Neutron Scattering

2
0 L Tr -0 L
2
ArDet
Figure 11 .1 : Spin rotations and setup of a generic NSE-spectrometer . Upper part : Spin
rotation, middle part : magnitude of the magnetic field, lower part : schematic setup of
the Jülich NSE-spectrometer .
rection) enter the spectrometer from the left . In the first so-called 7r/2-flipper the spin
is rotated such that on exit it is orthogonal to the longitudinal magnetic field of the
precession path . That defines the start of the "spin stop watch ", immediately after the
flipper a precession of the spins around the axial magnetic field begins .
The precession
frequency increases
during the approach to the centre of the main precession solenoid
where it reaches its maximum of up to a few MHz .
The accumulation of precession angle
continues-with decreasing frequency-until the neutrons reach the 7r-flipper close to the
sample (S) . The total precession angle at that point is :
where ~ = 27r x 2913 .06598 x 104 s-'/Tesla is the gyromagnetic ratio of the neutrons and
IBI is the modulus of the magnetic induction along the path 1 .
The "stop watch" does not proceed uniformly but with a position dependend frequency
that is proportional to the local magnetic field along the neutron path, see figure 11 .1 .
This may also be considered as a field dependend distance streching, which releases e .g .
the mechanical positioning accuracy requirements for the flippers since they are located
in low field regions .
The total number of precessions a neutron spin undergoes on passage
11-3