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

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25//
26// $Id: G4BetheHeitlerModel.cc,v 1.15 2010/10/25 19:02:32 vnivanch Exp $
27// GEANT4 tag $Name: emstand-V09-03-24 $
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// 20-02-07 SelectRandomElement is called for any initial gamma energy
46//          in order to have selected element for polarized model (VI)
47// 25-10-10 Removed unused table, added element selector (VI)
48//
49// Class Description:
50//
51// -------------------------------------------------------------------
52//
53//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
54//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
55
56#include "G4BetheHeitlerModel.hh"
57#include "G4Electron.hh"
58#include "G4Positron.hh"
59#include "G4Gamma.hh"
60#include "Randomize.hh"
61#include "G4ParticleChangeForGamma.hh"
62
63//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
64
65using namespace std;
66
67G4BetheHeitlerModel::G4BetheHeitlerModel(const G4ParticleDefinition*,
68                                         const G4String& nam)
69  : G4VEmModel(nam)
70{
71  fParticleChange = 0;
72  theGamma    = G4Gamma::Gamma();
73  thePositron = G4Positron::Positron();
74  theElectron = G4Electron::Electron();
75}
76
77//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
78
79G4BetheHeitlerModel::~G4BetheHeitlerModel()
80{}
81
82//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
83
84void G4BetheHeitlerModel::Initialise(const G4ParticleDefinition* p,
85                                     const G4DataVector& cuts)
86{
87  if(!fParticleChange) { fParticleChange = GetParticleChangeForGamma(); }
88  InitialiseElementSelectors(p, cuts);
89}
90
91//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
92
93G4double
94G4BetheHeitlerModel::ComputeCrossSectionPerAtom(const G4ParticleDefinition*,
95                                                G4double GammaEnergy, G4double Z,
96                                                G4double, G4double, G4double)
97// Calculates the microscopic cross section in GEANT4 internal units.
98// A parametrized formula from L. Urban is used to estimate
99// the total cross section.
100// It gives a good description of the data from 1.5 MeV to 100 GeV.
101// below 1.5 MeV: sigma=sigma(1.5MeV)*(GammaEnergy-2electronmass)
102//                                   *(GammaEnergy-2electronmass)
103{
104  static const G4double GammaEnergyLimit = 1.5*MeV;
105  G4double CrossSection = 0.0 ;
106  if ( Z < 0.9 || GammaEnergy <= 2.0*electron_mass_c2 ) { return CrossSection; }
107
108  static const G4double
109    a0= 8.7842e+2*microbarn, a1=-1.9625e+3*microbarn, a2= 1.2949e+3*microbarn,
110    a3=-2.0028e+2*microbarn, a4= 1.2575e+1*microbarn, a5=-2.8333e-1*microbarn;
111
112  static const G4double
113    b0=-1.0342e+1*microbarn, b1= 1.7692e+1*microbarn, b2=-8.2381   *microbarn,
114    b3= 1.3063   *microbarn, b4=-9.0815e-2*microbarn, b5= 2.3586e-3*microbarn;
115
116  static const G4double
117    c0=-4.5263e+2*microbarn, c1= 1.1161e+3*microbarn, c2=-8.6749e+2*microbarn,
118    c3= 2.1773e+2*microbarn, c4=-2.0467e+1*microbarn, c5= 6.5372e-1*microbarn;
119
120  G4double GammaEnergySave = GammaEnergy;
121  if (GammaEnergy < GammaEnergyLimit) { GammaEnergy = GammaEnergyLimit; }
122
123  G4double X=log(GammaEnergy/electron_mass_c2), X2=X*X, X3=X2*X, X4=X3*X, X5=X4*X;
124
125  G4double F1 = a0 + a1*X + a2*X2 + a3*X3 + a4*X4 + a5*X5,
126           F2 = b0 + b1*X + b2*X2 + b3*X3 + b4*X4 + b5*X5,
127           F3 = c0 + c1*X + c2*X2 + c3*X3 + c4*X4 + c5*X5;     
128
129  CrossSection = (Z + 1.)*(F1*Z + F2*Z*Z + F3);
130
131  if (GammaEnergySave < GammaEnergyLimit) {
132
133    X = (GammaEnergySave  - 2.*electron_mass_c2)
134      / (GammaEnergyLimit - 2.*electron_mass_c2);
135    CrossSection *= X*X;
136  }
137
138  if (CrossSection < 0.) { CrossSection = 0.; }
139  return CrossSection;
140}
141
142//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
143
144void G4BetheHeitlerModel::SampleSecondaries(std::vector<G4DynamicParticle*>* fvect,
145                                            const G4MaterialCutsCouple* couple,
146                                            const G4DynamicParticle* aDynamicGamma,
147                                            G4double,
148                                            G4double)
149// The secondaries e+e- energies are sampled using the Bethe - Heitler
150// cross sections with Coulomb correction.
151// A modified version of the random number techniques of Butcher & Messel
152// is used (Nuc Phys 20(1960),15).
153//
154// GEANT4 internal units.
155//
156// Note 1 : Effects due to the breakdown of the Born approximation at
157//          low energy are ignored.
158// Note 2 : The differential cross section implicitly takes account of
159//          pair creation in both nuclear and atomic electron fields.
160//          However triplet prodution is not generated.
161{
162  const G4Material* aMaterial = couple->GetMaterial();
163
164  G4double GammaEnergy = aDynamicGamma->GetKineticEnergy();
165  G4ParticleMomentum GammaDirection = aDynamicGamma->GetMomentumDirection();
166
167  G4double epsil ;
168  G4double epsil0 = electron_mass_c2/GammaEnergy ;
169  if(epsil0 > 1.0) { return; }
170
171  // do it fast if GammaEnergy < 2. MeV
172  static const G4double Egsmall=2.*MeV;
173
174  // select randomly one element constituing the material
175  const G4Element* anElement = SelectRandomAtom(aMaterial, theGamma, GammaEnergy);
176
177  if (GammaEnergy < Egsmall) {
178
179    epsil = epsil0 + (0.5-epsil0)*G4UniformRand();
180
181  } else {
182    // now comes the case with GammaEnergy >= 2. MeV
183
184    // Extract Coulomb factor for this Element
185    G4double FZ = 8.*(anElement->GetIonisation()->GetlogZ3());
186    if (GammaEnergy > 50.*MeV) { FZ += 8.*(anElement->GetfCoulomb()); }
187
188    // limits of the screening variable
189    G4double screenfac = 136.*epsil0/(anElement->GetIonisation()->GetZ3());
190    G4double screenmax = exp ((42.24 - FZ)/8.368) - 0.952 ;
191    G4double screenmin = min(4.*screenfac,screenmax);
192
193    // limits of the energy sampling
194    G4double epsil1 = 0.5 - 0.5*sqrt(1. - screenmin/screenmax) ;
195    G4double epsilmin = max(epsil0,epsil1) , epsilrange = 0.5 - epsilmin;
196
197    //
198    // sample the energy rate of the created electron (or positron)
199    //
200    //G4double epsil, screenvar, greject ;
201    G4double  screenvar, greject ;
202
203    G4double F10 = ScreenFunction1(screenmin) - FZ;
204    G4double F20 = ScreenFunction2(screenmin) - FZ;
205    G4double NormF1 = max(F10*epsilrange*epsilrange,0.); 
206    G4double NormF2 = max(1.5*F20,0.);
207
208    do {
209      if ( NormF1/(NormF1+NormF2) > G4UniformRand() ) {
210        epsil = 0.5 - epsilrange*pow(G4UniformRand(), 0.333333);
211        screenvar = screenfac/(epsil*(1-epsil));
212        greject = (ScreenFunction1(screenvar) - FZ)/F10;
213             
214      } else { 
215        epsil = epsilmin + epsilrange*G4UniformRand();
216        screenvar = screenfac/(epsil*(1-epsil));
217        greject = (ScreenFunction2(screenvar) - FZ)/F20;
218      }
219
220    } while( greject < G4UniformRand() );
221
222  }   //  end of epsil sampling
223   
224  //
225  // fixe charges randomly
226  //
227
228  G4double ElectTotEnergy, PositTotEnergy;
229  if (G4UniformRand() > 0.5) {
230
231    ElectTotEnergy = (1.-epsil)*GammaEnergy;
232    PositTotEnergy = epsil*GammaEnergy;
233     
234  } else {
235   
236    PositTotEnergy = (1.-epsil)*GammaEnergy;
237    ElectTotEnergy = epsil*GammaEnergy;
238  }
239
240  //
241  // scattered electron (positron) angles. ( Z - axis along the parent photon)
242  //
243  //  universal distribution suggested by L. Urban
244  // (Geant3 manual (1993) Phys211),
245  //  derived from Tsai distribution (Rev Mod Phys 49,421(1977))
246
247  G4double u;
248  const G4double a1 = 0.625 , a2 = 3.*a1 , d = 27. ;
249
250  if (9./(9.+d) >G4UniformRand()) u= - log(G4UniformRand()*G4UniformRand())/a1;
251  else                            u= - log(G4UniformRand()*G4UniformRand())/a2;
252
253  G4double TetEl = u*electron_mass_c2/ElectTotEnergy;
254  G4double TetPo = u*electron_mass_c2/PositTotEnergy;
255  G4double Phi  = twopi * G4UniformRand();
256  G4double dxEl= sin(TetEl)*cos(Phi),dyEl= sin(TetEl)*sin(Phi),dzEl=cos(TetEl);
257  G4double dxPo=-sin(TetPo)*cos(Phi),dyPo=-sin(TetPo)*sin(Phi),dzPo=cos(TetPo);
258   
259  //
260  // kinematic of the created pair
261  //
262  // the electron and positron are assumed to have a symetric
263  // angular distribution with respect to the Z axis along the parent photon.
264
265  G4double ElectKineEnergy = max(0.,ElectTotEnergy - electron_mass_c2);
266
267  G4ThreeVector ElectDirection (dxEl, dyEl, dzEl);
268  ElectDirection.rotateUz(GammaDirection);   
269
270  // create G4DynamicParticle object for the particle1 
271  G4DynamicParticle* aParticle1= new G4DynamicParticle(
272                     theElectron,ElectDirection,ElectKineEnergy);
273 
274  // the e+ is always created (even with Ekine=0) for further annihilation.
275
276  G4double PositKineEnergy = max(0.,PositTotEnergy - electron_mass_c2);
277
278  G4ThreeVector PositDirection (dxPo, dyPo, dzPo);
279  PositDirection.rotateUz(GammaDirection);   
280
281  // create G4DynamicParticle object for the particle2
282  G4DynamicParticle* aParticle2= new G4DynamicParticle(
283                      thePositron,PositDirection,PositKineEnergy);
284
285  // Fill output vector
286  fvect->push_back(aParticle1);
287  fvect->push_back(aParticle2);
288
289  // kill incident photon
290  fParticleChange->SetProposedKineticEnergy(0.);
291  fParticleChange->ProposeTrackStatus(fStopAndKill);   
292}
293
294//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
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