Neutron Scattering

8 .3 .1 . Pin-Hole SANS
The principle lay out of a pin-hole SANS is depicted in Fig . 8.2 . Alter the fission process,
thermalization, and a further moderation in the cold source the neutrons are guided through
neutron guides to the instrument . Monochromator and collimator are filters for neutrons with
a predetermined wave length and divergency . The collimator consists of two apertures of
Selector Apertures Sample Detector
Lc=1-20m
wiw
LD =1-20m
Figure 8 .2 : Principle design ofpin hole SANS
neutron absorbing material as fi . Cd and as monochromator one has a velocity selector,
delivering a monochromatic beam of wave lengths between 5 and 15 Â wich a relative mean
square deviation of about < 8 ;~ z > o.s / < ;, >=0.1 . After passing both apertures the neutrons
irradiate the sample and part of the neutrons are scattered. The thickness ofthe sample should
be adjusted in a way, Chat only about 10% of the neutrons are scattered in order to avoid
remarkable effects from multiple scattering . The scattered neutrons are counted in a two
dimensional local sensitive detector. The neutrons not scattered by the sample remain in the
primary beam and are absorbed in the beam stop in front of the detector. The resolution
function of this experiment is given as
Q 2 k [( dD ) 2 (dE ) 2 S ( 1 1 ) 2 2 ( 87,' ) 2 j .
12 LD L S L s LD <
(8 .5)
The symbols Ls und LD represent the distances between the two apertures and between
sample and detector, the symbols dD and dE the diameter of the two apertures . For a given
instrumental setting neutrons can be detected in a limited angular interval ; the setting is
adjusted by the distance between sample and detector between 1 .25 und 20 m leading for the
8-3