Triple axis spectrometers

In order to determine the dynamics of atoms we have to carry out an inelastic neutron scattering measurement. With a reactor source this can be done with a triple-axis spectrometer, which has an analyzer crystal. Tripleaxis refers to the three axes for the monochromator, sample, and analyzer, all moving independently and controlled by a computer. With a pulsed source we use a mechanical chopper, which is a rotating cylinder with a hole perpendicular to the rotating axis that allows neutrons with a chosen range of velocity to go through. The neutrons scattered by the sample are detected... 

Shirane, G., Shapiro, G., and Tranquada, J.M., Neutron Scattering with a Triple-Axis Spectrometer. Cambridge University Press, Cambridge, 2002. 

Triple-axis spectrometers are generally used in studies where there is a need of high resolution and longer data collection times can be tolerated. These spectrometers are most often used to study neutrons... 

Measurements were made by Meyers, Donovan, and King (27) on Marlex 6050 of approximately 90% crystallinity using a triple-axis spectrometer. The results are reproduced in Fig. 4. In addition to the... 

The neutron diffraction measurements were conducted at the High Flux Isotope Reactor of Oak Ridge National Laboratory using a triple-axis spectrometer operated in the diffractometer mode. The details of the experimental method have been described elsewhere [10,14], Slits of dimensions... 

The original INS spectrometer invented by Brockhouse and coworkers [15] at Chalk River, Canada, was the triple axis spectrometer. A modem example, INI [16] at the ILL, is shown in Fig. 3.12. Triple axis spectrometers are optimised for coherent INS spectroscopy and are rarely used for incoherent INS work. These instmments were designed to exploit the constant flux of reactor sources. A detailed description of the theory and practice of triple axis spectrometry is given in [17]. 

Fig. 3.12 Schematic of the triple axis spectrometer INI at the ILL. Reproduced from [16] with permission from the Institut Laue-Langevin.

Filter instruments offer good intensity at modest energy resolutions and crystal analyser instruments offer good resolution with modest intensities. While a ciystal analyser instrument is conceivable at a continuous source (such an instrument would be a specialised form of the triple axis spectrometer ( 3.4.1) [17]) none have yet been constructed to exploit the advantages of low final energies. 

Clearly both types of instruments are highly complementary and both have strengths and weaknesses. Ideally, the same sample would be run first on an indirect geometry instrument which would provide a rapid, but still fairly detailed overview of the subject. In many instances this would be sufficient. Subsequent measurements on a direct geometry instrument would allow detailed aspects of the spectroscopy to be probed. Table 3.2 gives a list of INS (excluding triple axis) spectrometers that have recently been in operation, are in operation, or are planned. 

Table 3.2 INS instruments. (Triple axis spectrometers are excluded, since they are mainly used for coherent INS. A list can be found in [17]). 

A triple axis spectrometer ( 3.4.1) was employed in these measurements [10,11,12,13]. The vibrational calculations were performed on the primitive cell once its calculated Helmholtz free energy, F, had been minimised using the quasi-harmonic approximation. Quantum effects are present due to the lightness of both atoms [14,15]. 

Alternatively, the variation of the logarithm of the observed intensity, IniS", with Ing can be used to determine the nature of a given transition. This function is linear and has a slope of about two for one-quantum transitions and about four for two-quantum transitions. This method has been used on data from triple-axis spectrometers. 

The classical method of using neutrons to measure these dispersion curves involves the use of single crystal samples and a triple axis spectrometer... 

Figure 8.11 Schematic diagram of the triple-axis spectrometer. The three axes are at the monochromator crystal, the sample holder, and the analyzer crystal.

Measurements of dispersion curves provide information about the interatomic forces in solids. In fact, a dispersion curve is a function of the vibration frequency V on the wavelength X. The methods of neutron spectroscopy based on the phenomenon of diffraction of heat neutrons by crystals enables one to graph the dispersion curves of solids. The most accurate measurements of these curves are obtained by the inelastic neutron scattering using triple axis spectrometers. 

Phase separation in two orthorhombic phases with x 7.0 and 6.7 but common orientation was found with XRD high-resolution scans in a triple-axis spectrometer using Ge (111) crystals as monochromator and analyzer (YouHoydoo et al. 1988). 

Now spin echo methods are used for a small angle neutron scattering instrument and a triple axis spectrometer to increase the momentum- and energy transfer sensitivities. However, these instruments are not typical because they require novel techniques. 

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