Extremely high precision of lattice parameters with neutrons, demonstrated for GaAs

A four-beam neutron method for the measurement of lattice parameters with extremely high accuracy of about ±10-7 of Δa/a has been developed. Its principle is similar to the double crystal method in parallel arrangement, which is well known from X-ray diffraction. The main difference consists in the backscattering geometry for both the sample and the reference crystal leading to the following advantages. The difference between the lattice parameters is measured by Doppler-shifting one crystal against the other thus keeping the line shape of the diffraction pattern highly symmetric and independent of the lattice parameter difference. By using four beams at the same time, misalignment errors are exactly compensated in first order and corrections of second order can be performed easily. In spite of the extremely high accuracy of this method the alignment can be relaxed at least by one order of magnitude. The method was applied to GaAs. (orig.)