NSLS U3c and X8a Beamline Descriptions and References

Jeffrey W. Keister

rev. 4/23/2007

Absolute detector responsivity (DC photocurrent) calibration of x-ray
detectors in the photon energy range of 50-5900 eV is performed at
National Synchrotron Light Source's (NSLS) beam lines U3C and X8A
\[1-3\] at the Brookhaven National Laboratory. Responsivity measurements
at beamlines U3c and X8a can typically be compared to model data for a
variety of detector types, including photodiode, photoconductor, and
photoemission type detectors. Filter transmission and mirror
reflectivity are also commonly measured, using the same reference
detectors to determine incident and transmitted (reflected) flux. Both
beamlines operate under ring-contiguous vacuum. Collimation of the x-ray
beam at either end station is typically accomplished by means of a 1/16"
diameter circular pinhole.

U3c operates on the NSLS-VUV storage ring (beam energy = 808 MeV),
taking approximately 10 mrad from a 1.41 T (1.91 m) bending magnet with
critical energy 622 eV. Monochromatic x-rays are provided by either of
two gratings in an "extended range grasshopper" (ERG) monochromator
\[4\]. Soft x-rays are refocused by a gold-coated bent cylindrical
mirror to maximize flux in the test chamber. Harmonic purity of the
x-ray beam is accomplished using both foil x-ray filters and a
four-bounce mirror set ("high-order trap") installed on the beamline.

X8a operates on the NSLS-XRAY storage ring (beam energy = 2.8 GeV),
taking approximately 4 mrad from a 1.36 T (6.875 m) bending magnet with
critical energy 7.1 keV. X-rays are focused to the end station by a
nickel-coated bent cylindrical mirror located 7.5 m from the source.
Monochromatic x-rays are provided by a dual crystal monochromator using
either Si(111) (d = 3.1355 Å) or W/Si (d = 25 Å) Bragg dispersion
elements. Harmonic purity of the x-ray beam is accomplished using foil
x-ray filters installed on the beamline, in addition to the upstream
mirror which provides a high-energy cutoff of 5.9 keV.

Responsivity of test detectors is measured in comparison to AXUV-100G
silicon photodiodes from IRD \[5\]. Diodes in use as reference detectors
are specified to have maximum 20 nm silicon dead layer and minimum 500
megaohm shunt resistance. Active layer thickness is typically 25 µm.
Absolute responsivity of the reference diode is determined using the
"self-calibration" method of Krumrey and Tegeler \[6,7\]. Optical
constants required for this approach are taken from references 8-10.
Spectroscopic responsivity of reference diodes installed at the
beamlines typically ranges from 0.15 to 0.27 A/W in the range of 50-5900
eV and is typically measured at these beamlines at specific energies
with an absolute accuracy of 2-5%.

1.  Beamline website: <http://www.bnl.gov/u3cx8a> and/or
    <http://www.nsls.bnl.gov/beamlines/beamline.asp?blid=U3C>

2.  R. J. Bartlett, W. J. Trela, F. D. Michaud, et al "Characteristics
    and performance of the Los Alamos VUV beam line at the NSLS," Nucl.
    Instr. Meth. A266 (1988) 199.

3.  R. H. Day, R. L. Blake, G. L. Stradling, W. J. Trela, R. J. Bartlett
    "Los Alamos X-ray characterization facilities for plasma
    diagnostics," SPIE Vol. 689 X-ray Calibration, Techniques, Sources
    and Detectors (1986) 208.

4.  F. C. Brown, J. P. Stott, S. L. Hulbert "First Operation of an
    Extended Range Grasshopper Monochromator on the Aladdin Storage
    Ring," Nucl. Instr. Meth. A246 (1986) 278.

5.  IRD website: <http://www.ird-inc.com/>

6.  M. Krumrey, E. Tegeler "Self-calibration of semiconductor
    photodiodes in the soft-x-ray region," Rev. Sci. Instrum. 63
    (No. 1) (1992) 797.

7.  M. Krumrey, E. Tegeler, R. Goebel, R. Köhler "Self-calibration of
    the same silicon photodiode in the visible and soft-x-ray ranges,"
    Rev. Sci. Instrum. 66 (No. 9) (1995) 4736.

8.  F. Scholze, H. Rabus, G. Ulm "Mean energy required to produce an
    electron-hole pair in silicon for photons of energies between 50 and
    1500eV," J. Appl. Phys. 84 (1998) 2926.

9.  B. L. Henke, E. M. Gullikson, J. C. Davis "X-ray interactions: photo
    absorption, scattering, transmission, and reflection at E = 50-30000
    eV, Z = 1-92," Atomic Data and Nucl. Data Tables Vol. 54
    (No. 2) (1993) 181 (also CXRO website:
    <http://www.cxro.lbl.gov/optical_constants/>)

10. J. Rife, J. Osantowski "Extreme ultraviolet optical properties of
    two SiO2 based low-expansion materials," J. Opt. Soc. Am. 70 (1980)
    1513.