High resolution Compton spectrometer

Compton scattered x-rays from a material provide information about the electron momentum distribution. Within the impulse approximation the energy of scattered x-rays is simply determined by the energy of the incident x-rays, the scattering angle and the primary electron momentum. The energy spectrum provides the momentum densities projected on the direction of the momentum transfer. In order to investigate momentum densities in materials, various kinds of x-ray spectrometers have been developed for Compton scattering experiments. Currently two methods are used for energy analysis of scattered radiation. One uses a Germanium solid-state detector and the other uses a crystal analyser which has 3-5 times better resolution. This improvement is very significant for examining solids, particularly in investigating Fermi surfaces in momentum space.

Compton spectrometers using a crystal analyser can be classified into either the Cauchois type or the scanning type. In the Cauchois type, all the components are basically stationary. A spectrum is recorded as a function of position on a position sensitive detector. In the scanning type, a spectrum is recorded as a function of the Bragg angle of the crystal analyser, where the analyser and the detector move along several axes, synchronized with analyser Bragg angle. If each component performs adequately, one can have similar resolutions and count rates using both types of spectrometers. However, for Cauchois type spectrometer problems arise in the high-energy region: position-sensitive detectors have poor efficiency and poor spatial resolution. Therefore, a scanning type spectrometer was built at ID15B. In the initial design of ID15B, it was anticipated that the main interest would be in conventional Compton scattering from light elements, and so the energies in the 30-60keV range were appropriate. It became clear that this choice was rather limiting, and a new monochromator and analyzer system was development to allow operation at 90keV using energy compensation scheme. Most high resolution Compton experiments use now 90keV option even for light materials due to better resolution (0.07a.u.) and higher count-rates.

Geometry and parameters of the dispersion compensating spectrometer, describing the x-ray source, the monochromator, the analyser and the detector.

The spectrometer is based on a novel idea, dispersion compensation. Figure 12 illustrates the spectrometer. A cylindrically bent Laue monochromator focuses 90keV synchrotron radiation at about 0.7 m before the sample, and produces a well defined energy gradient on the sample. A cylindrically bent Laue analyser almost perfectly compensates this wavelength gradient. For light elements the new spectrometer improves the counting rate by a factor of two compared with the previously constructed 30 and 60keV spectrometers. Because of reduced absorption owing to use of high-energy x-rays, the enhancement of the counting rate is spectacular for heavy-element materials.