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| 3 | <title>EXPGUI Alumina Tutorial, part #8</title> |
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| 17 | include("/var/www/include/utility.inc"); |
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| 19 | <blockquote> |
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| 20 | |
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| 21 | <TABLE BORDER BGCOLOR="#FFFF40" ALIGN=RIGHT> |
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| 22 | <TR><TH><A Href="index.html">Intro</A> |
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| 23 | </TH><TH><A Href="alumina9.html">Next page</A> |
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| 24 | </TH><TH><A Href="alumina7.html">Previous page</A> |
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| 25 | </TH></TR></TABLE><BR CLEAR=ALL> |
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| 26 | |
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| 27 | |
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| 28 | <CENTER> |
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| 29 | <H1><HR noshade width="75%" size="2" align="center"> |
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| 30 | GSAS/EXPGUI Alumina tutorial (part 8)<BR> |
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| 31 | Fitting the Diffractometer Zero Correction |
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| 32 | |
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| 33 | <HR noshade width="75%" size="2" align="center"> |
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| 34 | </H1> |
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| 35 | </CENTER> |
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| 36 | <P> |
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| 37 | Now that the unit cell parameters have been fit, it is now |
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| 38 | a good idea to also refine the diffractometer zero correction. |
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| 39 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 40 | The diffractometer zero correction refinement flag is found on the |
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| 41 | histogram panel, as seen below. Click on the check box for the zero correction |
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| 42 | near the middle of the window to refine the parameter. |
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| 43 | <P><img alt="tutorial screen dump" src="8a.gif" align=RIGHT> |
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| 44 | <BR clear=all> |
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| 45 | <blockquote><I> |
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| 46 | Note that the diffractometer zero correction is refined for parallel beam |
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| 47 | instruments, such as neutron and synchrotron diffractometers. It should |
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| 48 | not be used for Bragg-Brentano instruments |
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| 49 | (laboratory flat-plate parafocussing instruments). |
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| 50 | For Bragg-Brentano instruments, instead the sample displacement parameter |
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| 51 | (shft) should be refined instead. |
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| 52 | When needed, for Bragg-Brentano instruments, the sample transparency correction |
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| 53 | (trns) may also be refined. |
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| 54 | </I></blockquote> |
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| 55 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 56 | |
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| 57 | When GENLES is run, as shown below, a small but significant improvement is |
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| 58 | seen in the agreement factors. |
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| 59 | <P> |
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| 60 | <img alt="tutorial screen dump" src="8b.gif" align=RIGHT> |
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| 61 | <BR clear=all> |
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| 62 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 63 | |
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| 64 | Note that the zero correction has refined from the starting value of 0.04 |
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| 65 | (0.0004 degrees two-theta) to 1.73 (0.0173 degrees two-theta). This small |
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| 66 | correction is needed to obtain a good fit and accurate lattice parameters. |
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| 67 | <P><img alt="tutorial screen dump" src="8c.gif" align=RIGHT> |
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| 68 | <BR clear=all> |
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| 69 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 70 | <HR> |
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| 71 | Previous: <A HREF="alumina7.html"> |
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| 72 | Fitting the Unit Cell |
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| 73 | </A> |
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| 74 | <BR> |
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| 75 | Next step: <A HREF="alumina9.html"> |
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| 76 | Initial Fitting of Profile Parameters |
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| 77 | </A> |
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| 78 | |
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| 79 | <HR> |
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| 80 | </blockquote> |
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| 81 | <P><font size=-1><A HREF="MAILTO:crystal@NIST.gov?subject=WWW page <?=$PHP_SELF?>">Comments, corrections or questions: crystal@NIST.gov</A></font><BR> |
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| 82 | <font size=-1><? lastmod(); ?> |
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| 83 | <BR>$Revision: 627 $ $Date: 2009-12-04 23:09:20 +0000 (Fri, 04 Dec 2009) $</font> |
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| 84 | </body> |
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| 85 | </html> |
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