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Triple –axis spectrometer (Triax)

Triax is the only triple-axis spectrometer available on a university campus. Its flux on sample is roughly within a factor of six of the leading triple-axis spectrometer in the country (HB-3 at HFIR at ORNL). The primary application of such an instrument is to probe inelastic excitations in single crystals. The elegance of the three-axis design allows for independent determination of the incident and final energies as well as the incident and final moment of the scattered neutrons. In this way it is possible to map out, for example, dispersion curves of phonons and magnons in solids in addition to other excitations such as rotons in superfluid helium. The usefulness of this technique extends to quasi-elastic scattering which details the features of such phenomena as short range magnetic order, spin fluctuations and frustration in magnetic systems. A plethora of subject materials have been studied via this technique at MURR and around the world. The types of measurements that are particularly well suited for Triax are ones that require copious amounts of beamtime or are pilot experiments.

This instrument was originally built and installed at the Ames Laboratory Reactor and later moved to ORR at Oak Ridge National Laboratory (ORNL). When that reactor was closed, the spectrometer was transported to MURR. In the move, the incident beam aperture was enlarged for greater acceptance and a pneumatic wedge system of epoxy wedges loaded with B4C was built for the analyzer shield. This has resulted in tight shielding with very low background. In addition, the detector shield was rebuilt and a new detector, consisting of multiple ½"- diameter 3He tubes, was installed, allowing measurements using only those elements that are illuminated by the sample with further background reduction. There are three monochromator options: highly oriented pyrolytic graphite (PG), pressed mosaic Cu (220), both horizontally flat and vertically focusing, and doubly focusing bent perfect Si (111). Since 1997, when the triple-axis became fully operational, it has proven to be an outstanding, nationally competitive instrument in the demanding field of triple-axis spectrometry. In 2005 additional upgrades were implemented to reflect both a modernization in motor control and the computer interface.In the latter case, the SPICE software developed at ORNL was chosen so as to be compatible with triple-axis systems there. As a result, students and other users of Triax can easily transition to the three-axis instruments at HFIR.

Photo of the Triax Instrument
Triple-axis spectrometer (TRIAX) -- Uses unpolarized neutrons and is the only instrument of its kind on a university campus.
    • Three pre-monochromator filter options: Si, Be, Si/PG
    • Five pre-monochromator collimation options: Open (60’), 43’, 29’, 14’, Closed (Pb)
    • Three monochromator options; PG (002), Cu (220), and Si (111)
    • Collimation for mono-sample, sample-analyzer, or analyzer-detector ranging from 80’ to 10’
    • PG (002) analyzer
    • Runs on SPICE software
    • Closed cycle sample cooling to <5K in vacuum