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College Park, Maryland      June 6 - 10 , 2004

T2-C7 (3:00 PM): Novel time-of-flight diffractometer for very high resolution and extreme sample environment.

J. Peters, D. Clemens, K. Lieutenant (Hahn-Meitner-Institut Berlin), F. Mezei (Hahn-Meitner-Institut Berlin; Los Alamos Neutron Science Center, Los Alamos National Laboratory)

The Extreme Environment Diffractometer (EXED) powder diffractometer is under construction at the Hahn-Meitner-Institut (HMI) in Berlin. It is based on the time-of-flight (TOF) principle, which offers a number of advantages on a continuous source compared with the usual crystal monochromator instruments: a) It can provide higher resolutions, matching those achieved by now on synchrotron radiation sources; b) It makes small d spacing readily accessible; c) It is more efficient in terms of neutron intensity for conventionally high resolution neutron diffraction work and d) it facilitates the use of extreme sample environment equipment by providing a full coverage of the relevant Q domain at very limited angular access in scattering angles, for instance due to the magnet geometry. EXED will be installed on an innovative multispectral neutron guide, delivering both the thermal and the cold neutron spectrum to the sample at about 80 m from the neutron source. This will assure an exceptionally broad incoming wavelength band of 0.7 to 20 , i.e. d spacing from 0.35 to 10 , or a momentum transfer Q domain 0.63 -1 to 17.9 -1 can be studied in the high resolution backscattering configuration at scattering angles 155 to 178. The symmetric forward scattering range of 2 to 25 will make accessible Q values 0.01 -1 to 3.9 -1. With 54 m flight-path between the chopper and the sample, the highest resolution will be achieved using a 36000 RPM curved Fermi chopper for 6 μs FWHM pulse length and the resolution will be adjustable according to the needs of the experiment by extending the pulse length up to 4000 μs with the help of alternate choppers (lower resolution straight Fermi chopper and a disc chopper pair with variable slit width). The highest resolution in backscattering geometry will achieve δd ~ 3.5x10-4 , i.e. δd/d = 2.4x10-4 for common d spacing in the 2 range. The wavelength range to be explored will be defined by the phases and angular velocities of the choppers and can be scanned continuously.

The instrument is to be equipped with an immobile steady state magnet (25 T, later envisaged 40 T) of tapered solenoidal shape, which form allows us to achieve higher fields than the conventional split pair design. Low temperature and high pressure sample environments will also be available at high magnetic fields. Four position sensitive gas detectors with a space coverage of 50 x 80 cm2 each and a resolution of 1 cm x 1 cm will be arranged around the sample, to provide fairly full coverage of the forward and backward scattering angle domains left accessible by the form of the magnet. EXED can be further extended for small angle and inelastic scattering work in high magnetic fields.

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