Neutrons of a defined wavelength are selected from the "white" beam by means of a single crystal monochromator M (first axis). The wavelength or incident energy of neutrons depends on the monochromator material and the angle 2θ_M. The sample S diffracts by an angle 2θ_S (second axis). The scattered neutrons are further analyzed with regard to their energy (third axis) by the analyzer via further diffraction process by an angle 2θ_A and finally counted by the detector D. The scattering vector Q and the energy transfer hω are unequivocally determined by the angles 2θ_S and 2θ_A . Thereby , the wave vectors k_i and k_f are connected with vector Q and energy hω by Q = k_i - k_f and ΔE = h²(k_i²-k_f²)/2m . The sense of the energy transfer determines whether the neutron has gained ( k_i< k_f , ΔE < 0 ) or lost energy ( k_i< k_f, ΔE > 0 ) during the scattering process.



Scientific problems which can be studied on a triple-axis spectrometer:
In most cases one should have a single crystal. Then it will be possible to study many kinds of lattice vibrations like the phonon and/or magnon dispersion curves, study commensurate and incommensurate, structural and/or magnetic phase transitions and because of very good background conditions spin waves in thin layer systems. In nanocrystalline, polycristalline or even amorphous materials the magnetic stiffness or gaps can be also measured.