Running gudrun_dcs on GEM data

Running gudrun_dcs on GEM

Before running gudrun_dcs you must have already run purge_det .

The directory should already contain these two files: spectrum000.dat and sears91_gudrun.dat

Detector calibration files for running gudrun on GEM data are available here.

gudrun input files are normally named like this: sample_dcs.dat

A gudrun input file must be prepared before running gudrun. This should be done by copying and modifying a pre-existing file. This will have comments which will help to identify the parameters. For a typical run the essential parameters to change are as follows:

Block	Parameters to change
INSTRUMENT	The name of the directory where the .RAW files are stored The name of the Detector.dat calibration file (it is strongly recommended to use a calibration file, because GEM has been in a state of change for the whole of its life so far, and the calibration stored in the .RAW file cannot always be relied upon) Q_min, Q_step, Q_max
BEAM	Incident beam edges
VANADIUM	Number of vanadium files, and vanadium run numbers Number of vanadium background files, and vanadium background run numbers Radius of vanadium rod
SAMPLE BACKGROUND	Number of sample background files, and sample background run numbers
SAMPLE	Number of sample files, and sample run numbers Chemical composition of sample Radius and height of sample Effective density of sample (i.e. powder density)
CONTAINER	Number of empty can files, and empty can run numbers Inner and outer radius of can

Here is an example gudrun input file: example

gudrun creates many files, but the most important one is named like GEM12345.DCS01 - this contains the corrected differential cross-section for each detector group. This file may be read into OpenGenie (on a PC or on VMS) by a command like data=readfile(file="GEM12345.DCS01") if the GEMSQRAW software has been installed.

It is recommended to run gudrun from a command prompt window, not by double-clicking on the program's icon. This has the advantage that if the program encounters an error then the error messages are retained.

At the end of running gudrun you should note the message about the scattering level. In most cases the scattering level should be within a few percent of the expected value. If it is outside this range then it is probable that there is a problem with the input parameters in the sample_dcs.dat file.

A version of gudrun which uses logarithmic binning is available here: gudrun_dcs_log. If you use this version of gudrun it behaves exactly the same as the normal version, except that it is possible to give a negative value for the spacing delta-Q (qstep), in which case the data are binned with delta-Q/Q=-qstep.

Last updated on 22 Feb 2007 by Alex Hannon (a.c.hannon@rl.ac.uk)