Neutron Scattering

initial velocity-reassemble at the saine vertical position they had at the start point . The
rotation imposed by the second 7r/2-flipper couverts this back to the initial longitudinal
polarization that is fully restored .
The flippers limit the the two race tracks and realize
"start", "time reversai" and "stop" of the "spin stop watches" . The second 7r/2-flipper is
the last element used to manipulate the spins .
It couverts the average precession angle to
a longitudinal polarization component .
Since the field after the second 7r/2-flipper is again
longitudinal, further precessions do not influence the analyzed longitudinal polarization
component (the stop watch is stopped!) .
The analyzer accepts neutrons of one longitudinal
spin state for the detector .
After ensemble averaging this means that the cotait rate at
the detector is proportional to (1 ~_ cos(T))/2 s, where T is the expectation value of the
angle between spin and axial direction .
11 .2 .1 Flippers
Ignoring technical details the main elements needed to perform the spin operations necessary
for a NSE spectrometer are :
o 7r/2-flipper (start)
first precessions field
o 7r-flipper ("time reversal" )
e second precessions field
7r/2-flipper
While in the precession fields the spin vector (its expectation value) continuously rotates
around the field vector on a cone with constant angle (angle field-spin, ideal value =
90°) even if the field which the neutron experiences during its flight performs (sufficiently
slow, i .e .
adiabatic) direction changes', the flippers rely on a sudden change in the field
3 The sign in front of the cosine depends on the technical realization of poiarizer
and analyzer (both reflecting, transmitting, one reflecting one transmitting) and on the
orientation of flippers . It may be selected by choosing the signs of the flipper currents .
4 The cone of precession follows the direction of the precession field quite accurately
if the effective frequency of the field rotation is
much smaller than the local precession
11-5