[mcstas-users] Negative intensity reading on a PSD_monitor?

[Peter Kjær Willendrup]

Dear Serge,

On Jul 2, 2013, at 12:27 , Serge Duarte Pinto <S.C.DuartePinto at tudelft.nl<mailto:S.C.DuartePinto at tudelft.nl>>
 wrote:

I am trying to simulate a spin precession setup that yields a spatial modulation pattern after an analyzer. The modulations clearly show up on the PSD_monitor (see screenshot), but curiously the downward swings go below zero intensity! I wonder how that is possible, and what I could possibly do against it.

True, negative intensities does not sound completely right… :-) I will have a closer look at this together with Erik Knudsen who implemented most of the polarisation support.

Is it correctly understood that the two included comps are your own codes?

I also do not understand the units of intensity: I send a million neutrons to an area of 5 cm^2, to get intensities of the order 10^-13…

This one I should be able to explain:

You are using Source_simple which by default has an intensity of 1 neutrons/sec/cm^2/(4*PI)/Å.

In your instrument file I see that you assign a focusing to only 1x1cm at a distance of 100 m which brings your emitted intensity down to something like 1e-9, and distributed over your 1e6 rays, this easily gives the low intensities you are seeing.

I attach the instrument description, together with the components Foil_flipper_magnet and Analyser_ideal it depends on. And the result I get when I run it.

Thanks, I will have a closer look.

Best,

Peter

Peter Kjær Willendrup
Senior Research Engineer, Special Advisor

DTU Physics





Technical University of Denmark



[cid:image002.gif at 01CCCAF1.5E6331F0][cid:EC2A3CAB-AF57-4CEB-96B0-23F6DC35FE76 at fysik.dtu.dk]







Department of Physics
Neutrons and X-rays for Materials Physics
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pkwi at fysik.dtu.dk<mailto:pkwi at fysik.dtu.dk>



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