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// $Id: G4Mag_SpinEqRhs.cc,v 1.13 2008/11/21 21:18:26 gum Exp $
// GEANT4 tag $Name: geant4-09-02-cand-01 $
//
// This is the standard right-hand side for equation of motion.
// This version of the right-hand side includes the three components
// of the particle's spin.
//
//            J. Apostolakis, February 8th, 1999
//            P. Gumplinger,  February 8th, 1999
//            D. Cote-Ahern, P. Gumplinger,  April 11th, 2001
//
// --------------------------------------------------------------------

#include "G4Mag_SpinEqRhs.hh"
#include "G4MagneticField.hh"
#include "G4ThreeVector.hh"

G4Mag_SpinEqRhs::G4Mag_SpinEqRhs( G4MagneticField* MagField )
  : G4Mag_EqRhs( MagField ) 
{
   anomaly = 0.0011659208;
}

G4Mag_SpinEqRhs::~G4Mag_SpinEqRhs() {}

void
G4Mag_SpinEqRhs::SetChargeMomentumMass(G4double particleCharge, // in e+ units
                                       G4double MomentumXc,
                                       G4double particleMass)
{
   //  To set fCof_val 
   G4Mag_EqRhs::SetChargeMomentumMass(particleCharge, MomentumXc, particleMass);

   omegac = 0.105658387*GeV/particleMass * 2.837374841e-3*(rad/cm/kilogauss);

   ParticleCharge = particleCharge;

   E = std::sqrt(sqr(MomentumXc)+sqr(particleMass));
   beta  = MomentumXc/E;
   gamma = E/particleMass;

}

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
   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]);

   if (Spin.mag() > 0.) Spin = Spin.unit();

   G4ThreeVector dSpin;

   dSpin = ParticleCharge*omegac*(ucb*(Spin.cross(BField))-udb*(Spin.cross(u)));

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

   return ;
}
