Neutron Scattering - JUWEL - Forschungszentrum Jülich

8 W. Häußler
rf coil. Brf points perpendicular to Bz and rotates in the x-y-plane (around Bz) with the
frequency ωrf.
The neutrons entering the B0 coil start to precess with the Larmor frequency ωLar = γ Bz.
The frequency of the rf coil is adjusted to be in resonance with ωLar (being the origin of
the name NRSE):
ωrf ≡ ωLar. (14)
In addition to its precession around Bz, the neutrons precess around Brf as soon as they
enter the rf coil. This additional motion with frequency ωR is called Rabi oscillation
(figure 4). After a flight time of t and a precession of π = ωR · t, the neutrons leave the
magnetic field of the coil. By this, the spin of the neutrons is flipped with respect to the
field direction of Brf. This is the so-called resonant π-flip.
After the field free flight path the neutron enters the second NRSE coil (directions of
both magnetic Bz fields are identical) and the neutrons´ spin performs a second resonant
π-flip.
Figure 4: The spin s is initially in the x-y plane. As we consider the situation in the Larmor rotating
coordinate system, the spin rests without the presence of a rf field. If we add the rotating field Brf , with
rotating frequency being in resonance with the Larmor frequency, this field in the rotating coordinate
system and the spin precesses with the Rabi frequency around Brf. After a half-twist, it is back in the x y
plane, mirrored at Brf.
The naming π-flip becomes clear when the case is considered, where the initial spin
direction is parallel to Bz. In this case, the opening angle of the Larmor precession
around Bz is zero. However, the precession with respect to Brf takes place as discussed
above. The angle between spin and Brf is now exactly π/2 =90°, and the spin performs a
rotation by π =180° around the axis given by the Brf field. This corresponds just to a