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NASA NTRS1988

The scattered solar X-ray background in low earth orbit

In order to interpret X-ray observations of the sun-lit earth obtained with the IPC, a detailed model is developed that treats the radiative transfer of solar X-rays in single scattering approximation and employs the relevant scattering processes (elastic Thomson scattering and inelastic fluorescent scattering). The X-ray-bright earth, one of the strongest soft X-ray sources seen by the IPC, can then be understood in terms of solar X-rays scattered in the upper atmosphere. Using the CIRA 1972 Reference Atmosphere, it is possible to account for the observed 'bright earth' X-ray light curves under a variety of different viewing geometries. It is argued that the observed changes in hardness ratio of the scattered radiation can be interpreted as an indication of a change in the ratio of Thomson and fluorescently scattered photons as a function of zenith angle. The relevance of bright earth X-ray observations is further discussed in the context of operating X-ray telescopes in low-earth orbit, as well as in a broader astrophysical context.

Fink, H. H.↗

NASA NTRS1992

Raman Lidar Measurements of Pinatubo Aerosols over Southeastern Kansas During November-december 1991

The eruptions of the Philippine volcano Pinatubo during June 1991 produced large amounts of stratospheric aerosols that could significantly affect earth's climate as well as trigger stratospheric ozone depletion through heterogeneous chemical reactions. Information regarding the physical and optical properties of these aerosols is required to quantify those effects. By measuring both the elastically backscattered signal and the inelastic signal produced by Raman scattering from nitrogen molecules, Raman lidar can provide some of this information. In this presentation we discuss Raman lidar measurements of the scattering ratio, backscattering, extinction, extinction/backscattering ratio, and optical thickness of the Pinatubo aerosols over southeastern Kansas made on 10 nights during November and December, 1991. The Raman lidar developed at GSFC is a trailer-based system which uses an XeF excimer laser to transmit light at 351 nm. The light backscattered by molecules and aerosols at this wavelength is detected as well as Raman scattered light from water vapor, nitrogen, and oxygen molecules. Since background skylights interfere with the detection of the Raman signals the data discussed in this paper were acquired only at night.

Ferrare, R. A.↗

NASA NTRS1996

Thermodynamic measurements in a high pressure hydrogen-oxygen flame using Raman scattering from a broadband excimer laser

Raman scattering is an inelastic molecular scattering process in which incident radiation is reemitted at a fixed change in frequency. Raman spectroscopy can be used to measure the number density and temperature of the irradiated species. The strength of the Raman signal is inversely proportional to the wavelength raised to the fourth power. Consequently, high signal to noise ratios are obtained by using ultraviolet (UV) excitation sources. Using UV sources for Raman Spectroscopy in flames is complicated by the fact that some of the primary constituents in hydrogen-oxygen combustion absorb and reemit light in the UV and these fluorescence processes interfere with the Raman signals. This problem has been handled in atmospheric pressure flames in some instances by using a narrowband tunable excimer laser as a source. This allows for detuning from absorption transitions and the elimination of interfering fluorescence signals at the Raman wavelengths. This approach works well in the atmospheric pressure flame; however, it has two important disadvantages. First, injection-locked narrowband tunable excimer lasers are very expensive. More importantly, however, is the fact that at the high pressures characteristic of rocket engine combustion chambers, the absorption transitions are broadened making it difficult to tune to a spectral location at which substantial absorption would not occur. The approach taken in this work is to separate the Raman signal from the fluorescence background by taking advantage of the fact that Raman signal has nonisotropic polarization characteristics while the fluorescence signals are unpolarized. Specifically, for scattering at right angles to the excitation beam path, the Raman signal is completely polarized. The Raman signal is separated from the fluorescence background by collecting both horizontally and vertically polarized signals separately. One of the polarizations has both the Raman signal and the fluorescence background while the other has only the fluorescence signal. The Raman scatter is the difference between the signals. By choosing an appropriate optical setup, both signals can be obtained simultaneously with the same monochromator; hence, time resolved measurements are possible using this approach.

Hartfield, Roy, Jr.↗

NASA NTRS1972

Redistribution of resonance radiation. I - The effect of collisions.

The techniques of modern line-broadening theory are used to investigate the scattering of polarized radiation in the rest frame of an atom undergoing collisions. The formulation explicitly includes both elastic and inelastic (quenching) collisions. When the lower state has zero width, a form for the redistribution function similar to that of Zanstra is obtained, but with the redistribution in the neighborhood of the resonance line being caused solely by elastic collisions. In the limit of no collisions, but with both levels of finite lifetime, the result of Weisskopf and Woolley is obtained. The effect of level-degeneracy is also explicitly included; in this case the results are a function of the polarization of the light and the different relaxation rates for the multipolar components of the atomic states.

Omont, A.↗

NASA NTRSDate not supplied

Fractionation of Iron and Titanium Isotopes By Ilmenite and the Isotopic Compositions of Lunar Magma Ocean Cumulates

Basaltic volcanism on the Moon produced low- and high-Ti mare basalt suites that are also distinct with respect to their iron, titanium, and magnesium iso- topic compositions. Here, the equilibrium fractionation of Fe and Ti isotopes between ilmenite and melt was experimentally investigated in order to evaluate the role of ilmenite in generating the isotopic compositional variability among the lunar mare basalts. Ilmenite crystallization experiments were conducted using two bulk compositions: an ilmenite-saturated basaltic andesite and an ilmenite-saturated Apollo 14 black glass, and the Fe and Ti isotopic compo- sitions of the experimental ilmenites and glass (quenched melt) were analyzed using solution MC-ICPMS after hand-picking. Additionally, Nuclear Resonant Inelastic X-ray Scattering (NRIXS) measurements on synthetic ilmenite were conducted and compared to previous NRIXS measurements on synthetic lunar glasses in order to derive temperature-dependent equilibrium ilmenite-melt Fe isotopic fractionations. Experimentally determined ilmenite-melt fractionations were then incorporated into a lunar magma ocean crystallization model that tracks the major element and isotopic compositional evolution of lunar magma ocean cumulates and residual liquid. There is good agreement between the Fe equilibrium isotopic fractionation measured by NRIXS and the laboratory equili- bration experiments, and we find that the isotopic fractionation is sensitive to il- menite compositional differences (0 vs. 10% Fe3+). Further, the light Ti isotopic composition of ilmenite relative to the melt (∆49Ti ilmenite-melt = −0.09 ± 0.03h at 1100°C) is consistent with the higher coordination of Ti in ilmenite relative to melts and results of previous studies. The modeled Ti isotopic compositions for lunar magma ocean cumulates display Ti isotopic variability sufficient to explain the low- and high-Ti mare basalt sources. However, the difference in Fe isotopic composition between the low- and high-Ti mare basalts cannot be attributed solely to ilmenite fractionation. Instead, Fe isotopic fractionation by additional products of lunar magma ocean crystallization, such as clinopy- roxene, is required to generate the inferred Fe and Mg isotopic variability in the lunar mantle. Alternatively, the Fe and Mg isotopic compositions of the lunar mare basalts may indicate Fe-Mg interdiffusion has occurred in the Ti- rich component of the mare basalt source regions via reaction between ilmenite cumulates and the olivine- and pyroxene-rich lunar mantle.

Kelsey Prissel↗

NASA NTRS1977

Photon scattering cross sections of H2 and He measured with synchrotron radiation

Total (elastic + inelastic) differential photon scattering cross sections have been measured for H2 gas and He, using an X-ray beam. Absolute measured cross sections agree with theory within the probable errors. Relative cross sections (normalized to theory at large S) agree to better than one percent with theoretical values calculated from wave functions that include the effect of electron-electron Coulomb correlation, but the data deviate significantly from theoretical independent-particle (e.g., Hartree-Fock) results. The ratios of measured absolute He cross sections to those of H2, at any given S, also agree to better than one percent with theoretical He-to-H2 cross-section ratios computed from correlated wave functions. It appears that photon scattering constitutes a very promising tool for probing electron correlation in light atoms and molecules.

Ice, G. E.↗

NASA NTRS1978

Photon-scattering cross sections of H2 and He measured with synchrotron radiation

Total (elastic + inelastic) differential photon-scattering cross sections have been measured for H2 gas and for He, using an X-ray beam. Absolute measured cross sections agree with theory within the probable errors. Relative cross sections (normalized to theory at large S) agree to better than 1% with theoretical values calculated from wave functions that include the effect of electron-electron Coulomb correlation, but the data deviate significantly from theoretical independent-particle (e.g., Hartree-Fock) results. The ratios of measured absolute He cross sections to those of H2, at any given S, also agree to better than 1% with theoretical He-to-H2 cross-section ratios computed from correlated wave functions. It appears that photon scattering constitutes a very promising tool for probing electron correlation in light atoms and molecules. The degree of polarization of the synchrotron radiation beam has been measured by rotating the scattering plane about the beam axis; results are compared with theory.

Ice, G. E.↗