Neutron Scattering

distribution (current sheets) that leads to a homogeneous field B, r / z in thé inside of thé
flipper .
For a sufficient length of thé flipper thé stray field emerging from thé ends of
thé flipper coil is negligibly small in thé beam area in front of thé flipper . To obtain
thé desired operation thé 7r/2-flipper has to
be embedded in an external longitudinal
field The resulting field in thé interior of thé flipper Bi ,, t
has an angle of 45° to thé
longitudinal axis .
The magnitude of thé field must be set such that thé neutrons perform
a precession of exactly 180° during thé time they need to transverse thé flipper .
In this
way a longitudinal spin vector is rotated into an orientation perpendicular to thé axial
field . Then thé precession cone has thé maximum angle of 90°, i .e . it is a disc (dial of thé
"stop watch") .
For a typical flipper thickness of 1 cm thé interior field is in thé order of
0 .1 mTesla = 10 Gauss (For comparison thé earth's magnetic field is ~ 0 .5 Gauss) . Since a
fixed precession angle around Bi t=const must be accumulated during thé passage time of
thé neutron through thé flipper, thé flipper function is moderately wavelength dependent .
For thé ease of setup and stability of operation of thé spectrometer it is
advantageous
that thé flippers are embedded in a comparatively small external field
(< 10-3 of thé
maximum precession field) . Le . close to thé flippers thé "spin clock" runs slowly and
small path différences due to positioning inaccuracy and thermal expansion only lead to
very small errors in precession angle .
The technical realization of thé 7r-flipper is identical to thé one of thé 7r/2-flipper, however
its function-as indicated by its naine-is différent . It performs a 180° rotation around
an axis perpendicular to thé beam axis (e .g .
a vertical axis) which is virtually parallel to
thé axis of thé flipper coil .
To do so thé internal field has thé saine magnitude as in thé
7r/2-flipper however thé embedding field
is close to zero and therefore thé internal field
vector is virtually vertical 5 .
a
Unfortunately it is not possible to embed thé 7r-flipper in a zero field environnent, as
everywhere in thé beam volume this would lead to a violation of thé adiabatic condition .
While thé beam enters such a zero field région if would suffer uncontrolled inhomogeneous
tilts and rotations of thé precession cone that would effectively lead to a depolarization .
Smallest external stray fields would exert a big influence on thé signal .
A defined spin
operation would not be possible .
For that reason a minimal field in thé range of a few
(1 . . . 2 Gauss) is mandatory not to loose thé defined polarization . Without further action
thé 7r-flipper function is deteriorated by this finite external field . But by a slight tilt of
11-7