Physics and Engineering of Radiation Detection

12.3. Neutron Spectroscopy 699

escape depth of these electrons is not more than a few nanometers. This is a good
thing in the sense that one can then analyze the surfaces without worrying about
the interactions deep inside the material.
It is also possible to use electron beams for spectroscopic analysis. Theelectron
energy loss spectroscopyor EELS is one such technique. Here, the material under
investigation is bombarded with electrons. Some of these electrons suffer inelastic
scatterings with the atoms and cause electronic transitions. The resulting spectrum
can then be used to determine the chemical structure of the sample.

12.3NeutronSpectroscopy

Spectroscopy of neutrons is an emerging and a very active area of research. Its
usefulness in materials research and chemistry has been well established. The biggest
advantage of neutrons as a material probe is their high penetration power, owing
to their neutral electrical charge character. In this section we will look at different
techniques and instruments used in spectroscopy with neutrons. But before we do
that, let us quantitatively see why neutrons are useful for spectroscopy.

12.3.ANeutronsasMatterProbes

In the first chapter of this book we discussed the idea of wave-particle duality.
We saw that sometimes particles behave as waves and can actually be assigned
a wavelength. This wavelength is related to particle’s momentumpthrough the
relation

λ=

h

p

, (12.3.1)

wherehis the usual Planck’s constant. This relation can also be written in terms
of particle’s energy by noting that

E=

p^2

2 m

, (12.3.2)

wheremis the particle’s mass. Substitutingpfrom equation 12.3.5 in this relation
gives

E=

h^2

2 mλ^2

. (12.3.3)

For a neutron this relation reduces to

En =

(

6. 626 × 10 −^34

) 2

(2) (1. 6749 × 10 −^27 )λ^2

=

1. 3106 × 10 −^40

λ^2 n

J. (12.3.4)

whereλnis in units of meter. It is more convenient to transform this relation such
thatλncan be input in units of angstroms and the energy is obtained in units of
electron volts. For this, we multiply the above relation with joules to electron volts