Neutron Scattering

then additional deactivation of the 7r-flipper (maximum) .
An ideal spectrometer would
have zero transmission in the one case and 100% transmission of neutrons in the other
case .
The non-ideal behaviour is caused by depolarization effects at the technical elements
of the spectrometer, the wavelength dependence of the flipper operations and the finite
efficiency of polarizers and analyzers .
Also for an ideal spectrometer being free from the
above effects a finite minimum and less than 100% maximum intensity will result for spinincoherent
scattering which is always accompanied by spin flips for 2/3 of the incoherently
scattered neutrons .
To account for the polarization losses the difference between the thus determined "up"
and "down" cotait rates is used to normalize the echo amplitude (instead of taking just
the average intensity) .
11 .5 Field Integral Homogeneity and Resolution
Besides the decay of S(Q, t)
as a consequence of the dynamical processes in the sample
the measured echo amplitude suffers a further reduction due to resolution effects (different
from the above mentioned depolarization effects) that must be accounted for in the data
evaluation .
Up to this point we tacitly assumed that the values of Jl and JZ are the saine for all
neutrons in the primary and scattered beams .
This would however only be approximately
true for very narrow beams which therefore would carry only very few neutrons .
Useable
beams must have a width of several cm and contain neutrons of different direction (divergence)
.
In particular the use of a large area sensitive detector leads to rather divergent
rays in the secondary arm .
Note that a field of 1000 Gauss=0 .1 Tesla acting along a track
of 2 m yields 3000 Hz/Gauss x 1000 Gauss x 2 m / 400 m/s = 15000 full precessions for
neutrons with a velocity of 400 m/s (A = 1 .Onm) .
The condition that the precession angle
accumulated along different rays in the beam must be equal within 0 .1 precessions then
translates into the requirement that the field integrals along the different rays must be
saine within 1 : 10 5 . As soon as the precession angles resulting from different rays differ
by 180° the signal ist lost completely.
required homogeneity by a factor 100 .
Simple cylindrical precession coils fall behind the
Only by use of special correcting elements ("Fres-