= Triple-axis spectrometer
{wiki=Triple-axis_spectrometry}
A <triple-axis spectrometer> uses <Bragg's law> at an initial <crystal> <monochromator> to select $\lambda_i$ and at a second analyzing <crystal> to select $\lambda_f$. The sample sits between these selectors. The three axes refer to the <monochromator>, sample and analyzer rotations. The final counter measures accepted counts; the analyzer's <wavelength> selection makes those counts sensitive to outgoing <energy> even when the counter does not measure each neutron's <energy> directly.
\Image[/neutron-scattering-functional-modes.png]
{title=Functional routes for neutron diffraction and triple-axis energy analysis}
{description=Original functional schematic, with illustrative bends rather than historical angles or distances. The upper route omits the analyzer; interpreting its peaks as elastic requires a physical assumption. The lower route selects both projectile energies. Neither route specifies a historical detector construction.}
For a constant-$\mathbf Q$ scan, change incident or outgoing <energy> while coordinating sample and analyzer settings so that $\mathbf k_i-\mathbf k_f$ stays fixed. Counts versus $E_i-E_f$ then probe excitations at one <momentum transfer>. Changing only the analyzer angle generally changes both transferred quantities. Incident-beam monitoring, analyzer response and the instrumental resolution must enter quantitative interpretation; the diagram's arrows specify information flow, not an absolute <intensity (physics)> calibration.
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