Engineering topics

Ramsey, B. D.

Publications and source records attributed to Ramsey, B. D..

At least 55 records · Page 3

NASA NTRS1999

Preliminary Results from Small-Pixel CdZnTe and CdTe Arrays

We have evaluated 2 small-pixel (0.75 mm) Cadmium-Zinc-Telluride arrays, and one Cadmium-Telluride array, all fabricated for MSFC by Metorex (Finland) and Baltic Science Institute (Riga, Latvia). Each array was optimized for operating temperature and collection bias. It was then exposed to Cadmium-109 and Iron-55 laboratory isotopes, to measure the energy resolution for each pixel and was then scanned with a finely-collimated x-ray beam, of width 50 micron, to examine pixel to pixel and inter-pixel charge collections efficiency. Preliminary results from these array tests will be presented.

Ramsey, B. D.↗

NASA NTRS1999

The HERO Program: High-Energy Replicated Optics for a Hard-X-Ray Balloon Payload

We are developing high-energy replicated optics for a balloon-borne hard-x-ray telescope. When completed, the telescope will have around 150 square cm of effective collecting area up to 65 keV, and an angular resolution of around 30 arc seconds, half power diameter. When used in conjunction with an array of focal plane imaging detectors, for which Gas Scintillation Proportional Counters are under development, the payload will provide unprecedented sensitivity for pointed observations in the hard-x-ray band. We will present an overview of the High Energy Replicated Optics (HERO) program together with test data from the first mirror units. The overall sensitivity of the full payload, when flown on long- and ultra-long-duration balloon flights, will be compared with past and planned satellite-borne hard-x-ray missions.

Ramsey, B. D.↗

NASA NTRS1999

A High-Pressure Gas-Scintillation-Proportional Counter for the Focus of a Hard-X-Ray Telescope

We are developing a high-pressure Gas Scintillation Proportional Counter (GSPC) for the focus of a balloon-borne hard-x-ray telescope. The device has a total active diameter of 50 mm, of which the central 20 mm only is used, and is filled with xenon + 4% helium at a total pressure of 10 6 Pa giving a quantum efficiency of greater than 85% up to 60 keV. The detector entrance is sealed with a beryllium window, 3-mm thick, which provides useful transmission down to 6 keV, way below the atmospheric cut-off at balloon float altitudes. Scintillation light exits the detector via a UV transmitting window in its base and is registered by a Hamamatsu position-sensitive crossed-grid-readout photomultiplier tube. Initial testing is underway, quantifying light yield and energy resolution. Following that, the spatial resolution and absolute efficiency will be calibrated. Simulations show that a spatial resolution of better than 0.5 mm FWHM should be achievable up to 60 keV, and this is well matched to the angular resolution and plate scale of the mirror system. The energy resolution will be around 5% at 22 keV. Full details of the instrument design and its performance will be presented. A first flight is scheduled for the Fall of 99, on a stratospheric balloon to be launched from Fort Sumner, New Mexico.

Austin, R. A.↗

NASA NTRS1999

Origin of Gamma-Ray Emissions from the MeV Blazars

More than sixty gamma-ray emitting blazars have been detected by the Compton Gamma Ray Observatory (CGRO). A sub-class of these blazars is known as MeV blazars that are most luminous at the MeV energies. Spectra of these sources show a break somewhere between 1 MeV and 30 MeV with break amplitude as large as DELTA-GAMMA = 1.5. Different models like Compton cooling jet models, synchrotron self-Compton models, etc., have been suggested to explain the observed properties of the MeV luminous blazars. However these models are unable to account for spectral break amplitudes in excess of 0.5. Here we suggest an inverse-Compton model based on recent published experimental data where MeV gamma rays were produced via Compton backscattering of UV photons with 500 MeV electrons. We show that the high energy electrons of the relativistic jet can collide with the copious ultraviolet photons of the big blue bump of the blazar, and produce photons peaked in the MeV region. Also we show that the X rays and the soft gamma rays in these sources are produced through the synchrotron self-Compton process. Flux computed from our model have been used to compare with the observed X-ray/gamma-ray spectra of the MeV luminous blazars. Details of the model are discussed in the present paper.

Ghosh, K. K.↗

NASA NTRS1999

Major Optical Outburst of Two Blazars: 3C66A and OJ287

Differential CCD photometric observations of 3C66A and OJ287 were carried out on 16 and 35 nights between December 1995 and March 1996, at the Vainu Bappu Observatory, India, as part of the blazar monitoring program. During this period we detected major optical outbursts (brightness of 3C66A and OJ287 increased by 0.8 and 1 mag, respectively) of these two blazars on timescales of two months. Integrating the outburst profiles we find that both the blazars released around 10(exp 53) erg. Such large amount of energy may come from the release of binding energy of a compact star when tidally disrupted by a super-massive black hole at the center of these blazars. Also we extend this model to explain the observed multifrequency (radio through gamma-ray) outburst of these two blazars and show that this model will be able to explain the outburst phenomena of other blazars. These new results will be presented with a detailed discussion of the suggested model.

Ghosh, K. K.↗

NASA NTRS1999

Constellation-X Spectroscopy X-Ray Telescope Requirements and Development Program: MSFC Research Program

The Constellation-X Spectroscopy X-ray Telescope (SXT) will provide high-throughput, high-resolution spectroscopy of cosmic sources, from 0.25 keV to 10 keV. Key to this capability is the development of large (1.6 m diameter), lightweight optics for the SXT mirror assembly. Teams led by NASA's Marshall Space Flight Center (MSFC), by NASA's Goddard Space Flight Center (GSFC), and by Italy's Osservatorio Astronomico di Brera (OAB) are currently developing competing mirror technologies for this planned mission. Each team is making significant research progress in developing mirror technologies which satisfy the SXT requirements for lightweight optics, consistent with a system-level optical performance of better than 15 arcsec half-power diameter. The NASA Marshall Space Flight Center (MSFC), in collaboration with the Smithsonian Astrophysical Observatory (SAO), has focussed its efforts on full-shell replicated optics, of electroformed nickel alloys. Recent progress in identifying a surface treatment to effect low, controlled adhesion and, more significantly, in developing new high-strength nickel alloys make this a viable, low cost approach to satisfying the SXT requirements.

O'Dell, S. L.↗

NASA NTRS1999

Development of Constellation-X Optics Technologies at MSFC

One of the major technological challenges for the Constellation X-ray Mission is the development of light-weight, high-resolution, grazing-incidence optics. NASA's Marshall Space Flight Center is developing and evaluating candidate technologies, based upon full-shell replication off precision mandrels. Here we report on recent progress in meeting the weight and imaging-performance requirements, using very thin, high-strength electroformed nickel alloys. In addition, we briefly describe MSFC's optics fabrication, metrology, and x-ray test facilities.

Odell, S. L.↗

NASA NTRS1999

Optical Variability of BL Lacertae During the Major Outburst of 1997

We have undertaken an investigation of recent flux variability in BL Lac. We present optical observations taken over 12 nights documenting major as well as minor outbursts. This has been combined, for purposes of multifrequency analysis, with published X-ray and gamma ray data taken for an additional single night. On two nights in particular, which includes the night of the X-ray observations, a major outburst of about a full magnitude of variation was recorded. All the data have been analyzed with theoretical models. Attempts were made to use synchrotron self-Compton and external Comptonization models to explain the data; however both classes of models were found lacking. More satisfactory results were obtained using an analytical model proposed by Wang et al. that involves the evolution of synchrotron spectra in a homogeneous jet due to the injection of relativistic electrons, taking into account radiation losses during the outbursts. It is hoped that the results of this study of BL Lac, an archetype for the class of blazars in general, represent a more generic phenomenon applicable to the entire class.

Ghosh, K. K.↗

NASA NTRS1998

Rate and Gain Limitations of MSGC's and MGC's Combined with GEM and other Preamplification Structures

We have studied the rate and gain limits of diamond-coated Microstrip Gas Counters (MSGC's) and Micro-Gap Counters (MGC's) when combined with various preamplification structures: Gas Electron Multiplier (GEM), Parallel-Plate Avalanche Chamber (PPAC) or a MICROMEGAS-type structure. Measurements were done both with X rays and alpha particles with various detector geometries and in different gas mixtures at pressures from 0.05 to 10 atm. The results obtained varied significantly with detector design, gas mixture and pressure, but some general features can be identified. We found that in all cases, bare MSGC'S, MGC'S, PPAC's and MICROMEGAS, the maximum achievable gain drops with rate. The addition of preamplification structures significantly increases the gain of MSGC's and MGC'S, but this gain is still rate dependent. There would seem to be a general rate-dependent effect governing the usable gain of all these detectors. We speculate on possible mechanisms for this effect, and identify a safe, spark-free, operation zone for each system (detector + preamplification structure) in the rate-gain coordinate plane.

Fonte, P.↗

NASA NTRS1998

Breakdown Features of Various Microstrip-Type Gas Counter Designs and Their Improvements

Breakdown mechanisms and spurious pulses, the precursors to some breakdowns, were studied experimentally for both uncoated and coated Microstrip Gas Counters (MSGCs) of different geometries, as well as for MicroGap Counters (MGCs) and for the "Compteur A Trou" (CAT). It was found that in all cases the breakdowns occurred through surface streamers, although the exact mechanism of streamer formation depended on the particular detector design. Based on these studies, new designs of microstrip detectors, in which the role of the substrate was minimized, were elaborated and tested. In some of these detectors, especially with large pitches (greater than 2mm), gains up to 2-3 x 10(exp 5) were achieved together with good rate characteristics. The ultimate gain limit in all geometries was still set by spark-inducing streamers which appeared at some critical charge density in the avalanche. To avoid this, and particularly to enhance the performance of small-pitch MSGCs, preamplification structures can be used. Utilizing a parallel plate avalanche chamber as a front end to an MSGC resulted in an overall gain of approximately 10(exp 6), limited in this case only by charge saturation.

Peskov, V.↗

NASA NTRS1998

A Study of Breakdown Limits in Microstrip Gas Counters with Preamplification Structures

We have studied the charge and breakdown limits of Microstrip Gas Counters (MSGCs) with 2 different preamplification structures: the Gas Electron Multiplier (GEM) and the Parallel-Plate Avalanche Counter (PPAC). It was found that in both cases the breakdown limit was increased by 1-2 orders of magnitude compared to the bare MSGC that this is due to the spread of the primary electron cloud during pre-amplification. This spreading reduces the charge density in the final MSGC avalanche, permitting much higher total gains before streamers form. The real practical gain limitations in these two-stage detectors arose not from sparking, but from a loss of proportionality due to space charge effects.

Fonte, P.↗

NASA NTRS1998

Monte Carlo Simulations of Background Spectra in Integral Imager Detectors

Predictions of the expected gamma-ray backgrounds in the ISGRI (CdTe) and PiCsIT (Csl) detectors on INTEGRAL due to cosmic-ray interactions and the diffuse gamma-ray background have been made using a coupled set of Monte Carlo radiation transport codes (HETC, FLUKA, EGS4, and MORSE) and a detailed, 3-D mass model of the spacecraft and detector assemblies. The simulations include both the prompt background component from induced hadronic and electromagnetic cascades and the delayed component due to emissions from induced radioactivity. Background spectra have been obtained with and without the use of active (BGO) shielding and charged particle rejection to evaluate the effectiveness of anticoincidence counting on background rejection.

Armstrong, T. W.↗

NASA NTRS1998

Imaging Polarimetry of Six X-Ray Selected Blazars

Results of broad band V-filter linear optical polarization measurements of six X-ray selected B L Lac objects (XBLs) are presented. Four of these B L Lac objects (MS0737.9+7441, MS0950.9+4929, MS1458.8+2249 and MS 1534.2+0148) had not been measured before in this band. We have detected strong optical polarization in three B L Lac objects. Analysis of radio through X-ray spectra of these sources confirms that these objects are indeed XBLS. We have also detected several new polarized sources in the fields of these B L Lacs.

Ghosh, K. K.↗

NASA NTRS1998

Hard X-ray Observation of Cygnus X-1 By the Marshall Imaging X-ray Experiment (MIXE2)

The second generation of the Marshall Imaging X-ray Experiment (MIXE2) was flown from Fort Sumner, New Mexico on May 7-8, 1997. The experiment consists of coded-aperture telescope with a field of view of 1.8 degrees (FWHM) and an angular resolution of 6.9 arcminutes. The detector is a large (7.84x10(exp 4) sq cm) effective area microstrip proportional counter filled with 2.0x10(exp5) Pascals of xenon with 2% isobutylene. We present MIXE2 observation of the 20-80keV spectrum and timing variability of Cygnus X-1 made during balloon flight.

Minamitani, Takahisa↗

NASA NTRS1998

Near Simultaneous Spectroscopic and Polarimetric Observations of Be Stars

Near simultaneous optical spectroscopic (on four nights) and linear continuum (B, V, R and I bands) polarimetric (on seven nights) observations of 29 Be stars were carried out during November - December 1993. The program Be stars displayed wavelength dependence of intrinsic polarizations with no frequency dependence of position angles. Measured values of linear intrinsic polarizations of these Be stars were all less than 1.6%. The Be and Be-shell stars displayed no clear distinction in their polarization values. Observed polarizations at B, V, R and I bands were compared with the theoretically computed linear polarization values of the program stars. It has been suggested that these observed polarizations indicate that the envelopes of the Be stars may have a range of oblateness. Early-type Be stars displayed relatively higher polarizations than the late-type ones. Full widths at half intensity maximum of H(sub alpha) profiles and the intrinsic linear continuum polarizations are closely correlated with the projected rotational velocities of the program stars. Photospheric-absorption-corrected equivalent widths of H(sub alpha), profiles [W((alpha)] and the radii of H(sub alpha) emitting envelopes (R(sub e)) are non-linearly correlated with the intrinsic continuum polarizations of these stars. However, W((alpha) and R(sub e) are linearly correlated. Detail discussion of these results are presented in this paper.

Ghosh, K. K.↗

NASA NTRS1998

Calibration Results for the AXAF Flux Contamination Monitor

The AXAF Flux Contamination Monitor (FCM) serves the purposes of transferring the absolute flux calibration from the ground calibration at the X-Ray Calibration Facility (XRCF) to operation on orbit and of detecting any changes in molecular contamination of the High-Resolution Mirror Assembly (HRMA) between ground calibration and the post-launch activation. We describe the design, construction, and characterization of the FCM radioactive sources, and their placement on the Forward Contamination Cover (FCC). We present results from FCM measurements with the AXAF focal-plane instruments, particularly the AXAF CCD Imaging Spectrometer (ACIS), during the ground calibration phase at the XRCF in 1997. Finally, we describe the plans for FCM on-orbit measurements during observatory activation and for the subsequent analysis.

Elsner, R. F.↗

NASA NTRS1998

Correlation Between Radio-Millimeter and Gamma Ray Fluxes from Blazars

Using multifrequency databases, we have plotted radio (5 GHz, 22 GHz, and 37 GHz) and millimeter (90 GHz and 230 GHz) fluxes versus MeV and gamma-ray fluxes of the EGRET blazar sample. Strong correlations have been found between the emissions at the radio-millimeter wavelengths and they-rays. These correlations have been explained in the frame work of Synchrotron Self-Compton (SSC) processes between the radio-millimeter synchrotron emissions and the shock-accelerated electrons in the jets of blazars. Using this SSC model we are able to explain the GeV spectra of the EGRET blazars. The present model will also explain the physics of TeV flares in blazars and the time lag between the TeV and GeV photons from a blazar.

Ghosh, K. K.↗

NASA NTRS1998

Origin of Gamma-Ray Emissions from the MeV Blazars

More than sixty gamma-ray emitting blazars have been detected by the Compton Gamma Ray Observatory (CGRO). A sub-class of these blazars is known as MeV blazars that are most luminous at the MeV energies. Spectra of these sources show a break somewhere between 1 MeV and 30 MeV with break amplitude as large as Delta(Gamma) = 1.5. Different models like Compton cooling jet models, synchrotron self-Compton models, etc., have been suggested to explain the observed properties of the MeV luminous blazars. However these models are unable to account for spectral break amplitudes in excess of 0.5. Here we suggest an inverse-Compton model based on recent published experimental data where MeV gamma rays were produced via Compton backscattering of UV photons with 500 MeV electrons. We show that the high energy electrons of the relativistic jet can collide with the copious ultraviolet photons of the big blue bump of the blazar, and produce photons peaked in the MeV region. Also we show that the X rays and the soft gamma rays in these sources are produced through the synchrotron self-Compton process. Flux computed from our model have been used to compare with the observed X-ray/ gamma-ray spectra of the MeV luminous blazars. Details of the model are discussed in the present paper.

Ghosh, K. K.↗