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G4EqEMFieldWithSpin.cc
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25 //
26 // G4EqEMFieldWithSpin implementation
27 //
28 // Created: Chris Gong & Peter Gumplinger, 30.08.2007
29 // -------------------------------------------------------------------
30 
31 #include "G4EqEMFieldWithSpin.hh"
33 #include "G4ThreeVector.hh"
34 #include "globals.hh"
35 #include "G4PhysicalConstants.hh"
36 #include "G4SystemOfUnits.hh"
37 
39  : G4EquationOfMotion( emField ), charge(0.), mass(0.), magMoment(0.),
40  spin(0.), fElectroMagCof(0.), fMassCof(0.), omegac(0.),
41  anomaly(0.0011659208), beta(0.), gamma(0.)
42 {
43 }
44 
46 {
47 }
48 
49 void
51  G4double MomentumXc,
52  G4double particleMass)
53 {
54  charge = particleCharge.GetCharge();
55  mass = particleMass;
56  magMoment = particleCharge.GetMagneticDipoleMoment();
57  spin = particleCharge.GetSpin();
58 
60  fMassCof = mass*mass;
61 
62  omegac = (eplus/mass)*c_light;
63 
64  G4double muB = 0.5*eplus*hbar_Planck/(mass/c_squared);
65 
66  G4double g_BMT;
67  if ( spin != 0. ) g_BMT = (std::abs(magMoment)/muB)/spin;
68  else g_BMT = 2.;
69 
70  anomaly = (g_BMT - 2.)/2.;
71 
72  G4double E = std::sqrt(sqr(MomentumXc)+sqr(mass));
73  beta = MomentumXc/E;
74  gamma = E/mass;
75 }
76 
77 void
79  const G4double Field[],
80  G4double dydx[] ) const
81 {
82 
83  // Components of y:
84  // 0-2 dr/ds,
85  // 3-5 dp/ds - momentum derivatives
86  // 9-11 dSpin/ds = (1/beta) dSpin/dt - spin derivatives
87 
88  // The BMT equation, following J.D.Jackson, Classical
89  // Electrodynamics, Second Edition,
90  // dS/dt = (e/mc) S \cross
91  // [ (g/2-1 +1/\gamma) B
92  // -(g/2-1)\gamma/(\gamma+1) (\beta \cdot B)\beta
93  // -(g/2-\gamma/(\gamma+1) \beta \cross E ]
94  // where
95  // S = \vec{s}, where S^2 = 1
96  // B = \vec{B}
97  // \beta = \vec{\beta} = \beta \vec{u} with u^2 = 1
98  // E = \vec{E}
99 
100  G4double pSquared = y[3]*y[3] + y[4]*y[4] + y[5]*y[5] ;
101 
102  G4double Energy = std::sqrt( pSquared + fMassCof );
103  G4double cof2 = Energy/c_light ;
104 
105  G4double pModuleInverse = 1.0/std::sqrt(pSquared) ;
106 
107  G4double inverse_velocity = Energy * pModuleInverse / c_light;
108 
109  G4double cof1 = fElectroMagCof*pModuleInverse ;
110 
111  dydx[0] = y[3]*pModuleInverse ;
112  dydx[1] = y[4]*pModuleInverse ;
113  dydx[2] = y[5]*pModuleInverse ;
114 
115  dydx[3] = cof1*(cof2*Field[3] + (y[4]*Field[2] - y[5]*Field[1])) ;
116 
117  dydx[4] = cof1*(cof2*Field[4] + (y[5]*Field[0] - y[3]*Field[2])) ;
118 
119  dydx[5] = cof1*(cof2*Field[5] + (y[3]*Field[1] - y[4]*Field[0])) ;
120 
121  dydx[6] = dydx[8] = 0.;//not used
122 
123  // Lab Time of flight
124  dydx[7] = inverse_velocity;
125 
126  G4ThreeVector BField(Field[0],Field[1],Field[2]);
127  G4ThreeVector EField(Field[3],Field[4],Field[5]);
128 
129  EField /= c_light;
130 
131  G4ThreeVector u(y[3], y[4], y[5]);
132  u *= pModuleInverse;
133 
134  G4double udb = anomaly*beta*gamma/(1.+gamma) * (BField * u);
135  G4double ucb = (anomaly+1./gamma)/beta;
136  G4double uce = anomaly + 1./(gamma+1.);
137 
138  G4ThreeVector Spin(y[9],y[10],y[11]);
139 
140  G4double pcharge;
141  if (charge == 0.)
142  {
143  pcharge = 1.;
144  }
145  else
146  {
147  pcharge = charge;
148  }
149 
150  G4ThreeVector dSpin(0.,0.,0.);
151  if (Spin.mag2() != 0.)
152  {
153  dSpin = pcharge*omegac*( ucb*(Spin.cross(BField))-udb*(Spin.cross(u))
154  // from Jackson
155  // -uce*Spin.cross(u.cross(EField)) );
156  // but this form has one less operation
157  - uce*(u*(Spin*EField) - EField*(Spin*u)) );
158  }
159 
160  dydx[ 9] = dSpin.x();
161  dydx[10] = dSpin.y();
162  dydx[11] = dSpin.z();
163 
164  return;
165 }