Richard L. Blake

Design and characterization of x-ray multilayer analyzers for the 50–1000 eV region

Abstract This report describes a synthesis of more than 10 years of this programs development and application of multilayer analyzers for absolute Bragg spectrometry in the low-energy x-ray region of 50–1000 eV. Multilayers, defined here as systems of periodic layered structures parallel to the analyzer surface, have been applied principally in the diagnostics and application of the new, intense sources of synchrotron and high-temperature plasma x radiation. Detailed absolute reflectivity characterizations are presented for selected examples of these multilayers which have been semiempirically determined for mica, potassium acid phthalate, and the fabricated Langmuir-Blodgett and sputtered multilayer analyzers with d -spacings in the 10–200 A range. Design requirements for absolute spectrometry are established. Efficient analytical multilayer reflectivity models are derived and parameterized (based upon a modification of the Darwin-Prins model for the low-energy x-ray region), including, for the sputtered multilayers, parameters for denning interface structure. The dependence of the reflectivity characteristics, high-order Bragg diffraction suppression, and overall efficiency upon the model parameters is analyzed. A special spectrograph and procedure for the absolute measurement of the relevant reflectivity characteristics are described. Detailed measurements and semiempirical characterizations are presented. Programs for small laboratory computers have been developed that allow rapid and flexible spectral analysis, transforming measured spectra to absolute spectra.

Iridium optical constants for the Chandra X-ray Observatory from reflectance measurements of 0.05-12 keV

We present optical constants derived from synchrotron reflectance measurements of iridium-coated X-ray witness mirrors over 0.05-12 keV, relevant to the Chandra X-ray Observatory effective area calibration. In particular we present for the first time analysis of measurements taken at the Advanced Light Source Beamline 6.3.2 over 50-1000 eV, Chandras lower-energy range. Refinements to the currently tabulated iridium optical constants (B. L. Henke et al., At. Data Nucl. Data Tables 54, 181-343, 1993 and on the Web at http://www-cxro.lbl.gov/optical_constants/) will become important as the low-energy calibration of Chandras X-ray detectors and gratings are further improved, and as possible contaminants on the Chandra mirror assembly are considered in the refinement of the in-flight Ir absorption edge depths. The goal of this work has been to provide an improved tabulation of the Ir optical constants over the full range of Chandra using a self-consistent mirror model, including metallic layers, interface roughness, contaminating overlayer, and substrate. The low-energy data present us with a considerable challenge in the modeling of the overlayer composition, as the K-absorption features of C, O, and N are likely to be present in the ~10A overlayer. The haphazard contamination and chemical shifts may significantly affect optical constants attributed to this overlayer, which will distort the iridium optical constants derived. Furthermore, the witness mirror contamination may be considerably different from that deposited on the flight optics. The more complex modeling required to deal with low-energy effects must reduce to the simpler model applied at the higher energies, which has successfully derived optical constants for iridium in the higher energy range, including the iridium M-edges, already used in the Chandra calibration. We present our current results, and the state of our modeling and analysis, and our approach to a self-consistent tabulation.

Reflectance calibrations of AXAF mirror samples at absorption edges using synchrotron radiation

We are developing a system to calibrate reflectances of witness coupons to the AXAF flight mirrors at the National Synchrotron Light Source over the 0.05-12 keV energy range. These witness coupons will be coated in the same process as the AXAF mirror elements. One of the key issues is the accurate determination of mirror efficiencies across the absorption edges of the mirror coating elements. We present a series of reflectance measurements with 2 eV resolution of a nickel-coated flat mirror in the region of the Ni L-II (870 eV) and L-III (853 eV) absorption edges. Scans of reflectance versus grazing angle at fixed energies in this region show distinct interference fringes at grazing angles larger than the critical angle which are extinguished as the photon energy is increased beyond the low point of the L-III edge, indicating total absorption of the evanescent wave within the Ni film. At 51 arc minutes grazing angle, measured reflectance decreases smoothly by 35 percent and then recovers in an 8 eV band at the L-III edge. We have also measured reflectances in the M absorption edge region for gold, platinum, and iridium coated mirrors. We derive optical parameters n and k specific to the film for comparison to the existing data tables.

Los Alamos X-Ray Characterization Facilities For Plasma Diagnostics

A summary is given of characteristics of x-ray sources used by Los Alamos National Laboratory to calibrate various x-ray diagnostic packages and components. Included are D.C. sources in electron impact and fluorescence modes, a pulsed laser source for soft x rays with 100 ps time resolution, Febetron pulsed electron impact sources, and both EUV and x-ray synchrotron beamlines.

Modeling the Chandra high-energy transmission gratings below 2 keV

The High Energy Transmission Grating Spectrometer of the Chandra X-Ray Observatory is a high spectral resolution instrument utilizing gold X-ray transmission gratings. The gratings have been subjected to a rigorous program of calibration, including testing at synchrotron facilities for the purpose of refining and testing the grating model. Here we conclude our investigation of the optical constants of gold, extending it below 2 keV to complete the coverage over the Chandra energy range. We investigate the carbon, nitrogen, oxygen and chromium edge structures introduced by the grating support membrane. Finally, we summarize the state of the grating model, identifying those energy regions where the residuals are most significant and suggesting where the model might be improved.

Optical constants from synchrotron reflectance measurements of AXAF witness mirrors 2 to 12 keV

We report iridium optical constants fitted from synchrotron reflectance data. Specifically, we have used the NKFIT algorithm of D. L. Windt to derive (delta) (E) and (beta) (E) from 2 - 12 keV reflectance calibrations of AXAF witness mirrors. The model is applied at each energy separately, to fit four to nine data points from reflectance-versus-energy scans at selected grazing angles. The stability of the model in the presence of Gaussian noise has been tested extensively. We report the results of several bias studies, involving the generation and analysis of artificial data. Bias studies have been used to determine the optimal grazing angles to be scanned in the various x-ray energy ranges to condition the optical constants. They have also been used to investigate the effects of individual errant data points on the resulting fits and derived optical constants. The results will aid in eliminating systematic errors in the derived optical constants. We also present results of our investigation of the Debye-Waller and Nevot-Croce roughness correction algorithms as applied to our measurements. The Nevot-Croce method gives a much better representation of the data, however its rigorous justification in this experiment is lacking, and the roughness parameter derived is not constant with energy. A more self- consistent model for roughness correction is sought.

Iridium optical constants from synchrotron reflectance measurements over 0.05- to 12-keV x-ray energies

We present optical constants derived from synchrotron reflectance measurements of iridium-coated X-ray witness mirrors over 0.05-12 keV, relevant to the Chandra X-ray Observatory effective area calibration. In particular we present for the first time analysis of measurements taken at the Advanced Light Source Beamline 6.3.2 over 50-1000 eV, Chandras lower-energy range. Refinements to the currently tabulated iridium optical constants (B. L. Henke et al., At. Data Nucl. Data Tables 54, 181-343, 1993 and on the Web at http://www-cxro.lbl.gov/optical_constants/) will become important as the low-energy calibration of Chandras X-ray detectors and gratings are further improved, and as possible contaminants on the Chandra mirror assembly are considered in the refinement of the in-flight Ir absorption edge depths. The goal of this work has been to provide an improved tabulation of the Ir optical constants over the full range of Chandra using a self-consistent mirror model, including metallic layers, interface roughness, contaminating overlayer, and substrate. The low-energy data present us with a considerable challenge in the modeling of the overlayer composition, as the K-absorption features of C, O, and N are likely to be present in the ~10A overlayer. The haphazard contamination and chemical shifts may significantly affect optical constants attributed to this overlayer, which will distort the iridium optical constants derived. Furthermore, the witness mirror contamination may be considerably different from that deposited on the flight optics. The more complex modeling required to deal with low-energy effects must reduce to the simpler model applied at the higher energies, which has successfully derived optical constants for iridium in the higher energy range, including the iridium M-edges, already used in the Chandra calibration. We present our current results, and the state of our modeling and analysis, and our approach to a self-consistent tabulation.

ACIS UV/optical blocking filter calibration at the National Synchrotron Light Source

Measurements of the transmission properties of the AXAF CCD imaging spectrometer (ACIS) UV/optical blocking filters were performed at the National Synchrotron Light Source at Brookhaven Laboratories. The X-ray transmissions of two Al:Si/LEXAN/Al:Si three layer filters were measured between 260 and 3000 eV. The main purpose of the calibration was to determine a model transmission function with an accuracy of better than 1 percent. We present results from fits of model transmission functions to the measured x-ray transmission data. Detailed fine energy scans above the Al-K and C-K absorption edges revealed the presence of fine oscillations of the x-ray transmission. These features are most likely extended x-ray absorption fine structures (EXAFS). The amplitude of the EXAFS oscillations above the Al absorption edge is about 5 percent of the mean value of the x-ray transmission. EXAFS theory predicts a temperature dependence on the amplitude of the EXAFS oscillations. This dependence arises from the fact that thermal vibrations of the atoms in a solid produce a phase mismatch of the backscattered electron wave function. Since the ACIS filters will be at a much lower temperature on orbit we provide a prediction of the EXAFS component for the expected on orbit temperature of the ACIS filters.

Reflectance calibrations of AXAF witness mirrors using synchrotron radiation: 2 to 12 keV

For the past six years, a high-accuracy reflectance calibration system has been under development at the National Synchrotron Light Source at Brookhaven National Laboratory. The system utilizes Los Alamos National Laboratorys Beamlines X8A and X8C. Its purpose is to calibrate the reflection efficiencies of witness coupons associated with the coating of the eight mirror elements composing the High Resolution Mirror Assembly for NASAs Advanced X-ray Astrophysics Facility (AXAF). During the past year, measurements of reflectances of numerous iridium- coated witness flat mirrors have been obtained to a relative statistical precision of 0.4 percent, and an overall repeatability within 0.8 percent in the overlapping energy regions. The coating processes are strikingly repeatable, with reflectances in the 5-10 keV range for off-end witness flats nearly always being within 1 percent of one another, excluding interference fringes. The comparison reflectances between flats obtained from qualification coating runs and production runs of the Wolter Type I mirror elements are in turn nearly equal, indicating that the qualification run witness flats provide a good representation of the flight optics. Results will produce a calibration of AXAF with extremely good energy detail over the 2-12 keV range, which includes details of the M-absorption edge region for Ir. Development of the program to cover 0.05-2 keV continues.

Development of beamline U3A for AXAF synchrotron reflectivity calibrations

We discuss the development of beamline U3A at NSLS for AXAF telescope witness mirror reflectivity calibrations in the 1- 2 keV energy range. The beamline was originally constructed as a white light beamline and has been upgraded with the addition of a monochromator to meet the needs of the AXAF calibration program. The beamline consists of an upstream horizontally focussing gold coated elliptical mirror, a differential pumping section, a sample/filter chamber, a monochromator and a downstream filter set. The mirror is set at a 2 degree incident angle for a nominal high energy cutoff at 2 keV. The monochromator is a separated element, scanning, double crystal/multilayer design having low to moderate energy resolution. A fixed exit beam is maintained through the 7-70 degree Bragg angle range by longitudinal translation of the second scanning crystal. Tracking is achieved by computer control of the scan motors with lookup table positioning of the crystal rotary tables. All motors are in vacuum and there are no motional feedthroughs. Several different multilayer or crystal pairs are co-mounted on the monochromator crystal holders and can be exchanged in situ. Currently installed are a W/Si multilayer pair, beryl, and Na-(beta) alumina allowing energy coverage from 180 eV to 2000 eV. Measurements with Na-(beta) alumina and beryl show that beam impurity less than 0.1 percent can be achieved in the 1-2 keV energy range. Measured resolving powers are E/(Delta) E equals 60 for W/Si, 500-800 for (beta) alumina and 1500 to 3000 for beryl. Initial results suggest that signal to noise and beam purity are adequate in the 1-2 keV region to achieve the 1 percent calibration accuracy required by AXAF. This allows overlap of Ir MV edge data taken on x-ray beamline X8A and with low energy data taken on ALS beamline 6.3.2.