The optimum parameters and properties of a new type of neutron monochromator based on elastically bent silicon single crystals are theoretically evaluated. The use of a fully asymmetrical geometry permits the achievement of rocking‐curve widths exceeding 20' in the incident beam. The associated effect of the reflected‐beam widening is compensated for by the second crystal in the parallel (1,−1) setting in opposite geometry. Reflectivity calculations indicate that this double‐crystal system might be compared with the best single‐crystal mosaic monochromators. Besides the easy control of the effective mosaicity and the corresponding integrated reflectivity by variation of the bending radius, a further advantage of this system is its simultaneous action as a neutron filter improving the ratio of the thermal neutron signal to the background.

The optics of successive Bragg reflection by two bent crystals is considered in the lamellar approximation. The optimal curvatures ensuring minimal rockingcurve widths and good reflection efficiencies are determined. The conditions under which a rocking curve reproduces the reflectivity curve of a bent crystal are indicated. Analytical formulae for the rocking-curve width and peak intensity are derived for three simple limiting cases. The computations are supported by experimental results obtained with bent perfect silicon crystals.

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A thermal-neutron monochromator based on an elastically bent perfect silicon crystal in a strongly asymmetric diffraction geometry used for the spatial condensation of the output neutrons is presented and discussed. Its effective 'mosaic spread' 6hO can be easily controlled by a simple choice of the bending radius and the length of the irradiated part of the crystal plate. A theoretical analysis shows that a gain in the flux density of monochromatic neutrons in the fully asymmetric case in comparison with the symmetric one is equal to the ratio of the width of the incident polychromatic beam and the thickness of the crystal. The experimental integrated reflectivity and focusing obtained are found to be in good agreement with theoretical predictions. The asymmetric arrangement of the bent crystal seems to be suitable for use at a source with a large incident-beam cross section when 6hO may achieve a value of about 10-2 rad and an effective neutron flux at the sample position may achieve the fluxes obtained by the best mosaic monochromator.