TWO NEW TOF NEUTRON POWDER DIFFRAC-TOMETERS AT KENS. T. Kamiyama and K. Oikawa, Institute of Materials Science, University of Tsukuba, Tennodai, Tsukuba, Ibaraki 305, Japan, M. Furusaka and S. Satoh, National Laboratory for High Energy Physics, Tsukuba, Ibaraki 305, Japan, F. Izumi, National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305, Japan

A time-of-flight (TOF) neutron powder diffractometer, VEGA, has been operating at KENS since November, 1993. VEGA is situated 19 m apart from a solid-methane moderator at 30 K. Though upgrades of their hardware and software are ongoing, VEGA has already demonstrated itself as a better successor of the previous powder diffractometer HRP in both resolution and intensity.

VEGA has backscattering, 90 degree, and low-angle banks; each bank contains arrays of one-dimensional 3He position-sensitive detectors (PSD's) to cover wide solid angles two-dimensionally. Such a unique design was adopted because any arrangement with many short and long normal detectors along Debye-Scherrer rings inevitably produces wide inactive regions. Two-dimensional PSD's were not used because of their lower counting-rate limitations and difficulty in maintenance. This new geometry makes it possible to increase the efficiency of data collection and to improve resolution. The backscattering bank covers a solid angle of 1.2 str (0.9 str at present) with a resolution of delta(d)/d = 0.002. The typical measurement time with this bank for the Rietveld analysis is 4 h, which is four times as short as that with HRP. The 90 degree bank is suitable for in situ experiments under special environments such as high pressure and high temperature. The low-angle bank is utilized to cover long d ranges. Results of some Rietveld refinements with RIETAN-96T developed recently will be presented.

Further, another TOF neutron powder diffractometer with higher resolution is now under construction at KENS. The incident flight path for this diffractometer is about 40 m. It has a backscattering bank with 500 one-dimensional PSD's, which are 2 m apart from a sample position. The designed resolution is as high as 0.0009.