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The Value of Neutrons |
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The Value of Neutrons
Generally speaking neutrons are less available and more expensive than X-ray
photons. However there are many reasons why specific in-situ studies
ought rather to be carried out using neutrons than X-rays, or perhaps using
both neutrons and X-rays (see Combined Techniques). An introduction to neutrons
for powder diffractometry has already been given in
Neutrons Sources & Methods. Here we will
just concentrate on those properties of neutrons that can be exploited in
dynamic in-situ studies.
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Penetration: Although energetic X-rays are very penetrating, the
penetrating power of neutrons can be tailored to some degree by the choice of
element. For example aluminium is relatively transparent to neutrons and can
often be a convenient metal from which to make specimen cells and
enclosures; on the other hand vanadium is a negligible incoherent neutron
scatterer and can be a useful material for making resistive heaters. These
features can be exploited in the design of complex environmental sample cells
for difficult experiments.
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Increased atomic contrast: The scattering of neutrons by different
atoms appears to be unpredictable by comparison to X-rays where simply the
heaviest elements with the most electrons scatter the most. If one element in
the material scatters much more than the other elements then that element will
dominate the behaviour of the powder diffraction pattern. One example is oxygen
which for example scatters neutrons more strongly than many heavier elements,
such as V, Co, Cs, and comparably to others such as Mo, Ag and Au.
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Deuteration: This is a very special case of the above, often applied to
water. Hydrogen, 1H, scatters neutrons only about half as strongly
as Deuterium, 2D, its isotope, and with a different phase direction.
But in addition 1H scatters neutrons much more incoherently
than 2D; this means that materials containing hydrogen will produce
neutron diffraction patterns with a high background (due to the 1H's
incoherent scattering); the deuterated equivalent material will have a much
lower background. Again many elegant experiments, exchanging partially or
completely between 1H and 2D, have been carried out to
reveal features of the water or OH content of a material.
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Magnetic materials: This is outside the scope of the present course, but
it should be noted that powder diffraction has been a useful tool in the study
of magnetic materials due to the additional scattering of the neutron by the
magnetic dipoles within magnetic material.
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Inelastic scattering: Whereas conventional diffraction by X-rays is said
to be elastic (meaning the diffracted X-ray photon has the same
energy/wavelength as the incident photon), neutrons generally have a measurable
inelastic scattering due to the neutron's energy being similar to the thermal
energy of the atoms within the material. This can provide more information on
atomic motion and vibration. However this aspect is outside the scope of this
course
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Enhanced atomic contrast (isotopic substitution): This is a clever trick
whereby the contrast of a chosen element is increased by using its isotope. For
example whereas ordinary iron, 56Fe, has a neutron scattering power
of 10 fm, its isotopes, 54Fe, 57Fe have quite
different scattering powers of 4 and 2 fm respectively. By changing the
isotope, or comparing between patterns using different isotopes (the "isotopic
difference method"), some very elegant results have been achieved but these
have been far more widely applied to the field of liquids and amorphous
materials than to crystalline powders.
A few examples of in-situ studies using neutrons are now presented in which
some of the above features occur; see if you can spot them.
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