Neutron Scattering

11 . Neutron spin-echo spectrometer, NSE
Michael Monkenbusch, Reiner Zorn
11.1 Introduction
The attempt to increase the resolution of time-of-flight (TOF) instruments described in
chapter 10 far beyond the ~ 1% of the present realizations would lead to an unacceptable
loss of intensity (as expressed in terms of detector count rate) . E .g . a factor 0 .1 which had
to be applied as well to the monochromatization as to the pulse length (chopper opening
time) would on the one hand increase the resolution accordingly by a factor of about 10
but at the same time the intensity is reduced by O .lmonochr . x O .l eho pper = 10-2 . Still other
necessary measures to preserve the increased resolution as the reduction of sample size
(definition of flight path) are not contained in this reduction factor .
The same situation is
also given if monochromator and analyzer consist of crystals .
In general an improvement
of the spectral resolution requires the narrowing of the filter transmission functions before
and after the sample scattering by the desired improvement factor .
However this means
an intensity reduction by the square of the resolution improvement factor .
This situation
would immediately improve, if it would be possible to equip each neutron with an
individual stop watch which could be read in a way that the run time difference between
test tracks before and after the sample is obtained at detection .
If this stop watch has
a sufficient time resolution it would be possible to observe very small velocity changes
even if a beam with a wide range of initial neutron velocities is used .
This would allow
to escape the intensity trap .
In the neutron spin echo (NSE) spectrometer-with some restrictions (with important
consequences for the application)-it is indeed possible to use the neutron spin directions
as kind of individual stop watch pointers . The clockwork of this watch is then effected
by the precession of the neutron spins in an external magnetic field' .
The restrictions
1 It is somewhat involved to extract this analogy starting from a quantum mechanical
view with spin eigenstates and eigenvalues .
Implicitly we are talking about the behaviour
of the ensemble average of the spin vectors which obeys the "classical" Bloch equation