Spectrometer triple-axis neutron

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... 

Figure 1. Schematic of a triple-axis inelastic neutron spectrometer. The thermal-ized beam from the reactor is monochromated by Bragg reflection from crystal M. Neutrons that scatter from the sample S are energy analyzed by Bragg reflection from crystal A and enter detector D.

Inelastic neutron scattering, on the other hand, usually employs a monochromatic neutron beam and records the intensity of the scattered neutron beam as a function of neutron kinetic energy. Such inelastic collision spectra are monitored as a function of the applied field and the (usually low) temperature. The observed peaks then represent the energy differences of thermally populated and excited unpopulated multiplet states. Inelastic neutron scattering experiments can be conducted using triple-axis, backscattering, or time-of-flight spectrometers. 

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... 

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

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. 

Fig. tt.5-2tta,b BaTiOs. Triple-axis neutron spectrometer scans at constant frequency across the sheet of diffuse scattering at 150 °C. The path of the scans is shown in the inset, (a) shows the elastic scan (v = 0), where the high background level is due to nuclear incoherent scattering, (b) Ineleastic scan (v = 0.97 x 10 Hz)... 

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. 

These two experiments demonstrate the versatility of neutrons in probing the form of S(Q,a)) over a wide range of Q-(o space but the technique has not been more fully utilised due to the large experimental correction factors for multiple-scattering. Compared with other forms of spectroscopy, neutron studies with a triple axis spectrometer give very low data-collection rates. For this reason large samples are required and some compromise with respect of energy resolution is also necessary, it would seem that this technique is restricted due to the limitations in the incident, neutron flux. 

Neutron diffraction experiments were carried out by Triple Axis Spectrometer TAS-2 in addition to HRPD spectrometers equipped on JRR-3M installed on Japan Atomic Energy Research Institute (JAERI). Intensity distributions on the equator were measured at lOK, lOOK, ZOOK, and 300K by using X = 1.823 A. The ranges of... 

Shirane G, Shapiro SM, Tranquada JM (2002) Neutron scattering with a triple-axis spectrometer— basic techniques. Cambridge University Press, Cambridge... 

However, the neutron scattering cross sections depend on the momentum-transfer vector and vibrational displacements of scattering nuclei in normal vibrations, and the importance of measuring anisotropy of cross sections was emphasimd by Summerfield (1965). Accordingly, the anisotropy of down-scattering cross sections of a stretch-oriented sample of polyethylene was measured by Myeis, Summerfield, and King (1966) with a triple-axis spectrometer. As shown in Fig. VII.1, two peaks... 

Neutrons in this energy range are known as thermal neutrons, because typical energies correspond to room temperature see Table 1. Thermal-neutron INS spectroscopy was the first type to be studied and the instrument invented by Bertram Brockhouse in 1952, the triple-axis spectrometer, is stUl a mainstay of inelastic instrumentation at reactor sources. In 1994, Brockhouse, together with the pioneer of elastic neutron scattering (diffraction) Clifford Shull, received the Nobel prize for physics for his invention and subsequent work with it. 

Figure 8 General layout of the hot-neutron triple-axis spectrometer INI at the ILL.

The pulsed-source equivalent of a triple-axis spectrometer is demonstrated by PRISMA at ISIS. The difference is that a white beam of neutrons is incident on the sample, rather than a monochromated beam. As in the triple-axis spectrometer, Bragg reflection from analyser crystals is used to select the neutrons scattered by the sample that will ultimately be detected. However, PRISMA has an array of 16 independent analyser and detector arms instead of just one detector. Each arm of PRISMA measures along a parabolic path in (Q, w) space, so generating a two-dimensional scan through (Q, co) space for each ISIS pulse, in a single setting of the instrument and sample. 

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