The Spin Echo Spectrometer

The normalized dynamic structure factor thus gives the probability that a scattering event occurs at a certain wavelength change 8X = (m/2n) 3co at a given momentum transfer Q. The Fourier time t in the argument of the cosine is determined by the transformation from the phase angle Acp = p2 — cpi to co. The numerical relation reads 

The length L of the precession field is measured in meters, the guide field H in Oersted and the wavelength X in Angstrpm. 

This averaging process somewhat obscurses the relationship between Pf and S(Q,t). For many relaxation processes, however, where the quasi-elastic width varies with power laws in Q, the smearing of (12) is of no practical importance. For example, for internal relaxation of polymers in dilute solution, we have S(Q, t) = S(Q2t2/3) [34]. Since Q varies with /X and t with 3, the wavelength dependence drops out completely. For illustration, Fig. 3 shows a technical 

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In place of the diffractometer discussed in Section 2.5.3, a spectrometer is used, which allows measurement of the energy spectrum of scattered neutrons at different scattering angles. There are four main types of spectrometers in use today, the tripleaxis spectrometer, the time-of-flight spectrometer, the back-scattering spectrometer, and the spin-echo spectrometer, each of which is briefly described in the following section. 

The spin-echo spectrometer offers an exceptionally high-energy resolution (A E/E 10 5) and is based on a design first conceived by Mezei.10 Here the Larmor precession... 

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