706 Chapter 12. Radiation Spectroscopy
which implies that the energy resolution has dependence on angular resolution of
the system as well as wavelength of neutrons and the atomic plane spacing of the
crystal. Triple axis spectrometers can typically provide up to about 1meV of
resolution, which is good for most applications. Further enhancement in resolution
is possible by using lower energy neutrons, the so called cold neutrons.
B.2 High Flux Backscattering Spectrometer (HFBS)
As we saw in the previous section, the energy resolution of a triple axis spectrometer
has a dependence on the spread in the wavelength of neutrons (see equations 12.3.18
and 12.3.18). It is apparent from equation 12.3.18 that one could in principle obtain
perfect resolution ifθ=π/2, which will giveδλ=0andδE= 0. This is the case
when neutrons are backscattered. Of course the ideal case ofθ= 0 for all neutrons
is not practically achievable but one can design an instrument such that most of the
neutrons are backscattered. In this case we will have
δE→0ifθ→π
2Practically speaking, this condition requires higher flux of incident neutrons as com-
pared to the case of triple axis spectrometer. That is why the spectrometer based
on this condition is calledhigh flux backscattering spectrometerof HFBS. Such in-
struments are able to offer energy resolutions on the order ofμeV.
B.3 FilterAnalyzerSpectrometer(FAS)
Filter analyzer spectrometer is similar to the triple axis spectrometer with on excep-
tion: the energy analyzer part of the instrument is made of afilter analyzerinstead
of a crystal analyzer. The analyzer is still made of a crystalline structure but is used
such that it filters out all the Bragg scatterings. In other words, it is assured that
very minimal Bragg scatterings occur in the crystal. For this, the neutron energy
is chosen such that the wavelength does not correspond to any of the atomic lattice
plane spacings. Generally one chooses very low energy neutrons that have very long
wavelengths, far away from the Bragg wavelengths of the crystal. With possibility
of elastic scattering minimized, the other two processes that could lead to neutron
loss are absorption and inelastic scattering. To avoid absorption of neutrons, one
uses a material composed of atoms having very low neutron absorption cross sec-
tions. And to minimize the possibility of inelastic scattering, the whole instrument
is cooled to liquid nitrogen temperature. The operation of FAS consists of detecting
filtered neutrons at a fixed final energy. The experiment is performed at different
incident neutron energies.
A sketch of the principle of FAS design in shown in Fig.12.3.4. Note its similarity
to the triple axis spectrometer. The filter assembly is made of several layers of filters.
Common materials used for filter are bismuth, graphite, and beryllium. The filter
and detector assembly is generally constructed such that it can be moved around the
sample. However, use of two fixed wedge shaped filter-detector assemblies removes
the need for additional movement and alignment structures.