source: trunk/source/processes/electromagnetic/highenergy/src/G4AnnihiToMuPair.cc @ 836

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27// $Id: G4AnnihiToMuPair.cc,v 1.3 2006/06/29 19:32:34 gunter Exp $
28// GEANT4 tag $Name: geant4-09-01-patch-02 $
29//
30//         ------------ G4AnnihiToMuPair physics process ------
31//         by H.Burkhardt, S. Kelner and R. Kokoulin, November 2002
32// -----------------------------------------------------------------------------
33//
34//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......//
35//
36// 27.01.03 : first implementation (hbu)
37// 04.02.03 : cosmetic simplifications (mma)
38// 25.10.04 : migrade to new interfaces of ParticleChange (vi)
39//
40//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
41
42#include "G4AnnihiToMuPair.hh"
43
44#include "G4ios.hh"
45#include "Randomize.hh"
46
47#include "G4Positron.hh"
48#include "G4MuonPlus.hh"
49#include "G4MuonMinus.hh"
50#include "G4Material.hh"
51#include "G4Step.hh"
52
53//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
54
55using namespace std;
56
57G4AnnihiToMuPair::G4AnnihiToMuPair(const G4String& processName,
58    G4ProcessType type):G4VDiscreteProcess (processName, type)
59{
60 //e+ Energy threshold
61 const G4double Mu_massc2 = G4MuonPlus::MuonPlus()->GetPDGMass();
62 LowestEnergyLimit  = 2*Mu_massc2*Mu_massc2/electron_mass_c2 - electron_mass_c2;
63 
64 //modele ok up to 1000 TeV due to neglected Z-interference
65 HighestEnergyLimit = 1000*TeV;
66 
67 CrossSecFactor = 1.;
68}
69
70//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
71
72G4AnnihiToMuPair::~G4AnnihiToMuPair() // (empty) destructor
73{ }
74
75//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
76
77G4bool G4AnnihiToMuPair::IsApplicable(const G4ParticleDefinition& particle)
78{
79  return ( &particle == G4Positron::Positron() );
80}
81
82//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
83
84void G4AnnihiToMuPair::BuildPhysicsTable(const G4ParticleDefinition&)
85// Build cross section and mean free path tables
86//here no tables, just calling PrintInfoDefinition
87{
88   PrintInfoDefinition();
89}
90
91//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
92
93void G4AnnihiToMuPair::SetCrossSecFactor(G4double fac)
94// Set the factor to artificially increase the cross section
95{ 
96  CrossSecFactor = fac;
97  G4cout << "The cross section for AnnihiToMuPair is artificially "
98         << "increased by the CrossSecFactor=" << CrossSecFactor << G4endl;
99}
100
101//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
102
103G4double G4AnnihiToMuPair::ComputeCrossSectionPerAtom(G4double Epos, G4double Z)
104// Calculates the microscopic cross section in GEANT4 internal units.
105// It gives a good description from threshold to 1000 GeV
106{
107  static const G4double Mmuon = G4MuonPlus::MuonPlus()->GetPDGMass();
108  static const G4double Rmuon = elm_coupling/Mmuon; //classical particle radius
109  static const G4double Sig0  = pi*Rmuon*Rmuon/3.;  //constant in crossSection
110
111  G4double CrossSection = 0.;
112  if (Epos < LowestEnergyLimit) return CrossSection;
113   
114  G4double xi = LowestEnergyLimit/Epos;
115  G4double SigmaEl = Sig0*xi*(1.+xi/2.)*sqrt(1.-xi); // per electron
116  CrossSection = SigmaEl*Z;         // number of electrons per atom
117  CrossSection *= CrossSecFactor;   //increase the CrossSection by  (default 1)
118  return CrossSection;
119}
120
121//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
122
123G4double G4AnnihiToMuPair::GetMeanFreePath(const G4Track& aTrack,
124                                           G4double, G4ForceCondition*)
125
126// returns the positron mean free path in GEANT4 internal units
127
128{
129  const G4DynamicParticle* aDynamicPositron = aTrack.GetDynamicParticle();
130  G4double PositronEnergy = aDynamicPositron->GetKineticEnergy()
131                                              +electron_mass_c2;
132  G4Material* aMaterial = aTrack.GetMaterial();
133  const G4ElementVector* theElementVector = aMaterial->GetElementVector();
134  const G4double* NbOfAtomsPerVolume = aMaterial->GetVecNbOfAtomsPerVolume();
135
136  G4double SIGMA = 0 ;
137
138  for ( size_t i=0 ; i < aMaterial->GetNumberOfElements() ; i++ )
139  {
140    G4double AtomicZ = (*theElementVector)[i]->GetZ();
141    SIGMA += NbOfAtomsPerVolume[i] *
142      ComputeCrossSectionPerAtom(PositronEnergy,AtomicZ);
143  }
144  return SIGMA > DBL_MIN ? 1./SIGMA : DBL_MAX;
145}
146
147//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
148
149G4VParticleChange* G4AnnihiToMuPair::PostStepDoIt(const G4Track& aTrack,
150                                                  const G4Step&  aStep)
151//
152// generation of e+e- -> mu+mu-
153//
154{
155
156  aParticleChange.Initialize(aTrack);
157  static const G4double Mele=electron_mass_c2;
158  static const G4double Mmuon=G4MuonPlus::MuonPlus()->GetPDGMass();
159
160  // current Positron energy and direction, return if energy too low
161  const G4DynamicParticle *aDynamicPositron = aTrack.GetDynamicParticle();
162  G4double Epos = aDynamicPositron->GetKineticEnergy()+Mele;
163
164 if (Epos < LowestEnergyLimit)
165  { G4cout
166        << "error in G4AnnihiToMuPair::PostStepDoIt called with energy below"
167           " threshold Epos= "
168        << Epos << G4endl;       // shoud never happen
169        G4Exception(10);
170  }
171
172  if (Epos < LowestEnergyLimit)
173     return G4VDiscreteProcess::PostStepDoIt(aTrack,aStep);
174
175  G4ParticleMomentum PositronDirection = 
176                                       aDynamicPositron->GetMomentumDirection();
177  G4double xi = LowestEnergyLimit/Epos; // xi is always less than 1,
178                                        // goes to 0 at high Epos
179
180  // generate cost
181  //
182  G4double cost;
183  do cost = 2.*G4UniformRand()-1.;
184  while (2.*G4UniformRand() > 1.+xi+cost*cost*(1.-xi) ); 
185                                                       //1+cost**2 at high Epos
186  G4double sint = sqrt(1.-cost*cost);
187
188  // generate phi
189  //
190  G4double phi=2.*pi*G4UniformRand();
191
192  G4double Ecm   = sqrt(0.5*Mele*(Epos+Mele));
193  G4double Pcm   = sqrt(Ecm*Ecm-Mmuon*Mmuon);
194  G4double beta  = sqrt((Epos-Mele)/(Epos+Mele));
195  G4double gamma = Ecm/Mele;                    // =sqrt((Epos+Mele)/(2.*Mele));
196  G4double Pt    = Pcm*sint;
197 
198  // energy and momentum of the muons in the Lab
199  //
200  G4double EmuPlus   = gamma*(     Ecm+cost*beta*Pcm);
201  G4double EmuMinus  = gamma*(     Ecm-cost*beta*Pcm);
202  G4double PmuPlusZ  = gamma*(beta*Ecm+cost*     Pcm);
203  G4double PmuMinusZ = gamma*(beta*Ecm-cost*     Pcm);
204  G4double PmuPlusX  = Pt*cos(phi);
205  G4double PmuPlusY  = Pt*sin(phi);
206  G4double PmuMinusX =-Pt*cos(phi);
207  G4double PmuMinusY =-Pt*sin(phi);
208  // absolute momenta
209  G4double PmuPlus  = sqrt(Pt*Pt+PmuPlusZ *PmuPlusZ );
210  G4double PmuMinus = sqrt(Pt*Pt+PmuMinusZ*PmuMinusZ);
211
212  // mu+ mu- directions for Positron in z-direction
213  //
214  G4ThreeVector
215    MuPlusDirection ( PmuPlusX/PmuPlus, PmuPlusY/PmuPlus,  PmuPlusZ/PmuPlus  );
216  G4ThreeVector
217    MuMinusDirection(PmuMinusX/PmuMinus,PmuMinusY/PmuMinus,PmuMinusZ/PmuMinus);
218
219  // rotate to actual Positron direction
220  //
221  MuPlusDirection.rotateUz(PositronDirection);
222  MuMinusDirection.rotateUz(PositronDirection);
223
224  aParticleChange.SetNumberOfSecondaries(2);
225  // create G4DynamicParticle object for the particle1
226  G4DynamicParticle* aParticle1= new G4DynamicParticle(
227                         G4MuonPlus::MuonPlus(),MuPlusDirection,EmuPlus-Mmuon);
228  aParticleChange.AddSecondary(aParticle1);
229  // create G4DynamicParticle object for the particle2
230  G4DynamicParticle* aParticle2= new G4DynamicParticle(
231                     G4MuonMinus::MuonMinus(),MuMinusDirection,EmuMinus-Mmuon);
232  aParticleChange.AddSecondary(aParticle2);
233
234  // Kill the incident positron
235  //
236  aParticleChange.ProposeEnergy(0.); 
237  aParticleChange.ProposeTrackStatus(fStopAndKill);
238
239  return &aParticleChange;
240}
241
242//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
243
244void G4AnnihiToMuPair::PrintInfoDefinition()
245{
246  G4String comments ="e+e->mu+mu- annihilation, atomic e- at rest.\n";
247  G4cout << G4endl << GetProcessName() << ":  " << comments
248         << "        threshold at " << LowestEnergyLimit/GeV << " GeV"
249         << " good description up to "
250         << HighestEnergyLimit/TeV << " TeV for all Z." << G4endl;
251}
252
253//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
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