Three-axis inelastic neutron scattering

Abstract

The three-axis technique continues to be a basic neutron scattering technique for inelastic work on single-crystal specimens. As discussed in section 3, there is, at the moment, a fair degree of complementarity between TAS instruments on steadystate sources and TOF instruments on steady-state and pulsed sources. For the study of structural excitations, there is also complementarity with the synchrotronbased IXS technique whose capabilities have dramatically improved in recent years. In the long term, the availability of more powerful pulsed neutron sources will certainly tip the balance more in favour of the TOF technique and the same probably applies to synchrotron sources and the IXS technique. On the other hand, the capabilities of the TAS technique in certain fields, such as the study of low energy magnetic excitations, with or without polarization analysis, are likely to remain unchallenged for some time in the future. As seen in section 7, there are a number of possible extensions of the TAS technique, which are presently under consideration or under development and which should lead to major gains in data acquisition rate in the future. There are other developments which aim at improving the quality of the information accessible from TAS experiments. One such development is the spherical polarization analysis (CRYOPAD) technique, [44] which was briefly mentioned in section 2. Another important development, which could not be covered within the framework of this chapter, consists in combining the TAS and NSE (neutron spinecho) techniques. NSE is widely used to study slow relaxation phenomena in solids (see chapter by R. Cywinski in this volume). It was suggested long ago [45,46] that it could also be applied to the study of collective excitations and that it would then permit inelastic work to be done, such as phonon linewidth studies, with the equivalent of a meV energy resolution. The feasibility of such inelastic NSE experiments was demonstrated by the pioneering work of Mezei [47] on the roton linewidth in superfluid 4He and by several more recent experiments on 4He [48] and Ge [49]. The combined use of a polarized-beam TAS instrument, acting as background spectrometer, together with a NSE set-up optimized for inelastic work, may open new fields of research in the near future. Several TAS spectrometers at HMI, ILL and FRM-II are presently being equipped with NSE coils (using the precession [50] or resonance [51] method) in order to explore the potential applications of the technique. © 2006 Springer.