Neutron Scattering

10.2 .6 Crystal monochromators
The monochromator indicated in fig . 10 .2 has the function to select neutrons from a
narrow band of velocities (approx .
1%) out of the Maxwellian spectrum of the incoming
primary beam . It may be realized by different means . First the crystal monochromator
is described . It uses the matter wave properties of the neutrons to select neutrons of a
defined wavelength (i .e . velocity) by interference in a crystal lattice (Bragg reflection) .
Neutrons with a wavelength of
A - 2d sin(OA11/2) (10 .13)
n
are reflected in direction of the sample; here d is the distance of lattice planes of the
monochromator crystal (often pyrolytic graphite 002, d = 0.6708nm), n the diffraction
order and OAvt the angle of reflection . Le . a simple crystal monochromator rotates the
beam direction by Ont ;
if the wavelength should be changed the bulky rest of the spectrometer
must be rotated around the location of the monochromator crystal.
Sec figure
10 .9 in the following section . 7 In addition Eqn . 10 .13 implies that generally several
diffraction orders are reflected .
To suppress the unwanted orders mainly three methods
are used :
1 . Filter : if the desired wavelength A is long enough it is possible to use a block of
polycrystalline material (mostly beryllium) which has negligible neutron absorption .
The shorter wavelength neutrons are Bragg reflected by some crystallites
in the
block and removed from the beam direction while the long wavelength neutrons
with A > 2d,ax are transmitted with low losses .
2 . Curved slit choppers : by curving the slits of a Fermi chopper it is possible to
achieve that the chopper is only transparent for a certain band of neutron velocities .
This band may be selected such that only the selected diffraction order is included .
3 . Second (coarse) chopper : by a second (coarse) chopper at seine distance from
the main chopper it is possible to select the desired diffraction order via the TOF
TThis may be avoided by the use of t%vo crystals in a parallel arrangement . The second crystal performs
a reflection that restores the original beam direction and rotation and translation of the crystals in such
a "double monochromator" are performed such that the direction and position of the monochromatized
beam stays the came for all wavelengths . However this advantage is connected with an intensity loss .
More about crystal monochromators may be found in the chapter "Crystal spectrometers" .
10-13