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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Spectral Irradiance Calibration in the Infrared: New Composite Spectra, Comparison with Model Atmospheres, and Far-Infrared Extrapolations - Part 7

We present five new absolutely calibrated continuous stellar spectra constructed as far as possible from spectral fragments observed from the ground, the Kuiper Airborne Observatory (KAO), and the IRAS Low Resolution Spectrometer. These stars-alpha Boo, gamma Dra, alpha Cet, gamma Cru, and mu UMa-augment our six, published, absolutely calibrated spectra of K and early-M giants. All spectra have a common calibration pedigree. A revised composite for alpha Boo has been constructed from higher quality spectral fragments than our previously published one. The spectrum of gamma Dra was created in direct response to the needs of instruments aboard the Infrared Space Observatory (ISO); this star's location near the north ecliptic pole renders it highly visible throughout the mission. We compare all our low-resolution composite spectra with Kurucz model atmospheres and find good agreement in shape, with the obvious exception of the SiO fundamental, still lacking in current grids of model atmospheres. The CO fundamental seems slightly too deep in these models, but this could reflect our use of generic models with solar metal abundances rather than models specific to the metallicities of the individual stars. Angular diameters derived from these spectra and models are in excellent agreement with the best observed diameters. The ratio of our adopted Sirius and Vega models is vindicated by spectral observations. We compare IRAS fluxes predicted from our cool stellar spectra with those observed and conclude that, at 12 and 25 microns, flux densities measured by IRAS should be revised downwards by about 4.1% and 5.7%, respectively, for consistency with our absolute calibration. We have provided extrapolated continuum versions of these spectra to 300 microns, in direct support of ISO (PHT and LWS instruments). These spectra are consistent with IRAS flux densities at 60 and 100 microns.

Cohen, Martin↗
NASA NTRS1998

Detailed Mid- and Far- Ultraviolet Model Spectra for Accretion Disks in Cataclysmic Binaries

We present a large grid of computed far- and mid-ultraviolet spectra (850-2000 A) of the integrated light from steady-state accretion disks in luminous cataclysmic variables. The spectra are tabulated at 0.25 A intervals with an adopted FWHM resolution of 1.0 A, so they are suitable for use with observed spectra from a variety of modern space-borne observatories. Twenty-six different combinations of white dwarf mass M(sub wd) and mass accretion rate dot-m are considered, and spectra are presented for six different disk inclinations i. The disk models are computed self-consistently in the plane-parallel approximation, assuming LTE and vertical hydrostatic equilibrium, by solving simultaneously the radiative transfer, hydrostatic equilibrium, and energy balance equations. Irradiation from external sources is neglected. Local spectra of disk annuli are computed taking into account line transitions from elements 1-28 (H through Ni). Limb darkening as well as Doppler broadening and blending of lines are taken into account in computing the integrated disk spectra. The radiative properties of the models are discussed, including the dependence of ultraviolet fluxes and colors on M(sub wd), dot-m, and i. The appearance of the disk spectra is illustrated, with regard to changes in the same three parameters. Finally, possible future improvements to the present models and spectra are discussed.

Wade, Richard A.↗
NASA NTRS1999

Extending the McDonald Observatory Serendipitous Survey of UV/Blue Asteroid Spectra

Moderate resolution asteroid spectra in the 350 - 650 nm spectral range acquired randomly over many years (Cochran and Vilas, Icarus v 127, 121, 1997) identified absorption features in spectra of some of the asteroids. A feature centered at 430 nm was identified in the spectra of some low-albedo asteroids (C class and subclass), similar to the feature identified by Vilas et al. (Icarus, v. 102, 225,1993) in other low-albedo asteroid spectra and attributed to a ferric iron spin-forbidden transition in iron alteration minerals such as jarosite. Features at 505 nm and 430 nm were identified in the spectrum of 4 Vesta. The 505-nm feature is highly diagnostic of the amount and form of calcium in pyroxenes. This suggested further research on the sharpness and spectral placement of this feature in the spectra of Vesta and Vestoids (e.g., Cochran and Vilas, Icarus v. 134, 207, 1998). In 1997 and 1998, additional UV/blue spectra were obtained at the 2.7-m Harlan J. Smith telescope with a facility cassegrain spectrograph. These included spectra of low-albedo asteroids, the R-class asteroid 349 Dembowska, and the M-class asteroid 135 Hertha. These spectra will be presented and identified features will be discussed.

Vilas, Faith↗
NASA NTRS2003

Properties of the Hermean Regolith: New Optical Reflectance Spectra, Comparison with Lunar Anorthosites and Mineralogical Modelling - 5

We present new optical (0.4-0.65 micron) spectra of Mercury and lunar pure anorthosite locations, obtained quasi-simultaneously with the Nordic Optical Telescope (NOT) in 2002. A comparative study is performed with the model of Lucey et al. between iron-poor, mature, pure anorthosite (less than 90% plagioclase feldspar) Clementine spectra from the lunar farside and a combined 0.4-1.0 micron mercurian spectrum, obtained with the NOT, calculated for standard photometric geometry. Mercury is located at more extreme locations in the Lucey ratio-reflectance diagrams than any known lunar soil, specifically with respect to the extremely iron-poor mature anorthosites. Though quantitative prediction of FeO and TiO2 abundances cannot be made without a more generally applicable model, we find qualitatively that the abundances of both these oxides must be near zero for Mercury. We utilize the theory of Hapke, with realistic photometric parameters, to model laboratory spectra of matured mineral powders and lunar soils, and remotely sensed spectra of lunar anorthosites and Mercury. An important difference between fabricated and natural powders is the high value for the internal scattering parameter necessary to interpret the spectra for the former, and the requirement of rough and non-isotropically scattering surfaces in the modelling of the latter. The mature lunar anorthosite spectra were well modelled with binary mixtures of calcic feldspars and olivines, grain sizes of 25-30 micron and a concentration of submicroscopic metallic iron (SMFe) of 0.12-0.15% in grain coatings. The mercurian spectrum is not possible to interpret from terrestrial mineral powder spectra without introducing an average particle scattering function for the bulk soil that increases in backscattering efficiency with wavelength. The observed spectrum is somewhat better predicted with binary mixture models of feldspars and pyroxenes, that single-component regoliths consisting of either albite or diopside. Correct spectral reflectance values were predicted with a concentration of 0.1 wt% SMFe in coatings of 15-30 micron sized grains. Since reasonable cosmogonical formation scenarios for Mercury, or meteoritic infall, predict iron concentrations at least this high, we draw the conclusion that the average grain size of Mercury is about a factor of two smaller than for average returned lunar soil samples. The 0.6-2.5 micron spectrum of McCord and Clark is used to further limit the possible range of mineralogical composition of Mercury. It is found that an intimately mixed and matured 3 : 1 labradorite-to-enstatite regolith composition best matches both the optical and near-infrared spectra, yielding an abundance of approx. 1.2 wt% FeO and -approx. 0 wt% TiO2.

Warell, J.↗
NASA NTRS2011

Seasonal Variations of Stratospheric Age Spectra in GEOSCCM

There are many pathways for an air parcel to travel from the troposphere to the stratosphere, each of which takes different time. The distribution of all the possible transient times, i.e. the stratospheric age spectrum, contains important information on transport characteristics. However, it is computationally very expensive to compute seasonally varying age spectra, and previous studies have focused mainly on the annual mean properties of the age spectra. To date our knowledge of the seasonality of the stratospheric age spectra is very limited. In this study we investigate the seasonal variations of the stratospheric age spectra in the Goddard Earth Observing System Chemistry Climate Model (GEOSCCM). We introduce a method to significantly reduce the computational cost for calculating seasonally dependent age spectra. Our simulations show that stratospheric age spectra in GEOSCCM have strong seasonal cycles and the seasonal cycles change with latitude and height. In the lower stratosphere extratropics, the average transit times and the most probable transit times in the winter/early spring spectra are more than twice as old as those in the summer/early fall spectra. But the seasonal cycle in the subtropical lower stratosphere is nearly out of phase with that in the extratropics. In the middle and upper stratosphere, significant seasonal variations occur in the sUbtropics. The spectral shapes also show dramatic seasonal change, especially at high latitudes. These seasonal variations reflect the seasonal evolution of the slow Brewer-Dobson circulation (with timescale of years) and the fast isentropic mixing (with timescale of days to months).

Li, Feng↗
NASA NTRS2011

Visible and Near-IR Reflectance Spectra of Smectite Acquired Under Dry Conditions for Interpretation of Martian Surface Mineralogy

Visible and near-IR (VNIR) spectra from the MEx OMEGA and the MRO CRISM hyper-spectral imaging instruments have spectral features associated with the H2O molecule and M OH functional groups (M = Mg, Fe, Al, and Si). Mineralogical assignments of martian spectral features are made on the basis of laboratory VNIR spectra, which were often acquired under ambient (humid) conditions. Smectites like nontronite, saponite, and montmorillionite have interlayer H2O that is exchangeable with their environment, and we have acquired smectite reflectance spectra under dry environmental conditions for interpretation of martian surface mineralogy. We also obtained chemical, Moessbauer (MB), powder X-ray diffraction (XRD), and thermogravimetric (TG) data to understand variations in spectral properties. VNIR spectra were recorded in humid lab air at 25-35C, in a dynamic dry N2 atmosphere (50-150 ppmv H2O) after exposing the smectite samples (5 nontronites, 3 montmorillionites, and 1 saponite) to that atmosphere for up to approximately l000 hr each at 25-35C, approximately 105C, and approximately 215C, and after re-exposure to humid lab air. Heating at 105C and 215C for approximately 1000 hr is taken as a surrogate for geologic time scales at lower temperatures. Upon exposure to dry N2, the position and intensity of spectral features associated with M-OH were relatively insensitive to the dry environment, and the spectral features associated with H2O (e.g., approximately 1.90 micrometers) decreased in intensity and are sometimes not detectable by the end of the 215C heating step. The position and intensity of H2O spectral features recovered upon re-exposure to lab air. XRD data show interlayer collapse for the nontronites and Namontmorillionites, with the interlayer remaining collapsed for the latter after re-exposure to lab air. The interlayer did not collapse for the saponite and Ca-montmorillionite. TG data show that the concentration of H2O derived from structural OH was invariant to the dry N2 treatment for saponite and the montmorillionites, but the nontronites had additional structural OH after treatment. Upon exposure to dry N2, the VNIR spectra also acquired a red slope with decreasing albedo between approximately 0.4 and approximately 2.0 micrometers. The magnitude of the effects covaries with exposure time to dry N2 and heating temperature. Upon re-exposure to lab air, the slope and albedo do not completely recover to pre-exposure values. MB data show that these effects do not result from partial reduction of ferric to ferrous iron, and TG data show they do not result from loss of structural OH. Possible explanations include formation of small clusters of (superparamagnetic) ferric oxide and reduced smectite crystallinity. The difference in spectral properties between spectra acquired in humid lab air and under dry conditions are consequential for interpretation of CRISM and OMEGA spectra. For example, nontronite by itself and not nontronite plus ferrihydrite can account for the red spectral slope in martian spectra where nontronite is indicated by the Fe-OH spectral features.

Morris, Richard V.↗
DOE OSTI2022

Neutron Leakage Spectra Sensitivities for ICSBEP Benchmarks

Neutron leakage spectra have been measured, simulated, and investigated by many groups. These spectra have many uses, including determining shielding requirements and calculating dose for radiation protection purposes, validating nuclear data, and determining material composition. It has even been proposed to use measurements of neutron leakage spectra to determine the soil composition of Mars. As part of the Los Alamos National Laboratory (LANL) Experiments Underpinned by Computational Learning for Improvements in nuclear Data (EUCLID) Laboratory Directed Research Development (LDRD) project, the authors are working on developing methods to calculate sensitivities to neutron leakage spectra. This may allow nuclear data evaluators to better use neutron leakage spectra data to constrain or adjust nuclear data. This may also be particularly useful as many neutron leakage spectra measurements do not require fissile material and can be performed with well characterized neutron sources. Neutron leakage spectra measurements are usually performed by placing a detector at the outside of a nuclear system. This may be a reactor, a neutron generator, or a neutron source. Certain detection systems can use pulse height data to infer neutron energies, other detector systems rely on other ways of determining neutron energy (for example, a Bonner sphere with multiple moderator thicknesses can be used to measure neutron spectrum). These measurements typically rely on some unfolding of the measured results. For pulsed systems like the "Livermore Pulsed Spheres," time-of-flight information can also help to constrain the neutron energy spectra data.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Computational optimal transport for molecular spectra: The fully continuous case

Computational optimal transport is used to analyze the difference between pairs of continuous molecular spectra. It is demonstrated that transport distances which are derived from this approach may be a more appropriate measure of the difference between two continuous spectra than more familiar measures of distance under many common circumstances. Associated with the transport distances is the transport map which provides a detailed analysis of the difference between two molecular spectra and is a key component of our study of quantitative differences between two continuous spectra. The use of optimal transport for comparing molecular spectra is developed in detail here with a set of model spectra, so that the discussion is self-contained. The difference between the transport distance and more common definitions of distance is elucidated for some well-chosen examples and it is shown where transport distances may be very useful alternatives to standard definitions of distance. The transport distance between a theoretical and experimental electronic absorption spectrum for SO 2 is studied and it is shown how the theoretical spectrum can be modified to fit the experimental spectrum better adjusting the theoretical band origin and the resolution of the theoretical spectrum. In conclusion, this analysis includes the calculation of transport maps between the theoretical and experimental spectra suggesting future applications of the methodology.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

MS2Planner: improved fragmentation spectra coverage in untargeted mass spectrometry by iterative optimized data acquisition

Motivation: Untargeted mass spectrometry experiments enable the profiling of metabolites in complex biological samples. The collected fragmentation spectra are the metabolite’s fingerprints that are used for molecule identification and discovery. Two main mass spectrometry strategies exist for the collection of fragmentation spectra: data-dependent acquisition (DDA) and data-independent acquisition (DIA). In the DIA strategy, all the metabolites ions in predefined mass-to-charge ratio ranges are co-isolated and co-fragmented, resulting in multiplexed fragmentation spectra that are challenging to annotate. In contrast, in the DDA strategy, fragmentation spectra are dynamically and specifically collected for the most abundant ions observed, causing redundancy and sub-optimal fragmentation spectra collection. Yet, DDA results in less multiplexed fragmentation spectra that can be readily annotated. Results: We introduce the MS2Planner workflow, an Iterative Optimized Data Acquisition strategy that optimizes the number of high-quality fragmentation spectra over multiple experimental acquisitions using topological sorting. Our results showed that MS2Planner increases the annotation rate by 38.6% and is 62.5% more sensitive and 9.4% more specific compared to DDA. Availability and implementation MS2Planner code is available at https://github.com/mohimanilab/MS2Planner. The generation of the inclusion list from MS2Planner was performed with python scripts available at https://github.com/lfnothias/IODA_MS.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

The entropy of galaxy spectra: how much information is encoded?

Abstract The inverse problem of extracting the stellar population content of galaxy spectra is analysed here from a basic standpoint based on information theory. By interpreting spectra as probability distribution functions, we find that galaxy spectra have high entropy, thus leading to a rather low effective information content. The highest variation in entropy is unsurprisingly found in regions that have been well studied for decades with the conventional approach. We target a set of six spectral regions that show the highest variation in entropy – the 4000 Å break being the most informative one. As a test case with real data, we measure the entropy of a set of high-quality spectra from the Sloan Digital Sky Survey, and contrast entropy-based results with the traditional method based on line strengths. The data are classified into star-forming (SF), quiescent (Q), and active galactic nucleus (AGN) galaxies, and show – independently of any physical model – that AGN spectra can be interpreted as a transition between SF and Q galaxies, with SF galaxies featuring a more diverse variation in entropy. The high level of entanglement complicates the determination of population parameters in a robust, unbiased way, and affects traditional methods that compare models with observations, as well as machine learning (especially deep learning) algorithms that rely on the statistical properties of the data to assess the variations among spectra. Entropy provides a new avenue to improve population synthesis models so that they give a more faithful representation of real galaxy spectra.

Ferreras, Ignacio (ORCID:0000000345843127)↗
DOE OSTI2024

Novel principal component analysis tool based on python for analysis of complex spectra of time-of-flight secondary ion mass spectrometry

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a powerful surface analysis tool, which can simultaneously provide elemental, isotopic, and molecular information with part per million (ppm) sensitivity. However, each spectrum may be composed of hundreds of ion signals, which makes the spectra data complex. Principal component analysis (PCA) is a multivariate analysis technique that has been widely used to figure out the variances among samples in ToF-SIMS spectra data analysis and is showing great success in the explanation of complex ToF-SIMS spectra. So far, several software tools have been developed for PCA of ToF-SIMS spectra; however, none of them are freely available. Such a situation leads to some difficulties in extending applications of PCA to various research fields. More importantly, it has long been challenging for common researchers to understand PCA plots and extract chemical differences among samples. In this work, we developed a new and flexible software tool (named “advanced spectra pca toolbox”) based on python for PCA of complex ToF-SIMS spectra along with an easy-to-read manual. It can generate data analysis reports automatically to explain chemical differences among samples, allowing less experienced researchers to easily understand tricky PCA results. Moreover, it is expandable and compatible with artificial intelligence/machine learning functions. Pure goethite and different lignin adsorbed goethite samples were used as a model system to demonstrate our new software tool, proving that our software tool can be readily used in complex spectra data processing. Our new software tool is open-source, convenient, flexible, and expandable. We expect this open-source tool will benefit the ToF-SIMS community.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Automated Stellar Spectra Classification with Ensemble Convolutional Neural Network

Large sky survey telescopes have produced a tremendous amount of astronomical data, including spectra. Machine learning methods must be employed to automatically process the spectral data obtained by these telescopes. Classification of stellar spectra by applying deep learning is an important research direction for the automatic classification of high-dimensional celestial spectra. In this paper, a robust ensemble convolutional neural network (ECNN) was designed and applied to improve the classification accuracy of massive stellar spectra from the Sloan digital sky survey. We designed six classifiers which consist six different convolutional neural networks (CNN), respectively, to recognize the spectra in DR16. Then, according the cross-entropy testing error of the spectra at different signal-to-noise ratios, we integrate the results of different classifiers in an ensemble learning way to improve the effect of classification. The experimental result proved that our one-dimensional ECNN strategy could achieve 95.0% accuracy in the classification task of the stellar spectra, a level of accuracy that exceeds that of the classical principal component analysis and support vector machine model.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1976

Use of thin ionization calorimeters for measurements of cosmic ray energy spectra

The reliability of performing measurements of cosmic ray energy spectra with a thin ionization calorimeter was investigated. Monte Carlo simulations were used to determine whether energy response fluctuations would cause measured spectra to be different from the primary spectra. First, Gaussian distributions were assumed for the calorimeter energy resolutions. The second method employed a detailed Monte Carlo simulation of cascades from an isotropic flux of protons. The results show that as long as the energy resolution does not change significantly with energy, the spectral indices can be reliably determined even for sigma sub e/e = 50%. However, if the energy resolution is strongly energy dependent, the measured spectra do not reproduce the true spectra. Energy resolutions greatly improving with energy result in measured spectra that are too steep, while resolutions getting much worse with energy cause the measured spectra to be too flat.

Jones, W. V.↗
NASA NTRS1984

The dependence of reflectance spectra of Mercury on surface terrain

Reflectance spectra of Mercury covering the spectral range of 0.3-1.1 micron, obtained during 1963-1976, were examined for any correlations with surface terrain. Mercury's 6.1385-deg/day rotational rate, the phases of the planet around maximum elongations, and bidirectional-reflectance-spectroscopy theory were used to identify the surface area associated with each spectrum. Data from 1974-1975, rereduced with improved standard-star-flux ratios, show a weak absorption band in the near infrared not seen in earlier analyses. Older spectra suggest that the western longitudes of the unimaged side of Mercury are similar to the rest of the planet. Spectra of the intercrater plains in the 0-90-deg quadrant suggest a possible absorption band. Spectra of areas dominated by Caloris Basin with the encompassing smooth plains may show Fe(2+) abundances in the soil comparable to lunar-highlands soil. No striking differences between spectra of intercrater plains and smooth plains are found. The absorption features seen in spectra of Mercury are generally weaker than features seen in lunar spectra.

Vilas, F.↗
NASA NTRS1984

Spectra from pair-equilibrium plasmas

A numerical model of relativistic nonmagnetized plasma with uniform temperature and electron density distributions is considered, and spectra from plasma in pair equilibrium are studied. A range of dimensionless temperature (T) greater than about 0.2 is considered. The spectra from low pair density plasmas in pair equilibrium vary from un-Comptonized bremsstrahlung spectra at Thomson cross section tau(N) much less than one to Comptonized bremsstrahlung spectra with tau(N) over one. For high pair density plasmas the spectra are flat for T greater than about one, and have broad intensity peaks at energy roughly equal to 3T for T less than one. In the latter region the total luminosity is approximately twice the annihilation luminosity. All spectra are flat in the X-ray region, in contradiction to observed AGN spectra. For dimensionless luminosity greater than about 100, the cooling time becomes shorter than the Thomson time.

Zdziarski, A. A.↗
NASA NTRS1989

New LRS spectra for 356 bright IRAS sources

The low-resolution spectra of all IRAS point sources with F(nu) (12 microns) greater than 40 Jy that were not included in the Atlas of Low-Resolution Spectra are presented. These have been classified into eight groups based upon the spectral morphology. Silicate emission spectra and red-continuum spectra associated with H II region sources form about 60 percent of this sample. All types of spectra in the LRS Atlas are represented in the sample except for emission-line sources. The sample is used to test a recent classification scheme for IRAS sources based on broadband colors. The spectra is used to test a recent classification scheme for IRAS sources based on broadband colors. The spectra are consistent with the classifications from the colors in most cases.

Volk, Kevin↗
NASA NTRS1989

Characteristics of energetic solar flare electron spectra

A 55 event survey of energy spectra of 0.1-100 MeV interplanetary electrons originating from solar flares as measured by two spectrometers onboard the ISEE 3 (ICE) spacecraft for the years 1978-1982 has been completed. Spectra generated using the maximum flux of a given event in each energy channel were restricted to events with a well-defined flux rise time. Two broad groups of electron spectra are considered. In one group, the spectra are well represented by a single power law in rigidity with spectral index in the range 3-4.5. The spectra in the other group deviate from a power law in rigidity systematically in that they harden with increasing rigidity. Events with near power-law spectra are found to be correlated with long-duration soft X-ray events, whereas those with hardening spectra are correlated with short-duration events. The possible variation of acceleration and propagation processes with the properties of the flare site is discussed, using the duration of the soft X-ray flare emission as an indicator of the physical parameters of the flare site (flare volume, density, coronal height, and magnetic field geometry).

Moses, Dan↗
NASA NTRS1993

Retrieval of temperature profiles from Martian infrared spectra

In theory, a thermal emission spectrum of Mars (220-1600 cm(exp -1)) contains a wealth of information about the composition and thermal structure of the atmosphere and surface. However, in practice, the ability to retrieve quantitative information from the spectrum is limited by a number of factors including (1) the ill-posed nature of the spectral inversion problem and the resulting nonuniqueness of all solutions; (2) assumptions built into any spectral inversion program; (3) uncertainties in surface pressure, temperature, and emissivity; and (4) uncertainties in the optical properties of atmospheric aerosols. Below, factors (1) and (2) are discussed as fundamental limitations on temperature retrievals from Mariner 9 Infrared Interferometer Spectrometer (IRIS) spectra. In preparation for the Mars Observer mission and the return of tens of thousands of infrared spectra per day from the Thermal Emission Spectrometer (TES), we have developed a fast inversion algorithm to retrieve temperature structure and aerosol opacity from the infrared spectra. The derived atmospheric models will be used to provide the atmospheric contribution to the TES spectra, so that the thermal emission spectra of the underlying surface can be determined for making mineralogical identifications. As a test of our algorithm, we are undertaking a systematic analysis of the entire Mariner 9 Infrared Interferometric Spectrometer (IRIS) dataset of 21,000 plus spectra. While portions of the IRIS dataset have been previously analyzed, the lack of a speedy and robust algorithm to invert the IRIS spectra to retrieve temperature profiles and aerosol opacities has been a major impediment to a truly comprehensive analysis.

Mcmillan, W. Wallace↗