Neutron Scattering

through one arm of the spectrometer lies between 10 and some 10 4 .
300
0
Figure 11 .2 : Field along the axis of a main precession coil .
Close to the Sample (ideally : at the sample position) the so-called 7r-flipper is located, it
rotates the spins by 180° around a vertical axis . In this way the total precession angle is
transformed to `F1 = + ce --> -ce . The precession angle ce is-according to equation 11.1-
extremely dependent on velocity and therefore very different for different neutrons in
a beam with finite width of the wavelength distribution . As a consequence the spin
vectors at the sample position (7r-flipper) are evenly distributed on a disc orthogonal to
the field direction . If no velocity change occurs during scattering at the sample (elastic
scattering) each neutron enters the secondary arm of the spectrometer with unchanged
velocity. The precession field and path length of the secondary arm exactly match the
corresponding elements of the primary arm before sample and 7r-flipper . Accordingly, the
precession accumulated in the secondary arm is q~ 2 = +ce and the total precession angle
at the second 7r/2-flipper is q~ 1 + kP2 = - a + a = 0 . Le . all spins-irrespective of their
11- 4