Spin flipper

Figure 2.11 shows the profiles of the polarized beam, taken by a Stern-Gerlach dipole magnet, for the adiabatic (a) and (c) and the diabatic (b) and (d) setting of the spin flipper, each taken without (a) and (b) and with the central stop (c) and (d) at the polarizing sextupole exit. Evaluation of the profiles of Fig. 2.11c and 2.11d... 

Fig. 2.10 a Spin motion in the diabatic case (schematically). b Spin motion in the adiabatic case with an additional transverse field of 100 pT applied in the center of the spin flipper... 

Fig. 2.11 Beam profiles of the metastable He beam a without central stop, adiabatic mode of the spin flipper b without central stop, diabatic mode c with central stop, adiabatic mode d with central stop, diabatic mode...

Fig. 2.1 Spin history leading to the formation of the spin-echo. Longitudinally polarized neutrons enter from the left. Upper part spin motion. Lower part NSE setup, Ti/2-flipper between belonging current rings, primary main precession solenoid l symmetry scan...

The NSE principle - as described above - only works if the neutron spin is not affected by the scattering process (some exceptions hke complete spin-flip could be tolerated and would just replace the n-flipper). A related problem occurs if the scattering nuclei have a non-zero spin and their scattering power depends significantly on the relative orientation of nuclear and neutron spin. 

Fig. 2.2 A 7i/2-flipper effecively rotating the spin direction from longitudinal (S) to perpendicular to the path and to the external field. The vertical field B /2 generated inside the flipper adds to the embedding external field...

The most expensive parts of a conventional NSE instrument are the main solenoids providing the precession field. A closer look at Bloch s equation of motion for the spins (Eq. 2.11) shows that in a coordinate system that rotates with the precession frequency around Bg the spin is stationary, the coordinate system rotation is equivalent to the addition of - to all magnetic fields. By this means the large precession field inside the main coils may be transformed to zero - zero field spin-echo). The flippers are viewed as elements rotating... 

The complete instrument consists of two flippers with distance L before the sample and a symmetric set of two flippers after the sample. All flight paths between the first and the fourth (last) flipper are surrounded by magnetic shielding to yield the zero field condition that preserves the spin orientation. All RF flippers are operated synchronously, i.e. with the same current. Their field direction (i.e. rotation) is +, +. A neutron that enters the first flipper at a time... 

An interesting variant of the resonance NSE is the so-called MIEZE technique [ 19]. Using two RF-flippers that operate at different frequencies a neutron beam is prepared such that a special correlation between a time varying spin rotation co= Qi-Q2) the velocity of the neutrons is achieved. An analyser after the second RF-flipper translates the spin rotation into an intensity modulation. The... 

Initial wavelength selection is done by a mechanical (rotating) velocity selector, since energy resolution is not an important issue here. Polarized neutrons may be produced by reflection by a magnetic multilayer mirror. Essential parts are the so-called flippers. These devices, consisting of flat coils, can change the direction of the neutron spin (which is extremely sensitive to external magnetic fields). 

Si-crystal 3) phase-shrfter 4) detector 5) ancillary equipment or 1) monochromator 2) polarizer 3) pi/2-flipper 4) multi-layer spin splitter 5) pi-flipper 6) analyzer 7) detector 8) ancillary equipment 200,000 equipment ... 

Spin-echo 1) polarizing monochromator 2) pF2 flippers 3) pi-flipper 4) precession coils 5) analyzer 6) detector 1M 1) cooling system 2) magnetic coil system 3) ancillary equipment 1) magnetic coil system 2) computer system... 

The manipulation of the neutron spins through the spectrometer (8) is shown in Figure 3. After the beam is polarized, the neutron spins are rotated 90 by a jr/2 flipper, which begins the precession of the neutron in the main solenoidal field. 

Figure 4- SE signal as a function of the phase difference between the incident and scattered beams. The upper part of the figure shows the principle difference, in the case of deuterons and protons, between the scattering from a nucleus with and without nuclear spin. The lower part shows the NSE signals obtained in both cases—the count rate is plotted against the current of the phase correction coil. Acob und vdincoh ore the echo amplitudes for coherent and incoherent scattering, No is the average count rate outside the echo, N+ and iV are the maximal and minimal count rates with the ir/2-flippers on, and Nap and iVdown are the count rates of spin-up (rr-flipper off) and spin-down (ir-flipper on) measurements made with the nl2-flippers off.

Figure 13J A schematic diagram ofa neutron spin-echo spectrometer. The difTerence in velocities of the polarised neutron brfore and afta the scattering process can be observed by measuring the precession in the regions of uniform magnetic Md H. The difference in the precession is easily determined from the polarisation of the beam readiing the detector D if the fields before and after the sample S are symmetrical and the polarization is inverted with the flipper coil marked as n/2. The coils marked n/4 are used to provide magnetic fields that define the initial and final states of polarisation...

The action of the a-flipper rotates the neutron spin ensemble such that effectively Inserting the resulting total... 

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