2.2 Experimental Methods
55
selection rules, which limit the observation of modes in Raman and infrared spectra,
do not apply to INS spectroscopy.
5. Weak interactions with matter
INS therefore is inherently weighted towards measurement of the bulk properties of
a sample, whereas optical methods are weighted towards surface properties.
However, these advantages are accompanied by a number of complicating factors,
including:
1. Momentum transfer
INS does not measure scattering from the centre of the Brillouin zone. While this
does not tend to be a large effect for internal molecular modes, it can lead to some
changes in frequency as compared to optical probes, which only detect the Brillouin
zone centre (i.e. long-range order) vibrations.
2. Neutrons interact weaker with matter than protons
INS requires much larger sample size, and longer collection times to obtain
vibrational spectra as compared to optical techniques.
3. Availability
INS is only possible at dedicated beamlines, located at neutron sources.
4. Temperature range
Neutron scattering is much more sensitive to temperature than optical probes. Typical
INS spectra are therefore obtained at ca. 10 K.
2.2.2.1 Generation of Neutrons
Neutrons can be produced from a variety of nuclear reactions:[67] fusion, photofission, fission, and spallation. The latter two are most common for scattering experiments. In a fission reactor (such as the ILL in Grenoble), neutrons are produced
by thermal fission of fissionable isotopes, typically
235 U. Thermal fission of this
235 U generates a continuous stream of high energy neutrons, which can be moderated to produce thermal neutrons. At a spallation source, high energy protons are
generated using a synchrotron. These protons bombard a metal target (e.g. tungsten
or tantalum), which triggers emission of a cascade of high energy neutrons. As the
protons are generated in pulses, so too are the neutrons at a spallation source. All
of the INS spectra used in this thesis were obtained at a spallation source: the ISIS
Neutron and Muon Facility, STFC Rutherford Appleton Laboratory, UK. Hence this
process will be considered further.
At the ISIS Neutron Facility, high energy protons are generated using a
synchrotron, and are directed at a tantalum coated tungsten target (at Target Station
55
selection rules, which limit the observation of modes in Raman and infrared spectra,
do not apply to INS spectroscopy.
5. Weak interactions with matter
INS therefore is inherently weighted towards measurement of the bulk properties of
a sample, whereas optical methods are weighted towards surface properties.
However, these advantages are accompanied by a number of complicating factors,
including:
1. Momentum transfer
INS does not measure scattering from the centre of the Brillouin zone. While this
does not tend to be a large effect for internal molecular modes, it can lead to some
changes in frequency as compared to optical probes, which only detect the Brillouin
zone centre (i.e. long-range order) vibrations.
2. Neutrons interact weaker with matter than protons
INS requires much larger sample size, and longer collection times to obtain
vibrational spectra as compared to optical techniques.
3. Availability
INS is only possible at dedicated beamlines, located at neutron sources.
4. Temperature range
Neutron scattering is much more sensitive to temperature than optical probes. Typical
INS spectra are therefore obtained at ca. 10 K.
2.2.2.1 Generation of Neutrons
Neutrons can be produced from a variety of nuclear reactions:[67] fusion, photofission, fission, and spallation. The latter two are most common for scattering experiments. In a fission reactor (such as the ILL in Grenoble), neutrons are produced
by thermal fission of fissionable isotopes, typically
235 U. Thermal fission of this
235 U generates a continuous stream of high energy neutrons, which can be moderated to produce thermal neutrons. At a spallation source, high energy protons are
generated using a synchrotron. These protons bombard a metal target (e.g. tungsten
or tantalum), which triggers emission of a cascade of high energy neutrons. As the
protons are generated in pulses, so too are the neutrons at a spallation source. All
of the INS spectra used in this thesis were obtained at a spallation source: the ISIS
Neutron and Muon Facility, STFC Rutherford Appleton Laboratory, UK. Hence this
process will be considered further.
At the ISIS Neutron Facility, high energy protons are generated using a
synchrotron, and are directed at a tantalum coated tungsten target (at Target Station