Time-of-flight spin echo

Abstract

IN15 has been built in the framework of a collaboration between the ILL, HMI Berlin and FZ-Jiilich as an answer to the ever increasing need of higher resolution and higher intensity. NSE, unlike conventional quasielastic instruments (TOF or backscanering), measures S(q,t), the intermediate scattering hncrion. As described in ref. [1] this is achieved by using the Larmor precession as a clock attached to each neutron, thus decoupling monochromatization from energy resolurion. The generic layout of an NSE insrrument consists of a moderately (10–15% FWHM) monochromatic and polarized beam, a π flipper which flips the spins perpendicular to the magnetic field and starts the Larmor precession. In a first precession region (e.g. asolenoid) the spins accumulate a high number of turns (−l04). A π flipper close to the sample reverses the precession direction, followed by a second precession regon in which the spins wind back. Finally a second π flipper stops the precession and the scatrered beam goes through an analyzer (e.g. supermirrors) before reaching the detector. If the magnetic field integral of the two precession regions is the same, the initial polarization is recovered. A small energy exchange during the scattering process inthe sample will make the neutron precess more (energy loss) or less (energy gain) in the second half of the spectrometer. Integrating over all energy exchanges the recovered beam polarization isdiminished and becomes a measure of the quasielastic broadening. This way of measuring introducesan automatic Fourier transform of S(q,w) leading directly to S(q,t). The time constant is a function of the field inregral namely: