NIST - SURF III BL-9: XUV Detector Calibration Facility

The XUV Detector Calibration Beamline at SURF III

One of the beamlines (BL-9) at SURF III is dedicated to the calibration of transfer standard detectors in the XUV spectral region from 5 nm to 50 nm. The synchrotron radiation from SURF III is used as a bright source of continuum radiation which is dispersed by a grazing incidence toroidal grating monochromator. The monochromatic radiation passes into an experimental system that houses a rare gas ionization chamber (the absolute detector at the start of the calibration sequence) as well as NIST's working standard photodiodes and the transfer standards being calibrated for customers. NIST's working standards are periodically calibrated against the ionization chamber, and transfer standards are calibrated by intercomparison with the working standards.

Schematic Diagram of BL-9: Radiation from SURF enters from the left and is dispersed by the grating. The exit slit selects a narrow wavelength band, and monochromatic radiation is sent to the experiments and monitor diode.

Beamline Optics

The broadband continuum radiation from SURF III is dispersed in a vertical plane by a toroidal grating monochromator with an 83.5 degree angle of incidence at zero order. The "entrance slit" of the monochromator is the stored electron beam, which is maintained at a vertical height of 1 mm or less. Horizontally and vertically adjustable masks define the illuminated portion of the grating. The dispersed light is focused on a 1 mm exit slit by one of two ruled gold gratings. The short wavelength (5 nm to 17 nm) grating is ruled with 1200 lines/mm, while the long wavelength (18 nm to 50 nm) grating has 300 lines/mm. The gratings may be manually interchanged from outside the vacuum system. The resolution for a point source is 0.02 nm with the 1200 lines/mm grating and a 1 mm slit.

Absolute Photon Flux Measurements

Absolute measurements of photon flux in the monochromator exit beam are made by a rare gas ionization chamber. This detector measures photoionization currents in a low pressure neon gas cell. Photon flux can then be calculated from the measured photocurrent values, photoionization cross section, temperature and absorption cell length.

Working Standards Calibration

NIST's working standard photodiodes are calibrated by comparing them to an ionization chamber. The ionization chamber rare gas fills the experiment chamber while in use, and the presence of this absorbing gas near the working standard detectors renders directly intercomparing them to the ionization chamber impossible. To overcome this problem, an intermediate transfer photodiode in a high vacuum region is calibrated by direct intercomparison with the ionization chamber. After pumping the rare gas from the experiment chamber, the intermediate photodiode calibration is transferred to the working standards by direct intercomparison.

Transfer Standards Calibration

NIST maintains both windowless photoemissive and solid state working standards in the 5 nm to 50 nm region, and transfer standards are calibrated against a working standard of the same type. Outgoing transfer standards are calibrated by direct intercomparison with a working standard in a fixed beam.

References

For a more detailed description of the BL-9 detector calibration facility, refer to:

L. R. Canfield, "New far UV detector calibration facility at the National Bureau of Standards," Appl. Opt. 26, 18, (1987).
L. Randall Canfield and Nils Swanson, "Far Ultraviolet Detector Standards," Special Publication SP-250-2, National Bureau of Standards, (1987).

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Online: November 1994 - Last update: March 2001