High resolution powder diffraction

This instrument is operational .

View of HRPD’s sample position

View of HRPD’s sample position. The red shielding at the top surrounds the incident-beam guide and the backscattering detectors; the 90-degree banks sit underneath the platforms to left and right, and the low-angle bank is out of sight at the bottom right.
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HRPD, the High Resolution Powder Diffractometer, is the highest resolution neutron powder diffractometer of its type in the world.

Situated almost 100m from the ISIS target at the end of a neutron guide, its unprecedented resolution in the main backscattering detector bank, with a Dd/d resolution of ~4 × 10-4 which is effectively constant over the wide d-spacing range available, gives it unique power in the study of subtle structural details, for example in phase transitions. The ability to separate peaks resulting from small unit cell changes and to allow peaks at shorter d-spacings to be resolved from complex materials, where short wavelength epithermal neutrons from the ISIS target allow d-spacings below 0.3Å  to be recorded, has enabled HRPD to expand the technique of powder diffraction in novel directions.

In addition, the liquid methane moderator provides HRPD with an incident flux out to a maximum wavelength of some 10-12 Å. The highest resolution data at backscattering may therefore be recorded out to d-spacings of some 5-6 Å. Complementary detectors at 90° and low angles extend the measurable d-spacing range, albeit at lower resolution, to beyond 20 Å. The detector array at 90° has recently been enhanced substantially to provide a wider angular range in addition to an order-of-magnitude increase in count rate compared with the old bank.

Sections of the backscattering ZnS-scintillator detector in the equatorial and vertical planes may be decoupled from the Debye- Scherrer geometry typically used in powder studies so as to enable high-resolution single crystal investigations to be carried out.

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