Using phase-space transformation for an improved hybrid cold time-of-flight spectrometer

The concept and layout of a time-of-flight spectrometer for cold neutrons is presented which makes use of the principle of phase-space transformation. Close to the focal plane determined by a first-curved monochromator a second moving crystal is placed. If moving with the right velocity this crystal monochromatizes the neutron beam and reduces at the same time its horizontal divergence. With pyrolithic graphite as monochromator focusing can be realized for 5Å⩽λ⩽6.5Å. At λ=6Å an energy resolution δE∼60μeV is calculated. Compared to time-focusing instruments improved momentum transfer resolution and good energy resolution at all energy transfers is obtained. A reduction of intensity by the second monochromator will be most likely balanced by improved background. The wavelength dependence of the energy focusing velocity restricts in practice the instrument to one or two fixed wavelengths. The instrument does not need an end position on a neutron guide.