[mcstas-users] A new hand looking for help

[Andrew Wildes]

Hi new user,

I won’t comment on your request for the ‘.instr’ files.  I don't, however, understand how you plan to get Q-resolution from a vanadium sample?

Vanadium scatters isotropically.  The scattering cross-section is pretty much purely incoherent and elastic.  It’s a good choice to get the energy resolution of a spectrometer, but not the Q-resolution for a diffractometer.

Normally, I’d recommend using the Bragg peaks from a powdered crystalline sample to get the Q-resolution.  The sample component should give resolution-limited peaks.  Once you’ve simulated the scattering, you can then plot the widths of the peaks as a function of Q to get the resolution.  You can even fit the widths with an appropriate analytical function to get the resolution for all Q.

If you’re planning an instrument for liquids, you don’t need brilliant Q-resolution.  You should probably simulate for only one peak at a time, or with only a few peaks, to avoid problems with peaks merging at high Q.  I think a component like ‘Powder1’ will suffice, although it will take a longer time to simulate because you will need to run it for a number of peaks over the usable range of Q on your instrument.  Alternatively, you can choose something like ‘PowderN’ with, say beryllium as a sample, but edit the input file to only have ~10 peaks over the usable range of Q for your instrument.

I’m guessing that your instrument will be a TOF diffractometer.  The resolution should probably be pretty flat.  You can get more information on TOF resolution functions at:
	 <http://wwwisis2.isis.rl.ac.uk/Disordered/Manuals/ATLAS%20manual%20and%20SSG.pdf>http://wwwisis2.isis.rl.ac.uk/Disordered/Manuals/ATLAS%20manual%20and%20SSG.pdf <http://wwwisis2.isis.rl.ac.uk/Disordered/Manuals/ATLAS%20manual%20and%20SSG.pdf>

I’m also not sure what a "Q detector" is.  Personally, I would use a straight-forward monitor that measured the position and energy  (possibly as a time-of-flight) of the neutron and then calculate Q myself from there.  I see that there is one component (TOF2Q_cylPSD_monitor) that will also convert to Q, but I’ve never used it so I can’t vouch for it.

Sorry I can’t be more help.

Cheers,

Andrew


> On 12 Dec 2017, at 01:14, 韩泽华 <hanzh at ihep.ac.cn> wrote:
> 
> Dear all
>     I'm a new hand on McStas.I hope for some help from you.
> 
>     Recently I've been trying to make a series of simulations of a Total Scattering Diffractometer, which is almost like NIMROD. Now I'm facing two problems.
> 
>     First, I want to make a simulation with a D2O sample and then detect Q. I've got "D2O-coh.sqw" and "D2O-inc.sqw", but I couldn't get a correct result. Could you offer me some "*.instr" files or codes containing a D2O sample or H2O sample together with a Q detector?
> 
>     Second, I want to get a result of the Q-resolution of the Diffratometer with a Vanadium sample. Can I use a Detector Component to get the Q-resolution directly, even though at a certain angle? If it can be, please offer me some "*.instr" files or codes as well. If it can't be, then what should I do to get enough data to calculate the Q-resolution afterwards?
> 
>     Finally I'd like to remind that all my simulations are based on a Total Scattering Diffractometer.
> 
>     I'd appreciate all your help and suggestions.
> 
>     
>     PS: I may have sent it yesterday, but with no reply, I think I need to send it once again. May you forgive my disturbance.
> 
> 
> 
> 
>     A new user
> 
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