12.3. Neutron Spectroscopy 707
Ei
2θs
2θm
ki kf
Ef
Sample
Neutron Source
Monochoromator
Analyzer
Detector
Figure 12.3.4: Principle of filter analyzer spectrometer. The whole filter assembly
is kept at liquid nitrogen temperature to avoid inelastic scattering of neutrons. The
filter is generally made of layers of beryllium, graphite, and bismuth.B.4 DiskChopperSpectrometer..................
Disk chopper spectrometer is an instrument that uses the time-of-flight information
of neutrons to determine the inelastic scattering spectrum of the sample. Its working
principle is fairly simple: mono-energetic neutrons are allowed to pass through the
sample and the scattered neutrons are detected through a large array of detectors.
The times of arrival of the neutrons is used to determine the type of scatterings
they have gone through. The basic idea is that the neutrons that gain energy during
the scattering process arrive earliest. They are followed by the elastically scattered
neutrons. The neutrons that lose energy during collisions arrive latest. This implies
that the timing information can give insight into the dynamics of scattering.
The principle design of a disk chopper spectrometer is shown in Fig.12.3.5. Such
an instrument is generally designed to work on a nuclear reactor where high intensity
beam of neutrons is available. The experiment is performed in neutron bursts. The
reason is that the precise time or arrival of neutrons at the sample is required for
proper time-of-flight measurements. Since the neutron sources produce neutrons
continuously therefore DCS uses a chopper assembly to produce burst of neutrons,
hence the name disk chopper spectrometer.
A typical disc chopper spectrometer has about 1000 neutron detectors for time-
of-flight measurements. Since the detectors are used for timing measurements, their
timing resolution is of high importance.
An important practical consideration of disk chopper and similar instruments is
that they should have proper beam stop assemblies. The reason is that a typical
neutron source used in such an experiment produces an intense beam of neutrons.
Not all of these neutrons get reflected from the sample. In fact, a good fraction
of these neutrons simply pass through the sample without undergoing any interac-
tion. These neutrons have to be stopped for protection of personnel and safety. In
Fig.12.3.5 such a beam stopping assembly is symbolically represented by a black
circle.