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NASA NTRS1995

Radar investigation of barium releases over Arecibo Observatory, Puerto Rico

The NASA Combined Release and Radiation Effects Satellite (CRRES) El Coqui rocket campaign was successfully carried out in Puerto Rico during the period 18 May through 12 July 1992. This report describes five chemical release experiments in the upper ionosphere supported by Geospace Research, Inc. during the El Coqui campaign. Additional spin-off science is also discussed. The El Coqui releases are designated AA-1 (rocket 36-082), AA-2 (rocket 36-081), AA-3b (rocket 36-064), AA-4 (rocket 36-065), and AA-7 (rocket 36-083). Particular attention is paid to releases AA-2 and AA-4. These two experiments involved the illumination of ionospheric release regions with powerful high-frequency (HF) radio waves transmitted from the Arecibo HF facility. In the AA-2 experiment, microinstabilities excited by the HF wave in a Ba(+) plasma were examined. This release yielded a smooth plasma cloud that helped clarify several fundamental issues regarding the physics of wave plasma instabilities. During AA-2 extremely strong HF-induced Langmuir turbulence was detected with the Arecibo 430 MHz radar. CF3Br was released in the AA-4 study to create an ionospheric hole that focused the HF beam. This experiment successfully explored wave-plasma coupling in an O(+) ionosphere under conditions of very high HF electric field strengths.

Djuth, Frank T.↗

NASA NTRS1998

Stimulated Raman amplification, oscillation, and linewidth in barium nitrate

Measurements of Raman gain in a Ba(NO3)2 crystal are reported at 532 nm using a Raman oscillator/amplifier arrangement for differential absorption lidar measurements of ozone. The experimentally determined gain coefficient will be compared with theoretical results. The effect of single and multi-longitudinal mode pumping upon the amplification process will be discussed. Measurement of the Raman linewidth for 1st 2nd and 3d stokes shifts arc presented.

McCray, Christopher J.↗

NASA NTRS2000

Coherence Measurements for Excited to Excited State Transitions in Barium

Experimental studies concerning elastic and inelastic electron scattering by coherently ensembles of Ba (...6s6p (sub 1)P(sub 1)) atoms with various degrees of alignment will be described. An in-plane, linearly-polarized laser beam was utilized to prepare these target ensembles and the electron scattering signal as a function of polarization angle was measured for several laser geometries at fixed impact energies and scattering angles. From these measurements, we derived cross sections and electron-impact coherence parameters associated with the electron scattering process which is time reverse of the actual experimentally studied process. This interpretation of the experiment is based on the theory of Macek and Herte. The experimental results were also interpreted in terms of cross sections and collision parameters associated with the actual experimental processes. Results obtained so far will be presented and plans for further studies will be discussed.

Trajmar, S.↗

NASA NTRS1997

Barium Nitrate Raman Laser Development for Remote Sensing of Ozone

In order to understand the impact of anthropogenic emissions upon the earth's environment, scientists require remote sensing techniques which are capable of providing range-resolved measurements of clouds, aerosols, and the concentrations of several chemical constituents of the atmosphere. The differential absorption lidar (DIAL) technique is a very promising method to measure concentration profiles of chemical species such as ozone and water vapor as well as detect the presence of aerosols and clouds. If a suitable DIAL system could be deployed in space, it would provide a global data set of tremendous value. Such systems, however, need to be compact, reliable, and very efficient. In order to measure atmospheric gases with the DIAL technique, the laser transmitter must generate suitable on-line and off-line wavelength pulse pairs. The on-line pulse is resonant with an absorption feature of the species of interest. The off-line pulse is tuned so that it encounters significantly less absorption. The relative backscattered power for the two pulses enables the range-resolved concentration to be computed. Preliminary experiments at NASA LaRC suggested that the solid state Raman shifting material, Ba(NO3)2, could be utilized to produce these pulse pairs. A Raman oscillator pumped at 532 nm by a frequency-doubled Nd:YAG laser can create first Stokes laser output at 563 nm and second Stokes output at 599 nm. With frequency doublers, UV output at 281 nm and 299 nm can be subsequently obtained. This all-solid state system has the potential to be very efficient, compact, and reliable. Raman shifting in Ba(NO3)2, has previously been performed in both the visible and the infrared. The first Raman oscillator in the visible region was investigated in 1986 with the configurations of plane-plane and unstable telescopic resonators. However, most of the recent research has focused on the development of infrared sources for eye-safe lidar applications.

McCray, Christopher L.↗

NASA NTRS2002

Isotopic Composition of Barium in Single Presolar Silicon Carbide Grains

We have measured Ba isotope distributions in individual presolar SiC grains. We find that the Ba isotopic composition in mainstream SiC grains is consistent with models of nucleosynthesis in low to intermediate mass asymptotic giant branch (AGB) stars. Additional information is contained in the original extended abstract.

Savina, M. R.↗

NASA NTRS2003

Water Atomization of Barium Fluoride: Calcium Fluoride for Enhanced Flow Characteristics of PS304 Feedstock Powder Blend

PS304 is a plasma spray deposited solid lubricant coating with feedstock composed of NiCr, Cr2O3, Ag, and BaF2-CaF2 powders. The effects of rounded BaF2-CaF2 particles on the gravity-fed flow characteristics of PS304 feedstock have been investigated. The BaF2-CaF2 powder was fabricated by water atomization using four sets of process parameters. Each of these powders was then characterized by microscopy and classified by screening to obtain 45 to 106 micron particles and added incrementally from 0 to 10 wt% to the other constituents of the PS304 feedstock, namely nichrome, chromia, and silver powders. The relationship between feedstock flow rate, measured with the Hall flowmeter, and concentration of fluorides was found to be linear in each case. The slopes of the lines were between those of the linear relationships previously reported using angular and spherical fluorides and were closer to the relationship predicted using the rule of mixtures. The results offer a fluoride fabrication technique potentially more cost-effective than gas atomization processes or traditional comminution processes.

Stanford, Malcolm K.↗

NASA NTRS2003

Isotopic Composition of Molybdenum and Barium in Single Presolar Silicon Carbide Grains of Type A+B

Presolar SiC grains fall into several groups based on C, N, and Si isotopic compositions. Approximately 93% are defined as mainstream, having 10 less than C-12/C-13 less than 100 and N-14/N-15 ranging from 50 to 20,000. A number of studies have shown that the most likely sources of mainstream grains are low mass asymptotic giant branch stars. Models of nucleosynthesis in AGB stars reproduce the s-process enhancements seen in the heavy elements in mainstream SiC grains. Among the less common grains, A+B grains, which comprise approximately 3-4% of presolar SiC, are perhaps the least well understood. Recent studies by Amari et al. show that A+B grains can be divided into at least 4 groups based on their trace element concentration patterns. Of 20 grains studied, 7 showed trace element patterns consistent with condensation from a gas of solar system composition, while the rest had varying degrees of process enhancements. Our previous measurements on 3 A+B grains showed Mo of solar isotopic composition, but Zr with a strong enhancement in 96Zr, which is an r-process isotope but can be made in an sprocess if the neutron density is high enough to bridge the unstable Zr-95 (T(sub 1/2)= 64 d). The observation of Mo with solar system isotopic composition in the same grains is puzzling however. Meyer et al. have recently shown that a neutron burst mechanism can produce a high Zr-96/Zr-94 without enhancing Mo-100, however this model leads to enhancements in Mo-95 and Mo-97 not observed in A+B grains. We report here results of Mo measurements on 7 additional A+B grains, and Ba measurements on 2 A+B grains, and compare these to the previous studies.

Savina, M. R.↗

NASA NTRS2014

Stellar Laboratories: 3. New Ba 5, Ba 6, and Ba 7 Oscillator Strengths and the Barium Abundance in the Hot White Dwarfs G191-B2B and RE 0503-289

Context. For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Aims. Reliable Ba 5-7 oscillator strengths are used to identify Ba lines in the spectra of the DA-type white dwarf G191−B2B and the DO-type white dwarf RE 0503−289 and to determine their photospheric Ba abundances. Methods. We newly calculated Ba v-vii oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Ba lines exhibited in high-resolution and high-S/N UV observations of G191−B2B and RE 0503−289. Results. For the first time, we identified highly ionized Ba in the spectra of hot white dwarfs. We detected Ba vi and Ba vii lines in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of RE 0503−289. The Ba vi/Ba vii ionization equilibrium is well reproduced with the previously determined effective temperature of 70 000 K and surface gravity of log g=7.5. The Ba abundance is 3.5 +/- 0.5 × 10(exp−4) (mass fraction, about 23 000 times the solar value). In the FUSE spectrum of G191−B2B, we identified the strongest Ba vii line (at 993.41 Å) only, and determined a Ba abundance of 4.0 +/- 0.5 × 10(exp−6) (about 265 times solar). Conclusions. Reliable measurements and calculations of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed Ba vi-vii line profiles in two white dwarfs' (G191−B2B and RE 0503−289) far-ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. This allowed to determine the photospheric Ba abundance of these two stars precisely.

New Ba V↗

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