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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

Embedded Coplanar Strips Traveling-Wave Photomixers

The electric field distribution in photomixers with electrodes deposited on the surface has already been calculated. It was shown that the strength of the electric field diminishes rapidly with depth. It was argued that the resulting reduction of the effective interaction volume of the device lowers the optical-to-heterodyne conversion. In this paper, we will present the results of our investigation on the influence of the electrode placement on the performance of photomixers. We have fabricated and measured traveling-wave photomixer devices which have both embedded and surface electrodes - the nominal spacing between the electrodes was 2 micrometers. Devices were made using either low-temperature-grown (LTG)-GaAs or ErAs:GaAs as the photoconductive material. The dark current, photocurrent, and radio frequency (RF) emission were measured at nominally 1 THz. The experimental data show a surprising difference in the behavior of ErAs:GaAs devices when the electrodes are embedded. A factor of two increase in RF radiation is observed for electric fields < 20 kV/cm. No such improvement was observed for the LTG-GaAs devices. We argue that the distinctive behavior of the two photoconductive materials is due to differences in the crystal structure - LTG-GaAs is isotropic, while ErAs:GaAs is uniaxial. We find that the carrier mobility in-plane (parallel) to the ErAs layers in the ErAs:GaAs superlattice is larger than orthogonal to these layers. The data indicate that carrier velocity overshoot is responsible for the excess radiation produced for the embedded electrode ErAs:GaAs devices.

Wyss, R. A.↗
NASA NTRS2010

Status of the STIS Instrument After Repair During Sm4

The Space Telescope Imaging Spectrograph (STIS) was installed in the Hubble Space Telescope (HST) in February of 1997, and operated until August of 2004, when an electrical malfunction in a power supply forced cessation of operations. On May 17,2009, during the fourth EVA of SM4, astronauts Michael Good and Mike Massimino undertook an eight-hour spacewalk, during which they replaced the STIS LVPS-2 circuit board containing the failed component, successfully repairing STIS. We will review the scientific capabilities and operational status of STIS after this repair. In most respects, STIS after the 2009 repair operates in much the same way as it did prior to the 2004 failure. Most changes in performance are close to what had been expected. The degradation of the STIS CCD due to radiation damage and the modest changes in optical throughput are consistent with extrapolation of previously-observed trends. Internal and external alignments of the instrument are also similar to what they were in 2004. The biggest surprise is that the dark current for the NUV MAMA detector is several times larger than had been expected and is only slowly decreasing towards its expected range. We discuss how these changes will affect science with STIS now and in the future.

Proffitt, Charles R.↗
NASA NTRS1993

Optical characterization of LDEF contaminant film

Dark brown molecular film deposits were found at numerous locations on the Long Duration Exposure Facility (LDEF) and have been documented in great detail by several investigators. The exact deposition mechanism for these deposits is as yet unknown, although direct and scattered atomic oxygen, and solar radiation interacting with materials outgassing products have all been implicated in the formation process. Specimens of the brown molecular film were taken from below the flange of the experimental tray located at position D10 on the LDEF. The tray was one of two, comprising the same experiment, the other being located on the wake facing side of the LDEF satellite at position B4. Having access to both trays, we were able to directly compare the effect that orientation with respect to the atomic oxygen flux vector had on the formation of the brown molecular film deposits. The film is thickest on surfaces facing toward the exterior, i.e. the tray corner, as can be seen by comparing the lee and wake aspects of the rivets. The patterns appear to be aligned not with the velocity vector but with the corner of the tray suggesting that flux to the surface is due to scattered atomic oxygen rather than direct ram impingement. The role of scattered flux is further supported by more faint plume patterns on the sides of the tray. The angle of these plumes is strongly aligned with the ram direction but the outline of the deposit implies that incident atoms are scattered by collisions with the edges of the opening resulting in a directed, but diffuse, flux of atomic oxygen to the surface. Spectral reflectance measurements in the 2 to 10 micron (4000 to 1000 wavenumbers) spectral range are presented for the film in the 'as deposited' condition and for the free standing film. The material was analyzed by FTIR (Fourier Transform Infrared) microspectroscopy using gold as the reference standard. The 'as deposited' specimen was on an aluminum rivet taken from beneath the tray flange while the free film was obtained by chipping some of the material from the rivet. The transmission spectrum over the 2 to 10 micron range for the free film is presented. This spectrum appears to be essentially the same as that presented by Crutcher et.al. for films formed at vent sites which faced into the ram direction and suggested to originate from urethanes and silicones used on the LDEF. Banks et. al. state that silicones, when exposed to atomic oxygen, release polymeric scission fragments which deposit on surfaces and form a glassy, dark contaminant layer upon further atomic oxygen exposure and solar irradiation.

Blakkolb, Brian K.↗
NASA NTRS1971

Microwave evidence for interstellar molecules.

The present state of microwave observations of interstellar molecules is reviewed, with emphasis on the need for accurate theoretical understanding of their rates and modes of production. Typical abundances of molecular species in dark interstellar clouds are tabulated, and it is shown that in addition to abundance information observations of molecular lines can yield data on the isotope contents, velocities, and temperatures of the clouds in which they exist. Collisions, spontaneous emission, the isotropic radiation, radiation trapping, and higher-frequency radiation are evaluated as factors contributing to molecular excitation processes.

Townes, C. H.↗
NASA NTRS1975

The helium chromosphere, coronal holes, and stellar X-rays

The solar chromosphere at the limb seen in D3 is an irregular bright band 1000 km thick with a dark band 1000 km thick beneath. The D3 chromosphere disappears in coronal holes. It is shown that the D3 emission, as well as the other He I and He II lines, can be explained quantitatively by photoionization by coronal back-radiation. A Chapman layer with N(He)H = 5 times 10 to the 17th power is formed near tau = 1 in the He I and He II continua. The chromospheric He emission or absorption is weak in coronal holes because there is no coronal back-radiation. Based on this model, the soft X-ray flux from stars with He 10830-A absorption lines is estimated as proportional to the 10830-A equivalent width and the apparent area.

Zirin, H.↗
DOE OSTI2023

Coded-mask-based X-ray phase-contrast and dark-field imaging

Phase contrast and dark-field X-ray imaging enable imaging of objects that absorb or reflect very little X-ray light. Disclosed is a method and systems for performing coded-mask-based multi-contrast imaging (CMMI). The method includes providing radiation to a coded mask that has a known phase and absorption profile according to a pre-determined pattern. The radiation is then impingent upon a sample, and the radiation is detected to perform phase-reconstruction and image processing. The method and associated systems allow for the use of maximum-likelihood and machine learning methods for reconstruction images of the sample from the detected radiation.

Shi, Xianbo↗
NASA NTRS2018

Evaluation of Space Radiation Effects on HgCdTe Avalanche Photodiode Arrays for Lidar Applications

We report the results from proton and gamma ray radiation testing of HgCdTe avalanche photodiode (APD) arrays developed by Leonardo DRS for space lidar detectors. We tested these devices with both approximately 60 MeV protons and gamma rays, with and without the read out integrated circuit (ROIC). We also measured the transient responses with the device fully powered and with the APD gain from unity to greater than 1000. The detectors produced a large current impulse in response to each proton hit but the response completely recovered within 1 microsecond. The devices started to have persistent damage at a proton fluence of 7e10 protons/cm2, equivalent to 10 krad(Si) total ionization dose. The dark current became much higher after the device was warmed to room temperature and cooled to 80K again, but it completely annealed after baking at 85 C for several hours. These results showed the HgCdTe APD arrays are suitable for use in space lidar for typical Earth orbiting and planetary missions provided that provisions are made to heat the detector chip to 85 C for several hours after radiation damage becomes evident that system performance is impacted.

Sun, Xiaoli↗
NASA NTRSDate not supplied

Hunting Energetic Radiation from Thunderstorms

The true nature and mechanisms of terrestrial gamma-ray flashes (TGFs) have remained mysterious since their discovery in the 1990s. Despite decades of detections, their violent origins and irregularity make them particularly difficult to observe directly. Combined with the wide range of measured characteristics, this has kept the details of these energetic events in the dark. The recent ALOFT (Airborne Lightning Observatory for FEGS and TGFs) campaign has undertaken the challenge of hunting these elusive flashes from an airborne platform. This was performed by the NASA ER-2 aircraft in July 2023 equipped with a host of meteorological, electric field, and gamma-ray instruments to study intense tropical thunderstorms and their connection to gamma-ray production. Here we present on the campaign's success alongside some of its exciting preliminary results.

Lightning↗
NASA NTRS1975

The composition of the ultraviolet dark markings on Venus

The ultraviolet dark clouds are an ephemeral phenomenon in the Venus atmosphere, apparently just near the limit of stability. The UV dark material is moderately abundant, perhaps 10%, since the contrasts between light and dark material are some 20%. The material should absorb light between 3000-4000 A, and ideally should also have visible absorptions in the blue, as the overall spectral albedo of Venus indicates. Such a material is bromine dissolved in hydrobromic acid. Solar radiation near 2500 A is sufficient to partially photolyze HBr into Br2. HBr in the Venus atmosphere is inferred to have a mixing ratio of 0.0001 versus CO2. With a water vapor mixing ratio of 0.001, droplets of hydrobromic acid are possible. These droplets would eventually evaporate in the drier upper atmosphere. The refractive index of these hydrobromic acid droplets of 52% (by weight) composition is 1.46, within the limits set by polarization studies of Venus.

Sill, G. T.↗
NASA NTRS1980

Far-infrared observations of the globule B335

Far-infrared and submillimeter continuum emission from the dark globule B335 was observed. The spectrum is that of optically thin thermal emission from dust with temperature 13-16 K. The estimated dust mass is 0.07-0.17 solar masses. The observed luminosity, 5 solar luminosities, can be supplied by the interstellar radiation field.

Keene, J.↗
NASA NTRS1987

Physical interpretation of variations in total solar irradiance

Radiometry from the Solar Maximum Mission and Nimbus 7 satellites has demonstrated that the solar constant varies at a peak-to-peak level of up to 0.2 percent on time scales of weeks. The rotation and evolution of dark spots and bright faculae across the sun's disk accounts for most of that variation. Reasonable explanations have been put forward to explain how the spot-blocked heat flow might be stored, and to explain the source of the intense radiation that gives rise to the increased irradiance produced by the bright magnetic faculae. Time-dependent models of the response of the solar convection zone to small perturbations also indicate that slower variations in total solar irradiance of comparable magnitude are likely. More precise observations of the total solar irradiance and radius over long time scales are required to demonstrate the existence of such climatologically relevant changes, and to test models that would enable the interpretation and prediction of these changes.

Foukal, P.↗
NASA NTRS2020

Advanced Silicon Avalanche Photodiodes on NASA's Global Ecosystem Dynamics Investigation (GEDI) Mission

Silicon Avalanche Photodiodes (APDs) are used in NASA’s Global Ecosystem Dynamics Investigation (GEDI) which was launched in December 2018 and is currently measuring the Earth’s vegetation vertical structure from the International Space Station. The APDs were specially made for space lidar with a much lower hole-to-electron ionization coefficient ratio (k-factor ~0.008) than that of commercially available silicon APDs in order to reduce the APD excess noise from the randomness of the avalanche gain. A silicon heater resistor was used under the APD chip to heat the device up to 70°C and improve its quantum efficiency at 1064 nm laser wavelength while maintaining a low dark current such that the overall signal to noise ratio is improved. Special APD protection circuits were used to raise the overload damage threshold to prevent device damage from strong laser return by specular surfaces, such as still water bodies, and space radiation events. The APD and a hybrid transimpedance amplifier circuit were hermetically sealed in a package with a sufficiently low leak rate to ensure multi-year operation lifetime in space. The detector assemblies underwent a series of pre-launch tests per NASA Goddard Environmental Verification Standard for space qualification. They have performed exactly as expected with GEDI in orbit. A detailed description of the GEDI detector design, signal and noise model, and test results are presented in this paper.

Sun, Xiaoli↗
NASA NTRS2021

Near future MeV telescopes can discover asteroid-mass primordial black hole dark matter

Primordial black holes (PBHs), formed out of large over densities in the early Universe, are a viable dark matter (DM) candidate over a broad range of masses. Ultralight, asteroid-mass PBHs with masses around1017g are particularly interesting as current observations allow them to constitute the entire DM density. PBHs in this mass range emit∼MeV photons via Hawking radiation which can directly be detected by the gamma ray telescopes, such as the upcoming AMEGO. In this work we forecast how well an instrument with the sensitivity of AMEGO will be able to detect, or rule out, PBHs as a DM candidate, by searching for their evaporating signature when marginalizing over the Galactic and extra-Galactic gamma-ray back-grounds. We find that an instrument with the sensitivity of AMEGO could exclude nonrotating PBHs as the only DM component for masses up to7×1017g at 95% confidence level for a monochromatic mass distribution, improving upon current bounds by nearly an order of magnitude. The forecasted constraints are more stringent for PBHs that have rotation, or which follow extended mass distributions

Anupam Ray↗
NASA NTRS2014

Organism/Organic Exposure to Orbital Stresses (OOREOS) Satellite: Radiation Exposure in LEO and Supporting Laboratory Studies

We will present the results from the exposure of the metalloporphyrin iron tetraphenylporphyrin chloride (FeTPPCI), anthraufin (C(sub 14)H(sub 8)(O sub 4) (Anth) and Isoviolanthrene (C(sub 34H sub 18) (IVA) to the outher space environment, measured in situ aboard the Organism/Organic Exposure to Orbital Stresses nanosatellite. The compounds were exposed for a period of 17 months (3700 hours of direct solar exposure) including broad-spectrum solar radiation (approx. 122 nm to the near infrared). The organic films are enclosed in hermetically sealed sample cells that contain one of four astrobiologically relevant microenvironments. Transmission spectra (200-1000 nm) were recorded for each film, at first daily and subsequently every 15 days, along with a solar spectrum and the dark response of the detector array. In addition to analysis via UV-Vis spectroscopy, the laboratory controls were also monitored via infrared and far-UV spectroscopy. The results presented will include the finding that the FeTPPCI and IVA organic films in contact with a humid headspace gas (0.8-2.3%) exhibit faster degradation times, upon irradiation, in comparison with identical films under dry headspaces gases, whereas the Anth thin film exhibited a higher degree of photostability. In the companion laboratory experiments, simulated solar exposure of FeTPI films in contact with either Ar or CO(sub -2):O(sub -2):Ar (10:0.01:1000) headspace gas results in growth of a band in the films infrared spectra at 1961 cm(sup 1). Our assignment of this new spectral feature and the corresponding rational will be presented. The relevance of O/OREOS findings to planetary science, biomarker research, and the photostability of organic materials in astrobiologically relevant environments will also be discussed.

Radiation Exposure↗
DOE OSTI2022

SENSEI: Characterization of Single-Electron Events Using a Skipper Charge-Coupled Device

We use a science-grade skipper charge-coupled device (skipper CCD) operating in a low-radiation background environment to develop a semi-empirical model that characterizes the origin of single-electron events in CCDs. We identify, separate, and quantify three independent contributions to the single-electron events, which were previously bundled together and classified as "dark counts": dark current, amplifier light, and spurious charge. We measure a dark current, which depends on exposure, of (5.89±0.77)×10 -4 e - /pix/day, and an unprecedentedly low spurious charge contribution of (1.52±0.07)×10 -4 e - /pix, which is exposure independent. In addition, we provide a technique to study events produced by light emitted from the amplifier, which allows the detector's operation to be optimized to minimize this effect to a level below the dark-current contribution. Our accurate characterization of the single-electron events allows one to greatly extend the sensitivity of experiments searching for dark matter or coherent neutrino scattering. Moreover, an accurate understanding of the origin of single-electron events is critical to further progress in ongoing research and development efforts of skipper and conventional CCDs.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1985

Can spores survive in interstellar space?

Experimental evidence is presented for the effects of very low temperature and UV radiation, characteristic of the interstellar medium, on the survival of bacteria. In the most general space environment, 10 percent survival times are only of the order of hundreds of years, too short for panspermia to work. In a substantial fraction of space within dark clouds, however, it is shown that, even with conservative figures, survival times as long as millions to tens of millions of years are attainable. In such conditions, clouds could transport organisms from one solar system to another in times significantly shorter than the mean survival time. This occurs with significant probability.

Weber, P.↗
NASA NTRS1985

PIN photodiodes irradiated with 40- and 85-MeV protons

PIN photodiodes were bombarded with 40- and 85-MeV protons to a fluence of 1.5 x 10 to the 11th p/sq cm, and the resulting change in spectral response in the near infrared was determined. The photocurrent, dark current, and pulse amplitude were measured as a function of proton fluence. Changes in these three measured properties are discussed in terms of changes in the diode's spectral response, minority carrier diffusion length, and depletion width. A simple model of induced radiation effects is presented which is in good agreement with the experimental results. The model assumes that incident protons produce charged defects within the depletion region simulating donor type impurities.

Reft, C. S.↗
NASA NTRS1992

Eye-safe visible wavelength lidar

Recent technological advances on several fronts offer the possibility for relatively low-cost, eye-safe visible-wavelength lidar systems for autonomous aerosol/environmental monitoring applications. Improved silicon photodiodes and avalanche photodiodes that have become available offer high-quantum-efficiency detection at very low dark counts (10 to 1000 count/s) and can be used in a photon counting mode for signal plus background and dark current photoelectron count rates of megahertz. The essential requirements and features of a possible lidar system that capitalizes on technical advances on several fronts are outlined. A baseline lidar system is suggested for monitoring tropospheric and stratospheric aerosols. Sensitivity to wavelength, background radiation, detector characteristics, and other system parameters is discussed for several simulated data sets.

Cooley, T. W.↗