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| 3 | <title>EXPGUI Alumina Tutorial, part #3</title> |
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| 15 | <? |
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| 16 | include("/var/www/include/navigation.inc"); |
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| 17 | include("/var/www/include/utility.inc"); |
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| 18 | ?> |
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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="alumina4.html">Next page</A> |
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| 24 | </TH><TH><A Href="alumina2.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 3)<BR> |
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| 31 | Specifying Powder Diffraction Data (Adding a Histogram) |
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| 32 | <HR noshade width="75%" size="2" align="center"> |
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| 33 | </H1> |
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| 34 | </CENTER> |
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| 35 | <I> |
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| 36 | GSAS uses the term "histogram" to refer to a diffraction data set. |
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| 37 | A histogram can also be a set of "soft constraints," e.g. |
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| 38 | a set of target parameters, such as bond distances, that the |
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| 39 | model will also try to fit. |
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| 40 | GSAS can fit a model to up to 99 histograms simultaneously, |
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| 41 | although the majority of refinements done in GSAS |
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| 42 | use a single histogram or at most |
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| 43 | only a few histograms. |
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| 44 | GSAS can use single crystal or powder diffraction data, either |
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| 45 | neutron or x-ray. For neutron powder diffraction data, the data |
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| 46 | can be obtained from either time-of-flight (TOF) or constant wavelength (CW) |
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| 47 | instruments. GSAS can use x-ray data from synchrotron, laboratory alpha-1,2, |
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| 48 | and even energy-dispersive x-ray instruments. |
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| 49 | <P> |
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| 50 | Two files are needed to load a powder diffraction histogram. |
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| 51 | The first is a file containing the powder diffraction data, often called |
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| 52 | a GSAS raw data file (often using the extension .RAW, .GSA or .GSAS) |
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| 53 | and the second file is an |
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| 54 | instrument parameter file (.INS or .INST) that defines what type |
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| 55 | of data is included in the raw file (x-ray/neutron, CW/TOF/ED, etc.) |
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| 56 | as well as starting values |
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| 57 | for the diffractometer constants and peak shape parameters. |
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| 58 | There are a number of available formats for the raw data files |
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| 59 | and types of records in the instrument parameter file; this information |
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| 60 | is defined in the |
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| 61 | <A HREF="http://www.nist.gov/cgi-bin/exit_nist.cgi?url=ftp://ftp.lanl.gov/public/gsas/manual/GSASManual.pdf">GSAS documentation</A>. |
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| 62 | Note that raw data files can contain more than one set of data and that |
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| 63 | an instrument parameter file can contain more than one set of parameters. |
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| 64 | This feature is rarely used, with the exception of TOF instrumentation, |
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| 65 | where detectors are grouped into banks and the results for each bank |
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| 66 | are included in a single file. |
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| 67 | Software for translating diffraction data into a format accepted by |
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| 68 | GSAS is available at most user facilities or can be found at the |
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| 69 | <A HREF="http://www.nist.gov/cgi-bin/exit_nist.cgi?url=http://www.ccp14.ac.uk">CCP14 web site</A> |
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| 70 | Appropriate instrument parameter files can usually be |
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| 71 | provided by the instrument scientist at a user facility or prototypes |
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| 72 | can be found in the GSAS distribution files. |
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| 73 | </I><P> |
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| 74 | This web page demonstrates how the alumina powder diffraction data are |
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| 75 | now added to the experiment file. For this tutorial exercise, |
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| 76 | a special instrument parameter file that has peak shape values narrower than |
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| 77 | the actual instrument is provided. The tutorial would be less challenging |
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| 78 | if the appropriate instrument parameter file is used. |
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| 79 | |
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| 80 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 81 | |
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| 82 | The Histogram panel is selected by clicking on the Histogram tab, as is shown |
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| 83 | below. In this case, no data has been defined, as can be determined |
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| 84 | by the absence of entries in the histogram selection box, in the upper left. |
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| 85 | The "Add New Histogram" button, at the lower right, is used to add [additional] |
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| 86 | powder diffraction data sets to the refinement, as will be demonstrated in this |
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| 87 | page. The histogram panel is used |
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| 88 | to modify various parameters associated with |
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| 89 | each set of diffraction data, for example the diffractometer constants |
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| 90 | (such as wavelength), the background function and terms. |
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| 91 | <P><img alt="tutorial screen dump" src="3a.gif" align=RIGHT> |
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| 92 | <BR clear=all> |
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| 93 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 94 | |
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| 95 | <P><img alt="tutorial screen dump" src="3b.gif" align=RIGHT> |
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| 96 | Pressing the "Add New Histogram" button causes the "add new histogram" |
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| 97 | window, shown to the right, to be displayed. The entries on this |
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| 98 | window are usually considered from top to bottom. The "Dummy Histogram" |
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| 99 | option is used to simulate powder diffraction data, and is not |
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| 100 | used in this tutorial example. So the next item of interest is to select a |
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| 101 | data file. This is done by pressing the upper of the two "Select File" |
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| 102 | buttons. |
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| 103 | <BR clear=all> |
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| 104 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 105 | <img alt="tutorial screen dump" src="3c.gif" align=RIGHT> |
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| 106 | Pressing the "Select File" button creates a file open window, such as |
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| 107 | the one to the right (or slightly different in appearance in windows). |
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| 108 | Select the input file for this exercise, the file you |
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| 109 | <a href="index.html#downloads">downloaded earlier,</a> al2o3001.gsa. |
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| 110 | Double-click on the entry, or select is and press the "Open" button. |
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| 111 | This open window will then close. |
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| 112 | <BR clear=all> |
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| 113 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 114 | <img alt="tutorial screen dump" src="3d.gif" align=RIGHT> |
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| 115 | |
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| 116 | Selecting the raw data file in the open window causes the al2o3001.gsa file |
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| 117 | to be loaded into the upper box on the "add new histogram" window. This |
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| 118 | file is scanned to and check mark entries are created for each bank |
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| 119 | in the file. The al2o3001.gsa file also defines a default instrument |
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| 120 | parameter file, which is the bt1demo.ins that was |
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| 121 | <a href="index.html#downloads">downloaded earlier</a>, so this file name is |
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| 122 | entered into the "Instrument Parameter File" section. |
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| 123 | <P> |
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| 124 | The "Usable data limit" sets the maximum range of data to be used in fitting. |
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| 125 | This is usually determined by plotting the data to see where no further |
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| 126 | peaks are present. This can be done here with the GSAS RAWPLOT program. |
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| 127 | For this exercise, change the defaulted value (the entire data range) to 155 |
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| 128 | degrees, to exclude a single very broad high-angle peak. |
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| 129 | The press the "Add" button in the lower left. |
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| 130 | <BR clear=all> |
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| 131 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 132 | After the "Add" button is pressed, the EXPGUI program runs a GSAS program, |
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| 133 | EXPTOOL, that actually adds the data reference to the experiment. If an error |
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| 134 | occurs, this result is shown. If no error occurs, the histogram panel |
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| 135 | is redisplayed, but this time a histogram appears in the upper left, as |
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| 136 | seen below. |
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| 137 | <P> |
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| 138 | <img alt="tutorial screen dump" src="3e.gif" align=RIGHT> |
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| 139 | <BR clear=all> |
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| 140 | <HR> |
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| 141 | Previous: <A HREF="alumina2.html"> |
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| 142 | Adding a Phase |
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| 143 | </A> |
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| 144 | <BR> |
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| 145 | Next step: <A HREF="alumina4.html"> |
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| 146 | Change Background Function |
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| 147 | </A> |
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| 148 | |
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| 149 | <HR> |
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| 150 | </blockquote> |
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| 151 | <P><font size=-1><A HREF="MAILTO:crystal@NIST.gov?subject=WWW page <?=$PHP_SELF?>">Comments, corrections or questions: crystal@NIST.gov</A></font><BR> |
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| 152 | <font size=-1><? lastmod(); ?> |
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| 153 | <BR>$Revision: 622 $ $Date: 2009-12-04 23:09:15 +0000 (Fri, 04 Dec 2009) $</font> |
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| 154 | </body> |
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| 155 | </html> |
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