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DOE OSTI2024

Demonstrating the β-Ga 2 O 3 Schottky diodes for alpha radiation detection

Schottky barrier diodes were fabricated on (001) monoclinic β-Ga 2 O 3 wafers with low doped epitaxial layers of 7.0 × 10 15 cm −3 . Circular Ni Schottky contacts with area 2 mm 2 were deposited by electron beam evaporation. Devices were characterized electrically by performing forward and reverse current-voltage sweeps with a range of −100 V–2 V, as well as capacitance-voltage sweeps to −30 V. The breakdown voltage was also determined to be −180 V for the devices. Experiments measuring the electrical response from incident X-ray radiation was performed. A response time to X-ray radiation of less than 1 s was recorded and a decay time of approximately 2 s after removing X-ray source, which primarily attribute to X-ray switching on and off time. Energy spectra of alpha particles from a 0.9 μCi 241 Am button source was collected at various voltage biases using devices with the lowest measured leakage current while reverse biased. The total count rate was observed to increase linearly with increasing device bias. In conclusion, the peak channel number was observed to increase with increasing bias with the best resolution of 9.5% at −100 V reverse bias.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

NASA NTRS1983

The solar O III spectrum. II - Longer wavelengths, line widths, and the He II Lyman alpha radiation field

The solar O III spectrum above 900 A is analyzed, including several visible and infrared lines which are important in nebular studies. The dependence of the line intensities on the rate of photoexcitation by He Ly-alpha is determined, and the observability of these lines in the solar spectrum is studied. The impact approximation is employed to calculate the expected line widths of the stronger solar O III lines. The photoexciting field at 304 A calculated from the observed intensities of the O III lines below 900 A (Bhatia et al., 1982) is compared with the field predicted by a recent model (Avrett et al., 1976). It is shown that additional radiation trapping must be present beyond that given by this model.

Kastner, S. O.↗

NASA NTRS1968

Polarization of Lyman alpha radiation emitted by H/2S/ atoms in weak electric fields

Polarization prediction in modulated beam of ground state hydrogen atoms crossed by dc electron beam

Fite, W. L.↗

NASA NTRS1998

Polarization of Lyman-Alpha Radiation from Atomic Hydrogen Excited by Electron Impact form Near Threshold to 1800 eV

The polarization of Lyman-a radiation, produced by electron-impact excitation of atomic hydrogen, has been measured over the extended energy range from near threshold to 1800 eV. Measurements were obtained in a crossed-beam experiment using a silica-reflection linear polarization analyzer in tandem with a vacuum-ultraviolet monochromator to isolate the emitted line radiation. Comparison with various theoretical calculations shows that the present experimental results are in good agreement with theory over the entire range of electron-impact energies and, in particular, are in excellent agreement with theoretical convergent-close-coupling (CCC) calculations performed in the present work. Our polarization data are significantly different from the previous experimental measurements of Ott, Kauppila, and Fite.

James, G. K.↗

NASA NTRS1995

New analysis of the Voyager UVS H Lyman-alpha emission of Saturn

The limb to limb Lyman-alpha reflectivities observed with the Voyager ultraviolet spectrometer (UVS) instruments during the fly-by of Saturn are reanalyzed using a revised H Lyman-alpha sensitivity for the Voyager 1 instrument. The new sensitivity reconciles the measured intensities to those of Voyager 2 and gives a coherent set of data. To fit the UV airglow observations, four sources are considered: (1) H resonance and H2 Rayleigh scattering of solar Lyman-alpha radiation, (2) the interplanetary Lyman-alpha radiation, (3) a possible internal source of unknown origin, (4) the possibility of atmospheric turbulence recently proposed to explain the Lyman-alpha bulge of Jupiter. The analysis supports neither a dominant collisional excitation source for the UV emissions nor the presence of strong atmospheric turbulence. The best fit, in terms of brightness but also in terms of shape of the limb to limb profile (that is to say independent on the absolute calibrations), is obtained for pure resonance and Rayleigh scattering of solar and interstellar wind line in an atmosphere enriched in atomic hydrogen up to three times the standard model. Influx of water from the rings of Saturn may provide a means for producing such enhanced H densities in the upper atmosphere.

Jaffel, L. Ben↗

NASA NTRS1972

The degradation of alzak by short wavelength ultraviolet radiation

The changes in reflectance of thermal aluminum coating samples exposed to different irradiating utraviolet wavelengths are discussed. It is shown that the coating is damaged faster and further by 180 to 210 in radiation than by Lyman alpha radiation. On an equivalent incident energy basis, Lyman alpha does less damage than 180 to 210 nm radiation. Above 300 nm no degradation is observed for long exposures and below 300 nm increasing degradation with decreasing wavelength is found. It is concluded that Lyman alpha radiation need not be included in laboratory testing of this thermal coating for spacecraft structures.

Donohoe, M. J.↗

NASA NTRS2008

Radiation Transport of Heliospheric Lyman-alpha from Combined Cassini and Voyager Data Sets

Heliospheric neutral hydrogen scatters solar Lyman-alpha radiation from the Sun with '27-day' intensity modulations observed near Earth due to the Sun's rotation combined with Earth's orbital motion. These modulations are increasingly damped in amplitude at larger distances from the Sun due to multiple scattering in the heliosphere, providing a diagnostic of the interplanetary neutral hydrogen density independent of instrument calibration. This paper presents Cassini data from 2003-2004 obtained downwind near Saturn at approximately 10 AU that at times show undamped '27-day' waves in good agreement with the single-scattering models of Pryor et al., 1992. Simultaneous Voyager 1 data from 2003- 2004 obtained upwind at a distance of 88.8-92.6 AU from the Sun show waves damped by a factor of -0.21. The observed degree of damping is interpreted in terms of Monte Carlo multiple-scattering calculations (e.g., Keller et al., 1981) applied to two heliospheric hydrogen two-shock density distributions (discussed in Gangopadhyay et al., 2006) calculated in the frame of the Baranov-Malama model of the solar wind interaction with the two-component (neutral hydrogen and plasma) interstellar wind (Baranov and Malama 1993, Izmodenov et al., 2001, Baranov and Izmodenov, 2006). We conclude that multiple scattering is definitely occurring in the outer heliosphere. Both models compare favorably to the data, using heliospheric neutral H densities at the termination shock of 0.085 cm(exp -3) and 0.095 cm(exp -3). This work generally agrees with earlier discussions of Voyager data in Quemerais et al., 1996 showing the importance of multiple scattering but is based on Voyager data obtained at larger distances from the Sun (with larger damping) simultaneously with Cassini data obtained closer to the Sun.

Pryor, W.↗

NASA NTRS1975

Monte Carlo treatment of Lyman-alpha. II - Radiation in a spherical atmosphere

Intensity and state of polarization of solar L-alpha photons as they diffuse through an inhomogeneous, spherically symmetric, isothermal geocorona are theoretically determined. The fine structure of L-alpha and Doppler redistribution of frequencies are taken into account. The calculation use the Monte Carlo technique involving Stokes vectors. Comparison of the results with OGO-4 and OSO-4 observed intensities at an altitude of 650 km shows good agreement. Calculations of the polarization versus solar zenith angle show a residual polarization at large zenith angles which is mainly due to multiply scattered photons.

Modali, S. B.↗

NASA NTRS1972

New interpretations of extraterrestrial Lyman-alpha observations.

The solar Lyman-alpha radiation pressure affects the orbits and the velocities of the interstellar particles entering the solar system. This leads to enhanced particle losses in the heliosphere, since particles spend a longer time crossing it. This causes a stronger decrease of the density with decreasing distances from the sun than had been calculated without accounting for the radiation pressure. Furthermore, the emission pattern of the solar Lyman-alpha radiation is anisotropic and rotates with the sun in a 27-day period. This causes a temporal change in the location of the intensity extrema. At the same time it produces hydrogen density anisotropies with extrema deviating in their directions from those which had been calculated without consideration of the radiation pressure.

Blum, P. W.↗

NASA NTRS1972

Neutral hydrogen in cometary comas.

The strong L alpha radiation observed recently in comets Tago-Sato-Kosaka and Bennett can be explained in terms of the resonant scattering of solar L alpha radiation on neutral hydrogen formed by the photodissociation of H2O which is vaporized from a nucleus having an ice core. A complete hydrodynamic description of an atmosphere composed of H2O and its daughter products OH, H and O coupled through frictional interaction as well as production and loss processes is given. Numerical results are computed in a typical case, and it is found that a temperature of about 3000 K for the cometary atmosphere provides the best fit with observation.

Mendis, D. A.↗

NASA NTRS1988

Fermi acceleration of Lyman-alpha photons by shocks

The repeated scattering of Ly-alpha radiation across a shock front results in a systematic blueshift which may greatly exceed the shock velocity vs and is proportional to cube root of (Nvs), where N is the column density of hydrogen atoms on either side of the shock front. The blueshifting process is similar to the Fermi acceleration of cosmic rays and may be responsible for the blue asymmetric line profiles that have been observed in high-redshift Ly-alpha galaxies. The Ly-alpha line profile in 3C 326.1 is accounted for by a model in which shocks, driven into a population of interstellar clouds by a radio lobe, trigger the formation of ionizing stars and Fermi accelerate the Ly-alpha radiation emitted by H II regions surrounding those stars. Galaxy mergers, particularly between galaxies with low dust content, should produce Ly-alpha lines with strong blue wings.

Neufeld, David A.↗

NASA NTRS1978

Spectroscopic mapping of solar wind velocities

During the total solar eclipse of 1970, measurements of resonantly scattered Lyman-alpha radiation from the solar corona revealed a means to determine temperatures and densities in the solar corona beyond 1.5 R solar radii. A natural extension of this work is to use the Solar Probe to measure the spectral line profile of Lyman-alpha radiation backscattered toward the Sun from coronal regions between 4 R solar radii and 10 R solar radii. The backscattered profile would provide unique and quantitative determinations of the outlaw velocities of coronal material into the solar wind. Such information is of critical importance for understanding solar-wind formation and solar-terrestrial effects on the earth's climate. There is no known way to obtain this information without a Solar Probe-type mission.

Kohl, J. L.↗

NASA NTRS1998

Analysis of IUE Observations of Hydrogen in Comets

The 15-years worth of hydrogen Lyman-alpha observations of cometary comae obtained with the International Ultraviolet Explorer (IUE) satellite had gone generally unanalyzed because of two main modeling complications. First, the inner comae of many bright (gas productive) comets are often optically thick to solar Lyman-alpha radiation. Second, even in the case of a small comet (low gas production) the large IUE aperture is quite small as compared with the immense size of the hydrogen coma, so an accurate model which properly accounts for the spatial distribution of the coma is required to invert the infrared brightnesses to column densities and finally to H atom production rates. Our Monte Carlo particle trajectory model (MCPTM), which for the first time provides the realistic full phase space distribution of H atoms throughout the coma has been used as the basis for the analysis of IUE observations of the inner coma. The MCPTM includes the effects of the vectorial ejection of the H atoms upon dissociation of their parent species (H2O and OH) and of their partial collisional thermalization. Both of these effects are crucial to characterize the velocity distribution of the H atoms. This combination of the MCPTM and spherical radiative transfer code had already been shown to be successful in understanding the moderately optically thick coma of comet P/Giacobini-Zinner and the coma of comet Halley that varied from being slightly to very optically thick. Both of these comets were observed during solar minimum conditions. Solar activity affects both the photochemistry of water and the solar Lyman-alpha radiation flux. The overall plan of this program here was to concentrate on comets observed by IUE at other time during the solar cycle, most importantly during the two solar maxima of 1980 and 1990. Described herein are the work performed and the results obtained.

Combi, Michael R.↗

NASA NTRS1997

Atomic, Molecular, and Optical Physics: Optical Excitation Function of H(1s-2p) Produced by electron Impact from Threshold to 1.8 keV

The optical excitation function of prompt Lyman-Alpha radiation, produced by electron impact on atomic hydrogen, has been measured over the extended energy range from threshold to 1.8 keV. Measurements were obtained in a crossed-beams experiment using both magnetically confined and electrostatically focused electrons in collision with atomic hydrogen produced by an intense discharge source. A vacuum-ultraviolet mono- chromator system was used to measure the emitted Lyman-Alpha radiation. The absolute H(1s-2p) electron impact excitation cross section was obtained from the experimental optical excitation function by normalizing to the accepted optical oscillator strength, with corrections for polarization and cascade. Statistical and known systematic uncertainties in our data range from +/- 4% near threshold to +/- 2% at 1.8 keV. Multistate coupling affecting the shape of the excitation function up to 1 keV impact energy is apparent in both the present experimental data and present theoretical results obtained with convergent close- coupling (CCC) theory. This shape function effect leads to an uncertainty in absolute cross sections at the 10% level in the analysis of the experimental data. The derived optimized absolute cross sections are within 7% of the CCC calculations over the 14 eV-1.8 keV range. The present CCC calculations converge on the Bethe- Fano profile for H(1s-2p) excitation at high energy. For this reason agreement with the CCC values to within 3% is achieved in a nonoptimal normalization of the experimental data to the Bethe-Fano profile. The fundamental H(1s-2p) electron impact cross section is thereby determined to an unprecedented accuracy over the 14 eV - 1.8 keV energy range.

James, G. K.↗

NASA NTRS1997