[mcstas-users] Pol_simpleBfield : wrong intensity(?)

[André Kusmin]

Hi everyone,

I noticed that in a simple polarized neutron instrument
made of source, polarizer, precession field, analyzer
the resulting signal, as expected, oscillates as cos(const*lambda*B*L)
but at B=0, the intensity of the signal is less than 100% (see attached 
PNG, plotted intensity against two different lengths of magnetic field 
regions).

It turns out that intensity at B*L=0 is correct.

For 100 % polarized beam and zero precession field  the intensity after 
the analyzer is 4.2e-14
(with TestLarmor.out)
The same intensity I get with ./TestLarmorOnlysource.out
but when I set non zero field in precession region I get even larger 
intensity, which is impossible.

Test can be done with two  attached  instr files and commands:

./TestLarmor.out By1=0  -n 1e5  DLambda=0.000001
./TestLarmor.out By1=0.000001  -n 1e5   DLambda=0.000001
./TestLarmorOnlysource.out   -n 1e5   DLambda=0.000001

could anyone please check if this is reproducible?
perhaps I miss something.

I have McStas pre2.2 (compiled from SVN) on linux multilib 64 bit 
(Slackware)


best regards
Andre

  

-------------- next part --------------
/*******************************************************************************
*         McStas instrument definition URL=http://www.mcstas.org
*
* Instrument: TestLarmor
*
* %Identification
* Release: McStas 2.2
*
* %Description
* Toy model to test Larmor precessions
*
**
* %End
*******************************************************************************/

DEFINE INSTRUMENT TestLarmor(
	Lambda= 2.1,
    DLambda= 0.1, 
    sourcefocusdist = 100,
    detsize=0.5,
//polarization
	initialpol = 1.0,
    polarizerpos = 0.5,
//fields
    Bfield1start = 1,
    BsizeX = 0.5,
    BsizeY = 0.05,
    BsizeZ = 2,
// field strength
    By1 = 0.001
)

DECLARE
%{
	double Bfield1stop;
%}

INITIALIZE
%{
Bfield1stop=Bfield1start+BsizeZ;
%}


TRACE
COMPONENT Origin = Progress_bar()
AT (0, 0, 0) ABSOLUTE

COMPONENT Source = Source_simple(
	radius = 0.00,
	yheight = 0.01, 
	xwidth = 0.01,
    dist = sourcefocusdist, 
    focus_xw = 0.01, focus_yh = 0.01,
	lambda0 = Lambda, 
	dlambda = DLambda * Lambda)
  AT (0, 0, 0) RELATIVE Origin

COMPONENT polariser = Set_pol(px=initialpol)
AT (0, 0, polarizerpos) RELATIVE Origin

COMPONENT B_field1=Pol_simpleBfield(xwidth=BsizeX, yheight=BsizeY, zdepth=BsizeZ, Bx=0, By=By1, Bz=0)
AT (0, 0, Bfield1start) RELATIVE Origin

COMPONENT B_field_stop1=Pol_simpleBfield_stop(magnet_comp_stop=B_field1)
AT (0, 0, Bfield1stop) RELATIVE Origin


// *************************
//   analyzer and detectors
// *************************

COMPONENT analyser = Analyser_ideal(mx=1, my=0, mz=0)
AT (0, 0, Bfield1stop+1) RELATIVE Origin

COMPONENT PSD = PSD_monitor(nx=1, ny=1, filename="PSD", xwidth=detsize, yheight=detsize)
AT (0, 0, Bfield1stop+1.0002) RELATIVE Origin




FINALLY
%{
%}

END



-------------- next part --------------
/*******************************************************************************
*         McStas instrument definition URL=http://www.mcstas.org
*
* Instrument: TestLarmorOnlysource
*
* %Identification
* Release: McStas 2.2
*
* %Description
* only source and detector
*
*
* %End
*******************************************************************************/

DEFINE INSTRUMENT TestLarmorOnlysource(
	Lambda= 2.1,
    DLambda= 0.1, 
    sourcefocusdist = 100,
    detsize=0.5,
    polarizerpos = 0.5
)


DECLARE
%{
%}

INITIALIZE
%{
%}


TRACE
COMPONENT Origin = Progress_bar()
AT (0, 0, 0) ABSOLUTE

COMPONENT Source = Source_simple(
	radius = 0.00,
	yheight = 0.01, 
	xwidth = 0.01,
    dist = sourcefocusdist, 
    focus_xw = 0.01, focus_yh = 0.01,
	lambda0 = Lambda, 
	dlambda = DLambda * Lambda)
  AT (0, 0, 0) RELATIVE Origin


COMPONENT PSD = PSD_monitor(nx=1, ny=1, filename="PSD", xwidth=detsize, yheight=detsize)
AT (0, 0, polarizerpos+1.0002) RELATIVE Origin



FINALLY
%{
%}

END



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