Neutron Scattering - JUWEL - Forschungszentrum Jülich

RESEDA 5
various length values l according to the positions shown in figure (1). In practice, B1 is
scanned and L1 is kept constant, because it is easy to be technically realized.
After passing the second magnetic field B2 behind the sample, which is antiparallel to
B1, the spin phase can be written as:
ϕ(v) = ϕ1 (v) +ϕ 2 (v) = γ L1B ⎛ 1
⎝
⎜
v − L2B2 v
⎞
⎠
⎟
As shown in figure 1 the antiparallel field B2 leads to a back precession of the neutron
spin, denoted by the negative sign in equation (4).
Figure 3: Spin echo group measured at RESEDA. The spin echo point represents the point where the
magnitude of the magnetic field integrals and, consequently, the number of precessions in both magnetic
field regions are equal. In the ideal case, the polarization reaches the value 1 in the spin echo point.
In the case of L1B1 = L2B2 the total spin phase of the neutron vanishes (φ = 0). According
to equation (3) this leads to a polarization of Px = 1. Varying one of the magnetic field
integrals L1B1 or L2B2 and measuring for each set of field integrals the polarization
renders the so-called spin echo group. Figure 3 shows a spin echo group measured at
RESEDA. Here, the magnitude of the magnetic field B1 is varied, whereas B2, L1 and L2
are fixed. The center point, where L1B1 = L2B2 and the polarization Px reaches its
maximum, is called the spin echo point. At both sides of the spin echo group, the
polarization decreases, as the envelope of the spin echo group equals the envelope of the
spin rotation.
2.2 Inelastic and quasi-elastic scattering
In the case of inelastic scattering the kinetic energy and, consequently, the velocity of
the neutrons changes. Equation (4) then reads:
(4)