704 Chapter 12. Radiation Spectroscopy
S(Q, )
ω
ω
InelasticQuasielasticElasticFigure 12.3.2: Typical variation of neutron scattering func-
tion with respect to the energy transfer. The contributions of
elastic and inelastic scatterings are clearly visible as distin-
guishable peaks while that of quaielastic scattering appears
as broadening of the elastic peak.Time-of-Flight- Disk Chopper Spectrometer
- Fermi Chopper Spectrometer
Larmor Precession - Spin Echo Spectrometer
A comprehensive discussion on these spectrometers is out of the scope of this
book. However it is worthwhile to briefly introduce the reader to the principles of
their designs. The interested reader can then refer to advanced texts on neutron
spectroscopy to obtain further details.
B.1 Triple-AxisSpectrometry(TAS)
As the name suggests, this instrument is based on three axes to determine the wave
vectors. The principle of TAS is shown in Fig.12.3.3. The spectrometer consists of a
monochromator, a sample holder, an anlyzer, and a detector. All these components
can be rotated along their axes.
The neutrons emitted by the source are elastically scattered by the monochro-
mator, which works on the principle of Bragg diffraction. We have already studied
that the Bragg condition of constructive interference is given by
nλi=2dmsinθm, (12.3.12)wheredmis the atomic plane spacing of the monochromator,λiis the wavelength
of the neutrons scattered off the monochromator, andθmis the monochromator