Since the reflectivity curve changes by orders of magnitude in intensity as a function of Q, different scan points must be measured for different amounts of time. Before lines can be combined in the reflectivity calculation, they must be normalized to the same counting basis. Once they are normalized, the data can be scaled arbitrarily for display purposes. The monitor window lets you select how the data is displayed.
Because different data is taken with different monitors, and because the default display scale uses the monitor value of the first record displayed the actual counts displayed for a particular point may vary depending on which other records are displayed. To keep a consistent value no matter what the display, use a fixed scale. Fields are provided for fixed monitor counts and fixed seconds separately since some data is normalized by time and other by monitor.
The choice of normalization is inherent in the record. Ideally all records will contain a monitor column giving an estimate of the number of neutrons available. Sometimes the monitor is not available or not reliable and the record should be normalized against seconds rather than monitors. To do so it is necessary to leave the graphical user interface. First select the record that should be changed so that its contents are visible in the text window. Next type the following in the Tcl console:
monitor_set $rec(id) seconds
The data will now be normalized by the ::seconds_$rec(id) column. You can change all visible records with a single command:
monitor_set $::addrun seconds
These changes will last until you deselect and reselect the file. You can also force normalization by monitor by calling the following:
monitor_set $::addrun monitor
Normalizing against time assumes the variation in beam rate is small, which is not necessarily the case (e.g., power bumps due to weather conditions affecting the electrical supply), so use with caution.
Be careful when mixing monitor and time data in the graph. You can view them on the same scale by fixing the scale and entering the counts per second in counts and 1 in seconds. This does not change the value stored in the file, but it does allow you to view the data together. As of this writing the data for time and monitor will not be correctly combined. Instead, reduce time and monitor data separately and combine the resulting reflectivity curves.
In certain rare cases mixing monitor and time data will be unavoidable, but handling them is beyond the scope of the graphical user interface. Instead, use the Tcl console to construct specific monitor or seconds vectors. The function monitor_gen can generate either monitor or seconds given a monitor rate. For example, to generate a time column from a monitor column in the current record assuming the monitor measures 5132 counts measured over 3 seconds, use:
monitor_gen $rec(id) seconds 5132 3
2005-09-27