atlas logo.gif (621 bytes) Correcting GEM data with gudrun 

Why use gudrun?

gudrun is the new PC-based software for correcting and reducing GEM data. It is recommended to use gudrun for correcting GEM data in preference to the NORM/CORAL/ANALYSE stages of the ATLAS suite for two reasons:

  1. gudrun uses the azimuthal detector angle in calculating the corrections. ATLAS treats all detectors as though they are in the horizontal plane, which is not correct for GEM.
  2. gudrun calculates the corrections separately for each individual detector element. ATLAS calculates the corrections once for detector group, using the average parameters and wavelengths for the group. Hence the corrections used by gudrun are less of an approximation than those calculated by ATLAS, and gudrun offers the prospect of being able to achieve lower minimum Q-values than ATLAS.

Here you can see an example of GEM data which have been successfully analysed using gudrun: click here 

The two stages of gudrun

There are two stages involved in analysing data with gudrun:

  1. Run purge_det to determine which detectors are bad (i.e. unstable, dead or containing spikes) so that they can be excluded from the analysis. 
  2. Run gudrun_dcs to produce a final corrected I(Q) differential cross-section curve for each detector group. This program adds together all the runs of each type, and converts to a Q-scale. The data are corrected for absorption and multiple scattering, and they are normalised to vanadium to produce a result which is on an absolute cross-section scale.

Getting started with gudrun

At present the gudrun software runs only on a PC which is using a Windows operating system. Here is a zip file which can be downloaded and extracted to provide all of the standard files which are needed to run gudrun: gudrun_zip.zip 

The standard files which are needed to run gudrun are as follows:

purge_det.dat purge_det.exe spectrum000.dat sears91_gudrun.dat gudrun_dcs.exe 

In addition to these standard files, you will need to have the .RAW data files for your experiment and also a groups file and a calibration file.

Running the programs

Instructions for running purge_det are here 

Instructions for running gudrun_dcs are here

After running gudrun

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 this:
data=readfile(file="GEM12345.DCS01") if the GEMSQRAW software has been installed.

My own personal preference is currently to use the ATLAS suite to subtract the self scattering and to merge the different detector groups into one final i(Q) curve. In order to do this the .DCS01 file must first be converted into an ATLAS .DCS file. If the GEMSQRAW software has been installed then this may be done as follows: Download guddcs.zip and extract its files to \gemsqraw\gcl, overwriting previous versions of the files. Start OpenGenie, and set the working directory to the location of your .DCS01 file using the CD command. Type guddcs to create the required .DCS file. FTP the .DCS file to VMS - you must transfer it as binary. (This can also be done by FTPing the .DCS01 file to a VMS account and then converting to an ATLAS-type .DCS file by starting Genie v2 and typing @g_f:guddcs . However, the .DCS01 files have very long records and are not always transferred correctly by FTP.) Once this has been done you can use the ATLAS programs PLATOM, INTERFERE and MERGE to produce a final i(Q) curve. (Obviously it is not necessary to run the earlier stages of ATLAS; NORM, CORAL, ANALYSE, MUT0 and VANSM.)

Last updated on 08 May 2006 by Alex Hannon (a.c.hannon@rl.ac.uk)