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NASA NTRS1987

Charge distribution and response time for a modulation-doped extrinsic infrared detector

The electric charge distribution and response time of a modulation-doped extrinsic infrared detector are determined. First, it is demonstrated theoretically that the photoconductive layer is effectively depleted of ionized majority-impurity charges so that scattering is small and mobility is high for photogenerated carriers. Then, using parameters appropriate to an actual detector, the predicted response time is 10 to the -8th to about 10 to the -9th s, which is much faster than comparable conventional detectors. Thus, the modulation-doped detector design would be valuable for heterodyne applications.

Hadek, Victor↗

NASA NTRS1987

Shallow n(+) diffusion into InP by an open-tube diffusion technique

Very shallow n(+) layers have been obtained in InP by using gallium sulfide as a source for sulfur diffusion, and chemically vapor-deposited SiO2 as a cap. Diffusions were carried out from 585 to 725 C in an open-tube system with a nitrogen ambient. The doping profile of sulfur in InP is estimated to be of the complementary error function type with a surface concentration of 5.6 x 10 to the 18th/cc and a diffusion constant of 1.1 x 10 to the -14th sq cm/s at 670 C. Diodes made on n(+)-p junctions obtained by this diffusion technique show ideality factors close to unity and saturation current densities as low as 3.4 x 10 to the -15th A/sq cm, signifying the presence of a defect-free junction. These diffusions, with junction depths in the 400-700 A range, are ideal for solar cell applications.

Ghandhi, Sorab K.↗

NASA NTRS1987

Rocket observation of the N II 2143 A emission in an aurora

Using a rocket-mounted spectrometer with a multichannel detector, the existence of the N II 2143 A doublet in the auroral ultraviolet spectrum was confirmed. The measured intensity is in agreement with previous auroral observations and is consistent with a peak electron-impact cross section of 1-2 x 10 to the -18th sq cm. By comparing an altitude profile of the 2143 A emission to the N2(+) 3914 A emission, a deactivation rate for the N(+)(5S) state was obtained. The most likely quencher is N2, although the possibility of a reaction with O2 cannot be ruled out. For N2, the deduced value of the quenching coefficient equals 5.5 x 10 to the -10th cu cm/s.

Siskind, David E.↗

NASA NTRS1987

Energy transfer of O(D-1) atoms in collision with O(P-3) atoms

Calculations are carried out of the elastic scattering and excitation exchange cross-sections in collisions of O(D-1) and O(P-3) atoms, which determine the degree of thermalization of the O(D-1) atoms produced by dissociative recombination in the thermosphere. The effective elastic scattering and excitation exchange cross-sections are calculated to be 1.55 x 10 to the -15th and 6.25 x 10 to the -16th sq cm, respectively, at a relative collision energy of 1.0 eV. The mutual diffusion coefficient between O(D-1) and O(P-3) atoms is also presented.

Yee, Jeng-Hwa↗

NASA NTRS1987

The production of NH(A 3Pi i) through photodissociative excitation of NH3

The NH(A-X) emission produced by photodissociation of NH3 is examined using a line emission source in the 600-1340 A region. It is observed that the production of the NH(A 3Pi i) photofragment involves a spin-forbidden transition. Fluorescence spectra of NH3 are derived and analyzed. The relationship between fluorescence cross sections and fluorescence count rates and gas pressure is studied. The fluorescence cross sections of NH(A-X) are calculated as 2.1 x 10 to the -22nd, 3.1 x 10 to the -22nd, 3.0 x 10 to the -21st, 1.5 x 10 to the -20th, and 3.4 x 10 to the -20th sq cm for the primary photon wavelength of 1239, 1184, 1065, 1037, and 976 A, respectively; and the corresponding fluorescence quantum yields are 0.00002, 0.00003, 0.0012, 0.0085, and 0.015.

Wu, C. Y. Robert↗

NASA NTRS1987

A systematic search for long-term variability in a large sample of X-ray sources

A method which uses the correct measurement error distribution (Poisson) and which naturally takes into account detections as well as upper limits has been developed to search for flux variability in X-ray sources. The method is applied to 256 sources observed at least twice with the Einstein Observatory Imaging Proportional Counter. These sources have, in the very large majority of cases, an X-ray flux in the range 1 x 10 to the -13th to 6 x 10 to the -12th erg/sq cm/s (0.2-3.5 keV). Thirty-one sources show flux variations with amplitude ranging from about 20 percent to a factor of 6. Flux variations of small amplitude seem to be fairly common, but can be detected only in bright sources or in sources observed with very long exposures. Variations of a factor of 2 or more are seen in about 15 percent of the sources for which variations of such an amplitude are detectable.

Maccacaro, Tommaso↗

NASA NTRS1987

Search for X-rays from the region of the Aries-Perseus flasher

The region containing the Perseus flasher was observed for 5.4 h with the Exosat observatory. Upper limits to a point source with a steady X-ray flux were 2 x 10 to the -12th erg/sq cm s and 6 x 10 to the 12th erg/sq cm s. Upper limits to X-ray flashes of about 1 s duration were 4 x 10 to the 9th erg/sq cm s and 7 x 10 to the -10th erg/sq cm s. In view of the growing number of negative optical observations, it appears that either the optical flashes do not have a celestial origin, or that the flasher has a transient character and turns off for extended periods of time.

Lewin, Walter H. G.↗

NASA NTRS1987

Snow load effect on earth's rotation and gravitational field, 1979-1985

A global, monthly snow depth data set has been generated from the Nimbus 7 satellite observations using passive microwave remote-sensing techniques. Seven years of data, 1979-1985, are analyzed to compute the snow load effects on the earth's rotation and low-degree zonal gravitational field. The resultant time series show dominant seasonal cycles. The annual peak-to-peak variation in J2 is found to be 2.3 x 10 to the -10th, that in J3 to be 1.1 x 10 to the -10th, and believed to decrease rapidly for higher degrees. The corresponding change in the length of day is 41 micro-s. The annual wobble excitation is (4.9 marc sec, -109 deg) for the prograde motion component and (4.8 marc sec, -28 deg) for the retrograde motion component. The excitation power of the Chandler wobble due to the snow load is estimated to be about 25 dB less than the power needed to maintain the observed Chandler wobble.

Chao, B. Fong↗

NASA NTRS1987

Ultraviolet and visual spectroscopy of DB white dwarfs

Visual wavelength and ultraviolet spectroscopy and model atmospheres of DB white dwarfs are reported. The results yield no evidence for lines of C or any other element besides H or He in the spectra. Upper limits for carbon are in the range C:He less than 10 to the -5th to the -7th for the ultraviolet data and C:He less than 0.01-0.001 for the visual. The upper limits are consistent with the convective dredging theory for the origin of the atmospheric carbon observed in the cooler DQ members of the helium-rich white dwarf sequence. Additional new DBA stars are reported.

Wegner, G.↗

NASA NTRS1986

Impact on rock, water, and air

It is argued that the meteorite-impact accretion is a process vital to the formation of the earth and terrestrial planets and that the evolution of the surfaces with time is affected by impacts. The paper reviews the previous calculations of Ahrens and O'Keefe of the effect of meteorite impacts on the rock surface of the earth, on the ocean, and the atmosphere, and presents some new work on the mechanism of impact-induced atmospheric escape. Using the similarity solution, the mass of atmosphere lost due to the impacts of 1 to 5 kg radius projectiles is calculated. It is shown that no atmosphere is lost for surface sources with energies less than 10 to the 27th erg. Impact of objects in the energy range 10 to the 27th to 10 to the 30th ergs causes gas losses of 10 to the 11th to 10 to the 14th kg (i.e., 10 to the -8th to 10 to the -5th of the total present atmospheric budget). Impact energies of greater than 10 to the 30th ergs cause little increase in atmospheric loss.

Ahrens, Thomas J.↗

NASA NTRS1987

A study of the infrared emission from the O I (3d 3D0) and (4s 5S0) states produced by electron impact excitation of O2

The absolute emission cross section values for the O I(3p 3P-3d 3D0, lambda = 11,287 A) and the (3p 5P-4s 5S0, lambda = 11,299 A) multiplets excited by electron impact on O2 have been measured. These infrared oxygen emission features appear prominently in auroral spectra and were observed in electron bombardment of O2. The former cross section has a value of 3.05 x 10 to the -19th + or - 15 percent at 100 eV, while the latter's value is 4.14 x 10 to the -19th sq cm + or - 15 percent. The quintet emission at lambda = 11,299 A is the only deexcitation channel for the 4s 5S0 state and thus represents the total excitation cross section for that state. However, the lambda = 11,287 A branch from the O I (3d 3D0) state only constitutes about 25 percent of the total 3d 3D0 cross section, with the major branch from this state being the transition to the ground state observed at lambda = 1027 A. The branching ratio is in good agreement with the theoretical calculation of Padhan and Saraph (1977).

Erdman, P. W.↗

NASA NTRS1987

Channel size influence on the heat flux density at zero net mass flow in the non-linear transport regime between 1.2 and 2.1 K

Porous media and narrow ducts of simple shape at zero net mass flow (ZNMF) are used to investigate the influence of pore size on the entropy/heat convection rate at ZNMF. The study is relevant to the development of specific types of phase separators. Previous work on heat transport by convection is extended to porous media without mass loss. The experimental results show the influence of pore size on heat flux for permeabilities between 10 to the -8th and 10 to the -6th sq cm. ZNMF plug data are found to be similar to results obtained for vapor liquid phase separation.

Frederking, T. H. K.↗

NASA NTRS1987

Kinetics of the reactions of SH radicals with NO2 and O2

The SH + NO2 reaction rate constant was measured by monitoring SH concentrations with long-path optical absorption at 324 nm in a laser photolysis system. At 298 K, the rate constant was found to be (4.8 + or - 1.0) x 10 to the -11th cu cm/molecule per sec. An upper limit of 4 x 10 to the -19th cu cm/molecule per sec was inferred for the SH + O2 reaction, indicating that the atmospheric oxidation of the SH radical may occur via reaction with species such as NO2 and O3.

Stachnik, R. A.↗

NASA NTRS1988

Chemical constraints on the water and total oxygen abundances in the deep atmosphere of Jupiter

The mechanisms responsible for the observed H2O/H2 mixing ratio (HMR) in the Jovian troposphere are analyzed, with a focus on the relationship between HMR and O2 abundance and the abundance of the nonequilibrium trace gases CO and SiH4. The results of computations using a Jupiter atmosphere model consistent with recent observations (Bjoraker et al., 1986) are presented graphically and discussed in detail. It is found that the observed mixing ratios of CO (about 10 to the -9th) and SiH4 (upper limit 2-4 x 10 to the -9th) preclude O2 depletion and the presence of significant global H2O. The low observed HMRs (0.00003 at 6 bar) are tentatively attributed to local condensation in cloud-forming regions.

Fegley, Bruce, Jr.↗

NASA NTRS1988

Mass loss from S stars

The mass-loss process in S stars is studied using 65 S stars from the listing of Wing and Yorka (1977). The role of pulsations in the mass-loss process is examined. It is detected that stars with larger mass-loss rates have a greater amplitude of pulsations. The dust-to-gas ratio for the S stars is estimated as 0.002 and the average mass-loss rate is about 6 x 10 to the -8th solar masses/yr. Some of the properties of the S stars, such as scale height, surface density, and lifetime, are measured. It is determined that scale height is 200 pc; the total duration of the S star phase is greater than or equal to 30,000 yr; and the stars inject 3 x 10 to the -6th solar masses/sq kpc yr into the interstellar medium.

Jura, M.↗

NASA NTRS1987

X-ray surveys as tools to investigate the cosmological evolution of quasars, BL Lac objects and clusters of galaxies

A progress report on a major extension of the Einstein Observatory Medium Sensitivity Survey (MSS) is presented. The MSS consists of X-ray sources serendipitously discovered with the IPC on board the Einstein Observatory in the energy range 0.3-3.5 keV, at fluxes between 10 to the -11th and 10 to the -13th ergs/sq cm/s. The basic properties of the extragalactic sources identified with AGN and clusters of galaxies are discussed.

Gioia, Isabella M.↗

NASA NTRS1988

Light echoes - Novae

The sudden brilliance of a nova eruption will be reflected on surrounding dust grains to create a phantom nebula. Previous searches for these light echoes have used relatively short exposures with photograhic detectors. This paper reports on a search around eight recent novae with long exposures using a CCD camera. Despite an increase of sensitivity by over an order of magnitude, no light echoes were detected. It is found that the average grain density must be less than about 10 to the -9th per cu cm for distances from 0.1 pc to 1000 pc from the novae. The light echo around Nova Persei 1901 was caused by reflection off clouds with grain densities of several times 10 to the -9th per cu cm which are at distances between 0.1 pc and 10 pc. Echoes from dust in a circumstellar shell or ejected during a previous eruption will be effectively unobservable.

Schaefer, Bradley E.↗

NASA NTRS1988

The X-ray spectra of the extragalactic sources in the Einstein extended medium sensitivity survey

The X-ray energy distribution of a large sample of faint extragalactic X-ray sources is studied. Two independent methods are used to determine that the average spectral index for the sources is about 0.95. The intrinsic dispersion of the spectral index distribution is about 0.36. The sources are characterized by a range of power law slopes in the soft X-ray band with a mean of 1.03 + 0.05 or - 0.06 and an intrinsic dispersion of about 0.36. The present analysis extends knowledge of the AGN spectral properties to sources which are about three times more distant and more than 10 times fainter than those previously studied. There is no evidence in the data for any change in the mean spectral index of either extragalactic sources as a whole or AGN in particular, as a function of the X-ray flux in the flux range 10 to the -11th to 10 to the -13th ergs/sq cm/s.

Maccacaro, Tommaso↗