Focusing effects in triple-axis neutron spectrometers

The theory of triple-axis spectrometers is discussed, and expressions are given for peak height and integrated intensities which show that the instrument possesses focusing properties which serve to sharpen line widths and increase peak heights. Five focusing conditions are derived and an operational procedure is suggested to exploit these effects. The use of focusing promises to give considerable improvements in the accuracy of measurements of phonon and magnon line widths.