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Note Re: cold neutron spin tracking 

Keywords: spin, neutrons, magnetic field
Forum: Fields: Magnetic and Otherwise
Re: Question cold neutron spin tracking (Emil Frlez)
Re: Question Re: cold neutron spin tracking (Emil Frlez)
Date: 03 Feb, 2006
From: Emil Frlez <Emil Frlez>

 Hi all,

 my thanks to Peter and John for helping me out with
 an advice. Here is the summary:

 Peter was right: in order to do a neutral particle
 spin tracking needs to be modified.

 The simplest way I find to accomplish that was:

 (i) copy the geant4.6.2.p01 sources into geant4.6.2.p02
 (ii) in  geant4.6.2.p02 modify the G4Transportation,

   if( (particleCharge != 0.0) )
   { ... }


   fNeutron = G4ParticleTable::GetParticleTable()->FindParticle("neutron");
   if( (particleCharge != 0.0 || pParticle->GetDefinition() == fNeutron ) )
   { ... }

 (iii) Modify, change 

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


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

   or, for nonrelativistic case:     
   dSpin = (3.8260837/2.) * omegac*(Spin.cross(BField));

   Set the anomaly for neutron, set the straight propagation:

   anomaly = -2.913;
   if ( particleCharge == 0. ) {dydx[3] = 0.;   dydx[4] = 0.;   dydx[5] = 0.;}

 (iv) recompile the sources and set 
      setenv G4INSTALL $HOME/geant4.6.2.p1

 With these changes I get the correct neutron spin precession.

                                               Cheers, Emil

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