Neutron spin-echo spectrometers

Fig. 2. Schematic representation of a neutron spin-echo spectrometer. (Reprinted with permission from [12]. Copyright 1987 Vieweg and Sohn Verlagsgemeinschaft, Wiesbaden)...

Fig. 3. The neutron spin echo spectrometer IN11 at the ILL Grenoble. The two large coils producing the precession fields are clearly visible (Photograph was kindly given to us by H. Biittner, Scientific Coordination Office of the Institute Laue-Langevin Grenoble).

Figure 8.14 Schematic drawing of a neutron spin-echo spectrometer.

Having been acquainted with the components of neutron scattering instruments, two types of complete instruments will be described briefly here one is simple, a standard powder diffractometer, while the other is one of the most complicated, a neutron spin echo spectrometer. 

The main point about neutron spin echo is the manipulation with neutron spins, which has to be carried out rather precisely this requirement dominates the instrumental realization. (Note that neutron spin echo spectrometers, at present, can only be found at reactor sources.) A schematic drawing of a generic spin echo instrument is given inO Fig. 29.7. 

Higgins et al. measured f) for polytetrahydrofuran in CSj and in the melt. Measurements were taken on both the conventional back-scattering spectrometer, giving S(Q,w), and the new neutron spin echo spectrometer (NSE), which measures S Q,t) directly. The agreement between the two sets of data was good and it was found that in solution and Q J- - - in the melt, with the melt values... 

Figure 3 (a) Schematic of a Neutron Spin Echo spectrometer. The various components are discussed in the text, (b) Motion of the neutron spins in the precession region, looking downstream along the beam direction, (c) Motion of the neutron spins looking perpendicular to the beam direction. (Reproduced with permission from Reference (9J.)... 

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...

Figure 7 Schematic diagram of a neutron spin echo spectrometer. The direction of the neutron spin Is shown as it traverses the spectrometer. Reproduced with permission of Adenine Press from Ferrand M (1997) Neutron instrumentation in studying dynamics of biomolecules. In Cusack S, Buttner H, Ferrand M, Langan P and Timmins P (eds) Biological Macromolecular Dynamics. Schenectady Adenine Press.

Nicholson LK (1981) The neutron spin-echo spectrometer a new high resolution technique in neutron scattering. Contemporary Physics 22 451-475. 

In place of the diffractometer discussed in Section 2.5.3, a spectrometer is used, which allows measurement of the energy spectrum of scattered neutrons at different scattering angles. There are four main types of spectrometers in use today, the tripleaxis spectrometer, the time-of-flight spectrometer, the back-scattering spectrometer, and the spin-echo spectrometer, each of which is briefly described in the following section. 

Colub, R. Gahler, R. A neutron resonance spin-echo spectrometer for quasielastic and inelastic scattering. Phys. Lett.. A 1984. 123, 13-48. 

Fig. 2. Time (x) and scattering vector (Q) raqge for various scattering experiments on dynamics SANS ILL spectrometers Dll and DI7 relaxation after stepstrain, cyclic experiments, steady couette shear. Elastic neutron scattering ILL spectrometer D20, also real time experiment. Neutron Spin Echo (NSE) ILL spectrometer, inelastic measurement and classical quasielastic light scattering (QELS) (from Ref. )...

Three high-resolution inelastic instruments are nearing completion at the NIST Center for Neutron Research (NCNR) (1) a disk chopper time-of-fiight spectrometer (2), a backscattering spectrometer (3), and a neutron spin echo (NSE) spectrometer. All three instruments will be available to researchers through reviewed proposals (4). Both the NSE and Backscattering spectrometers will be operation in early 1999 and the Disk Chopper Spectrometer shortly thereafter. 

Thus rapid motions require relaxed resolution, while slower motions require high resolution. Note that these are relative terms typically an energy resolution of better than 1% AE/E and 100 peV is required. The timescale to be probed can be separated into three regimes, each of which uses a different technique for r 10" s, AE is 10-100 peV and direct-geometry time-of-flight is used, for t 10" s, AE is 0.3- 20 peV and a backscattering crystal analyser is used, for t 10" s, AE is 0.005-1 peV and neutron spin echo is used. Examples of each type of spectrometer will be considered. 

Another feature that is unique to neutron spin echo is that the signal, which is the neutron polarization at the echo point, is directly proportional to the intermediate scattering function, I Q, t), which is the Fourier transform of the scattering function S Q, a)). This is in contrast to the other types of spectrometer that measure 5(Q, ). It is often advantageous to calculate I Q, t) rather than 5(Q, w). 

Figure 7 This figure maps out the length and energy scales that are accessible with the different inelastic neutron scattering techniques. In the text, we have limited our discussion to the time-of-flight (TOF) spectrometers, backscattering (BS) spectrometers, and spin echo spectrometers.

Michael Monkenbusch studied physics at the University of Munster and finished with a thesis work on neutron scattering on absorbed molecules. After 4 yeare at the Institute of Macromolecular Chemistry in Freibui working on electrically conducting polymere, he entered the Institute for Solid State Research (IFF) in Jiilich as a researcher. There he was responsible for building the neutron spin echo (NSE) spectrometer and its exploitation for a variety of soft matter problems from polymer melt dynamics to protein motion. 

Now spin echo methods are used for a small angle neutron scattering instrument and a triple axis spectrometer to increase the momentum- and energy transfer sensitivities. However, these instruments are not typical because they require novel techniques. 

---