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Polynomial fit coefficients have been obtained for the energy dependences of the photoelectric absorption cross sections of 17 astrophysically important elements. These results allow the calculation of X-ray absorption in the energy range 0.03-10 keV in material with noncosmic abundances.
Cr is a ubiquitous and relatively abundant minor element in basaltic, planetary magmas. At the reduced oxidation states (<FMQ) of many planetary basalts Cr is present in melts as both divalent and trivalent forms. The ratio of trivalent to divalent Cr present in the melt has many consequences for the stability and Cr concentration of magmatic phases such as spinel, clinopyroxene, and olivine. However, understanding the Cr valence in quenched melts has historically been plagued with analytical issues, and only recently has reliable methodology for quantifying Cr valence in quenched melts been developed. Despite this substantial difficulty, the pioneering works of Hanson and Jones and Berry and O'Neill provided important insights into the oxidation state of Cr in in silicate melts. Here we present a series of 1-bar gas mixing experiments performed with a Fe-rich basaltic melt in which have determined the Cr redox ratio of the melt at over a range of fO2 values by measuring this quantity in olivine with X-ray Absorption Near Edge Spectroscopy (XANES). The measured Cr redox ratio of the olivine phenocrysts can be readily converted to the ratio present in the conjugate melt via the ratio of crystal-liquid partition coefficients for Cr3+ and Cr2+. We have applied these results to modeling Cr spinel stability and Cr redox ratios in a primitive, iron-rich martian basalt.
Limits to the first-order Compton scattering, brightness temperature, and angular size of the compact radio source AO0235+164 were calculated on the basis of a flux density of less than about 1 microJy at 10 to the 18th Hz calculated from observations of the source with the rotating modulation collimator aboard the SAS 3 X-ray observatory. It was found that the maximum brightness temperature at this frequency is less than 2.4 times 10 to the 11th K, corresponding to a minimum angular size of 0.38 msec of arc, assuming the synchrotron self-absorption turnover coefficient is 0.5. On the basis of the distance implied by the largest redshift present, the energy contained in relativistic particles and the magnetic field are calculated, and the ratio of particle to magnetic energy is greater than 10, so the dynamics must be particle dominated. This indicates relativistic expansion if the mean energy per particle is comparable to or greater than the rest of those particles that neutralize the electron plasma.
The construction of composition profiles from X-ray intensity bands was investigated. The intensity band-to-composition profile transformation utilizes a solution which can be easily evaluated. The technique can be applied to thin films and thick speciments for which the variation of lattice parameters, linear absorption coefficient, and reflectivity with composition are known. A deconvolution scheme with corrections for the instrumental broadening and ak-alfadoublet is discussed.
A sealed Ti window proportional counter sensitive to X radiation in the energy range 0.35-0.45 keV and above 1.5 keV is described. Measurements of the Ti mass absorption coefficients and a graphical summary of the literature values are presented. For a proportional counter with a 930 microgram/sq cm (2.1-micron) Ti window, the peak efficiency at 0.45 keV is found to lie between 4.6% and 7.1%. An application in X-ray astronomy involving a rocket observation of Sco X-1 is discussed.
A fine grain diamond film has been developed by microwave plasma assisted chemical vapor deposition. Various analytical techniques, including Rutherford backscattering, proton recoil analysis, Raman spectroscopy, and X-ray diffraction, were utilized to characterize the diamond films. The grain size of the film was determined from bright and dark field electron micrographs, and found to be 200-1000 A. The films exhibited good optical transmission between 2.5 and 10 microns, with a calculated absorption coefficient of 490/cm. The friction coefficients of this film were found to be 0.035 and 0.030 at dry nitrogen and humid air environments, respectively, and the films had low wear rates.
In this study, the solidus curve of the Cd0.8Zn0.2Te homogeneity range was constructed from the partial pressure measurements by optical absorption technique which provided the information of the melt growth parameters to process crystals with the required electrical resistivity. The melt growth of Cd0.8Zn0.2Te crystals were then processed by directional solidification under controlled Cd overpressure. During the growth experiments, several procedures have been developed to improve the crystalline quality: (1) minimizing the contamination of impurities (2) improving the structural defects and (3) minimizing the Te-precipitates within the grown crystals to enhance charge transport properties. Additionally, the thermal conductivity, electrical conductivity, and Seebeck coefficient of a vapor-grown CdTe and two melt-grown Cd0.8Zn0.2Te crystals were measured between 190oC and 780oC to provide an in-depth understanding of the thermal and electrical conduction mechanisms of the crystals as well as the prospect of its thermoelectric applications.
In order to search for such fluid inclusions in carbonaceous chondrites, a nondestructive technique using x-ray micro-absorption tomography combined with FIB sampling was developed and applied to a carbonaceous chondrite. They found fluid inclusion candidates in calcite grains, which were formed by aqueous alteration. However, they could not determine whether they are really aqueous fluids or merely voids. Phase and absorption contrast images can be simultaneously obtained in 3D by using scanning-imaging x-ray microscopy (SIXM). In refractive index, n=1-sigma+i(beta), in the real part, 1-sigma is the refractive index with decrement, sigma, which is nearly proportional to the density, and the imaginary part, beta, is the extinction coefficient, which is related to the liner attenuation coefficient, mu. Many phases, including water and organic materials as well as minerals, can be identified by SIXM, and this technique has potential availability for Hayabusa-2 sample analysis too. In this study, we examined quantitative performance of d and m values and the spatial resolution in SIXM by using standard materials, and applied this technique to carbonaceous chondrite samples. We used POM ([CH2O]n), silicon, quartz, forsterite, corundum, magnetite and nickel as standard materials for examining the sigma and mu values. A fluid inclusion in terrestrial quartz and bi-valve shell (Atrina vexillum), which are composed of calcite and organic layers with different thickness, were also used for examining the spatial resolution. The Ivuna (CI) and Sutter's Mill (CM) meteorites were used as carbonaceous chondrite samples. Rod- or cube-shaped samples 20-30 micron in size were extracted by using FIB from cross-sectional surfaces of the standard materials or polished thin sections of the chondrites, which was previously observed with SEM. Then, the sample was attached to a thin W-needle and imaged by SIXM system at beamline BL47XU, SPring-8, Japan. The slice thickness was 109.3 nm and the pixel size was mostly 100 nm.
The oxidation state of Cr in 200-micron regions within individual lunar olivine and pyroxene grains from lunar basalt 15555 was inferred using X-ray Absorption Near Edge Structure (XANES). Reference materials had previously been studied by optical absorption spectroscopy and included Cr-bearing borosilicate glasses synthesized under controlled oxygen fugacity and Cr-doped olivines. The energy dependence of XANES spectral features defined by these reference materials indicated that Cr is predominantly divalent in the lunar olivine and trivalent in the pyroxene. These results, coupled with the apparent f(02)-independence of partitioning coefficients for Cr into olivine, imply that the source magma was dominated by divalent Cr at the time of olivine crystallization.
The metal-silicate partitioning behavior of molybdenum has been used as a test for equilibrium core formation hypotheses [for example, 1-6]. However, current models that apply experimental data to equilibrium core-mantle differentiation infer the oxidation state of molybdenum from solubility data or from multivariable coefficients from metal-silicate partitioning data [1,3,7]. Molybdenum, a multi-valent element with a valence transition near the fO2 of interest for core formation (approx.IW-2) will be sensitive to changes in fO2 of the system and silicate melt structure. In a silicate melt, Mo can occur in either 4+ or 6+ valence state, and Mo(6+) can be either octahedrally or tetrahedrally coordinated. Here we present X-ray absorption near edge structure (XANES) measurements of Mo valence in basaltic run products at a range of P, T, and fO2 and further quantify the valence transition of Mo.
Striations in Czochralski (CZ) grown crystals have been observed in materials such as GaAs, silicon, photorefractive crystals used for data storage, potassium titanyl phosphate crystals and LiNbO3. Several techniques have been used for investigating these defects including electron microscopy, laser scanning tomography, selective photoetching, X-ray diffuse scattering, interference orthoscopy, laser interferometry and micro-Fourier transform infrared spectroscopy mapping. A 2mm thick sample of the material to be investigated is illuminated with light that is absorbed and non-absorbed by the ion concentration to be observed. The back surface of the sample is focused onto a solid-state image detector and images of the input beam and absorbed (and diffracted) beams are captured at two wavelengths. The variation of the coefficient of absorption asa function of distance on the sample can be derived from these measurements. A Big Sky Software Beamcode system is used to capture and display images. Software has been written to convert the Beamcode data files to a format that can be imported into a spreadsheet program such as Quatro Pro. The spreadsheet is then used to manipulate and display data. A model of the intensity map of the striae collected by the imaging system has been proposed and a data analysis procedure derived. From this, the variability of the attenuation coefficient alpha can be generated. Preliminary results show that alpha may vary by a factor of four or five over distances of 100 mu m. Potential errors and problems have been discovered and additional experiments and improvements to the experimental setup are in progress and we must now show that the measurement techniques and data analysis procedures provide 'real' information. Striae are clearly visible at all wavelengths including white light. Their basic spatial frequency does not change radically, at least when changing from blue to green to white light. Further experimental and theoretical work can be done to improve the data collection techniques and to verify the data analysis procedures.
The IR absorption in polycrystalline diamond films deposited in oxygen-acetylene flames is characterized using FTIR. The one-phonon absorption coefficient in the region from 7 to 12 microns that is related to extrinsic defects in the diamond films shows a strong dependence on the flame conditions as well as the substrate temperature. A high degree of diamond crystalline perfection, as judged from the undetectable one-phonon absorption, is achieved under the optimized deposition conditions for our flame setup. This is further supported by the sharp Raman peak at 1332/cm as well as the high purity in crystal orientation according to the X-ray diffraction pattern measured for the high quality diamond films.
XMM-Newton and Chandra observations of active galactic nuclei (AGN) show rich spectra of X-ray absorption lines. These observations have detected a broad unresolved transition array (UTA) between approx. 15-17 A. This is attributed to inner-shell photoexcitation of M-shell iron ions. Modeling these UTA features is currently limited by uncertainties in the low-temperature dielectronic recombination (DR) data for M-shell iron. In order to resolve this issue, and to provide reliable iron M-shell DR data for plasma modeling, we are carrying out a series of laboratory measurements using the heavy-ion Test Storage Ring (TSR) at the Max-Plank-Institute for Nuclear Physics in Heidelberg, Germany. Currently, laboratory measurements of low temperature DR can only be performed at storage rings. We use the DR data obtained at TSR, to calculate rate coefficients for plasma modeling and to benchmark theoretical DR calculations. Here we report our recent experimental results for DR of Fe XIV forming Fe XIII.
We develop a general expression for the gamma - gamma absorption coefficient, alpha(sub gamma(gamma)) for gamma-rays propagating in an arbitrary direction at an arbitrary point in space above an X-ray-emitting accretion disk. The X-ray intensity is assumed to vary as a power law in energy and radius between the outer disk radius, R(sub 0), and the inner radius, R(sub ms) which is the radius of marginal stability for a Schwarzschild black hole. We use our result for alpha(sub gamma(gamma)) to calculate the gamma - gamma optical depth, tau(sub gamma(gamma)) for gamma - rays created at height z and propagating at angle Phi relative to the disk axis, and we show that for Phi = 0 and z greater than or approx equal to R(sub 0), tau(sub gamma(gamma)) proportional to Epsilon(sup alpha)z(sup -2(alpha) - 3), where alpha is the X-ray spectral index and Epsilon is the gamma - ray energy. As an application, we use our formalism to compute the minimum distance between the central black hole and the site of production of the gamma-rays detected by EGRET during the 1991 June flare of 3C 279. In order to obtain an upper limit, we assume that all of the X-rays observed contemporaneously by Ginga were emitted by the disk. Our results suggest that the observed gamma - rays may have originated within less than or approx equal to 45 GM/sq c from a black hole of mass greater than or approx equal to 10(exp 9) solar mass, perhaps in active plasma located above the central funnel of the accretion disk. This raises the possibility of establishing a direct connection between the production of the observed gamma - rays and the accretion of material onto the black hole. We also consider the variation of the optical depth as a function of the angle of propagation Phi. Our results indicate that the "focusing" of the gamma - rays along the disk axis due to pair production is strong enough to explain the observed degree of alignment in blazar sources. If the gamma - rays are produced isotropically in gamma - ray blazars, then these objects should appear as bright MeV sources when viewed along off-axis lines of sight.
The properties of single crystals of cadmium sulfide as radiation detectors are described. It has been found possible to select crystals such that: (a) The ratio of increase of conductivity under irradiation to the rate of absorption of energy in the crystal is substantially independent of particle energy (over the examined ranges of 500 ev to 80 kev for electrons and 5 kev to 180 kev for protons) and of the magnitude of energy flux (over the range from.005 to 10 ergs/cm(sup 2 -sec); and (b) The above ration is substantially the same for protons, electrons, alpha particles, x-rays, and gamma-rays. For a driving voltage of 100 volts, typical crystal yield currents of 10(sup -7) to 10(sup- 6) amperes for each erg/cm(sup 2-sec) of energy absorbed by the crystal. The threshold of such crystal detectors (resulting from dark currents of the order of 10(sup 10 amp) is typically 10(sup -3) ergs/cm(sup 2- sec). For the selected crystals a response-temperature coefficient of -0.25% per degree centigrade is found for the temperature range -50 deg C to + 50 deg C. A description is given of a complete CdS total corpuscular energy detector for the study of geomagnetically trapped radiation by means of a satellite. The detector described has a dynamic range great than 10(sup 4), a solid angle of 10(exp -3) steradian, and a detection threshold of approximately 1 erg/cm(sup 2-sec-sterad). A similar detector employing a small magnet for the selective exclusion of electrons is also described. Noteworthy practical features of these detectors for satellite and space probe experiments are: (a) Use of bare crystals, without covering foils, in order to detect charged particles having energies as low as hundreds of electron volts. (b) Simplicity of electronic auxiliaries. (c) Compactness, lightweight and nechanical ruggedness. (d) Low electrical power requirements; and (e) Conversion of conduction current to the rate of a twostate relaxation oscillator in order to facilitate telemetric transmission of data. A pair of such detectors was flown as part of the s-46 satellite payload on March 23, 1960, but due to vehicular failure an orbit was not achieved and the operation of the CdS detectors was observed for only, six minutes of flight.