[mcstas-users] Optimization of alpha-Fe thickness

[farhi]

Hi,

it could also be that the quick estimate (46mm) only takes into account
the absoption. In reality, scattering also counts, and it highly depends
on the incoming wavelength (Bragg edges). So the actual optimal
thickness can only be smaller than 46mm. The best would be that you
measure the scattered intensity on your instrument, with a set of iron
plates. Please tell us back what it gives.

Emmanuel.

Le 2013-10-28 10:34, Peter Willendrup a écrit : 

> Hi Tom, 
> 
> It could well be that the Fe.laz provided with McStas has too high F2 values and hence produces too much scattering. 
> 
> Via the attached .CIF from the ICSD database (from the publication Physical Review (1,1893-132,1963/141,1966-188,1969) (1917) 10, p661-p696), I have generated a .lau file through Crystallographica, also attached. 
> 
> Running a quick simulation with the McStas 2.0 PSI_DMC instrument and this .lau (remember to adjust the BARNS flag accordingly) yields lower scattering than what the Fe.laz from the McStas 2.0 distribution, see attachments. 
> 
> Could this be the cause of your reported issue? 
> 
> Best, 
> 
> Peter 
> 
> On 28 Oct 2013, at 09:09 , 陈彦舟 <wokaoyan1981 at 126.com> wrote: 
> 
>> Hello all, 
>> The thickness of plate-shaped sample α-Fe on Residual Stress Neutron Diffractometer has been optimized by McStas. According to the result, best thickness corresponding to the maximal intensity is around 9mm. see picture 1 and models pdc1 and pdc2 in enclosure. 
>> 
>> <picture1.png> 
>> However, the theoritical optimized thickness calculated from the following formula is around 46mm which is 5 times as large as the simulated one. 
>> Topt=1/Σa=1/Nσa=V/nσa 
>> Topt:Optimized thickness 
>> Σa:macroscopic absorption cross section 
>> N:Atomic number density 
>> σa: micro absorption cross section 
>> n:number of atoms per unit cell 
>> V: Volume of each unit cell 
>> As for α-Fe, V=23.55Å3, n=2, σa=2.56 barn 
>> Then Topt=46mm 
>> Will anybody please explain the difference between two results? 
>> 
>> Best wishes, 
>> Tom 
>> 
>> <picture1.png><pdc2.instr><pdc1.instr>_______________________________________________
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-- 
FARHI Emmanuel <farhi at ill.fr>
Groupe DS/CS, ILL4/156, Tel (33) 4 76 20 71 35
ILL, Grenoble
 

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