[mcstas-users] Optimization of alpha-Fe thickness

[陈彦舟]

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.

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
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