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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Experience gained in operation of the VLF ATD lightning location system

The United Kingdom (UK) Meteorological Office's Very Low Frequency (VLF) Arrival Time Difference (ATD) System for long-range location of lightning flashes started automatic international issue of lightning-location products on 17 Jun. 1988. Data from before and after this formal start-date were carefully scrutinized to judge performance. Techniques for estimating location accuracy include internal consistency and comparisons against other systems. Other areas studied were range (up to several thousand km); detection efficiency, saturation effects in active situations, and communication difficulties (for this redundant system); and spurious fix rate. Care was taken to assess the potential of the system, in addition to identifying the operational difficulties of the present implementation.

Lee, Anthony C. L.↗
NASA NTRS1991

Coincidence probabilities for spacecraft gravitational wave experiments - Massive coalescing binaries

Massive coalescing binary systems are candidate sources of gravitational radiation in the millihertz frequency band accessible to spacecraft Doppler tracking experiments. This paper discusses signal processing and detection probability for waves from coalescing binaries in the regime where the signal frequency increases linearly with time, i.e., 'chirp' signals. Using known noise statistics, thresholds with given false alarm probabilities are established for one- and two-spacecraft experiments. Given the threshold, the detection probability is calculated as a function of gravitational wave amplitude for both one- and two-spacecraft experiments, assuming random polarization states and under various assumptions about wave directions. This allows quantitative statements about the detection efficiency of these experiments and the utility of coincidence experiments. In particular, coincidence probabilities for two-spacecraft experiments are insensitive to the angle between the directions to the two spacecraft, indicating that near-optical experiments can be done without constraints on spacecraft trajectories.

Tinto, Massimo↗
NASA NTRS1990

Photoelectron energy spectra of opaque photocathodes in the extreme and far ultraviolet

Measurements of the EUV quantum detection efficiency (QDE) of opaque RbBr, CsBr, and KBr photocathodes are described and investigations of their photoemission characteristics over the 44-1560 A wavelength range are reported. The results show that high QDEs can be obtained in the EUV. Narrow QDE peaks at soft X-ray wavelengths occur at slightly different wavelengths for each of the materials studied. The long-wavelength thresholds vary according to the material band gap. Data on the photoemission from the photocathode layer on the microchannel plate interchannel web area are used to determine the number and energy distribution of the emitted photoelelectrons as a function of wavelength.

Siegmund, Oswald H. W.↗
NASA NTRS1991

A hard X-ray polarimeter utilizing Compton scattering

The paper describes a 50-cm-diam prototype of a novel Compton-scattering-type polarimeter for hard X-rays in the energy range 30-100 keV. The characteristics of the prototype polarimeter were investigated for various conditions. It was found that, with polarized X-rays from a simple polarizer, the detection efficiency and the modulation factor of the polarimeter with a 40-mm thick scatterer were 3.2 percent and 0.57 percent, respectively, at about 60 keV.

Sakurai, H.↗
NASA NTRS1992

The AXAF CCD imaging spectrometer

The current status of the instrument design and the status of the CCDs being fabricated for the AXAF CCD Imaging Spectrometer (ACIS) are summarized. The instrument consists of an image recording array of CCDs and a linear arrangement of CCDs to record the spectra formed by the objective grating spectrometer. Both arrays employ CCDs with pixel dimensions which correspond to about 0.5 arcsec samples of the image. The CCDs provide moderate spectral resolution and good detection efficiency over the energy range 0.5 to 10 keV. Spectral resolution of 200 or more is achievable using the objective grating with the grating array. Radiation damage effects are shown to degrade the energy resolution of CCDs. Specially designed CCD pixel architecture is employed together with shielding and low temperature operation to slow the effects of radiation damage.

Garmire, G. P.↗
NASA NTRS1993

Balloon observations of interstellar CII (158 microns) and OI (63 microns) forbidden lines

Interstellar CII and OI forbidden lines were observed by the Balloon-Borne Infrared Telescope (BIRT) with a Fabry-Perot spectrometer. Two balloon flights were successfully made. With a method of 'frequency switching', diffuse CII forbidden-line emission was efficiently detected and mapped in extended regions around HII/molecular cloud complexes and in a wide area of the Galactic plane. It has been shown that the CII forbidden-line emission is very strong and ubiquitously distributed in interstellar space in the Galaxy.

Shibai, H.↗
NASA NTRS1992

Characteristics of square pore and low noise microchannel plate stacks

An evaluation is conducted of several square-pore microchannel plates (MCPs) with either 25- or 85-micron diameter pores and 80:1 or 50:1 channel length/diameter ratio. Flat field measurements show that the 25-micron-pored MCPs, unlike those with 85-micron pores, exhibit periodic modulation; this may be due to the MCP stacking configurations. Attention is given to the relative quantum detection efficiency advantages of the two MCPs.

Siegmund, Oswald H. W.↗
NASA NTRS1993

Development efforts to improve curved-channel microchannel plates

Curved-channel microchannel plate (C-plate) improvements resulting from an ongoing NASA STIS microchannel plate (MCP) development program are described. Performance limitations of previous C-plates led to a development program in support of the STIS MAMA UV photon counter, a second generation instrument on the Hubble Space Telescope. C-plate gain, quantum detection efficiency, dark noise, and imaging distortion, which are influenced by channel curvature non-uniformities, have all been improved through use of a new centrifuge fabrication technique. This technique will be described, along with efforts to improve older, more conventional shearing methods. Process optimization methods used to attain targeted C-plate performance goals will be briefly characterized. Newly developed diagnostic measurement techniques to study image distortion, gain uniformity, input bias angle, channel curvature, and ion feedback, will be described. Performance characteristics and initial test results of the improved C-plates will be reported. Future work and applications will also be discussed.

Corbett, M. B.↗
NASA NTRS1992

Performance of the EGRET astronomical gamma ray telescope

On April 5, 1991, the Space Shuttle Atlantis carried the Compton Gamma Ray Observatory (CGRO) into orbit, deploying the satellite on April 7. The EGRET instrument was activated on April 15, and the first month of operations was devoted to verification of the instrument performance. Measurements made during that month and in the subsequent sky survey phase have verified that the instrument time resolution, angular resolution, and gamma ray detection efficiency are all within nominal limits.

Nolan, P. L.↗
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.↗
NASA NTRS1992

Microchannel Plate Imaging Detectors for the Ultraviolet

There has been significant progress over the last few years in the development of technologies for microchannel plate imaging detectors in the Ultraviolet (UV). Areas where significant developments have occurred include enhancements of quantum detection efficiency through improved photocathodes, advances in microchannel plate performance characteristics, and development of high performance image readout techniques. The current developments in these areas are summarized, with their applications in astrophysical instrumentation.

Siegmund, O. H. W.↗
NASA NTRS1993

New room temperature high resolution solid-state detector (CdZnTe) for hard x rays and gamma rays

The new CdZnTe high 'Z' material represents a significant improvement in detectors for high energy photons. With the thicknesses available, photons up to 100 keV can be efficiently detected. This material has a wide band gap of 1.5 - 2.2 eV which allows it to operate at room temperature while providing high spectral resolution. Results of resolution evaluations are presented. This detector can be used for high resolution spectral measurements of photons in x-ray and gamma-ray astronomy, offering a significant reduction in the weight, power, and volume of the detector system compared to more conventional detector types such as scintillation counters. In addition, the detector will have the simplicity and reliability of solid-state construction. The CdZnTe detector, as a new development, has not yet been evaluated in space. The Get Away Special program can provide this opportunity.

Stewart, Amyelizabeth C.↗
NASA NTRS1993

The polarization sensitivity of the liquid xenon imaging telescope

The properties and the expected performance of a liquid xenon (LXe) gamma ray imaging telescope, optimized for the MeV energy region, are presented. The unique potential of this telescope as a Compton polarimeter is particularly emphasized. Based on Monte Carlo simulations we show that the modulation factor is as high as 40 percent at 1 MeV with a detection efficiency close to 20 percent. These figures of merit combined with the excellent background suppression capability of the three dimensional position sensitive LXe detector yield sensitivity at the three sigma level to polarization fractions as small as a few percent for strong sources, even in a balloon flight.

Aprile, Elena↗
NASA NTRS1995

Summary of LET spectra and dose measurements on ten STS missions

A comparison of linear energy transfer (LET) spectra measurements made with plastic nuclear track detectors (PNTD's) from area passive dosimeters (APD's), was made for ten different STS missions under similar shielding. The results show that integral flux, dose rate and equivalent dose rate values follow a general increase with respect to increasing orbital inclination and altitude but that there are large variations from a simple relationship. This is to be expected since it has been shown that Shuttle attitude variations, combined with the anisotropic particle flux at the South Atlantic Anomaly (SAA), can result in differences of a factor of 2 in dose rate inside the Shuttle (Badhwar et al., 1995). Solar cycle and shielding differences also result in variations in radiation dose between STS missions. Spaceflight dosimeters from the STS missions are also being used in the development of a method for increasing LET spectra measurement accuracy by extending LET measurements to particle tracks of ranges 10-80 microns. Refinements in processing and measurement techniques for the flight PNTD's have yielded increased detection efficiencies for the short tracks when LET spectra determined by using the standard and refined methods were intercompared.

Source record↗
NASA NTRS1995

Fission foil detector calibrations with high energy protons

Fission foil detectors (FFD's) are passive devices composed of heavy metal foils in contact with muscovite mica films. The heavy metal nuclei have significant cross sections for fission when irradiated with neutrons and protons. Each isotope is characterized by threshold energies for the fission reactions and particular energy-dependent cross sections. In the FFD's, fission fragments produced by the reactions are emitted from the foils and create latent particle tracks in the adjacent mica films. When the films are processed surface tracks are formed which can be optically counted. The track densities are indications of the fluences and spectra of neutrons and/or protons. In the past, detection efficiencies have been calculated using the low energy neutron calibrated dosimeters and published fission cross sections for neutrons and protons. The problem is that the addition of a large kinetic energy to the (n,nucleus) or (p,nucleus) reaction could increase the energies and ranges of emitted fission fragments and increase the detector sensitivity as compared with lower energy neutron calibrations. High energy calibrations are the only method of resolving the uncertainties in detector efficiencies. At high energies, either proton or neutron calibrations are sufficient since the cross section data show that the proton and neutron fission cross sections are approximately equal. High energy proton beams have been utilized (1.8 and 4.9 GeV, 80 and 140 MeV) for measuring the tracks of fission fragments emitted backward and forward.

Benton, E. V.↗
NASA NTRS1994

The polarization sensitivity of the liquid xenon imaging telescope

The properties and the expected performance of a liquid xenon (LXe) gamma-ray imaging telescope, optimized for the MeV energy region, are presented. The unique potential of this telescope as a Compton polarimeter is particularly emphasized. Based on Monte Carlo simulations, we show that the modulation factor is as high as 40% at 1 MeV with a detection efficiency close to 20%. These figures of merit, combined with the excellent background suppression capability of the three-dimensional position sensitive LXe detector, yield sensitivity at the 3 sigma level to polarization fractions as small as a few percent for strong sources, even in a balloon flight.

Aprile, E.↗
NASA NTRS1995

Inferring the spatial and energy distribution of gamma-ray burst sources. 1: Methodology

We describe Bayesian methods for analyzing the distribution of gamma-ray burst peak photon fluxes and directions. These methods fit the differential distribution, and have the following advantages over rival methods: (1) they do not destroy information by binning or averaging the data (as do, say, chi squared, the averaged value of V/V(sub max), and angular moment analyses); (2) they straightforwardly handle uncertainties in the measured quantities; (3) they analyze the strength and direction information jointly; (4) they use information available about nondetections; and (5) they automatically identify and account for biases and selection effects given a precise description of the experiment. In these methods, the most important information needed about the instrument threshold is not its value at the times of burst triggers, as is used in the average value of V/V(sub max) analyses, but rather the value of the threshold at times when no trigger occurred. We show that this information can be summarized as an average detection efficiency that is similar to the product of the exposure and efficiency reported in the First Burst and Transient Source Experiment (BATSE) Burst (1B) Catalog, but significantly different from it at low fluxes. We also quantify an important bias that results from estimating the peak flux by scanning the burst to find the peak number of counts in a window of specified duration, as was done for the 1B Catalog. When the duration of the peak of the light curve is longer than the window duration, a simple flux estimate based on the peak counts significantly overestimates the peak flux in a nonlinear fashion that distorts the shape of the log(N)-log(P) distribution. This distortion also corrupts analyses of the V/V(sub max) distribution that use ratios of counts above background to estimate V/V(sub max). The Bayesian calculation specifies how to account for this bias. Implementation of the Bayesian approach requires some changes in the way burst data are reported that we describe in detail. Subsequent papers will report analyses of the 1B Catalog data using the methods described here.

Loredo, Thomas J.↗
NASA NTRS1996

Preselected Sub-Poissonian Correlations

The simplest possible photon-number-squeezed states containing only two photons and exhibiting sub-poissonian statistics with the Fano factor approaching 0.5 have been used for a proposal of a loophole-free Bell experiment requiring only 67 percent of detection efficiency. The states are obtained by the fourth order interference first of two downconverted photons at an asymmetrical beam splitter and thereupon of two photons from two independent singlets at an asymmetrical beam splitter. In the latter set-up, the other two photons which nowhere interacted and whose paths never crossed appear entangled in a singlet-like correlated state.

Pavicic, Mladen↗