Hard x-ray spectrometer calibrations using a portable 120 kV x-ray source.

A Cauchois transmission-crystal hard x-ray spectrometer was calibrated by using a portable, compact, battery-powered tungsten x-ray source having 120 peak kilovoltage. The source emission region was characterized by recording high-resolution 2D x-ray images and was found to be composed of three emission regions having a 400 µm overall extent. The absolutely calibrated source fluence was measured by using a calibrated silicon drift detector and was in good agreement with the spectrum calculated by the SpekPy code. High-resolution spectra of the W Kα and Kß lines in the 57-70 keV energy range were recorded on image plate detectors by the Cauchois spectrometer and provided excellent calibrations of the spectrometer's dispersion and spectral resolution. The minimal effect of the source size in the spectral lines recorded on the spectrometer's Rowland circle and the source-size broadening of the spatial lines recorded well beyond the Rowland circle were analyzed. The integrated reflectivity of the spectrometer's quartz (101) crystal was measured by using the absolutely calibrated 59.318 keV W Kα1 spectral line emission and was in agreement with previous integrated reflectivity measurements performed at the National Institute of Standards and Technology. The well-characterized portable 120 kV x-ray source provides a convenient and cost-effective way to accurately calibrate the sensitivity, dispersion, spectral resolution, and source-size broadening in the spectra recorded by high-resolution x-ray spectrometers operating in the hard x-ray range. The absolutely calibrated source fluence can also be used to calibrate x-ray detectors at energies in the 40-100 keV energy range.

Gigagauss magnetic field measurements using Zeeman broadening of Ne-like transitions in highly charged ions.

The measurement of gigagauss magnetic fields using Zeeman broadening of Ne-like transitions in highly charged ions in high energy density plasmas is investigated. The transition in Xe44+ from the (2p5 1/23d3/2)J=1 level to the (2p6)J=0 ground level, designated 3C and having 4.858 keV transition energy, is considered in detail. Ne-like Xe spectra were previously recorded from compressed Xe-filled capsules, and the plasma conditions were determined from spectral line ratios and atomic code modeling. Using the Paschen-Back expression for Zeeman broadening produced by strong magnetic fields and the plasma conditions from the compressed Xe-filled capsule experiments to estimate the competing Stark, Doppler, and turbulence broadenings, it is found that >0.34 GG fields can be determined from spectra recorded by a high-resolution spectrometer having 1 eV resolution (5000 resolving power). By scaling the plasma conditions to higher temperatures and densities, the minimum detectable magnetic field is determined for Zeeman broadening of the 3C transition in Ne-like ions up to U82+. Recently developed transmission-crystal spectrometers, employing a cylindrically bent crystal in the Cauchois configuration, have sufficient resolution to determine gigagauss magnetic fields from Zeeman broadening of Ne-like transitions in the 4-18 keV range in Xe44+ to U82+.

High x-ray resolving power utilizing asymmetric diffraction from a quartz transmission crystal measured in the 6 keV to 22 keV energy range.

A Cauchois-type spectrometer utilizing the (203) lattice planes at an oblique angle of 11.53° to the normal to the surface of a quartz transmission crystal recorded the Kα and Kß spectral lines of six elements from Fe to Ag in the 6-22 keV energy range from a laboratory x-ray source. After deconvolving the natural lifetime widths and the image plate detector broadening from the observed spectral linewidths, the intrinsic crystal resolving power was determined to be 4000 at the lower energies and decreasing to 1000 at the higher energies. Previously, a Si wafer crystal exhibited twice this resolving power when the (331) planes had been used in asymmetric geometry. The investigation of diffraction with this quartz crystal, with a very similar lattice spacing and therefore spectral coverage, was motivated by the larger integrated reflectivity of quartz due to its well-known quasimosaicity upon elastic bending. The measured spectral linewidths were in good agreement with the widths calculated by accounting for various broadening mechanisms, including source size, crystal thickness, crystal height, crystal rocking curve width, geometrical aberrations, and possible spectrometer configuration errors. This is the first, to the best of our knowledge, demonstration of high resolving power achieved by asymmetric diffraction over a wide energy range (6-22 keV) and with detailed comparisons with theoretical broadenings. Based on these results, Cauchois spectrometers employing asymmetric planes of perfect quartz and silicon crystals can be reliably designed and optimized for high-resolution spectroscopy in the >6 keV energy range.

X-ray spectrometer having 12 000 resolving power at 8 keV energy.

An x-ray spectrometer employing a thin (50 µm) silicon transmission crystal was used to record high-resolution Cu Kα spectra from a laboratory x-ray source. The diffraction was from the (331) planes that were at an angle of 13.26° to the crystal surface. The components of the spectral lines resulting from single-vacancy (1s) and double-vacancy (1s and 3d) transitions were observed. After accounting for the natural lifetime widths from reference double-crystal spectra and the spatial resolution of the image plate detector, the intrinsic broadening of the transmission crystal was measured to be as small as 0.67 eV and the resolving power 12 000, the highest resolving power achieved by a compact (0.5 m long) spectrometer employing a single transmission crystal operating in the hard x-ray region. By recording spectra with variable source-to-crystal distances and comparing to the calculated widths from various geometrical broadening mechanisms, the primary contributions to the intrinsic crystal broadening were found to be the source height at small distances and the crystal apertured height at large distances. By reducing these two effects, using a smaller source size and vignetting the crystal height, the intrinsic crystal broadening is then limited by the crystal thickness and the rocking curve width and would be 0.4 eV at 8 keV energy (20 000 resolving power).

High-efficiency echelle gratings for MIGHTI, the spatial heterodyne interferometers for the ICON mission.

Development of a new generation of low-groove density-blazed echelle gratings optimized for MIGHTI, a space-borne spatial heterodyne interferometer operating in the visible and near infrared is described. Special demands are placed on the wavefront accuracy, groove profile, and efficiency of these gratings. These demands required a new ruling for this application, with significant improvements over existing gratings. Properties of a new generation of highly efficient, plane gratings with 64 grooves/mm blazed at 8.2° are reported.

Tunable hard X-ray spectrometer utilizing asymmetric planes of a quartz transmission crystal.

A Cauchois type hard x-ray spectrometer was developed that utilizes the (301) diffraction planes at an asymmetric angle of 23.51° to the normal to the surface of a cylindrically curved quartz transmission crystal. The energy coverage is tunable by rotating the crystal and the detector arm, and spectra were recorded in the 8 keV to 20 keV range with greater than 2000 resolving power. The high resolution results from low aberrations enabled by the nearly perpendicular angle of the diffracted rays with the back surface of the crystal. By using other asymmetric planes of the same crystal and rotating to selected angles, the spectrometer can operate with high resolution up to 50 keV.

Ultra-thin curved transmission crystals for high resolving power (up to E/ΔE = 6300) x-ray spectroscopy in the 6-13 keV energy range.

Ultra-thin curved transmission crystals operating in the Cauchois spectrometer geometry were evaluated for the purpose of achieving high spectral resolution in the 6-13 keV x-ray energy range. The crystals were silicon (111) and sapphire R-cut wafers, each 18 µm thick, and a silicon (100) wafer of 50-µm thickness. The W Lα(1) spectral line at 8.398 keV from a laboratory source was used to evaluate the resolution. The highest crystal resolving power, E/ΔE=6300, was achieved by diffraction from the (33-1) planes of the Si(100) wafer that was cylindrically bent to a radius of curvature of 254 mm, where the (33-1) planes have an asymmetric angle of 13.26° from the normal of the crystal surface facing the x-ray source. This work demonstrates the ability to measure highly resolved line shapes of the K transitions of the elements Fe through Kr and the L transitions of the elements Gd through Th using a relatively compact spectrometer optical system and readily available thin commercial wafers. The intended application is as a diagnostic of laser-produced plasmas where the presence of multiple charged states and broadenings from high temperature and density requires high-resolution methods that are robust in a noisy source environment.

Efficiency calibrations of cylindrically bent transmission crystals in the 20 to 80 keV x-ray energy range.

Two quartz (10-11) crystals were cylindrically bent to a 25.4 cm radius of curvature and were mounted in identical Cauchois-type transmission spectrometers, and the crystal diffraction efficiencies were measured to 5% absolute accuracy using narrow bandwidth x-ray source fluences in the 20 to 80 keV energy range. The measured integrated reflectivity values were compared to calculations performed using a computational model that accounts for the diffraction geometry of the bent transmission crystal. These crystal calibrations enable the accurate measurement of absolute hard x-ray emission levels from laser-produced plasmas and other laboratory sources.

Spatial resolution of a hard x-ray CCD detector.

The spatial resolution of an x-ray CCD detector was determined from the widths of the tungsten x-ray lines in the spectrum formed by a crystal spectrometer in the 58 to 70 keV energy range. The detector had 20 microm pixel, 1700 by 1200 pixel format, and a CsI x-ray conversion scintillator. The spectral lines from a megavolt x-ray generator were focused on the spectrometer's Rowland circle by a curved transmission crystal. The line shapes were Lorentzian with an average width after removal of the natural and instrumental line widths of 95 microm (4.75 pixels). A high spatial frequency background, primarily resulting from scattered gamma rays, was removed from the spectral image by Fourier analysis. The spectral lines, having low spatial frequency in the direction perpendicular to the dispersion, were enhanced by partially removing the Lorentzian line shape and by fitting Lorentzian curves to broad unresolved spectral features. This demonstrates the ability to improve the spectral resolution of hard x-ray spectra that are recorded by a CCD detector with well-characterized intrinsic spatial resolution.

Radiometry and metrology of a phase zone plate measured by extreme ultraviolet synchrotron radiation.

The diffraction efficiency, focal length, and other radiometric and metrology properties of a phase zone plate were measured by using monochromatic synchrotron radiation in the 7-18.5 nm wavelength range. The zone plate was composed of molybdenum zones having a 4 mm outer diameter and 70 nm nominal thickness and supported on a 100 nm thick silicon nitride membrane. The diffraction efficiency was enhanced by the phase shift of the radiation passing through the zones. The measured first-order efficiency was in good agreement with the calculated efficiency. The properties of the zone plate, particularly the small variation of the efficiency with off-axis angle, make it suitable for use in a radiometer to accurately measure the absolutely calibrated extreme ultraviolet emission from the Sun.

Performance optimization of Si/Gd extreme ultraviolet multilayers.

We compare the performance, stability and microstructure of Si/Gd multilayers containing thin barrier layers of W, B(4)C, or SiN(x), and determine that multilayers containing 0.6 nm thick W barrier layers at each interface provide the best compromise between high peak reflectance in the extreme ultraviolet near lambda=60 nm and good stability upon heating. The Si/W/Gd films have sharper interfaces and also show vastly superior thermal stability relative to Si/Gd multilayers without barrier layers. We find that these structures have relatively small compressive film stresses, and show good temporal stability thus far. We measured a peak reflectance of 29.7% at lambda=62.5 nm, and a spectral bandpass of Deltalambda=9 nm (FWHM), for an optimized Si/W/Gd multilayer having a period d=32.0 nm.

Measurement of dysprosium optical constants in the 2-830 eV spectral range using a transmittance method, and compilation of the revised optical constants of lanthanum, terbium, neodymium, and gadolinium.

The optical constants beta, delta of the complex refractive index (ñ = 1-delta+ibeta) of Dy were obtained in the 2-830 eV energy range using a novel transmittance method. Si/W/Dy/W films were deposited by dc-magnetron sputtering on Si photodiode substrates, and the transmittance was characterized using synchrotron radiation. The extinction coefficients beta of Dy and the transmittance of a Si capping layer and two W interface barrier layers as functions of energy were solved simultaneously using a nonlinear optimization routine. The measured transmittances of the capping and barrier layers were primarily used as indicators for any flaws in the transmittance results. The dispersion coefficients delta of Dy were calculated using the Kramers-Kronig integral, and a complete set of beta values required for this integral was obtained by combining the present data with data from the literature. Sum rule tests on Dy show some deficiencies in the present data, which may be attributed to lower film density compared with the bulk value. Similar procedures were applied to previously measured transmittances of B4C/La, Si/Tb, Si/Nd, and Si/Gd films, where B4C or Si were used as capping layers on those reactive rare-earth films. The improved sets of transmittance values of B(4)C and Si capping layers were used as input in the optimization routine to solve for more accurate beta values of La, Tb, Nd, and Gd. The revised optical constants of these materials, tested for consistency with partial sum rules, are also reported.

High-resolution imaging spectrometer for recording absolutely calibrated far ultraviolet spectra from laser-produced plasmas.

An imaging spectrometer was designed and fabricated for recording far ultraviolet spectra from laser-produced plasmas with wavelengths as short as 155 nm. The spectrometer implements a Cassegrain telescope and two gratings in a tandem Wadsworth optical configuration that provides diffraction limited resolution. Spectral images were recorded from plasmas produced by the irradiation of various target materials by intense KrF laser radiation with 248 nm wavelength. Two pairs of high-resolution gratings can be selected for the coverage of two wavebands, one grating pair with 1800 grooves/mm and covering approximately 155-175 nm and another grating pair with 1200 grooves/mm covering 230-260 nm. The latter waveband includes the 248 nm KrF laser wavelength, and the former waveband includes the wavelength of the two-plasmon decay instability at 23 the KrF laser wavelength (165 nm). The detection media consist of a complementary metal oxide semiconductor imager, photostimulable phosphor image plates, and a linear array of 1 mm(2) square silicon photodiodes with 0.4 ns rise time. The telescope mirrors, spectrometer gratings, and 1 mm(2) photodiode were calibrated using synchrotron radiation, and this enables the measurement of the absolute emission from the laser-produced plasmas with temporal, spatial, and spectral resolutions. The spectrometer is capable of measuring absolute spectral emissions at 165 nm wavelength as small as 5x10(-7) J/nm from a plasma source area of 0.37 mm(2) and with 0.4 ns time resolution.

Enhanced x-ray resolving power achieved behind the focal circles of Cauchois spectrometers.

Maintaining high resolving power is a primary challenge in hard x-ray spectroscopy of newly developed bright and transient x-ray sources such as laser-produced plasmas. To address this challenge, the line widths in x-ray spectra with energies in the 17 keV to 70 keV range were recorded by positioning the detectors on and behind the focal circles of Cauchois type transmission-crystal spectrometers. To analyze and understand the observed line widths, we developed a geometrical model that accounts for source broadening and various instrumental broadening mechanisms. The x-ray sources were laboratory Mo or W electron-bombarded anodes, and the spectra were recorded on photostimulable phosphor image plates. For these relatively small x-ray sources, it was found that when the detector was placed on or near the focal circle, the line widths were dominated by the effective spatial resolution of the detector. When the detector was positioned beyond the focal circle, the line widths were determined primarily by source-size broadening. Moreover, the separation between the spectral lines increased with distance behind the focal circle faster than the line widths, resulting in increased resolving power with distance. Contributions to line broadenings caused by the crystal thickness, crystal rocking curve width, geometrical aberrations, and natural widths of the x-ray transitions were in all cases smaller than detector and source broadening, but were significant for some spectrometer geometries. The various contributions to the line widths, calculated using simple analytical expressions, were in good agreement with the measured line widths for a variety of spectrometer and source conditions. These modeling and experimental results enable the design of hard x-ray spectrometers that are optimized for high resolving power and for the measurement of the x-ray source size from the line widths recorded behind the focal circle.

Normal-incidence silicon-gadolinium multilayers for imaging at 63 nm wavelength.

Si/Gd multilayers designed as narrowband reflective coatings near 63 nm were developed. The highest peak reflectance of 26.2% at a 5 degrees incident angle was obtained at 62 nm, and the spectral bandwidth was 7.3 nm FWHM. The fits for x-ray and extreme ultraviolet reflectance data of Si/Gd multilayers indicate the possibility of silicide formation at the Si-Gd interfaces. B(4)C, W, and SiN were deposited as interface barrier layers to improve the reflectance of Si/Gd multilayers. More than an 8% increase in reflectance was observed from the interface-engineered Si/W/Gd and Si/B(4)C/Gd multilayers.

X-ray modulation transfer functions of photostimulable phosphor image plates and scanners.

The modulation transfer functions of two types of photostimulable phosphor image plates were determined in the 10 keV to 50 keV x-ray energy range using a resolution test pattern with up to 10 line pairs per mm (LP/mm) and a wavelength dispersive x-ray spectrometer. Techniques were developed for correcting for the partial transmittance of the high energy x rays through the lead bars of the resolution test pattern, and the modulation transfer function (MTF) was determined from the measured change in contrast with LP/mm values. The MTF was convolved with the slit function of the image plate scanner, and the resulting point spread functions (PSFs) were in good agreement with the observed shapes and widths of x-ray spectral lines and with the PSF derived from edge spread functions. The shapes and the full width at half-maximum (FWHM) values of the PSF curves of the Fuji Superior Resolution (SR) and Fuji Maximum Sensitivity (MS) image plate detectors, consisting of the image plate and the scanner, determined by the three methods gave consistent results: The SR PSF is Gaussian with 0.13 mm FWHM, and the MS PSF is Lorentzian with 0.19 mm FWHM. These techniques result in the accurate determination of the spatial resolution achievable using image plate and scanner combinations and enable the optimization of spatial resolution for x-ray spectroscopy and radiography.

Optics and mechanisms for the Extreme-Ultraviolet Imaging Spectrometer on the Solar-B satellite.

The Extreme-Ultraviolet Imaging Spectrometer (EIS) is the first of a new generation of normal-incidence, two-optical-element spectroscopic instruments developed for space solar extreme-ultraviolet astronomy. The instrument is currently mounted on the Solar-B satellite for a planned launch in late 2006. The instrument observes in two spectral bands, 170-210 A and 250-290 A. The spectrograph geometry and grating prescription were optimized to obtain excellent imaging while still maintaining readily achievable physical and fabrication tolerances. A refined technique using low ruling density surrogate gratings and optical metrology was developed to align the instrument with visible light. Slit rasters of the solar surface are obtained by mechanically tilting the mirror. A slit exchange mechanism allows selection among four slits at the telescope focal plane. Each slit is precisely located at the focal plane. The spectrograph imaging performance was optically characterized in the laboratory. The resolution was measured using the Mg iii and Ne iii lines in the range of 171-200 A. The He ii line at 256 A and Ne iii lines were used in the range of 251-284 A. The measurements demonstrate an equivalent resolution of ~2 arc sec? on the solar surface, in good agreement with the predicted performance. We describe the EIS optics, mechanisms, and measured performance.

Laboratory calibration of the Extreme-Ultraviolet Imaging Spectrometer for the Solar-B satellite.

The laboratory end-to-end testing of the Extreme-Ultraviolet Imaging Spectrometer (EIS) for the Solar-B satellite is reported. A short overview of the EIS, which observes in two bands in the extreme-ultraviolet wavelength range, is given. The calibration apparatus is described, including details of the light sources used. The data reduction and analysis procedure are outlined. The wavelength calibration using a Penning source to illuminate the aperture fully is presented. We discuss the aperture determination using a radiometrically calibrated hollow-cathode-based source. We then give an account of the predicted and measured efficiencies from consideration of the efficiencies of individual optical elements in first order, an account of efficiencies out of band when radiation incident in one band is detected in the other, and efficiencies in multiple orders. The efficiencies measured in first order for in band and out of band are compared with the predictions and the sensitivity, and its uncertainties are derived. Application of the radiometric calibration is discussed.

SiC/Tb and Si/Tb multilayer coatings for extreme ultraviolet solar imaging.

Narrowband SiC/Tb and Si/Tb multilayers are fabricated with as much as a 23% normal-incidence reflectance near a 60 nm wavelength and spectral bandpass (FWHM) values of 9.4 and 6.5 nm, respectively. The structural properties of the films are investigated using extreme ultraviolet and x-ray reflectometry and transmission electron microscopy. Thermal stability is investigated in films annealed to as high as 300 degrees C. Because of their superior thermal stability, relatively high reflectance, and narrower spectral bandpass, Si/Tb multilayers are identified as optimal candidates for solar physics imaging applications, where the peak response can be tuned to important emission lines such as O v near 63.0 nm and Mg x near 61.0 nm. We describe our experimental procedures and results, discuss the implications of our findings, and outline prospects for improved performance.

Response of a SiC photodiode to extreme ultraviolet through visible radiation.

The responsivity of a type 6H-SiC photodiode in the 1.5-400 nm wavelength range was measured using synchrotron radiation. The responsivity was 0.20 A/W at 270 nm and was less than 0.10 A/W in the extreme ultraviolet (EUV) region. The responsivity was calculated using a proven optical model that accounted for the reflection and absorption of the incident radiation and the variation of the charge collection efficiency (CCE) with depth into the device. The CCE was determined from the responsivity measured in the 200-400 nm wavelength range. By use of this CCE and the effective pair creation energy (7.2 eV) determined from x-ray absorption measurements, the EUV responsivity was accurately modeled with no free parameters. The measured visible-light sensitivity, although low compared with that of a silicon photodiode, was surprisingly high for this wide bandgap semiconductor.