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3   <title>EXPGUI Alumina Tutorial, part #3</title>
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22<TR><TH><A  Href="index.html">Intro</A> 
23</TH><TH><A Href="alumina4.html">Next page</A>
24</TH><TH><A Href="alumina2.html">Previous page</A>
29<H1><HR noshade width="75%" size="2" align="center">
30GSAS/EXPGUI Alumina tutorial (part 3)<BR>
31Specifying Powder Diffraction Data (Adding a Histogram)
32<HR noshade width="75%" size="2" align="center">
36GSAS uses the term "histogram" to refer to a diffraction data set.
37A histogram can also be a set of "soft constraints," e.g.
38a set of target parameters, such as bond distances, that the
39model will also try to fit.
40GSAS can fit a model to up to 99 histograms simultaneously,
41although the majority of refinements done in GSAS
42use a single histogram or at most
43only a few histograms.
44GSAS can use single crystal or powder diffraction data, either
45neutron or x-ray. For neutron powder diffraction data, the data
46can be obtained from either time-of-flight (TOF) or constant wavelength (CW)
47instruments. GSAS can use x-ray data from synchrotron, laboratory alpha-1,2,
48and even energy-dispersive x-ray instruments.
50Two files are needed to load a powder diffraction histogram.
51The first is a file containing the powder diffraction data, often called
52a GSAS raw data file (often using the extension .RAW, .GSA or .GSAS)
53and the second file is an
54instrument parameter file (.INS or .INST) that defines what type
55of data is included in the raw file (x-ray/neutron, CW/TOF/ED, etc.)
56as well as starting values
57for the diffractometer constants and peak shape parameters.
58There are a number of available formats for the raw data files
59and types of records in the instrument parameter file; this information
60is defined in the
61<A HREF="">
62GSAS documentation</A>.
63Note that raw data files can contain more than one set of data and that
64an instrument parameter file can contain more than one set of parameters.
65This feature is rarely used, with the exception of TOF instrumentation,
66where detectors are grouped into banks and the results for each bank
67are included in a single file.
68Software for translating diffraction data into a format accepted by
69GSAS is available at most user facilities or can be found at the
70<A HREF="">CCP14 web site</A>
71Appropriate instrument parameter files can usually be
72provided by the instrument scientist at a user facility or prototypes
73can be found in the GSAS distribution files.
75This web page demonstrates how the alumina powder diffraction data are
76now added to the experiment file. For this tutorial exercise,
77a special instrument parameter file that has peak shape values narrower than
78the actual instrument is provided. The tutorial would be less challenging
79if the appropriate instrument parameter file is used.
81<P><HR noshade width="50%" size="4" align="center"><P>
83The Histogram panel is selected by clicking on the Histogram tab, as is shown
84below. In this case, no data has been defined, as can be determined
85by the absence of entries in the histogram selection box, in the upper left.
86The "Add New Histogram" button, at the lower right, is used to add [additional]
87powder diffraction data sets to the refinement, as will be demonstrated in this
88page. The histogram panel is used
89to modify various parameters associated with
90each set of diffraction data, for example the diffractometer constants
91(such as wavelength), the background function and terms.
92<P><img alt="tutorial screen dump" src="3a.gif" align=RIGHT>
93<BR clear=all>
94<P><HR noshade width="50%" size="4" align="center"><P>
96<P><img alt="tutorial screen dump" src="3b.gif" align=RIGHT>
97Pressing the "Add New Histogram" button causes the "add new histogram"
98window, shown to the right, to be displayed. The entries on this
99window are usually considered from top to bottom. The "Dummy Histogram"
100option is used to simulate powder diffraction data, and is not
101used in this tutorial example. So the next item of interest is to select a
102data file. This is done by pressing the upper of the two "Select File"
104<BR clear=all>
105<P><HR noshade width="50%" size="4" align="center"><P>
106<img alt="tutorial screen dump" src="3c.gif" align=RIGHT>
107Pressing the "Select File" button creates a file open window, such as
108the one to the right (or slightly different in appearance in windows).
109Select the input file for this exercise, the file you
110<a href="index.html#downloads">downloaded earlier,</a> al2o3001.gsa.
111Double-click on the entry, or select is and press the "Open" button.
112This open window will then close.
113<BR clear=all>
114<P><HR noshade width="50%" size="4" align="center"><P>
115<img alt="tutorial screen dump" src="3d.gif" align=RIGHT>
117Selecting the raw data file in the open window causes the al2o3001.gsa file
118to be loaded into the upper box on the "add new histogram" window. This
119file is scanned to and check mark entries are created for each bank
120in the file. The al2o3001.gsa file also defines a default instrument
121parameter file, which is the bt1demo.ins that was
122<a href="index.html#downloads">downloaded earlier</a>, so this file name is
123entered into the "Instrument Parameter File" section.
125The "Usable data limit" sets the maximum range of data to be used in fitting.
126This is usually determined by plotting the data to see where no further
127peaks are present. This can be done here with the GSAS RAWPLOT program.
128For this exercise, change the defaulted value (the entire data range) to 155
129degrees, to exclude a single very broad high-angle peak.
130The press the "Add" button in the lower left.
131<BR clear=all>
132<P><HR noshade width="50%" size="4" align="center"><P>
133After the "Add" button is pressed, the EXPGUI program runs a GSAS program,
134EXPTOOL, that actually adds the data reference to the experiment. If an error
135occurs, this result is shown. If no error occurs, the histogram panel
136is redisplayed, but this time a histogram appears in the upper left, as
137seen below.
139<img alt="tutorial screen dump" src="3e.gif" align=RIGHT>
140<BR clear=all>
142Previous: <A HREF="alumina2.html">
143Adding a Phase
146Next step: <A HREF="alumina4.html">
147Change Background Function
152<P><font size=-1><A HREF=" page <?=$PHP_SELF?>">Comments, corrections or questions:</A></font><BR>
153<font size=-1><? lastmod(); ?> 
154<BR>$Revision: 714 $ $Date: 2009-12-04 23:10:47 +0000 (Fri, 04 Dec 2009) $</font>
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