Streamlining Detector-based Radiometry at SURF III
NIST has maintained the Synchrotron Ultraviolet Radiation Facility SURF for nearly 6 decades as a light source for research and radiometry. Radiometric services include detector calibrations, based on an absolute-cryogenic radiometer on beamlines 4 and 7, detector calibrations based on ionizations chambers on beamline 9, and lamp calibrations on beamline 3 and spectrometer calibrations on beamline 2, based the calculability of synchrotron radiation.
The ultraviolet radiation group is establishing a new center for absolute detector-based ultraviolet radiometry at the Synchrotron Ultraviolet Radiation Facility SURF III. This new center will reduce the uncertainty of calibrations through the implementation of cryogenic radiometry covering the wavelength range from 3 nm to 400 nm, thus establishing a single calibration station that covers the extreme ultraviolet, vacuum ultraviolet, and ultraviolet spectral ranges. This new core competence will for the first time cover the wavelength region between 92 nm to 116 nm and down below 5 nm to about 3 nm, spectral regions currently not covered at NIST. Construction of the new center for ultraviolet radiometry will consolidate several existing experimental stations, covering different parts of the ultraviolet spectral region, thus freeing space for other purposes in the future.
Description of the NRL beamline
The NRL beamline X24C is a grating-crystal monochromator beamline that covers a wide spectral range from the visible to the soft x-ray. It served as a calibration and optical component test facility for the NRL Space Sciences Division.
The synchrotron radiation is collimated by the first mirror and directed into the monochromator section. In the monochromator, a pair of optical elements – normal-incidence gratings, grazing incidence gratings, crystals, or mirrors (in any combination) – disperse the radiation. A second mirror focuses the radiation onto an exit slit. End-station instrumentation consists of a small reflectometer and a large instrument calibration chamber capable of testing optical components and assembled spacecraft instruments. NRL purchased a cryogenic radiometer that can be used for making absolute measurements of optical power. Available tests include e.g. reflectivity, transmission, and detector responsivity.
The two mirrors are focusing reflectors with a 2° grazing angle (88° angle of incidence with respect to the mirror normal) and 10-m focal length. Ideally, the mirror figure is an off-axis paraboloid, but elliptical or toroidal mirrors can be used with modest degradation in monochromator performance.
Current NRL Programs
The NRL program on X24C centers around the calibration of optical components – both flight hardware and research and development samples – and of assembled spacecraft instrumentation. The beamline is capable of making these measurements in a very wide spectral range from the visible (about 600 nm; 2 eV) into the soft x-ray (about 1.7 nm; 730 eV). While NRL makes use of the entire spectral range, the principle focus of the programs is in the EUV from 9 nm (138 eV) to 200 nm (6.2 eV).
Recent NRL research activities include the development and characterization of novel multilayer optics (e.g. Sc/Si, MoRu/Be for use below the Si L edge); measurement of the normal-incidence efficiency of transmission gratings; characterization of the transmission of filter materials directly deposited onto Si photodiodes; measurement of the responsivity of coated and uncoated Si photodiodes and of SiC wide-bandgap photodiodes; temperature dependance of the responsivity of Si and PtSi photodiodes; and timing studies of synchrotron radiation and the single-pulse response of fast Si photodiodes. Staff from NRL have received funding for further multilayer-coated grating development and will soon submit a proposal to develop EUV Fresnel zone plates.
Calibration of flight hardware has been performed for NASA/GSFC Solar Extreme-Ultraviolet Rocket Telescope and Spectrograph (SERTS), a NOAA transmission grating spectrometer, NASA Extreme Ultraviolet Imaging Spectrometer (EIS), and Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI). Test gratings have been characterized for the Constellation-X mission.
The relocation of X24C to SURF, in combination with our strong effort in absolute source-based irradiance calibrations, will establish SURF as the prime location for the calibration of space-borne ultraviolet and extreme-ultraviolet instruments in the United States.
Specifications / Capabilities:
Enable cryogenic radiometry with 1% uncertainty from 4 nm to 400 nm
Optical power provided by the new beamline in comparison to existing beamlines at SURF.
Name: Thomas Lucatorto