Neutron Scattering

10000 -
8000 -
co 6000 -
C
ô 4000-
U
2000 -
0
-2 0 2 4
Phase current (40 turns) / A
Figure 11 .4 : Echo shape : count rate as function of the magnetic symmetry (oc phase coil
current) .
11.4 Experimental Procedures and Evaluation
In principle the information on S(Q, t) according to equation 11 .10 is contained in the ratio
of the intensities at the symmetry point and the average intensity (17/2) S(Q) . However
there are practical reasons that prevent the reliable setting of the symmetry point alone .
The location of the symmetry point (i .e . phase zero current in the phase coil) is extremely
sensitive to tiny variations of the magnetic environment caused e .g . by displacement of
larger iron parts at neighbouring instruments, movement of the crane of the instrument
hall and thermal displacements of coils .
Therefore, the position of the symmetry point has
to be measured as well as the intensity for each Q, t setting .
In figure 11 .5 the minimum
of single countings is indicated, intensity must be determined for three points Pl . . . P3
separated by a symmetry change corresponding to a quarter precession each . From these
three values it is possible to extract the average intensity I(Q, 0), the echo amplitude