Jonathan J. Fitch

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.

Molecular contamination study of iridium-coated x-ray mirrors

We have completed extensive synchrotron reflectivity measurements on several iridium mirrors which were intentionally coated with thin layers (100 angstroms or less) of polyethylene, a hydrocarbon contaminant. The purpose was to verify theoretical predictions of alterations in reflection efficiency of an iridium surface for various thicknesses of hydrocarbon contamination, and to evaluate the acceptability of attainable upper limits of such contamination for the mirrors aboard NASAs Advanced X-ray Astrophysics Facility (AXAF). Although the deposition of such thin layers is problematic with no systematic guarantee of uniform thickness or density, successful analysis by modeling the contaminant as a uniform surface layer may be done, within a limited X-ray energy range. The M-edges of iridium are significantly affected by the polyethylene layers. For the most part, contamination increases the reflectance in the M-edge range over that of bare iridium, although cross-over points between contaminated and uncontaminated mirrors occur at several angles relevant to AXAF. However, calibratability of the reflectance is a more significant issue than X-ray mirror efficiency. We present the modeling results for three thicknesses of polyethylene, and discuss the implications for the performance of AXAF mirrors and their calibratability.

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.

Reflectometer end station for synchrotron calibrations of Advanced X‐ray Astrophysics Facility flight optics and for spectrometric research applications

Preparations have been underway to construct and test a facility for grazing incidence reflectance calibrations of flat mirrors at the National Synchrotron Light Source. The purpose is to conduct calibrations on witness flats to the coating process of the flight mirrors for NASA’s Advanced X‐ray Astrophysics Facility (AXAF). The x‐ray energy range required is 50 eV–12 keV. Three monochromatic beamlines (X8C, X8A, U3A) will provide energy tunability over this entire range. The goal is to calibrate the AXAF flight mirrors with uncertainties approaching 1%. A portable end station with a precision‐positioning reflectometer has been developed for this work. We have resolved the vacuum cleanliness requirements to preserve the coating integrity of the flats with the strict grazing‐angle certainty requirements placed on the rotational control system of the reflectometer. A precision positioning table permits alignment of the system to the synchrotron beam to within 10 arcsec; the reflectometer’s rotational control system can then produce grazing angle accuracy to within less than 2 arcsec, provided that the electron orbit is stable. At 10–12 keV, this degree of angular accuracy is necessary to achieve the calibration accuracy required for AXAF. However the most important energy regions for the synchrotron calibration are in the 2000–3200 eV range, where the M‐edge absorption features of the coating element, iridium, appear, and the 300–700 eV range of the Ir N edges. The detail versus energy exhibited in these features cannot be traced adequately without a tunable energy source, which necessitates a synchrotron for this work. We present the mechanical designs, motion control systems, detection and measurement capabilities, and selected procedures for our measurements, as well as reflectance data.

Modeling of synchrotron reflectance calibrations of AXAF iridium-coated witness mirrors over 2 to 12 keV

As calibrations of AXAF mirrors in the 2-12 keV range are near completion, we can report that the archive is complete for the scheduled number of mirrors over the 5-12 keV range, and also over the 2010-6200 eV range. Analysis of the results for optical constants and coating layer parameters is now proceeding rapidly. To date, we have derived optical constants from 39 mirrors over 5-12 keV using our existing Fresnel Equation model, with uniform layers of Ir, Cr, and Zerodur, and the roughness algorithm of Nevot and Croce. The analysis method has been presented in our earlier papers, but its application has been expanded to all three varieties of witness mirrors use in AXAFs qualification and production coating runs. For the first and most consistent variety, which have 1 angstrom roughness, reflectances are indistinguishable from mirror to mirror save for thickness variations between coating runs. Residuals of the fits for optical constants become large when reflectance values below 18 percent are included in the fits. If such data points are ignored, values of (delta) (E) and (beta) (E) very much like those of Henke et al. over 5-11 keV are obtained. Residuals are at the 0.6-0.8 percent level, which meets calibration requirements but exceeds experimental noise. For the second variety of witness mirror, polished to obtain roughness specification similar to the flight mirrors, fits approximately within the noise level of the measurements may be obtained over 5-11 keV, for reflectance values down to 5 percent. The (delta) (E) is essentially the same as that obtained from the 1 angstrom flats; however (beta) (E) is higher by approximately 3-4 percent systematically, and the mirror-to-mirror variation is larger. The third variety of mirror was obtained with 5-7 angstrom nominal roughness. Residuals to the fits are large for the entire range of angles, in some case exceeding 2 percent with a sinusoidal character through the critical angle. Coating layer depths are similar to those found for the other mirror types, with good precision. We discuss results along with possible improvements to the model and experiments to verify it.

Feasibility study of ALS beamline 6.3.2 in the calibration of AXAF: initial reflectivity results

AXAF telescope witness mirror calibrations have been carried out on ALS Beamline 6.3.2 in July 1997 and Jan-Feb 1998 to ascertain whether sufficient beam purity and signal to noise were available over the energy range 50-1000 eV. An overall accuracy of 1 percent is necessary on reflectivities to meet AXAF program requirements. Some beamline variations were incorporated - based on two previous test runs. Various filter/order-sorter combinations were used to control beam purity. A test involving strength of the N-edge dips in reflectivity versus sample angle to the beam showed the NVII, NVI, NIII, NII, and NI edges in roughly expected proportions. This verified adequate purity in all filter/order-sorter channels except the 260-454 eV channel that includes the NV and NIV edges just above the carbon K- edge. We discuss a future solution of this puzzle by other test for evaluation of the carbon contamination layer. Oxygen contamination is also ubiquitous on Ir mirrors and foils. These problems can also be handled through optimization of the data acquisition and use of control samples. In reflection Ir NIII appears out of the noise as two doublet-like bumps. Ir NII has so far been observed near the noise level < 2 percent. Averaged over many scans it too appears to be doublet-like. Scans through the NI edge near the noise level are compatible with a doublet-like or broad feature from 685-707 eV. transmission data give the NI and NIII edges better. Ir NIV and NV edges are not yet measurable for systematic reasons. NVI and NVII appear to be normal edges. Angle scans to derive the Ir and Cr layer thicknesses are feasible at approximately 900-920 eV. Reflectivities can be reliably measured at small angles to 1300 eV. This will be useful for overlap with measurements at NSLS. Optical constants derived from reflectivities in most segments of this energy range will meet requirements for AXAF. Transmission measurements are being pursued in addition, for confirmation and to augment some segments.

AXAF synchrotron witness mirror calibrations 2 to 12 keV

We have completed another full year of reflectance calibrations of AXAF witness mirrors at the National Synchrotron Light Source. At the NSLS, we have used beamlines X8C (5 - 12 keV) and X8A (2 - 6 keV), sponsored by Los Alamos National Laboratory. All of the flats have been calibrated in the 5 - 12 keV range, and approximately 1/4 of all our flats have been calibrated in the 2 - 6.2 keV range. The repeatability in the coating processes reported in Denver has continued with the measurement of additional mirrors. Optical constants from reflectances have been derived for six of the eight AXAF mirror elements, and a degree of spatial uniformity information exists for three of these six. The addition of a semitransparent monitor has markedly increased efficiency of measurements in the 5 - 12 keV range, and efforts are being made to provide such a monitor detector for the lower energy ranges. We report the progress in reflectance data acquisition and optical constant derivations, and discuss implications of the results for the AXAF program.

Determination of optical constants for AXAF mirrors from 0.05 to 1.0 keV through reflectance measurements

We discuss calibration of the Advanced X-ray Astrophysics Facility (AXAF) high resolution mirror assembly (HRMA) through the use of surrogate coating process witness flats. Reflectance measurements of representative witness flats have been made at the Advanced Light Source (ALS) Synchrotron Facility over an energy range of 60 - 940 eV. We discuss the procedures used for these measurements and some preliminary results of our studies. The initial results show, for some energy regions, a reduction in reflectance expected from a pure iridium coating layer. The observed decrease in mirror reflectance is believed to be the combined result of the presence of an organic thin film on the mirror surfaces, plus the effects of carbon on the ALS beamline optics. It appears that the tested mirror surfaces have a maximum level of molecular contamination amounting to an effective carbon thickness of from 5 - 10 angstroms. The source of this contamination is not identified, although this amount is not surprising.

Analysis of iridium reflectance measurements for AXAF witness mirrors from 2 to 12 keV

For the best flexibility in ground and on-orbit calibration modeling of the AXAF telescope over its entire field of view, including off-axis calibration evaluations, AXAF synchrotron reflectance calibrations require that the measured reflectance data be reduced to optical parameters analogous to n and k. We have developed a method for AXAF witness mirror analysis which is a modification of the NKFIT optical constants algorithm published by D.L. Windt. The algorithm assumes uniform layer thicknesses using a recursive, exact formation of Fresnels equations, with a modified Debye-Waller roughness correction factor. The recursion formula has been modified to include an explicit double-precision formulation. The results of most of the fits of AXAF calibration measurements yield residuals less than 1 percent of the reflectance value levels down to R approximately .03. The precision of the measurements is smaller still, which compromises the (chi) 2 fitting algorithm; however, the results will most likely prove adequate for AXAF witness mirrors calibrated in the 5-12 keV range. Coating density determined from the refractive index n is approximately 98.5 percent of the bulk for iridium. Derived coating thicknesses are extremely consistent with the photon energy, giving still more significant calibration information to the program.