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<p>Hi,<br /><br />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.<br /><br />Emmanuel.<br /><br />Le 2013-10-28 10:34, Peter Willendrup a écrit :</p>
<blockquote type="cite" style="padding-left:5px; border-left:#1010ff 2px solid; margin-left:5px"><!-- html ignored --><!-- head ignored --><!-- meta ignored -->Hi Tom,
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<div>It could well be that the Fe.laz provided with McStas has too high F2 values and hence produces too much scattering.</div>
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<div>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.</div>
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<div>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.</div>
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<div>Could this be the cause of your reported issue?</div>
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<div>Best,</div>
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<div>Peter</div>
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<div>On 28 Oct 2013, at 09:09 , 陈彦舟 <<a href="mailto:wokaoyan1981@126.com">wokaoyan1981@126.com</a>> wrote:</div>
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<div style="margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;">Hello all,<!-- o ignored --></span></span></div>
<div style="margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;">The thickness of plate-shaped sample </span></span><span style="mso-ascii-font-family: Calibri; mso-hansi-font-family: Calibri;">α</span><span><span style="font-family: Calibri;">-Fe on Residual Stress Neutron Diffractometer has been optimized by McStas. <span style="mso-spacerun: yes;"> </span>According to the result, best thickness corresponding to the maximal intensity is around 9mm. see picture 1 and models pdc1 and pdc2 in enclosure.<!-- o ignored --></span></span></div>
<div style="text-align: center; margin: 0cm 0cm 0pt;"><span><!-- v ignored --><!-- v ignored --><!-- v ignored --><!-- v ignored --><!-- v ignored --><!-- v ignored --><!-- v ignored --><!-- v ignored --><!-- v ignored --><!-- v ignored --><!-- v ignored --><!-- v ignored --><!-- v ignored --><!-- v ignored --><!-- v ignored --><!-- v ignored --><!-- o ignored --><!-- o ignored --></span></div>
<div style="text-align: left; margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;"><picture1.png></span></span></div>
<div style="text-align: left; margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;">However, the theoritical optimized thickness calculated from the following formula is around 46mm which is 5 times as large as the simulated one.<!-- o ignored --></span></span></div>
<div style="text-align: center; margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;">T<sub>opt</sub>=1/</span></span><span>Σ</span><span style="font-family: Calibri;"><sub><span>a</span></sub><span>=1/N</span></span><span>σ</span><span style="font-family: Calibri;"><sub><span>a</span></sub><span>=V/n</span></span><span>σ</span><sub><span><span style="font-family: Calibri;">a<!-- o ignored --></span></span></sub></div>
<div style="text-align: left; margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;"><span style="mso-spacerun: yes;"> </span>T<sub>opt</sub>:Optimized thickness<!-- o ignored --></span></span></div>
<div style="text-align: left; margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;"><span style="mso-spacerun: yes;"> </span><span style="mso-spacerun: yes;"> </span></span></span><span>Σ</span><span style="font-family: Calibri;"><sub><span>a</span></sub><span>:macroscopic absorption cross section<!-- o ignored --></span></span></div>
<div style="text-align: left; margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;"><span style="mso-spacerun: yes;"> </span>N:Atomic number density<!-- o ignored --></span></span></div>
<div style="text-align: left; margin: 0cm 0cm 0pt;"><span><span style="mso-spacerun: yes;"><span style="font-family: Calibri;"> </span></span></span><span>σ</span><span style="font-family: Calibri;"><sub><span>a</span></sub><span>: micro absorption cross section<!-- o ignored --></span></span></div>
<div style="text-align: left; margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;"><span style="mso-spacerun: yes;"> </span>n:number of atoms per unit cell<!-- o ignored --></span></span></div>
<div style="text-align: left; margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;"><span style="mso-spacerun: yes;"> </span>V: Volume of each unit cell<!-- o ignored --></span></span></div>
<div style="text-align: left; margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;">As for </span></span><span style="mso-ascii-font-family: Calibri; mso-hansi-font-family: Calibri;">α</span><span style="font-family: Calibri;"><span>-Fe, V=23.55</span><span style="mso-bidi-font-family: Calibri;">Å</span><sup><span>3</span></sup><span>, n=2, </span></span><span>σ</span><span style="font-family: Calibri;"><sub><span>a</span></sub><span>=2.56 barn<!-- o ignored --></span></span></div>
<div style="text-align: left; margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;">Then T<sub>opt</sub>=46mm<!-- o ignored --></span></span></div>
<div style="text-align: left; margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;">Will anybody please explain the difference between two results?<!-- o ignored --></span></span></div>
<div style="text-align: left; margin: 0cm 0cm 0pt;"><span><!-- o ignored --><span style="font-family: Calibri;"> </span></span></div>
<div style="text-align: left; margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;">Best wishes,<!-- o ignored --></span></span></div>
<div style="text-align: left; margin: 0cm 0cm 0pt;"><span><span style="font-family: Calibri;">Tom<!-- o ignored --></span></span></div>
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<pre>-- <br />FARHI Emmanuel <farhi@ill.fr>
Groupe DS/CS, ILL4/156, Tel (33) 4 76 20 71 35
ILL, Grenoble</pre>
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