A plane-grating monochromator for circularly polarized undulator radiation at BESSY II

At BESSY II, a third-generation 1.7 GeV storage ring is under construction. A planar elliptical undulator will be installed as a source of X-rays that have a high degree of circular polarization. Radiation in the first, third and fifth harmonics will cover the energy range 87–1330 eV. The beamline will essentially consist of a plane-grating monochromator working with collimated light in the dispersion plane. A single set of optical elements can be used to handle the two angularly separated beams of opposite helicity from the double undulator. The degree of circular polarization ranges from 73 to 100%, and a flux of up to 5 × 1013 photons s−1 (100 mA)−1 can be achieved. A maximum spectral resolution of about 13 000 will be possible at 100 eV using a 20 µm slit.

Post-Spectrometer Instrumentation for STEM

A charge-coupled device (CCD) is being used for parallel recording on the EELS spectrometer of the V-G HB501 STEM in the Cavendish. Three quadrupole lenses are provided for focussing the EELS spectrum and, unlike in some other similar systems, the spectrometer slit has been retained. This was done so that the spectrometer could still be used for serial acquisition of EELS spectra and for energy-filtered imaging, although it would be preferable, and was envisaged as a later development, to have the first quadrupole positioned before the dispersion plane of the spectrometer; the slit would have been simulated by a suitably masked scintillator for the bright field detector.However it has proved advantageous to keep the slit so that microdiffraction patterns can be imaged through the spectrometer. The recording of high quality diffraction patterns in the HB501 STEM has not been straightforward: Grigson recording is far too slow, and although the standard microscope is provided with a phosphor screen and TV and film cameras, the speed and resolution are rather poor.

A Photodiode, Parallel Detection System for Energy Loss Spectrometry

The advantages of parallel detection systems for energy loss spectometry are certainly clear. A variety of approaches are possible ranging from simple film recording to elaborate intensified television cameras. We are in the process of evaluating one approach to such a parallel detection system, which consists of; a magnetic post-spectrometer lens to increase dispersion, a phosphor conversion plate, glass coupling lenses, a dual channeltron image intensifier, and a cooled photodiode array (Reticon RL256C/17). A schematic diagram of the system is shown in Fig. 1.The post-spectrometer lens is an RCA-3G intermediate lens which typically magnifies (∼15x) the dispersion to ∼25μm/ev with a rotation of ∼90°. Although the dispersion plane of the straight edge sector magnet used is tilted at 30° from the beam axis, pre-spectrometer optics reduce the angular divergence of the beam entering and leaving the spectrometer to ∼1 mr and the consequently large depth of focus assures that the dispersed beam is in focus across the conversion plate, perpendicular to the beam axis.