Small angle X-ray scattering measurements for expanded fluid Se near the critical point

Small angle X-ray scattering (SAXS) measurements have been carried out to i nvestigate density fluctuations near the critical point of expanded fluid S e. The experiments were performed using a polycrystalline sapphire cell of our design and a high pressure vessel. The vessel has a diamond window for an incident X-ray beam and a Be window for the scattered beams. The absolut e scattering intensity was deduced from the measurements on the He gas used as the pressure transmitting medium. The pressure variation of the SAXS in tensity for fluid Se at 1630 degrees C shows a maximum intensity at 401 bar just below the isochor of the critical density. This is the first observat ion of critical scattering for fluid Se and a correlation length of 19 +/- 3 Angstrom was obtained from an Ornstein-Zernike plot. (C) 1999 Elsevier Sc ience B.V. All rights reserved.