PURPOSE: Integral to the success of several federally funded research programs is the absolute calibration of optical components used for x-ray detection, imaging, and spectroscopy as diagnostics in those programs. It is also important for synchrotron radiation facilities to have some beamline(s) dedicated to the testing and calibration of optics and detectors for use on the other beamlines. O/C/M is used to address several areas in physics and optical instrumentation:

• Nuclear physics (fusion plasma) diagnostics
• Astronomical spectroscopy and imaging, including remote detection of x-rays
• Synchrotron diagnostics and testing of beamline optical components
• Measurement and verification of x-ray optical data

HOW THE TECHNIQUE WORKS: Four basic types of x-ray optical components and measurements are considered: detectors (responsivity), mirrors (reflectivity), filters (transmission), and gratings (efficiency). Absolute responsivity (amperes per watt) is measured for test detectors against calibrated standard x-ray detectors that are maintained on site. In some cases, a detector under test may be a two-dimensional array, providing image data. Gratings and mirrors (including multilayers) are calibrated under varying conditions of incident and collection angles, with efficiency or reflectivity given as the ratio of the measured signal to the signal for the incident beam. Similarly, filter calibrations consist of measuring the ratio of x-ray beam intensity with the test component inserted into the beam path to that without.

UNIQUENESS: Synchrotron radiation is important for providing high flux onto small optical surfaces, with tunability over an extremely wide range of photon energies. The photon energy range needed, 5 eV to 50 keV, is not available using laboratory x-ray sources. Precision positioning and automation are also important, in particular for angle-resolved or surface-uniformity calibrations of custom components, which must simultaneously be kept in an ultrahigh-vacuum environment. At each beamline, individual calibration standard detectors are maintained, often in collaboration with other laboratories.

EXAMPLES:

Characterizing Chandra’s Iridium Mirror Coating
Diagnostics at Z
Diagnostic System for the National Ignition Facility

Chandra X-Ray Observatory with galactic center background (CXC/NGST illustration).

NASA's Chandra X-Ray Observatory is a space telescope designed to observe cosmic x-rays from some of the most exotic and explosive environments in the universe: supernovas, quasars, pulsars, and black holes — extreme phenomena that have characterized the universe from the beginning of its existence. To accomplish its mission, Chandra’s science support team selected iridium as the coating of choice for the mirrors in its telescope system. However, the use of iridium, a gold-like metal that was found to be the best reflector of x-rays over a wide range of energies, would require a trade-off in the comfort and experience that had already been gained using gold or nickel in previous missions. A portion of this trade-off was to be recovered by means of a separate coating calibration program, wherein a tunable, narrow-bandwidth source (e.g. synchrotron radiation) would be used to characterize the coatings in terms of their optical constants. What ensued was a major programmatic effort to characterize iridium optical constants thoroughly in the energy range of interest, 0.05–12 keV.Hence, in the energy range in question, this material is now understood as well as (or better than) any of its neighboring mirror materials in the periodic table, namely gold, platinum, osmium, rhenium, or tungsten.

D.E. Graessle, R. Soufli, A.J. Nelson, C.L. Evans, A.L. Aquila, E.M. Gullikson, R.L. Blake, and A.J. Burek, “Iridium optical constants from synchrotron reflectance measurements over 0.05- to 12-keV x-ray energies,” Proc. SPIE 5538, 72 (2004). Link:chandra.harvard.edu.

The “Z” machine at Sandia National Laboratory uses 100-nanosecond, 200-million-ampere pulses of current to generate powerful magnetic fields and x-rays that can be used to simulate nuclear weapon explosions, test materials under extreme conditions, or develop possible technologies for peacetime fusion energy. It is surrounded by an extensive set of diagnostics. Instruments that are routinely used to measure plasma temperature include a filtered five-channel x-ray diode (XRD) array, a filtered six-channel photoconducting detector (PCD) array, a bolometer, a transmission grating spectrometer, and a filtered silicon photodiode array. Filtered XRD detectors are used as primary radiation flux diagnostics. Vitreous carbon photocathodes are used to reduce the effect of hydrocarbon contamination present in the Z-machine vacuum system.Calibration of these devices is performed periodically (between accelerator shots) to track spectrally dependent changes in the sensitivity of these detectors with exposure to the Z environment. Pre- and postcalibration data taken indicate spectrally dependent changes in the sensitivity of these detectors by factors of up to 2 or 3.

G.A. Chandler, C. Deeney, M. Cuneo, D.L. Fehl, J.S. McGurn, R.B. Spielman, J.A. Torres, J.L. McKenney, J. Mills, and K.W. Struve, “Filtered x-ray diode diagnostics fielded on the Z accelerator for source power measurements,” Rev. Sci. Instrum. 70, 561 (1999).

The interior of the NIF target chamber, which weighs one million pounds and measures 30 feet in diameter (June 2000).

The National Ignition Facility (NIF) in Livermore, California, is a 192-beam experimental laser facility the size of a sports stadium. The lasers (with 1000 times the instantaneous electric generating power of the United States) focus on a target the size of a BB-gun pellet for a few billionths of a second. Experiments in NIF will allow studies of high-energy-density and fusion regimes with direct applications to stockpile stewardship, energy research, science, and astrophysics.The soft x-ray power diagnostic (SXSS) component of the NIF diagnostic system measures the x-ray emission from a fusion target through a lined hole in its wall, giving the time history of the radiation temperature inside. The diagnostic is based on calibrated x-ray filters, mirrors, and x-ray diodes (XRDs). Transmission gratings may also be used for time-resolved spectroscopy of the NIF shot.

R.J. Leeper, G.A. Chandler, et al., “Target diagnostic system for the National Ignition Facility,” Rev. Sci. Instrum. 68, 868 (1997).