Neutron backscattering spectroscopy

Abstract

Neutron backscattering (BS) spectroscopy is an important scattering technique for achieving high energy resolution and thus for accessing long times. What does backscattering mean? The term "backscattering" signifies, that the neutron energy is determined by Bragg reflection from crystals under a Bragg angle Q of 90' in order to minimize the energy resolution. To avoid misunderstanding among newcomers, we note that backscattering is not related to the scattering process at the sample (BS measurements are of course possible at small scattering angles). Thus "backscattering" concerns only the neutron optics such as monochromators and analyzers. As we will see later, the principle is to define the incident wave vector | ki | precisely and to vary its length for energy analysis, whereas the final wave vector | kf | is always kept constant. The neighbouring neutron instruments of BS in phase space are neutron spin echo (NSE) and time-of-flight (TOF) instruments. Neutron-BS, like TOF, measures in frequency space, thus determines S(Q,w), whereas NSE measures in time space and determines S(Q, t). Neutron-BS is a unique technique for measuring low lying, in frequency peaked excitations down to energy transfers of 0.2 meV and it is complementary to NSE with respect to quasielastic scattering. Concerning inelastic X-ray spectrometers, there is no equally versatile spectrometer with similar energy resolution today. We show in figure 1 the energy-momentum-range (phase space or (Q,w)-range), which can be covered by neutron backscattering. The different dark grey areas belong to the already available regions on reactor-BS (r-BS) instruments. The lightgrey areas indicate possible extensions. The Q-range is normally covered for BS in one run simultaneously (e.g. not so for IN15) and one can potentially access a time range between 0.01 microseconds and 100 picoseconds. All regions are limited by the instrumental resolution towards the low energies. On TOF spectrometers, in order to achieve a high energy resolution, one has to work with a long wavelength, which consequently limits the maximum Q strongly. We compare the NSE spectrometer IN15 in its longest time resolution mode and IN11 optimized for short times. These spectrometers cover with an excellent energy resolution a wide time range. © 2006 Springer.