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Question Why electron increases in energy when traveling in uniform magnetic field? 

Forum: Fields: Magnetic and Otherwise
Date: 30 Aug, 2002
From: Emil Frlez <Emil Frlez>

 Dear GEANT4 experts,

 before trying a simulation of detector in a combined
 electromagnetic field, I tried a simpler case I am
 interested in:

 Particle: 1.0 MeV e-
 Field: Uniform magnetic field along x axis, 4.0 tesla
 Detector: 2mm silicon

 I modify somewhat Em5 example, and also have a look
 at N04 novice example for the magnetic field definition.

 My problem: the energy deposited in Si detector is
 greater then the starting kinetic energy of electron.
 I print out the kinetic energy of e- and its position
 at every step and see the electron gains energy in 
 the uniform magnetic field. Is this a bug? I get
 the same behavior using 

void Em5Field::GetFieldValue(const double Point[3],double *Bfield) const

 to set a field value (as in N04), and

void Em5DetectorConstruction::SetMagField(G4double fieldValue)

 to set a field value (as in Em5 example).

 The increase in energy apparently depend on the initial
 momentum direction of e-.

 I am trying to see what is a reason for (unphysical) boost
 in kinetic energy. Is this due to rounding errors in tracking
 or ...? Should I set some step size to a smaller
 value to increase a tracking precision?

 Because I will be eventually interested in simulating the
 neutron beta decay electron (with a maximum energy 0.8 MeV) detected
 in a Si counter after traveling ca. 1.5 meters in ~4 tesla
 magnetic field, this behavior obviously renders the 
 simulation unusable...

 Am I missing something?
					Cheers, Emil
					frlez@virginia.edu

 P.S. Here is kinetic energy in MeV in first column followed
      by position of e- in mm, until e- stops in 2 mm of Si 
      centered at x=150 cm
      Initial energy: 1.0 MeV, deposited energy: 1.08308 MeV

---> Begin of Event: 0 1 KinEn e- (0.70713,0.62626,0.28086) mm 1.0001 KinEn e- (1.4142,0.83297,0.93536) mm 1.0001 KinEn e- (2.1213,0.48164,1.525) mm 1.0002 KinEn e- (2.8284,-0.1924,1.6548) mm 1.0002 KinEn e- (3.5354,-0.73762,1.2377) mm 1.0003 KinEn e- (4.2424,-0.78879,0.55319) mm 1.0003 KinEn e- (4.9494,-0.31162,0.059663) mm 1.0003 KinEn e- (5.6564,0.37432,0.087722) mm 1.0004 KinEn e- (6.3634,0.80963,0.6186) mm 1.0004 KinEn e- (7.0703,0.70277,1.2968) mm 1.0005 KinEn e- (7.7772,0.12529,1.6681) mm 1.0005 KinEn e- (8.4841,-0.53615,1.484) mm 1.0005 KinEn e- (9.191,-0.83866,0.86757) mm 1.0006 KinEn e- (9.8978,-0.57967,0.23164) mm 1.0006 KinEn e- (10.605,0.067433,0.0019294) mm 1.0007 KinEn e- (11.311,0.66946,0.33224) mm 1.0007 KinEn e- (12.018,0.82339,1.0015) mm 1.0008 KinEn e- (12.725,0.42617,1.5617) mm 1.0008 KinEn e- (13.432,-0.25635,1.6379) mm 1.0008 KinEn e- (14.138,-0.76736,1.179) mm 1.0009 KinEn e- (14.845,-0.76485,0.49222) mm 1.0009 KinEn e- (15.552,-0.25048,0.037082) mm 1.001 KinEn e- (16.258,0.43157,0.11819) mm 1.001 KinEn e- (16.965,0.82489,0.68129) mm 1.001 KinEn e- (17.672,0.66631,1.3496) mm 1.0011 KinEn e- (18.378,0.061903,1.676) mm 1.0011 KinEn e- (19.085,-0.58398,1.4421) mm 1.0012 KinEn e- (19.791,-0.83925,0.80435) mm 1.0012 KinEn e- (20.498,-0.53312,0.18935) mm 1.0013 KinEn e- (21.204,0.12966,0.0084996) mm 1.0013 KinEn e- (21.911,0.70578,0.38279) mm 1.0013 KinEn e- (22.617,0.8099,1.0619) mm 1.0014 KinEn e- (23.324,0.37237,1.5917) mm 1.0014 KinEn e- (24.03,-0.31423,1.6178) mm 1.0015 KinEn e- (24.736,-0.79077,1.1228) mm 1.0015 KinEn e- (25.443,-0.73858,0.4376) mm 1.0015 KinEn e- (26.149,-0.19254,0.020422)

..... deleted .....

1.0831 KinEn e- (1498.4,0.51728,1.6226) mm 
1.0831 KinEn e- (1499,-0.11084,1.7409) mm 
0.83218 KinEn e- (1499.7,-0.36039,1.6732) mm 
0.6459 KinEn e- (1499.9,-0.80863,1.2929) mm 
0.43104 KinEn e- (1499.7,-0.49025,1.396) mm 
0.27809 KinEn e- (1499.8,-0.70194,1.4364) mm 
0.13182 KinEn e- (1499.9,-0.79351,1.544) mm 
2.6926e-11 KinEn e- (1499.9,-0.8145,1.5381) mm 
Run terminated.
Run Summary
  Number of events processed : 1
  User=0.23s Real=0.29s Sys=0s
 ================== run summary =====================
 end of Run TotNbofEvents = 1
    mean charged track length   in absorber=2.83012 +- 0  mm  

            mean energy deposit in absorber=1.08308 +- 0  MeV 

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