THE FOUR CIRCLE DIFFRACTOMETER AT ELETTRA R. Spagna, L. Barba, M. Camalli, A. Cassetta, M. Catricala', C. Marciante, A. Pifferi, Istituto di Strutturistica Chimica, Area della Ricerca, CNR, C.P. 10, 00016 Monterotondo Stazione, M. Colapietro, G. Portalone, Dip. di Chimica, Universita' di Roma "La Sapienza", Pl. Aldo Moro, 00100 Roma

ELETTRA is a low emittance synchrotron radiation (SR) facility at Trieste. The radiation source of the beamline for X-ray diffraction is the 4.5m long, 57 poles permanent magnet wiggler which widens the range of wavelength available for experiments to a short wavelength limit of about 0.5Å. The SR can be monochromatized in the range 4-25keV (0.5-3.0Å) by means of a fixed-exit double-crystal monochromator containing two interchangeable pairs of Si(111) or Si(220) crystals in a nondispersive parallel setting. In order to obtain a higher flux on the sample a mirror collects 2.8 mrad horizontally and 0.23 mrad vertically and focuses X-rays up to 25keV with a demagnification factor of 1.5 (S.Bernstorff et al., Rev.Sci.Instrum. (1995) 66,1661-1664).

The experimental station is equipped with both a imaging plate detector system from Marresearch (180mm) and a Mod. 5020 Huber 4-circle diffractometer. The last instrument designed for operation with a vertical scattering plane is supported on a kinematic table to allow the alignment of the instrument with respect to the beam. The diffractometer is controlled by a personal computer through a plug-in board interface and the program system CS has been made to carry out single crystal diffraction experiments. A silicon crystal was used to plan the strategies to handle the very sharp diffracted beams. 19 reflections were located and centred. The rocking curves of these reflections show full widths at half-maxima of about 0.01. A first oriantation matrix gives a average value of the cell parameter a = 5.431(2) in very good agreement with the value of 5.43088(4) calculated by powder diffraction of NBS Standard Reference Material No. 640. Afterwards a spherical crystal of calcium fluride and then one of corundum, Al2O3, were mounted on the diffractometer and data collections were performed.

Results will be presented during the conference.