Neutron Scattering

is performed without Jacobian due to an asymmetry in definitions of S(Q, w), namely
du(20)/d52 = S(Q(20)) !
10.2 .4 Fermi chopper
One possibility to open the beam for a short time to create the required neutron pulse
consists in the placement of a short rotating collimator in the beam such that it transmits
neutrons only for a narrow interval of the rotation angle . The collimator consists of of
a parallel arrangement of neutron absorbing sheets (cadmium, gadolinium, boron) . The
gaps are filled with a material which is transparent for neutrons (aluminum) . Such a
chopper is sketched in fig . 10 .1, this type of chopper is called "Fermi chopper" . The
divergence Aa is determined by the distance between neigbouring sheets D and their
length l in beam direction, OcxFWHAI = arctan(D/l) (typical values are 1° . . . 2°) . The
duration of the opening OT is given by the rotation frequency 52
OaFWHM
Note that the neutron pulse frequency for a chopper with straight collimator slits is 252,
because the collimator axis is twice per revolution parallel to the beam axis . Silice the
neutrons have a finite velocity, the chopping collimator must not be too long, because otherwise
the transmission direction drops too muck even before the neutrons that entered,
rohen the collimator orientation corresponded to transmission, have emerged from the
collimator . It is also possible to use a "collimator" with curved slits, which accounts for
the above mentioned effect, however such a curved-slit-chopper exhibits a wavelength dependent
transmission characteristic . Sometimes this is a desired characteristic to suppress
unwanted orders of monochromator Bragg reflections .
10.2 .5 Disc choppers
Besides the Fermi choppers also so called "dise choppers" are used . Figure 10 .7 shows a
front view with respect to the beam direction of such a chopper dise .
The dise is covered by a neutron absorbing layer (dark grey) and contains -for counterbalancing
two- window zones, A and B . The beam cross section is indicated as striped area
in A . The typical diameter of such dises is 0 .5 . . . 1m . From the figure it is immediately
visible that the ratio pulse width to pulse-pulse distance for a copper of this type is rather