ATHOS

ATHOS - Triple-Axis Spectrometer Athos

Instrument responsible: Gyula Török, Alex Szakál

The cold neutron three-axis spectrometer (TAS) at BNC is installed on the 17 m position of the first neutron guide (NG1). The beamline is a borofloat glass substrate NiTi coated supermirror (m=2) guide. This instrument was in extensive use on a thermal beam in the period of 1972-86. Since the reactor reconstruction and the cold source installation this instrument – with a major upgrade – has been relocated on this cold neutron guide and it was named ATHOS.

The ATHOS instrument provides moderate resolution (0.01-1.0 meV) with sufficient intensity for use in a wide range of problems. It is ideally suited for the study of phonon dispersion curves in single crystals, tunneling modes of energies greater than » 0.025 meV, quasielastic scattering studies of rotational and non-local diffusion in the time regime of picoseconds, vibrations of surfaces or molecules adsorbed on surfaces and phonon density of states for that large class of materials which contain hydrogen. Specific mention of the applicability of neutron scattering to the study of hydrogenous materials should be emphasized here. The hydrogen nucleus has the largest cross section (scattering interaction) for neutron scattering and is predominantly incoherent. Hydrogen vibrations have been detected in samples containing as little as 0.01 mol. of total hydrogen in the sample. Because the instrument is energy sensitive, it can also be used to measure purely elastic scattering whether it be due to coherent (nuclear or magnetic) or incoherent events. Information on the time-averaged structure of the atomic and molecular constituents of the sample is therefore accessible. Finally, the ability of producing and analyzing polarized neutrons allows more detailed measurement of the magnetic properties of the sample. These magnetic properties can be static, i.e., a structural description of the magnetic moments, or dynamic such as magnons.

The range of energies (0.025-14 meV) of excitations accessible to this instruments is substantially larger (although with poorer resolution) than available with the spin-echo and backscattering spectrometers. Independent control of the momentum (Q) and energy transfer (E) is routine if required as opposed to the time of flight spectrometer in which Q and E are related by the instrumental configuration. The monochromatic beam is provided by a 90mm high focusing multi-blade ZYA grade pyrolytic graphite monochromator.

The movable part of the monochromatic shielding has a chain type construction. Changing the incident wavelength the whole chain is driven by the monochromator-sample arm. This construction automatically provides the most effective shielding near the detector area (see Fig.1). This enables very low background conditions (1 neutron/300s).

For higher order filtering in the incident monochromatic beam a multidisc neutron velocity selector can be installed in front  of the sample goniometer, or Ge analyzer used. The beam divergence is determined by thin film Soller-type Mylar collimators coated with gadolinium-oxide paint. A 200x200 mm2 two dimensional position sensitive 3He detector in medium resolution mode was installed in 1999. Using this detector the efficiency of data collection has been raised 40 times in quasielastic mode.

Although this tree-axis spectrometer has been designed for structural and dynamical studies of condensed matter – because of the limited number of other operational equipment – the instrument is extensively used in a multi purpose regime, e.g. for high-resolution diffractometry, strain analysis, reflectometry, quasielastic and inelastic scattering. This spectrometer has been served also for testing polarization setups, detectors, monitors, any other neutron beam components developed in our Laboratory. For example a neutron spin-echo setup was also realized on this instrument (Fig.1). This “mini-spin-echo” can be used for training and methodical development purposes (Török Gy, Lebedev VT, Nagy A, Gordeev GP, Zsigmond G,  Physica Status Solidi C 1, No 11, 3182 (2004).

The main parameters of the spectrometer:

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Figure 1. Neutron spin-echo setup installed on the TAS