Application of synchrotron radiation to anomalous scattering for structure analysis with a four-circle diffractometer

The present paper describes a newly built four-circle diffractometer for X-ray structure analysis with synchrotron radiation and particularly with the use of anomalous scattering due to a relatively heavy atom or atoms. This system is installed on beamline 10A in the Photon Factory (PF) in the National Laboratory for High Energy Physics, and its 2θ arm is rotated in the plane perpendicular to the floor. In this system, a one-crystal monochromator of the horizontal dispersion type is used. When the 111 reflection from a Si monochromator is used, the energy resolution is about 14 eV for X-rays of about 10 keV. The four-circle diffractometer, a monochromator and an alignment carriage for the four-circle diffractometer can be controlled by a computer and, therefore, once the optical system has been adjusted for X-rays of a certain wavelength then the diffraction intensity values from a single-crystal sample can be automatically measured at any wavelength chosen. An NaI(Tl) scintillation counter is used for both the intensity measurements of the diffraction beam and the monitoring of the incident-beam intensity. In order to make the counting rate high, 50 MHz counters are used for the above two intensity measurements. In this set-up, about 1% counting loss occurs at a counting rate of 55000 s⁻¹. With this diffractometer, diffraction intensities have been measured for a hemimorphite single-crystal at two energy values on both sides of the Zn K absorption edge. The variation of the intensities, after being normalized by the standard reflection and the monitor counts, was somewhat large in comparison with the data measured in the usual laboratory. From these data measured at two energy values, a so-called two-wavelength anomalous-scattering difference Patterson function was calculated, where the Zn–Zn vector peaks were twice as high as in an ordinary Patterson.