[mcstas-users] Virtual experiment problem

Peter Kjær Willendrup pkwi at fysik.dtu.dk
Tue May 28 16:25:26 CEST 2013

Hi Tom,

Is it correctly understood that your first instrument is run with 1e8 neutrons? - Doing so gives me only ~ 8000 neutrons in the virtual output file, which in any case is a very low number. As a rule of thumb, I would never go below ~ 1e5-1e6 events in a virtual input/output file, you risk imposing bias on your results because of the poor statistics.

I the attached, I have improved on your instrument files

* To follow loss of the neutrons through the instrument, I have added various PSD's and wavelength-monitors.
* The bandwidth of a Monochromator Bragg reflection is a few percent - so simulating the full range of 1-2 Å kills a lot of your neutrons. I suggest using [1.57 1.63]  - then 1e8 neutrons produces ~ 135000 events in your virtual_output file

* Using both a SPLIT and repeat_count is meaningless - they do the same, namely repeat incoming neutrons. This means that you repeated your 8000 neutrons above 10000 times each. I would rather look at the number of reflections in your Fe.laz and use that number as an over bound on the repetition ~ 50.
* I have used the d_phi parameter of PowderN to restrict the Debye-Scherer cones to what the detector sees (180*atan(30/120)/PI ~ 14 degrees - have made it 30 for safety.
* I have put frac and tfrac parameters of PowderN to zero, thereby suppressing incoherent scattering and description of the "direct beam".
* I have added another banana-shaped detector BEFORE the radial collimator, with restore_neutron=1.
* I have added the input parameter Div that parametrizes the collimator divergence.

The added banana-shaped detector shows that your radial collimator is what suppresses the peak, see attached graphics for the r=2 mm sample case

My feeling is that 12 minutes of arch becomes much to restrictive when you are studying the small sample - the natural divergence from the 4 mm wide sample is 180*atan(0.4/120)/PI ~ around 11 minutes of arch, which is around what your collimator accepts, so only the strongest reflections stand a chance.

Hope this helps,


On May 27, 2013, at 11:53 , 闄堝溅鑸?<wokaoyan1981 at 126.com<mailto:wokaoyan1981 at 126.com>> wrote:

Hi all,
I am conducting a virtual experiment by McStas. The sample is a column-shaped alpha-Fe. I change the radius of it and compare the diffraction patterns. The diffraction patterns change along with the varied radius value. For example the peak around 85掳in figure 1 (radius=10mm,in enclosure) vanishes in figure 2 (radius=2mm) while other diffraction peaks are still visible. This phenomenon is not expected.You may have a look at the pdc1.instr and pdc2.instr in enclosure.

The other question is that we can find dozens of small peaks besides the diffraction peaks(See banana.th<http://banana.th> file).What are they? The background?
The Ncount=10E8 and nrepeat=1000.
I鈥檓 looking forward your reply. Thank you.

Best wishes,

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mcstas-users at mcstas.org<mailto:mcstas-users at mcstas.org>

Peter Kj忙r Willendrup
Senior Research Engineer, Special Advisor

DTU Physics

[cid:1fac4d9c-d86e-416b-af82-089d71b91043 at win.dtu.dk]

Technical University of Denmark

[cid:8670423b-4e07-4f82-855c-a7a5937ee27c at win.dtu.dk]
[cid:2875a0a6-bc25-4dd8-8d50-57e5519d957a at win.dtu.dk]

Department of Physics
Neutrons and X-rays for Materials Physics

Building 307

DK-2800 Kongens Lyngby

Direct +45 2125 4612

Mobil +45 2125 4612
Fax +45 4593 2399

pkwi at fysik.dtu.dk<mailto:pkwi at fysik.dtu.dk>

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