[mcstas-users] Guide_gravity Problem

Thu Feb 7 16:53:08 CET 2019

Hi all,

	I checked the supermirror_m3.rfl file and it ranges from 0 to 0.09 [Angs-1] so I’m really confused with the k value in the first column of reflectivity file. I thought it represent the total momentum transferred. Can you explain more on “better the k values in the reflectivity file to be around e.g. 1.0 Angs-1.” Does the guide_gravity component have a different definition on reflectivity file? My actual source wavelength ranges from 0 to 10 [Angs].

       And also for Peter, my actual problem is to optimize a polarizer which is built by stack of supermirror. I’m trying to find a way to get the most transmission through that setup. So I want to calculate the ratio neutron_out/neutron_in. Now it seems that I should just divide the intensity instead of event numbers?

Happy Lunar New Year,
Lingnan Shen

From: Peter Kjær Willendrup
Sent: Tuesday, February 5, 2019 4:20 AM
To: Lingnan Shen
Cc: Emmanuel Farhi
Subject: Re: [mcstas-users] Guide_gravity Problem

Hello again, 

A better starting point for understanding Guide_gravity and reflectivity files may be the instrument file Test_Guides which is included with your McStas installation, in combination with one of the example reflectivity files from the data folder - e.g. supermirror_m3.rdl.

Also, for a pencil beam with controllable divergence, an option is to use Source_div.

Hope this helps - otherwise you are welcome to write again, preferably with a good description of what exact problem you are trying to solve. :-)

Best,

Peter

On 5 Feb 2019, at 10.12, Peter Kjær Willendrup <pkwi at fysik.dtu.dk> wrote:

Hello Lingnan, 

Agree with Emmanuel. 

Further, the combination of a pencil-beam, i.e. almost non-divergent source and using 500 channels in a rotated Guide_gravity means that transport through the guide becomes close to impossible. 

With regard to your second question you should always consider the transported intensity I. The event count is simply a measure of statistics (of "neutron rays", not “neutrons”).

Best,

Peter Willendrup

On 5 Feb 2019, at 10.01, Emmanuel FARHI <farhi at ill.fr> wrote:

Hello Lingnan,
Your reflectivity file is given for k=0 to 0.14 Angs-1. This is very small and covers neutrons around lambda=60 Angs, which are not even created by your source. Change either the lambda setting at the source, or better the k values in the reflectivity file to be around e.g. 1.0 Angs-1.
Cheers, Emmanuel.

On 2/5/19 7:17 AM, Lingnan Shen wrote:
Hi all, 

        I was trying to learn how to use the guide_gravity component. I tried to define the reflectivity curve but I get zero neutrons at the wavelength monitor in the end of guide. I have attached my .instr and .rfl file. I also found that even if the neutron beam just go through the guide. The detector at the end still see nothing.
        Also another question about transmission of certain setup. To calculate the ratio of neutron_in / neutron_out. I want to know how much neutron remained after passing through that setup. Should I use the flux or the events count?

Sincerely,
Lingnan Shen

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Peter Kjær Willendrup
Forskningsingeniør, Specialkonsulent
Næstformand for DTU Fysik LSU

DTU Physics

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Technical University of Denmark

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Department of Physics
Fysikvej
Building 307
DK-2800 Kongens Lyngby
Direct +45 2125 4612
Mobil +45 2125 4612
Fax +45 4593 2399
pkwi at fysik.dtu.dk 

Peter Kjær Willendrup
Forskningsingeniør, Specialkonsulent
Næstformand for DTU Fysik LSU

DTU Physics

Technical University of Denmark

Department of Physics
Fysikvej
Building 307
DK-2800 Kongens Lyngby
Direct +45 2125 4612
Mobil +45 2125 4612
Fax +45 4593 2399
pkwi at fysik.dtu.dk 

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