G4Mag_SpinEqRhs.cc

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//
// G4Mag_SpinEqRhs implementation
//
// Authors: John Apostolakis (CERN) & Peter Gumplinger (TRIUMF), 08.02.1999
// --------------------------------------------------------------------
 
#include "G4Mag_SpinEqRhs.hh"
#include "G4PhysicalConstants.hh"
#include "G4SystemOfUnits.hh"
#include "G4MagneticField.hh"
#include "G4ThreeVector.hh"
 
G4Mag_SpinEqRhs::G4Mag_SpinEqRhs( G4MagneticField* MagField )
  : G4Mag_EqRhs( MagField )
{
}
 
void
G4Mag_SpinEqRhs::SetChargeMomentumMass(G4ChargeState particleCharge,
                                       G4double MomentumXc,
                                       G4double particleMass)
{
   G4Mag_EqRhs::SetChargeMomentumMass( particleCharge, MomentumXc, mass);
 
   charge = particleCharge.GetCharge();
   mass      = particleMass;
   magMoment = particleCharge.GetMagneticDipoleMoment();
   spin      = particleCharge.GetSpin();
 
   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
G4Mag_SpinEqRhs::EvaluateRhsGivenB( const G4double y[],
                                    const G4double B[3],
                                          G4double dydx[] ) const
{
   G4double momentum_mag_square = sqr(y[3]) + sqr(y[4]) + sqr(y[5]);
   G4double inv_momentum_magnitude = 1.0 / std::sqrt( momentum_mag_square );
   G4double cof = FCof()*inv_momentum_magnitude;
 
   dydx[0] = y[3] * inv_momentum_magnitude;       //  (d/ds)x = Vx/V
   dydx[1] = y[4] * inv_momentum_magnitude;       //  (d/ds)y = Vy/V
   dydx[2] = y[5] * inv_momentum_magnitude;       //  (d/ds)z = Vz/V
 
   if (charge == 0.)
   {
      dydx[3] = 0.;
      dydx[4] = 0.;
      dydx[5] = 0.;
   }
   else
   {
      dydx[3] = cof*(y[4]*B[2] - y[5]*B[1]) ;   // Ax = a*(Vy*Bz - Vz*By)
      dydx[4] = cof*(y[5]*B[0] - y[3]*B[2]) ;   // Ay = a*(Vz*Bx - Vx*Bz)
      dydx[5] = cof*(y[3]*B[1] - y[4]*B[0]) ;   // Az = a*(Vx*By - Vy*Bx)
   }
 
   G4ThreeVector u(y[3], y[4], y[5]);
   u *= inv_momentum_magnitude; 
 
   G4ThreeVector BField(B[0],B[1],B[2]);
 
   G4double udb = anomaly*beta*gamma/(1.+gamma) * (BField * u); 
   G4double ucb = (anomaly+1./gamma)/beta;
 
   // Initialise the values of dydx that we do not update.
   dydx[6] = dydx[7] = dydx[8] = 0.0;
 
   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)));
   }
 
   dydx[9] = dSpin.x();
   dydx[10] = dSpin.y();
   dydx[11] = dSpin.z();
 
   return;
}