Tables for
Volume C
Mathematical, physical and chemical tables
Edited by E. Prince

International Tables for Crystallography (2006). Vol. C, ch. 4.4, pp. 442-443

Section Flipping of the polarization direction

I. S. Andersona and O. Schärpff Flipping of the polarization direction

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The term `flipping' was originally applied to the situation where the beam polarization direction is reversed with respect to a guide field, i.e. it describes a transition of the polarization direction from parallel to antiparallel to the guide field and vice versa. A device that produces this 180° rotation is called a π flipper. A π/2 flipper, as the name suggests, produces a 90° rotation and is normally used to initiate precession by turning the polarization at 90° to the guide field.

The most direct wavelength-independent way of producing such a transition is again a non-adiabatic fast passage from the region of one field direction to the region of the other field direction. This can be realized by a current sheet like the Dabbs foil (Dabbs, Roberts & Bernstein, 1955[link]), a Kjeller eight (Abrahams, Steinsvoll, Bongaarts & De Lange, 1962[link]) or a cryoflipper (Forsyth, 1979[link]).

Alternatively, a spin flip can be produced using a precession coil, as described above, in which the polarization direction makes a precession of just π about a direction orthogonal to the guide field direction (Mezei, 1972[link]). Normally, two orthogonally wound coils are used, where the second, correction, coil serves to compensate the guide field in the interior of the precession coil. Such a flipper is wavelength dependent and can be easily tuned by varying the currents in the coils.

Another group of flippers uses the non-adiabatic transition through a well defined region of zero field. Examples of this type of flipper are the two-coil flipper of Drabkin, Zabidarov, Kasman & Okorokov (1969[link]) and the line-shape flipper of Korneev & Kudriashov (1981[link]).

Historically, the first flippers used were radio-frequency coils set in a homogeneous magnetic field. These devices are wavelength dependent, but may be rendered wavelength independent by replacing the homogeneous magnetic field with a gradient field (Egorov, Lobashov, Nazarento, Porsev & Serebrov, 1974[link]).

In some devices, the flipping action can be combined with another selection function. The wavelength-dependent magnetic wiggler flipper proposed by Agamalyan, Drabkin & Sbitnev (1988[link]) in combination with a polarizer can be used as a polarizing monochromator (Majkrzak & Shirane, 1982[link]). Badurek & Rauch (1978[link]) have used flippers as choppers to pulse a polarized beam.

In neutron resonance spin echo (NRSE) (Gähler & Golub, 1987[link]), the precession coil of the conventional spin-echo configuration is replaced by two resonance spin flippers separated by a large zero-field region. The radio-frequency field of amplitude [B_{1}] is arranged orthogonal to the DC field, [B_{0}], with a frequency [\omega = \omega _{L}], and an amplitude defined by the relation [\omega _{1}\tau] = π, where τ is the flight time in the flipper coil and [\omega _{1} = \gamma {B}_{1}]. In this configuration, the neutron spin precesses through an angle π about the resonance field in each coil and leaves the coil with a phase angle [\varphi]. The total phase angle after passing through both coils, [\varphi =2\omega L/v], depends on the velocity v of the neutron and the separation L between the two coils. Thus, compared with conventional NSE, where the phase angle comes from the precession of the neutron spin in a strong magnetic field compared with a static flipper field, in NRSE the neutron spin does not precess, but the flipper field rotates. Effectively, the NRSE phase angle [\varphi] is a factor of two larger than the NSE phase angle for the same DC field [{B}_{0}]. Furthermore, the resolution is determined by the precision of the RF frequencies and the zero-field flight path L rather than the homogeneity of the line integral of the field in the NSE precession coil.


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