geant4/source/geometry/magneticfield/src/G4EqEMFieldWithSpin.cc

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// G4EqEMFieldWithSpin implementation
//
// Created: Chris Gong & Peter Gumplinger, 30.08.2007
// -------------------------------------------------------------------

#include "G4EqEMFieldWithSpin.hh"
#include "G4ElectroMagneticField.hh"
#include "G4ThreeVector.hh"
#include "globals.hh"
#include "G4PhysicalConstants.hh"
#include "G4SystemOfUnits.hh"

G4EqEMFieldWithSpin::G4EqEMFieldWithSpin(G4ElectroMagneticField *emField )
  : G4EquationOfMotion( emField ), charge(0.), mass(0.), magMoment(0.),
    spin(0.), fElectroMagCof(0.), fMassCof(0.), omegac(0.), 
    anomaly(0.0011659208), beta(0.), gamma(0.)
{
}

G4EqEMFieldWithSpin::~G4EqEMFieldWithSpin()
{
} 

void  
G4EqEMFieldWithSpin::SetChargeMomentumMass(G4ChargeState particleCharge,
                                           G4double MomentumXc,
                                           G4double particleMass)
{
   charge    = particleCharge.GetCharge();
   mass      = particleMass;
   magMoment = particleCharge.GetMagneticDipoleMoment();
   spin      = particleCharge.GetSpin();

   fElectroMagCof =  eplus*charge*c_light ;
   fMassCof = mass*mass;

   omegac = (eplus/mass)*c_light;

   G4double muB = 0.5*eplus*hbar_Planck/(mass/c_squared);

   G4double g_BMT;
   if ( spin != 0. ) g_BMT = (std::abs(magMoment)/muB)/spin;
   else g_BMT = 2.;

   anomaly = (g_BMT - 2.)/2.;

   G4double E = std::sqrt(sqr(MomentumXc)+sqr(mass));
   beta  = MomentumXc/E;
   gamma = E/mass;
}

void
G4EqEMFieldWithSpin::EvaluateRhsGivenB(const G4double y[],
                                       const G4double Field[],
                                             G4double dydx[] ) const
{

   // Components of y:
   //    0-2 dr/ds,
   //    3-5 dp/ds - momentum derivatives
   //    9-11 dSpin/ds = (1/beta) dSpin/dt - spin derivatives

   // The BMT equation, following J.D.Jackson, Classical
   // Electrodynamics, Second Edition,
   // dS/dt = (e/mc) S \cross
   //              [ (g/2-1 +1/\gamma) B
   //               -(g/2-1)\gamma/(\gamma+1) (\beta \cdot B)\beta
   //               -(g/2-\gamma/(\gamma+1) \beta \cross E ]
   // where
   // S = \vec{s}, where S^2 = 1
   // B = \vec{B}
   // \beta = \vec{\beta} = \beta \vec{u} with u^2 = 1
   // E = \vec{E}

   G4double pSquared = y[3]*y[3] + y[4]*y[4] + y[5]*y[5] ;

   G4double Energy   = std::sqrt( pSquared + fMassCof );
   G4double cof2     = Energy/c_light ;

   G4double pModuleInverse  = 1.0/std::sqrt(pSquared) ;

   G4double inverse_velocity = Energy * pModuleInverse / c_light;

   G4double cof1 = fElectroMagCof*pModuleInverse ;

   dydx[0] = y[3]*pModuleInverse ;                         
   dydx[1] = y[4]*pModuleInverse ;                         
   dydx[2] = y[5]*pModuleInverse ;                        

   dydx[3] = cof1*(cof2*Field[3] + (y[4]*Field[2] - y[5]*Field[1])) ;
   
   dydx[4] = cof1*(cof2*Field[4] + (y[5]*Field[0] - y[3]*Field[2])) ; 
 
   dydx[5] = cof1*(cof2*Field[5] + (y[3]*Field[1] - y[4]*Field[0])) ;  
   
   dydx[6] = dydx[8] = 0.;//not used

   // Lab Time of flight
   dydx[7] = inverse_velocity;
   
   G4ThreeVector BField(Field[0],Field[1],Field[2]);
   G4ThreeVector EField(Field[3],Field[4],Field[5]);

   EField /= c_light;

   G4ThreeVector u(y[3], y[4], y[5]);
   u *= pModuleInverse;

   G4double udb = anomaly*beta*gamma/(1.+gamma) * (BField * u);
   G4double ucb = (anomaly+1./gamma)/beta;
   G4double uce = anomaly + 1./(gamma+1.);

   G4ThreeVector Spin(y[9],y[10],y[11]);

   G4double pcharge;
   if (charge == 0.)
   {
     pcharge = 1.;
   }
   else
   {
     pcharge = charge;
   }

   G4ThreeVector dSpin(0.,0.,0.);
   if (Spin.mag2() != 0.)
   {
      dSpin = pcharge*omegac*( ucb*(Spin.cross(BField))-udb*(Spin.cross(u))
                           // from Jackson
                           // -uce*Spin.cross(u.cross(EField)) );
                           // but this form has one less operation
                     - uce*(u*(Spin*EField) - EField*(Spin*u)) );
   }

   dydx[ 9] = dSpin.x();
   dydx[10] = dSpin.y();
   dydx[11] = dSpin.z();

   return;
}