YB66- a new soft X-ray monochromator for synchrotron radiation. II. Characterization

YB66, a complex boron‐rich man‐made crystal, has been singled out as a potential monochromator material to disperse synchrotron soft X‐rays in the 1–2 keV region. Results of a series of systematic property characterizations pertinent for this application are presented in this paper. These include Laue diffraction patterns and high‐precision lattice‐constant determination, etch rate, stoichiometry, thermal expansion, soft X‐ray reflectivity and rocking‐curve measurements, thermal load effects on monochromator performance, nature of intrinsic positive glitches and their reduction. The 004 reflection of YB66 has a reflectance of ∼3% in this spectral region. The width of the rocking curve varies from 0.25 eV at 1.1 keV to 1.0 eV at 2 keV, which is a factor of two better than that of beryl(1010) in the same energy range, and enables measurements of high‐resolution XANES spectra at the Mg, Al and Si K‐edges. The thermal bump on the first crystal arising from the low thermal conductivity of YB66 causes an energy drift of a few eVs with storage‐ring current and necessitates periodic energy calibration with metal foils. The positive glitches in the transmission function just above the Mg K‐edge have substantially been reduced using an Si or SiC mirror which suppresses the sharp reflectivity increases associated with anomalous scattering for the YB66 006 reflection at the Y L3‐ and L2‐edges. Continual operation over the past five years of a YB66 double‐crystal monochromator installed on the JUMBO beamline at Stanford Synchrotron Radiation Laboratory (SSRL) indeed proves the long‐term stability of this material in synchrotron radiation under ultrahigh vacuum conditions as indicated by the invariance in rocking‐curve characteristics after being exposed to an accumulative power level of ∼3 × 108 J over this period of time.