Efficiency of a grazing-incidence off-plane grating in the soft-x-ray region.

Efficiency measurements of a grazing-incidence diffraction grating in the off-plane mount were performed using polarized synchrotron radiation. The grating had 5000 grooves/mm, an effective blaze angle of 14 degrees, and was gold coated. The efficiencies in the two polarization orientations (TM and TE) were measured in the 1.5-5.0 nm wavelength range and were compared with the efficiencies calculated using the PCGrate-SX code. The TM and TE efficiencies differ, offering the possibility of performing unique science studies of astrophysical, solar, and laboratory sources by exploiting the polarization sensitivity of the off-plane grating.

Flash x-ray observations of cavitation in cadaver thighs caused by high-velocity bullets.

BACKGROUND: The purpose of this study was to record flash x-ray images of cavitation in human cadaver thighs caused by the passage of high-velocity bullets. The images are an initial step for understanding the cavitation process in human tissue and for implementing a better definition of extensive tissue injury. METHODS: Bullets were fired through the mid-thighs of 13 cadaver legs. The bullets were of two calibers, 7.62-mm full metal jacket boat tail with strike velocities in the range of 794 m/s to 880 m/s (10 thighs) and 5.70 mm full metal jacket with velocities in the range of 973 m/s to 992 m/s (3 thighs). Short duration (35 ns) x-ray images were recorded at various selected times after the bullets passed near the femurs. This study was carried out at the Armed Forces Institute of Pathology under approved human subject protocols. RESULTS: The cavity sizes and shapes were observed for the two types of bullets and at a number of times during the expansion and collapse of the cavities. As the bullets passed through the thighs, narrow cavities behind the bullets were observed. At later times, large expanded cavities were observed that encompassed the entire mid-thigh region. The observed cavities are at variance with those which were reported previously in gelatin tissue simulants. CONCLUSION: Flash x-ray radiography is an effective technique for the observation of internal cavitation in cadaver thighs caused by high-velocity bullets. These observations suggest that gelatin is not a proven simulant for human cadaver tissue in the study of cavitation subsequent to high-velocity missile impact.

Calibration of an extreme-ultraviolet transmission grating spectrometer with synchrotron radiation.

The responsivity of an extreme-ultraviolet transmission grating spectrometer with silicon photodiode detectors was measured with synchrotron radiation. The spectrometer was designed to record the absolute radiation flux in a wavelength bandpass centered at 30 nm. The transmission grating had a period of 200 nm and relatively high efficiencies in the +1 and the -1 diffraction orders that were dispersed on either side of the zero-order beam. Three photodiodes were positioned to measure the signals in the zero order and in the +1 and -1 orders. The photodiodes had aluminum overcoatings that passed the desired wavelength bandpass centered at 30 nm and attenuated higher-order radiation and wavelengths longer than approximately 80 nm. The spectrometer's responsivity, the ratio of the photodiode current to the incident radiation power, was determined as a function of the incident wavelength and the angle of the spectrometer with respect to the incident radiation beam. The spectrometer's responsivity was consistent with the product of the photodiode responsivity and the grating efficiency, both of which were separately measured while removed from the spectrometer.

Dual-energy bone densitometry using a single 100 ns x-ray pulse.

A pulsed, portable hard x-ray source has been developed for medical imaging and flash x-ray absorptiometry. The source is powered by a Marx generator that drives a field emission x-ray tube which produces a 30-300 keV x-ray pulse of 100 ns duration. The x-ray fluence has dual-energy properties. The x-ray energy is relatively high early in the pulse and lower later in the pulse. The feasibility of using a single x-ray pulse for precision bone densitometry was analyzed. A computer simulation model was developed for the x-ray source, the filtration that enhances the dual-energy distribution, the absorption of the energy distribution by bone mineral and soft tissue, and the dual-energy detection system. It is feasible to determine the bone mineral density (BMD) of axial sites such as the lumbar spine and proximal femur with 2% precision over an area that is 15-20 mm in size, depending on the bone mineral and soft tissue thicknesses. An algorithm for determining the absolute BMD, to an accuracy of 2%, using a Plexiglas/TiO2 calibration phantom is discussed. At a distance of 50 cm from the source, the patient exposure is 3.7 mR. The average absorbed bone and tissue doses are 0.6 and 4.3 mrem, respectively. Factors that facilitate diagnostic measurements in clinical settings are the short patient observation time and the portability of the x-ray source.

Uniformity of the Soft-X-Ray Emissions from Gold Foils Irradiated by OMEGA Laser Beams Determined by a Two-Mirror Normal-Incidence Microscope with Multilayer Coatings.

A two-mirror normal-incidence microscope with multilayer coatings was used to image the soft-x-ray emissions from planar foils irradiated by OMEGA laser beams. The bandpass of the multilayer coatings was centered at a wavelength of 48.3 ? (257-eV energy) and was 0.5 ? wide. Five overlapping OMEGA beams, without beam smoothing, were typically incident on the gold foils. The total energy was 1500 J, and the focused intensity was 6 x 10(13) W cm(-2). The 5.8x magnified images were recorded by a gated framing camera at various times during the 3-ns laser pulse. A pinhole camera imaged the x-ray emission in the energy range of >2 keV. On a spatial scale of 10 mum, it was found that the soft-x-ray images at 257 eV were quite uniform and featureless. In contrast, the hard-x-ray images in the energy range of >2 keV were highly nonuniform with numerous features of size 150 mum.

Effect of energetic electron and proton bombardment on the reflectance of silicon-carbide mirrors in the extreme-ultraviolet region.

Chemical-vapor-deposited silicon-carbide mirrors were exposed to bombardment by 95-keV electrons and 100-keV protons with accumulated fluxes comparable with those expected in low-altitude Earth orbit for simulated periods of as many as 5 years. The reflectances of four mirrors were measured at five wavelengths (58.4, 73.6, 104.8, 121.6, and 161 nm) and at 11 angles of incidence from 5° to 80°. The electron and proton exposure resulted in no significant change in the reflectances of the mirrors.

Effect of oxygen atom bombardment on the reflectance of silicon carbide mirrors in the extreme ultraviolet region.

Chemical-vapor-deposited silicon carbide mirrors were exposed to bombardment by 8-km/s oxygen atoms that simulated the effects of exposure in low Earth orbit for periods up to 7.5 yr. The reflectances of four mirrors were measured before and after exposure at five wavelengths (58.4, 73.6, 104.8, 121.6, and 161 nm) and at 11 angles of incidence from 5 degrees to 80 degrees . The oxygen exposure reduced the normal-incidence reflectances by factors of 1.5-4.5 in the wavelength rate of 58.4-121.6 nm but had no effect on the visual appearance. The optical constants and the thicknesses of the thin surface layers present on the SiC substrates were determined from the reflectance measurements. This analysis indicated that before exposure the surface layers were composed of SiO(x) (where x is ~ 1.5) with thicknesses of 0.8-1.8 nm. After exposure to 8-km/s oxygen atoms, the surface layers were composed of SiO(x) with thicknesses of 3.5-4.5 nm. There were no systematic differences between the measured reflectances after simulated space ex osures of 1.5, 4.5, and 7.5 years. This implied that most of the growth in thickness of the SiO(x). layers occurred early in the exposure and stabilized at thicknesses of 3.5-4.5 nm. The optical results were consistent with x-ray photoelectron spectroscopy of the four mirrors after oxygen exposure.

Dual-waveband operation of a multilayer-coated diffraction grating in the soft x-ray range at near-normal incidence.

A concave diffraction grating (2400 grooves/mm) coated with a Si/Mo multilayer has an efficiency of 2.5% at 290 A and a resolving power of 14,000 in third order of 156 A.

On-blaze operation of a Mo/Si multilayer-coated, concave diffraction grating in the 136-142-A wavelength region and near normal incidence.

The efficiency and resolving power of a concave, 2400-groove/mm, blazed diffraction grating that had a Mo/Si multilayer coating were determined. The multilayer coating had a peak reflectance of 55% at 140-A near normal incidence. The efficiency of the multilayer grating for wavelengths in the 136-139-A range was 2% near normal incidence. This efficiency was a factor of 150 greater than the efficiency of a sister replica Au-coated grating in the same wavelength region. The resolving power of the multilayercoated grating in the third order of a V viii transition with a first-order wavelength of 140.451 A was 9100. Comparisons with the Au-coated grating indicated that the application of the multilayer coating did not affect the resolving power or the blaze angle.

High-resolution imaging of laser-produced plasmas at a wavelength of 130 A by a normal-incidence multilayer-mirror microscope.

Laser-produced plasmas were imaged by a microscope consisting of spherical, primary, and secondary mirrors in a Cassegrain-type optical configuration. The mirrors were coated with 40 periods of Mo and Si and had a peak reflectance of ∼60% at a wavelength of 130 Å. Nb, Au, and Al targets were irradiated by as many as 24 beams of the Omega laser at the Laboratory for Laser Energetics at the University of Rochester. The images were recorded on Kodak 101 film, and the resolution was limited by the 10-µm emulsion grain size. Two different secondary mirrors produced images with a magnification of 0.8 or 2.2, and a variety of plasma emission features were recorded with 5-µm (0.6-arcsec) resolution in the target plane. The possibility of operating a similar microscope in the wavelength region of 34-50 A is discussed.