Dear Colleagues, Sorry for this non-rietveld or crystallography related question but an input on this may be of great help for us to diagnose our problem.
We have a 7-years old Rigaku ULTIMAIV powder diffractometer with Cu sealed tube source equipped with CBO system for paralell beam optics, a curved Ge crystal monochromator and NaI scintallator detector. Over the last two weeks we are experiencing a dramatic fall of intensity of 80-90% after a change in geometry from paralell to divergent beam. We were running on a rather old tube that may be releasing less and less photons, but the step-like fall of intensity makes us think it´s not necessarily a tube problem. Alignment has been checked, just in case something went awry with the system and the intensity after re-alignment is still far from normal. Indeed all the alignment can be done with apparently normal intensity in the detector, maybe a slightly smaller intensity in the Si alignment plate scan is found, but the real problem shows when the detector HV scan is performed to find the best value, there is no peak whatsoever in that scan, so the original (reasonable HV) value is kept. Working without the monocrhomator but a Ni filter gets us with a higher intensity but still below normal, background is 7 times higher and a peak intensity twice as large than observed with the curved Ge monochromator. The detector is counting since we can align and reach very high counts when collecting data for 20 seconds or so per step (removing the monochromator) but it is like it has lost its efficiency for weaker signals. Have you ever seen something like this with this kind of detectors? could this be a problem with the electronics? or the detector is dying...? Another thing that is probably unrelated but called our attention and confused us a bit during the trials to diagnose the problem is that, when the Ni filter is installed and the gap between the RS and the detector is not covered with a lead sheet, we observe a much higher background (3 times the one with covered gap) and two broad peaks that look like Kalfa and Kbeta from a diffracting body somewhere between the source and the detector. We are guessing it comes from the CBO but we are unsure. These peaks changed positions towards higher 2theta when we tested a Co tube, that´s why we suppose they are the result of some diffraction. The peaks also seem to be reaching the detector through the gap with the RS because the peaks are there all the time when the gap is open, no matter if there is a sample in the holder. Indeed the peaks overlap with sample peaks when we run the samples... We thank any comment on this "phenomenon" also. Thanks in advance for your help, Leo -- Dr. Leopoldo Suescun Prof. Agr (Assoc. Prof.) de Física Tel: (+598) 29290705/29249859 Cryssmat-Lab./DETEMA Fax: (+598) 29241906* Facultad de Quimica, Universidad de la Republica ,_. | \ | v- ,' \ | ( \__Montevideo, Uruguay En pleno disfrute del Año Internacional de la Cristalografía 2014 ( http://www.iycr2014.org ), (http://www.cristalografia2014.fq.edu.uy)
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