Scanning X-ray spectrometer for high-resolution Compton pro®le measurements at ESRF

A scanning-type crystal spectrometer for high-resolution Compton pro®le measurements has been constructed at the High Energy Inelastic Scattering Beamline (ID15B) of the ESRF. Radiation from a seven-period asymmetrical permanent-magnet wiggler or from a superconducting wavelength shifter is focused horizontally onto the sample by a bent-crystal monochromator. Typical energies are 30, 50 and 60 keV, the ̄ux on the sample is 10 photons sÿ1, and the relative energy bandwidth is 3 10ÿ4. The spectrometer operates in the Rowland circle geometry, where the sample is ®xed and the cylindrically bent analyser crystal and the detector move on the focusing circle by synchronized translations and rotations. The main detector is a large-diameter NaI scintillation counter, the incident beam is monitored by an Si diode, and scattering from the sample is detected using a Ge detector. The recorded spectrum is corrected for the energy-dependent response of the spectrometer, background and multiple scattering, and converted to the momentum scale. The resolution of the spectrometer is calculated from the geometrical factors and the re ̄ectivity curve of the analyser crystal, and the result is checked against the widths of the elastically scattered line and ̄uorescent lines. So far, 0.1 a.u. resolution in electron momentum has been achieved. The typical average count rate over the Compton pro®le is about 1000 counts sÿ1 from a weakly absorbing sample.