ON THE USE OF LABORATORY EXAFS SPECTROMETER WITH SIEMENS 18 kW ROTATING ANODE. B.Mierzwa, Z.Kaszkur, J.Pielaszek - Institute of Physical, Chemistry, Polish Academy of Sciences, Warsaw, Poland.

EXAFS laboratory spectrometer was constructed for using in preliminary laboratory studies of catalysts. Standard Q - 2Q goniometer has been adapted with monochromator mounted at the goniometer axis and two especially build proportional counters ( with the sample placed in between ) on the arm of the goniometer. The radius of the monochromator curvature is changeable during the measurements to keep the setting close to the Rowland geometry. Silicon, about 200mm thick, (110) and (111) oriented crystals were used as monochromators.

Initially the system was used with a standard 1.5 kW sealed-off X-ray tube. The measurements of Fe Ka absorption edge performed on Fe - foils showed that the data obtained were comparable to synchrotron data but at the significant expense of time of measurements.

To increase the intensity of the measured spectra and shorten the time of measurements the spectrometer was installed on Siemens 18 kW rotating anode with Mo target. As it will be shown the obtained spectra were of much inferior quality than expected from possible lower energy resolution. Therefore a series of long - lasting tests was performed showing how instabilities of the performance of the rotating anode affect the quality of the measurements.

Special attention was paid to the possibility of X-ray focus displacement, total intensity fluctuations and thermal expansion of the rotating anode target itself. As results show the most important are the long term beam fluctuations rising up to 10% of the overall intensity of the emitted X-rays which are highly correlated with minute instabilities the effective interphase voltage of the mains, in spite of the very high stability of each of the phases of the main.