The BT-1 high-resolution neutron powder diffractometer is used to to obtain neutron powder diffraction data for crystallographic analysis by the Rietveld method or other characterization purposes. It is a 32 detector instrument that can be used with three different monochromators and two different incident Soller collimators, allowing the instrument response to be tailored to the needs of the experiment. The instrument can be used with furnaces, refrigerators and cryostats so that data may be collected at temperatures from 0.3 to 2000 K, and with magnet fields. For room-temperature data collection, a six-position sample changer is available.

Choice of Monochromator and Collimator

The choice of three focusing monochromators, Ge(311), Cu(311), and Ge(733), and either a 15' or a 7' in-pile Soller collimator allows optimization of the instrument's resolution minimum versus relative diffraction intensities. All monochromators give data up to 165 degrees in 2 theta range.

• The Ge(311) monochromator yields the highest neutron intensity and best resolution at low scattering angles. This monochromator has a takeoff angle of 75^o and produces neutrons with wavelength 2.079 Å. Diffraction intensities are approximately three times that obtained with the Cu(311) monochromator. Typical data collection times range from 15 minutes to a few hours, depending on sample size and diffraction properties. The Ge(311) monochromator has been used for zeolite materials where high-angle diffraction measurements are not needed. It is also useful for studies of magnetic ordering and phase changes.
• The Cu(311) monochromator is appropriate for the majority of samples, offering an optimal balance between intensity and resolution and a perfect gaussian line shape. This monochromator has a takeoff angle of 90^o and produces neutrons with a wavelength 1.540 Å. Typical data collection times range from 1 to 12 h, averaging 3-4 h. The Cu(311) monochromator has been used with 15' collimation to collect data for Rietveld refinement of the structures of a wide variety of materials, including high T_c superconductors, fullerenes, ceramics, and intermetallics, as well as for determining phase fractions in multiphase powders.
• The Ge(733) monochromator provides the best resolution at high scattering angles, but longer data collection times are needed, often from 4 to 24 h. This monochromator has a takeoff angle of 120^o and produces neutrons with wavelength 1.197 Å. Diffraction intensities are approximately 25% of what is obtained with the Cu(311) monochromator. The Ge(733) monochromator is used for study of materials such as perovskites where a subtle lowering of the unit cell symmetry requires separation of reflections at high angles or where precise determination of oxygen vacancies is desired. With 7' collimation, gaussian line shapes with widths (FWHM) as small as 10' (delta d/d = 8 x 10^-4) are observed but for most samples the intrinsic sample broadening exceeds the instrumental resolution. Thus, use of the Ge(733) monochromator is restricted to those applications where highest resolution is clearly needed and where samples are available in sufficient quality and quantity.

Resolution in Each Operational Mode

The shape of the BT-1 instrument response function for the six different operational modes can be viewed in angular units or Q (Å^-1) and relative intensities are tablulated in BT-1 Specifications. (Note that these curves show peak widths observed from a reference sample.)

Ideal Sample Size

The ideal sample size for BT-1 is 10 cm³ (typically 5-15 g) of material, although data have been collected on samples as small as 200 mg.

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Related Links:

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• BT-1 Home Page
  
• BT-1 Specifications
  
• Sample Requirements and Safety Considerations for BT-1 Users
  
• Available Furnace Options at the NCNR
  
• Available Refrigerator Options at the NCNR
  
• Available Magnet Options at the NCNR
  
• Request Beam Time on BT-1
  
• Contact a BT-1 Scientist
  
  

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  Additional Links:
  
  
  NCNR Home Page.
  NCNR Sample Environment Page.
  Safety Guidelines for Working at the NCNR
  
  

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  Last modified 21-February-2008