ECHIDNA - High Resolution Powder Diffractometer


ECHIDNA - High Resolution Powder Diffractometer

ECHIDNA is the name of the new neutron High Resolution Powder Diffractometer at Australia's new research reactor OPAL, ANSTO.

The instrument serves to determine the crystalline structures of materials using neutron radiation analogical to X-ray techniques.

It will operate with thermal neutrons. One of the main features is the array of 128 collimators and position sensitive detectors for rapid data acquisition. ECHIDNA will allow structure determinations, texture measurements and reciprocal space mapping of single crystals in most different sample environments serving the physics, chemistry, materials, minerals and earth-science communities. ECHIDNA is part of the Bragg Institute's park of neutron scattering instruments.

The Australian instrument, "mother of all monsters", is named after the Australian monotreme "echidna". Spiny peaks allude to the high resolution of the instrument.

Components

* Neutron guide
*: The instrument is located on the TG1 thermal neutron guide of the OPAL reactor. The distance from the reactor is 58 m. The position is the second on the guide after the WOMBAT instrument. The size of the guide is 300 mm height X 50 mm width and it is plated with supermirror coatings.
* Primary collimator
*: There are Söller collimators prior to the monochromator in order to reduce the divergence of the beam and to increase the angular resolution of the instrument. Since this is an intensity compromise, two items of 5' and 10', respectively, can be interchanged or fully removed by an automated mechanism. The collimators cover the full size of the beam delivered by the neutron guide.
* Monochromator
*: The monochromator is made by slabs of [115] oriented Germanium crystals which are inclined towards each other in order to focus down the Bragg reflected beam. The device has been acquired from the Brookhaven National Laboratory in the USA after the shutdown of their neutron facility.
* Secondary collimator
*: Optionally a secondary collimator with 10' angular acceptance and 200 mm X 20 mm can be placed in the monochromatic beam between the monochromator and the sample, which again influences the resolution function of the instrument.
* Slit system
*: Two automated sets of horizontal and vertical pairs of absorbing plates allow to cut down the size of the monochromatic beam prior to the secondary collimator and sample size. They remove unwanted netrons and reduce the background near the detector. In addition, they allow to select the sample position to be studied.
* Beam monitor
*: A 235U fission monitor measures the amount of neutrons incident to the sample. The efficiency is 10-4 and most neutrons traverse the device undisturbed. The monitor counts are important to correct for beam flux variations due to changes in the reactor or at the upstream instrument.
* Sample stageThe sample is supported by a heavy load goniometer consisting of a 360° vertical omega rotation axis, x-y translation tables and a chi - phi cross tilt stage of +/-20° range. It can hold a few hundred kilograms in order to support heavier sample environment, such as cryostats, furnaces, magnets, load frames, reaction chambers and others. A typical powder sample is filled into vanadium cans which give little, unstructured background. The mentioned sample environment allows to measure changes of the sample as a function of external parameters, like temperature, presser, magnetic field, etc. The goniometer stage is redundant for most powder diffraction measurements, but will be important for single crystal and texture measurements, where the orientation of the sample plays a role.
* Detector collimatorsA set of 128 detectors each equipped which a 5' collimator in front are arranged in a 160° sector focusing to the sample. The collimators select the scattered radiation into the well defined ranges of 128 angular positions. All the collimator and detector setup is mounted on a common table which is scanned in finer steps around the sample, to be combinded further to a continuous diffraction pattern.
* Detector tubesThe 128 linear position sensitive 3He gas detector tubes cover the full opening height of 300 mm behind the collimators. They determine the position of the neutron event by charge division over the resistive anode towards each end of the detector. Overall and local count rates lie in the several 10000 Hz range.

Applications

* Structure determination
** Powder diffraction
** Single crystal diffraction
** Diffuse scattering
* Structural changes
** Temperature dependence
** Magnetic fields
** Electric fields
* Texture analysis
** Texture measurements
** Texture evolution under stress
** Behavior of texture through a phase transition
** Texture engineering
* Geology
** Geological deformations studied by texture analysis
** Structures of rocks and minerals
** Simulation of deformation processes in the earth crust
* Phase transitions
** Structural transformations
** Solidification of liquids
** Precipitation processes
** Segregation processes
* Neutron optics
** Crystal quality tests
** Special setups

Literature

* [http://www.kdliss.de/KDL/papers/LissEchidnaICNS2005.pdf K.-D. Liss, B. Hunter, M. Hagen, T. Noakes, S. Kennedy: "ECHIDNA - The new high-resolution powder diffractometer being built at OPAL", Physica B, in press (2005)]

External links

* [http://www.ansto.gov.au/ansto/bragg/2005/hrpd/instrument_hrpd.html ECHIDNA home page]


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