UV/VUV radiometric calibrations at SURF II
Since 1981 more than 150 instrument calibrations have been performed on the radiometric instrumentation calibration beamline at the Synchrotron Ultraviolet Radiation Facility (SURF II) at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland. This 300 MeV electron storage ring operates routinely at electron currents greater than 250 mA. The spectrometer calibration beamline provides a continuum of radiation from 4 nm through the visual spectral region in the form of an intense beam with total angular divergence of 1.2 mrad. The probable uncertainty for the flux ranges from about 5 percent at 4 nm to less than 2 percent above 20 nm. The new system will radiometrically trace to SURF II, and thus is expected to reduce the calibration uncertainty for diodes in this region by a factor of five to the 1-2 percent level.