Neutron Scattering - JUWEL - Forschungszentrum Jülich

RESEDA 11
The neutrons are counted in 3 He detectors. RESEDA uses four points of an echo group
to analyze the polarization. These four points are separated by a π/2-rotation of the spin
phase. The measured signals do not necessarily correspond to the exact maximum or
minimum of the amplitude. Nevertheless this method allows to obtain the polarization
in a short measurement time with good statistics and without knowing the exact phase
of the neutron spin.
The measurements of the phase of the neutron spin may be disturbed by external
magnetic stray fields. Therefore, the spectrometer arms and sample region of RESEDA
is surrounded by a double Mu-metal shielding (very high permeability of μr ≫ 10000)
which compensates small stray fields.
4.2 The samples under study
At RESEDA, quasi-elastic dynamics in the spin echo time range 1.5 ps to 2 ns is
routinely studied. In the laboratory course, two samples will be used. A standard
elastically scattering sample is used to verify the operation of the instrument, and, in
particular, to check the polarization of the spin rotation signal and the spin echo signal.
The quasi-elastic measurements are performed on a magnetically scattering sample
which exhibits helical magnetic ordering (figure 8) below the temperature of 28.5 K.
The quasi-elastic dynamics are studied slightly above this temperature.
Figure 8: Schematic magnetic ordering structure of MnSi below 28.5 K. The spins form a helix with
wavelength λ (therefore the name „helical phase“). a is the lattice spacing of the MnSi crystal.
5 Preparatory Exercises
1. Read this instruction carefully.
2. Calculate the energy and velocity of the neutrons accessible at RESEDA (cold
source!). Calculate the time the neutrons need to pass the spectrometer arms
(total length 8 m).
3. Look up the typical value of the earth‘s magnetic field. Calculate the precession
frequency of the neutron spin in this field (gyromagnetic ratio!). Conclude with
2. on the spin phase accumulated at RESEDA, if there was no mu-metal
shielding.