------------------------------------
51//
52//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
53//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
54
55#include "G4BetheHeitlerModel.hh"
56#include "G4Electron.hh"
57#include "G4Positron.hh"
58#include "G4Gamma.hh"
59#include "Randomize.hh"
60#include "G4DataVector.hh"
61#include "G4PhysicsLogVector.hh"
62#include "G4ParticleChangeForGamma.hh"
63#include "G4LossTableManager.hh"
64
65//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
66
67using namespace std;
68
69G4BetheHeitlerModel::G4BetheHeitlerModel(const G4ParticleDefinition*,
70                                         const G4String& nam)
71  : G4VEmModel(nam),
72    theCrossSectionTable(0),
73    nbins(10)
74{
75  fParticleChange = 0;
76  theGamma    = G4Gamma::Gamma();
77  thePositron = G4Positron::Positron();
78  theElectron = G4Electron::Electron();
79}
80
81//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
82
83G4BetheHeitlerModel::~G4BetheHeitlerModel()
84{
85  if(theCrossSectionTable) {
86    theCrossSectionTable->clearAndDestroy();
87    delete theCrossSectionTable;
88  }
89}
90
91//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
92
93void G4BetheHeitlerModel::Initialise(const G4ParticleDefinition*,
94                                     const G4DataVector&)
95{
96  if(!fParticleChange) {
97    if(pParticleChange) {
98      fParticleChange = reinterpret_cast<G4ParticleChangeForGamma*>(pParticleChange);
99    } else {
100      fParticleChange = new G4ParticleChangeForGamma();
101    }
102  }
103
104  if(theCrossSectionTable) {
105    theCrossSectionTable->clearAndDestroy();
106    delete theCrossSectionTable;
107  }
108
109  const G4ElementTable* theElementTable = G4Element::GetElementTable();
110  size_t nvect = G4Element::GetNumberOfElements();
111  theCrossSectionTable = new G4PhysicsTable(nvect);
112  G4PhysicsLogVector* ptrVector;
113  G4double emin = LowEnergyLimit();
114  G4double emax = HighEnergyLimit();
115  G4int n = nbins*G4int(log10(emax/emin));
116  G4bool spline = G4LossTableManager::Instance()->SplineFlag(); 
117  G4double e, value;
118
119  for(size_t j=0; j<nvect ; j++) { 
120
121    ptrVector  = new G4PhysicsLogVector(emin, emax, n);
122    ptrVector->SetSpline(spline);
123    G4double Z = (*theElementTable)[j]->GetZ();
124    G4int   iz = G4int(Z);
125    indexZ[iz] = j;
126 
127    for(G4int i=0; i<nbins; i++) {
128      e = ptrVector->GetLowEdgeEnergy( i ) ;
129      value = ComputeCrossSectionPerAtom(theGamma, e, Z); 
130      ptrVector->PutValue( i, value );
131    }
132
133    theCrossSectionTable->insert(ptrVector);
134  }
135}
136
137//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
138
139G4double G4BetheHeitlerModel::ComputeCrossSectionPerAtom(
140                                                   const G4ParticleDefinition*,
141                                              G4double GammaEnergy, G4double Z,
142                                              G4double, G4double, G4double)
143// Calculates the microscopic cross section in GEANT4 internal units.
144// A parametrized formula from L. Urban is used to estimate
145// the total cross section.
146// It gives a good description of the data from 1.5 MeV to 100 GeV.
147// below 1.5 MeV: sigma=sigma(1.5MeV)*(GammaEnergy-2electronmass)
148//                                   *(GammaEnergy-2electronmass)
149{
150  static const G4double GammaEnergyLimit = 1.5*MeV;
151  G4double CrossSection = 0.0 ;
152  if ( Z < 1. ) return CrossSection;
153  if ( GammaEnergy <= 2.0*electron_mass_c2 ) return CrossSection;
154
155  static const G4double
156    a0= 8.7842e+2*microbarn, a1=-1.9625e+3*microbarn, a2= 1.2949e+3*microbarn,
157    a3=-2.0028e+2*microbarn, a4= 1.2575e+1*microbarn, a5=-2.8333e-1*microbarn;
158
159  static const G4double
160    b0=-1.0342e+1*microbarn, b1= 1.7692e+1*microbarn, b2=-8.2381   *microbarn,
161    b3= 1.3063   *microbarn, b4=-9.0815e-2*microbarn, b5= 2.3586e-3*microbarn;
162
163  static const G4double
164    c0=-4.5263e+2*microbarn, c1= 1.1161e+3*microbarn, c2=-8.6749e+2*microbarn,
165    c3= 2.1773e+2*microbarn, c4=-2.0467e+1*microbarn, c5= 6.5372e-1*microbarn;
166
167  G4double GammaEnergySave = GammaEnergy;
168  if (GammaEnergy < GammaEnergyLimit) GammaEnergy = GammaEnergyLimit ;
169
170  G4double X=log(GammaEnergy/electron_mass_c2), X2=X*X, X3=X2*X, X4=X3*X, X5=X4*X;
171
172  G4double F1 = a0 + a1*X + a2*X2 + a3*X3 + a4*X4 + a5*X5,
173           F2 = b0 + b1*X + b2*X2 + b3*X3 + b4*X4 + b5*X5,
174           F3 = c0 + c1*X + c2*X2 + c3*X3 + c4*X4 + c5*X5;     
175
176  CrossSection = (Z + 1.)*(F1*Z + F2*Z*Z + F3);
177
178  if (GammaEnergySave < GammaEnergyLimit) {
179
180    X = (GammaEnergySave  - 2.*electron_mass_c2)
181      / (GammaEnergyLimit - 2.*electron_mass_c2);
182    CrossSection *= X*X;
183  }
184
185  if (CrossSection < 0.) CrossSection = 0.;
186  return CrossSection;
187}
188
189//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
190
191void G4BetheHeitlerModel::SampleSecondaries(std::vector<G4DynamicParticle*>* fvect,
192                                            const G4MaterialCutsCouple* couple,
193                                            const G4DynamicParticle* aDynamicGamma,
194                                            G4double,
195                                            G4double)
196// The secondaries e+e- energies are sampled using the Bethe - Heitler
197// cross sections with Coulomb correction.
198// A modified version of the random number techniques of Butcher & Messel
199// is used (Nuc Phys 20(1960),15).
200//
201// GEANT4 internal units.
202//
203// Note 1 : Effects due to the breakdown of the Born approximation at
204//          low energy are ignored.
205// Note 2 : The differential cross section implicitly takes account of
206//          pair creation in both nuclear and atomic electron fields.
207//          However triplet prodution is not generated.
208{
209  const G4Material* aMaterial = couple->GetMaterial();
210
211  G4double GammaEnergy = aDynamicGamma->GetKineticEnergy();
212  G4ParticleMomentum GammaDirection = aDynamicGamma->GetMomentumDirection();
213
214  G4double epsil ;
215  G4double epsil0 = electron_mass_c2/GammaEnergy ;
216  if(epsil0 > 1.0) return;
217
218  // do it fast if GammaEnergy < 2. MeV
219  static const G4double Egsmall=2.*MeV;
220
221  // select randomly one element constituing the material
222  const G4Element* anElement = SelectRandomAtom(aMaterial, theGamma, GammaEnergy);
223
224  if (GammaEnergy < Egsmall) {
225
226    epsil = epsil0 + (0.5-epsil0)*G4UniformRand();
227
228  } else {
229    // now comes the case with GammaEnergy >= 2. MeV
230
231    // Extract Coulomb factor for this Element
232    G4double FZ = 8.*(anElement->GetIonisation()->GetlogZ3());
233    if (GammaEnergy > 50.*MeV) FZ += 8.*(anElement->GetfCoulomb());
234
235    // limits of the screening variable
236    G4double screenfac = 136.*epsil0/(anElement->GetIonisation()->GetZ3());
237    G4double screenmax = exp ((42.24 - FZ)/8.368) - 0.952 ;
238    G4double screenmin = min(4.*screenfac,screenmax);
239
240    // limits of the energy sampling
241    G4double epsil1 = 0.5 - 0.5*sqrt(1. - screenmin/screenmax) ;
242    G4double epsilmin = max(epsil0,epsil1) , epsilrange = 0.5 - epsilmin;
243
244    //
245    // sample the energy rate of the created electron (or positron)
246    //
247    //G4double epsil, screenvar, greject ;
248    G4double  screenvar, greject ;
249
250    G4double F10 = ScreenFunction1(screenmin) - FZ;
251    G4double F20 = ScreenFunction2(screenmin) - FZ;
252    G4double NormF1 = max(F10*epsilrange*epsilrange,0.); 
253    G4double NormF2 = max(1.5*F20,0.);
254
255    do {
256      if ( NormF1/(NormF1+NormF2) > G4UniformRand() ) {
257        epsil = 0.5 - epsilrange*pow(G4UniformRand(), 0.333333);
258        screenvar = screenfac/(epsil*(1-epsil));
259        greject = (ScreenFunction1(screenvar) - FZ)/F10;
260             
261      } else { 
262        epsil = epsilmin + epsilrange*G4UniformRand();
263        screenvar = screenfac/(epsil*(1-epsil));
264        greject = (ScreenFunction2(screenvar) - FZ)/F20;
265      }
266
267    } while( greject < G4UniformRand() );
268
269  }   //  end of epsil sampling
270   
271  //
272  // fixe charges randomly
273  //
274
275  G4double ElectTotEnergy, PositTotEnergy;
276  if (G4UniformRand() > 0.5) {
277
278    ElectTotEnergy = (1.-epsil)*GammaEnergy;
279    PositTotEnergy = epsil*GammaEnergy;
280     
281  } else {
282   
283    PositTotEnergy = (1.-epsil)*GammaEnergy;
284    ElectTotEnergy = epsil*GammaEnergy;
285  }
286
287  //
288  // scattered electron (positron) angles. ( Z - axis along the parent photon)
289  //
290  //  universal distribution suggested by L. Urban
291  // (Geant3 manual (1993) Phys211),
292  //  derived from Tsai distribution (Rev Mod Phys 49,421(1977))
293
294  G4double u;
295  const G4double a1 = 0.625 , a2 = 3.*a1 , d = 27. ;
296
297  if (9./(9.+d) >G4UniformRand()) u= - log(G4UniformRand()*G4UniformRand())/a1;
298  else                            u= - log(G4UniformRand()*G4UniformRand())/a2;
299
300  G4double TetEl = u*electron_mass_c2/ElectTotEnergy;
301  G4double TetPo = u*electron_mass_c2/PositTotEnergy;
302  G4double Phi  = twopi * G4UniformRand();
303  G4double dxEl= sin(TetEl)*cos(Phi),dyEl= sin(TetEl)*sin(Phi),dzEl=cos(TetEl);
304  G4double dxPo=-sin(TetPo)*cos(Phi),dyPo=-sin(TetPo)*sin(Phi),dzPo=cos(TetPo);
305   
306  //
307  // kinematic of the created pair
308  //
309  // the electron and positron are assumed to have a symetric
310  // angular distribution with respect to the Z axis along the parent photon.
311
312  G4double ElectKineEnergy = max(0.,ElectTotEnergy - electron_mass_c2);
313
314  G4ThreeVector ElectDirection (dxEl, dyEl, dzEl);
315  ElectDirection.rotateUz(GammaDirection);   
316
317  // create G4DynamicParticle object for the particle1 
318  G4DynamicParticle* aParticle1= new G4DynamicParticle(
319                     theElectron,ElectDirection,ElectKineEnergy);
320 
321  // the e+ is always created (even with Ekine=0) for further annihilation.
322
323  G4double PositKineEnergy = max(0.,PositTotEnergy - electron_mass_c2);
324
325  G4ThreeVector PositDirection (dxPo, dyPo, dzPo);
326  PositDirection.rotateUz(GammaDirection);   
327
328  // create G4DynamicParticle object for the particle2
329  G4DynamicParticle* aParticle2= new G4DynamicParticle(
330                      thePositron,PositDirection,PositKineEnergy);
331
332  // Fill output vector
333  fvect->push_back(aParticle1);
334  fvect->push_back(aParticle2);
335
336  // kill incident photon
337  fParticleChange->SetProposedKineticEnergy(0.);
338  fParticleChange->ProposeTrackStatus(fStopAndKill);   
339}
340
341//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
342
343
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