New Results on 'Quasi-Elastic' Scattering from Some Hydrogenous Liquids

A study of the 'quasi-elastic' scattering of cold neutrons fmm pentane and pentanol by use of the inverted filter technique is reported. The total energy resolution of the spectrometer in this investigation was varied from 0.10 to 0.13 m eV at the beryllium oxide cut-off. It is found that in no case hitherto studied has the total 'quasi-elastic' cross-section the shape of a single Lorentzian but is made up of two components, a narrow central peak and a broader underlying part. From their intensity variation it is concluded that the narrow peak is of an elastic nature while the broader underlying distribution has an inelastic origin. The width of the central peak for liquid pentane and for pentanol in the low viscosity region is understood from measured or extrapolated translational diffusion constants, while for lower temperatures another diffusion mechanism, probably rotational diffusion, is responsible for the width. A comparison is made with results obtained from two time-of-flight spectrometers, using the whole or only a part of the cold spectrum to define the energies of the neutrons hitting the sample. It is found that both of the two components observed in the inverted filter investigation are included in the quasi-elastic peak in the time-of-flight cases. It is shown that both the width dependence and the intensity dependence of k of the time-of-flight results can be fully reproduced by an appropriate sum of the new results. The quasi-elastic peak is thus probably of a true almost elastic origin as well as of an inelastic origin. It is concluded that very high resolution both on the monochromator and on the analyser side is required to reveal the true nature of the near elastic neutron scattering spectra. (author)