Neutron Scattering

possible neutron wave lengths between 5 and 15 A to a Q inteival of 10-'Â-'-0 .3Â-I . The
resolution has always been adjusted to the instrumental setting and which is determined by the
length of collimation L ., The space between selector and first aperture is always bridged by
Figure 8 .3 : Sample intensity for different length ofcollimation
neutron guides which in segments of lm length can be posed in or out the beam; in this way
the primary intensity can be remarkably enhanced by a beam with larger divergency . This is
shown in Fig .8 .3 where the measured primary intensity is depicted versus the length of
collimation . The optimized conditions of the instrument are achieved, when all elements of
resolution in Eq .(8 .5) contribute the same amount to the ,desmearing" of the scattered
intensity . Optimal conditions are accordingly obtained for the following instrumental setting :
LD =Ls und dE =d D =2d s . (8 .6)
One always tries to perform measurements with as much intensity as possible with
sufficiently good resolution . Such an optimized instrument has a resolution of
8Q opt.=(k/ -~3 ) - d,/LD (sec Egs.(8 .5) und (8 .6)) and delivers an intensity at the sample
according to
AI, =LAS2 L2 LD =L(Q) 4 L D . (8 .7)
D
The last relationship shows that the intensity at the sample is proportional to the square of its
length and is the reason of 40m long pin-hole SANS instruments . The upper limit is
8-4