NRSE method
To obtain the highest possible energy resolution, the neutron spin echo method (NSE) uses the Larmor precession of neutrons. If the NRSE alternative to the spin echo method is used, two RF flippers with a static magnetic field B, located at a distance L from each other, act like a single homogeneous magnetic field with an effective field integral 2LB. By doubling the number of compact RF flippers (so-called bootstrap coils) the field integral can be doubled again. The polarisation vector of the neutrons after the coils depends on how long the neutron stayed in the magnetic field: The polarisation vector is precessed by a certain angle [1]. By combining the precession fields before and after the scattering process at the sample, the energy transfers can be measured with the highest precision.
To measure the line widths of dispersive excitations, the boundaries of the precessions fields have to be rotated relative to the neutron beam (phonon focussing). With the help of the NRSE method, these tilt fields can easily be realised, since NRSE coils define field boundaries precisely and can easily be rotated on goniometers. The high flexibility in rotating the field boundaries also allows for high resolution elastic measurements. Depending on the relative orientation of the fields, the so-called Larmor diffraction methods allows for high resolution measurements of the spacing between the lattice planes or the mosaicity of the sample [2].
The main components of the combined set-up of the triple axis spectrometer with NRSE option are shown in Fig. 1 (instrument before the upgrade). Inside the closed shielding boxes are the tilting RF flippers. Due to the coils used so far, the maximum beam cross section is limited to 30x30 mm2 (at a maximum tilt angle of 45°). The transmission of the polariser limits the lowest available wavelength for polarised neutrons to 3.7 Å. A rotating PG filter or a Be filter can be placed into the incoming beam and serves as a filter for higher orders.
References
[1] Mezei, F. ed. “Neutron Spin Echo”, Lecture Notes in Physics, Springer (1980)
[2] M. T. Rekveldt et al., Europhysics Letters, Vol. 54, pp. 342 (2001)
