NEW CRYSTAL SPECTROMETER FOR NEUTRONS

A new feature of the spectrometer is the monochromator unit, which contains two parallel reflecting copper crystals and delivers a beam of monoenergetic neutrons through a fixed collimator which is parallel to the incident beam from the reactor. About half the neutrons reflected by the first crystal are reflected by the second crystal, though the fraction declines at the upper end of our energy scale (about 60 meV). The beam from the monochromator is free from fast neutrons, and the background extremely low. The material, thickness, and mosaic width appropriate to efficient reflection in a monochromator crystal are discussed. The sample and analyzer form a compact separate unit on a stationary base. All the angles of the spectrometer (including those of the monochromator) are continuously variable over wide ranges, with either manual control or automatic control from a programming tape. Two spectrometers which are mirror images of one another are mounted side by side at the same reactor beam hole. They are mainly intended for measurements on phonons with comparatively high resolution. An example of the accuracy attained is given. © 1969 The American Institute of Physics.