A NEW POWDER NEUTRON DIFFRACTOMETER AT THE JEEP II REACTOR AT KJELLER IN NORWAY. Bjorn C. Hauback, Helmer Fjellvaag and Olav Steinsvoll, Department of Physics, Institute for Energy Technology, P.O. Box 40, N-2007 Kjeller, Norway

A new high-resolution powder neutron diffractometer has been installed at the 2 MW JEEP II reactor at Institute for Energy Technology. Special emphasis has been put on designing a flexible instrument where the utilization of useful neutrons

is maximized. At a low-flux reactor, like JEEP II, great efforts are required to optimize the relationship between intensity (signal/noise ratio) and resolution. In that respect, the most important components of the diffractometer system are the collimators, the monochromator, the detector and the shielding. A new in-pile collimator with increased height has been installed. Different Soller slit collimators in the incident neutron beam allow either a high intensity or a high resolution mode. The flux at the sample position is enhanced by using a vertically focussing composite germanium monochromator. Wavelengths in the range 0.7 - 2.6 A are available by using different reflections from Ge (h11), h=3,5,7 and by varying the monochromator take-off angle in the range 55-100 degrees. Each of the two detector-units, containing seven position sensitive He-3 detectors stacked on top of each other, covers 20 degrees in 2theta. A complete powder pattern of 120 degrees can be collected by moving both detector-units together in three steps. All parts of the instrument is controlled by software running on a PC-486 computer using the OS-9000 operative system. A HP work-station is connected to the instrument with graphics and programs for data treatment, Rietveld refinements and general crystallographic calculations. Examples of possible applications of the new powder neutron diffractometer will be shown.

The financial support from The Research Council of Norway is gratefully acknowledged.