NRSE method

To achieve an extraordinarily high energy resolution, exceeding the energy resolution of the standard triple-axis mode by two orders of magnitude, the neutron spin-echo (NSE) method uses the Larmor precession of polarized neutrons [1]. Two magnetic field regions one in front of the sample and a second one behind the sample are used. A key advantage of NSE spectroscopy is that the energy resolution does not depend on the beam monochromaticity since the energy transfer at the sample is encoded in the difference in Larmor precession angles before and after the scattering process.

In contrast to conventional inelastic neutron scattering techniques NSE spectroscopy accesses the intermediate scattering function thus collecting information about the dynamics of the scattering system in the time-domain rather than in the energy domain. The important consequence is that the instrumental resolution is not convoluted with the scattering function. Resolution effects are thus corrected for by dividing the NSE data by the resolution function. 

The neutron resonance spin-echo (NRSE) variant of the spin-echo method employs two radio frequency (RF) flippers each providing a static magnetic field B0 and a rotating RF field operated at a frequency in resonance with the neutron Larmor precession frequency in B0. Two RF flippers separated by a distance L from each other, act like a single homogeneous magnetic field oriented perpendicular to the scattering plane resulting in an effective field integral 2LB0. By doubling the number of compact RF flippers (so-called bootstrap coils) the field integral can be increased by another factor of two. To minimize additional rotation of the neutron polarization vector the RF flippers are housed in µ-metal shielding external magnetic stray fields.

To measure the linewidths of dispersive excitations with finite group velocity, the boundaries of the precessions fields have to be tilted relative to the neutron beam paths. This methodological approach is known as spin-echo focusing or phonon focusing [2]. Assuming that the excitation mode under investigation has zero linewidth, the inclination of the field boundaries results in identical Larmor precession angles for all inelastically scattered neutrons, irrespective of the finite slope of the dispersion. With the NRSE apparatus, these tilted fields can be realized very easily. The surfaces of the static field coils (B0-coils) define precise field boundaries. Rotating the RF flipper coils, which are installed on individually controllable rotation stages with vertical axes, allows to reach tilt angles exceeding 45° while retaining high transmission.

The high flexibility in adjusting the field boundaries also provides opportunities for elastic measurements with very high wavevector resolution. The Larmor diffraction method [3] allows to extract distributions of lattice spacings from the modulation of the neutron beam polarization with a very high sensitivity. Likewise relative changes in the lattice constants, e.g. as a function of temperature, are obtained from the phase shift of the modulated polarization signal. Choosing an opposite relative orientation of the precession fields, the mosaicity of the sample is precisely probed, an important input parameter also for the interpretation of inelastic NRSE data.

Fig. 1 shows the main components of the combined set-up of the triple-axis spectrometer FLEXX with the NRSE option. For the installation of the NRSE option two modules are additionally inserted, one between the monochromator and the sample and a second between the sample and the analyzer. These modules host the µ-metal shielding boxes (closed during operation) with the rotatable RF spin flipper coils. During the upgrade of the NRSE option newly designed coils have been installed, which allow for a larger beam cross section, higher frequencies and larger tilt angles.

NRSE Option at FLEXX

Fig. 1: Triple-axis spectrometer FLEXX with the NRSE option. Two pairs of resonant spin flippers in front of and behind the sample position are located in µ-metal shielding boxes (closed during operation). Tilted precession field regions which are required for dispersive excitations are easily realized by rotating the spin flippers around their vertical axes

References

[1] F. Mezei, The Principles of Neutron Spin Echo, in ‘Neutron Spin Echo’, Lecture Notes in Physics, Vol. 128, ed. by F. Mezei, 3-26, Springer, Berlin (1980).

[2] K. Habicht, R. Golub, R. Gähler, T. Keller, Space-Time View of Neutron Spin Echo, Correlation Functions and Phonon Focusing, in ‘Neutron Spin Echo Spectroscopy’, Lecture Notes in Physics, F. Mezei, C. Pappas, T. Gutberlet (Eds.), 116-132, Springer, Berlin (2003).

[3] M.T. Rekveldt, T. Keller, R. Golub, Larmor precession, a technique for high-sensitivity neutron diffraction, Europhys. Lett. 54 (2001) 342-346.