Neutron Scattering

determined by the maximum divergence of the neutrons according to the total angle of
reflection.
A further important criterion for the quality of a SAS diffractometer is the sharpness of
the primait' neutron beam prepared by the collimator . In Fig.8 .4 primary beam for a given
72
N
10-3
" Si~
:- , 2F,
r2
1 1 1
,
2F,/2
-20 -15 -10 -5 0 5 10 15 20
Position [cm]
Figure 8 .4 : Resolution curvefor a given configuration
configuration bas been depicted in a semi-logarithmic presentation. One recognizes, that at
twice the full half width the primary intensity has been decreased to values between
10 -5 und 10 -6 . This result demonstrates that the instrument is in a good condition ; the
background near the primary beam is sufficiently small, so that also in thfs region scattered
neutrons can be sensitively detected and analt'zed.
8 .3 .2 . Focusing SANS
The principle of a focusing SANS has been depicted in Fig.8 .5 . The monochromatic neutrons
enter the instrument through an aperture with a diameter of about lmm and enter the focusing
mirror with the full divergence of the neutron guide . From the mirror the neutrons are
reflected and in the focal point the neutrons are detected from a local sensitive detector . The
resolution element of this detector has the saine size as the aperture of about lmm. Just
behind the minor is the position of the sample . The instrument with the lengths given in the
8-5