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The instrumentation developed for poly crystalline diffractometry using the storage ring at the Stanford Synchrotron Radiation Laboratory is described. A pair of automated vertical scan diffractometers was used for a Si (111) channel monochromator and the powder specimens. The parallel beam powder diffraction was defined by horizontal parallel slits which had several times higher intensity than a receiving slit at the same resolution. The patterns were obtained with 2:1 scanning with’ a selected monochromatic beam, and an energy dispersive diffraction method in which the monochromator is step-scanned, and the specimen and scintillation counter are fixed. Both methods use the same instrumentation.
At the lowest radio frequencies (≤30 MHz), the Earth's ionosphere transmits poorly or not at all. This relatively unexplored region of the electromagnetic spectrum is thus an area where high resolution, high sensitivity observations can open a new window for astronomical investigations. Also, extending observations down to very low frequencies brings astronomy to a fundamental physical limit where the Milky Way becomes optically thick over relatively short path lengths due to diffuse free-free absorption.
The Hat Creek mm-wavelength array, operated by the Berkeley-Illinois-Maryland Association (BIMA), is undergoing an expansion to nine telescopes. The first six of these telescopes will be operational in January, 1993. The telescopes are relatively small, 6 m in diameter, and are moved on rubber-tired dollies so it will be possible to transport them on roads in the vicinity of the main array. Initially, we plan to place two of the telescopes at outrigger sites located 0.5 and 1.0 km from the main array. Use of these outrigger telescopes will yield angular resolutions of 0.45 arc-sec at 2.7 mm wavelength and 0.21 arc-sec at 1.2 mm wavelength. When all nine telescopes are operational, reasonable image quality will be obtained with a single 12-hour observation. The maximum sidelobe amplitude depends upon source declination, but is about 16%. Figures 1, 2, and 3 show the uv coverage and beamshapes which will be achieved at various declinations.
In 1984 Perley and Erickson proposed a 73.8 MHz [4 meter wavelength] observing system at the VLA site (NRAO Scientific Memo #146). They proposed a stand-alone antenna system that would feed its signals into an existing spare channel of the VLA waveguide and utilize a separate correlator. Over the 35 km VLA baselines this system would produce images with 20 arcsec resolution, unprecedented at this frequency. The major technical problems are ionospheric refraction and interference. Some doubt existed as to whether or not it would be possible to cope with the large, rapidly-changing ionospheric phase fluctuations to be expected over 35 km baselines. Thus it was proposed, as a first step in the development, that 73.8 MHz feeds be installed in the present VLA dishes and that trial observations be made to prove that techniques such as self-calibration can be successful. Eight dishes now have 73.8 MHz instrumentation and a number of radio source images have been made with this initial system.
Decametric wavelength imaging has been largely neglected in the quest for higher angular resolution because ionospheric structure limited interferometric imaging to short (< 5 km) baselines. The long wavelength (LW, 2—20 m or 15—150 MHz) portion of the electromagnetic spectrum thus remains poorly explored. The NRL-NRAO 74 MHz Very Large Array has demonstrated that self-calibration techniques can remove ionospheric distortions over arbitrarily long baselines. This has inspired the Low Frequency Array (LOFAR)—-a fully electronic, broad-band (15—150 MHz)antenna array which will provide an improvement of 2—3 orders of magnitude in resolution and sensitivity over the state of the art.
(Solar Phys.). The positional analysis of solar bursts at meter and decameter wavelengths during the period July 31–August 7, 1972 is presented. The observations were taken with two arrays – a log periodic array of 16 elements situated on an E–W base line of 3.3 km and portions of the new Clark Lake array in the form of a Tee (an E–W arm of 32 log spiral antennas and a N–S arm of 16 similar antennas). The new array operates over the frequency range 10–120 MHz and has angular resolutions of approximately 3.′5 at 100 MHz and 8.′5 at 40 MHz in the E–W direction.
A total of 18 radio sources selected on the basis of steep low-frequency radio spectra have been searched for the presence of millisecond pulsars using the Molonglo Observatory synthesis telescope. The search covered pulsar periods down to 2 ms with a limiting sensitivity of approximately 10 mJy. No pulsars were detected.
The ALFA mission is designed to map the entire sky at frequencies between approximately 0.3 and 30 MHz with angular resolution limited by interstellar and interplanetary scattering. Most of this region of the spectrum is inaccessible from the ground because of absorption and refraction by the Earth’s ionosphere. A wide range of astrophysical questions concerning solar system, galactic, and extragalactic objects could be answered with high resolution images at low frequencies, where absorption effects and coherent emission processes become important and the synchrotron lifetimes of electrons are comparable to the age of the universe.
73.8 MHz instrumentation for the VLA is being developed and is currently installed on 8 VLA dishes. Test observations of strong radio sources have been made. We describe techniques that we have developed to analyze these test observations and we present examples of our first maps.
We have assembled a new sample of some of the most FIR-luminous galaxies in the Universe and have imaged them in 1.1 mm dust emission and measured their redshifts 1 < z < 4 via CO emission lines using the 32-m Large Millimeter Telescope / Gran Telescopio Milimétrico (LMT/GTM). Our sample of 31 submm galaxies (SMGs), culled from the Planck and Herschel all-sky surveys, includes 14 of the 21 most luminous galaxies known, with LFIR > 1014L⊙ and SFR > 104M⊙/yr. These extreme inferred luminosities – and multiple / extended 1.1 mm images – imply that most or all are strongly gravitationally lensed, with typical magnification μ ~ 10 × . The gravitational lensing provides two significant benefits: (1) it boosts the S/N, and (2) it allows investigation of star formation and gas processes on sub-kpc scales.
Airborne observations of the Trapezium region of the Orion Nebula in the 60–300 μ range have been made from 13.7 km altitude using a Michelson interferometer with the Rice University 12-in. Flying Infrared Telescope. Fourier analysis of five interferometer scans provided spectra with resolution ranging from 7 cm–1 to 20 cm–1. These spectra were compared with lunar spectra taken with the same instrument at the same altitude to correct for instrumental and atmospheric effects. A weighted combination of these scans provides a low resolution spectrum. The radiation per unit frequency interval at 190 μ was found to be at least 40% of that measured between 75 and 90 μ. Neglecting possible features in the spectrum, its general shape is consistent with a blackbody near 70 K. The resolution is inadequate to resolve spectral lines.
Approximately 1000 observations of neutral hydrogen have been obtained with the 54-channel H-line receiver and the Würzburg antenna of the Carnegie Institution of Washington. H-line profiles have been observed at 10-degree intervals along the ±20-, ±30-, and ± 40-degree parallels of galactic latitude; at 20-degree intervals along the ± 50- and ±60-degree parallels; at 40-degree intervals along the ±70- and ±80-degree parallels and at the poles. Approximately two dozen observations have been taken at points near the galactic plane in order to correlate these observations with the Leiden survey . The beamwidth of the Würzburg antenna was about 2 degrees. The observations were taken in two series, one series during the summer of 1957, and the other series during 1958 January. The video frequency bandwidth of the receiver is 12 kc/s. The profiles consist of averages of from two to six scans with integration times from 4.8 to 7.5 minutes.
A radio spectrometer has been built on Bruny Island, south of Hobart, for the study of solar bursts in the rarely observed frequency range from 3 to 20 MHz. This spectrometer is an adaptive device that employs digital techniques to avoid most of the strong terrestrial interference prevalent in this frequency range. The residual interference that cannot be avoided is excised during off-line processing. As a result, successful observations are made down to the minimum frequency that can propagate through the ionosphere to the antenna. This minimum frequency depends upon the zenith distance of the Sun and it is usually between 4 and 8 MHz.
Major depressive disorder (MDD) is associated with deficits in recalling specific autobiographical memories (AMs). Extensive research has examined the functional anatomical correlates of AM in healthy humans, but no studies have examined the neurophysiological underpinnings of AM deficits in MDD. The goal of the present study was to examine the differences in the hemodynamic response between patients with MDD and controls while they engage in AM recall.
Participants (12 unmedicated MDD patients; 14 controls) underwent functional magnetic resonance imaging (fMRI) scanning while recalling AMs in response to positive, negative and neutral cue words. The hemodynamic response during memory recall versus performing subtraction problems was compared between MDD patients and controls. Additionally, a parametric linear analysis examined which regions correlated with increasing arousal ratings.
Behavioral results showed that relative to controls, the patients with MDD had fewer specific (p=0.013), positive (p=0.030), highly arousing (p=0.036) and recent (p=0.020) AMs, and more categorical (p<0.001) AMs. The blood oxygen level-dependent (BOLD) response in the parahippocampus and hippocampus was higher for memory recall versus subtraction in controls and lower in those with MDD. Activity in the anterior insula was lower for specific AM recall versus subtraction, with the magnitude of the decrement greater in MDD patients. Activity in the anterior cingulate cortex was positively correlated with arousal ratings in controls but not in patients with MDD.
We replicated previous findings of fewer specific and more categorical AMs in patients with MDD versus controls. We found differential activity in medial temporal and prefrontal lobe structures involved in AM retrieval between MDD patients and controls as they engaged in AM recall. These neurophysiological deficits may underlie AM recall impairments seen in MDD.
A comparative study of Pt and Pt/Si contacts to p-type 6H-SiC in terms of various processing conditions and interlayer specifications was performed. Deposition temperature, the thickness of the Si layer, and B-dopant incorporation in the Si were found to significantly affect the specific contact resistivity (SCR) values. In addition, pre-etching of the SiC surface in SF6 + Ar was found to consistently reduce the SCR's. The lowest average SCR values were 3 × 10−5 Ωcm2 for Pt/Si/SiC contacts deposited on pre-etched SiC surfaces (7.0 × 1018 cm−3 doping concentration) and annealed at 1100 °C for 5 min.
Aluminum-titanium contacts also showed dependence on the thicknesses of the Al and Ti layers and on the locations of the layers. Differences in both the SCRs and surface morphology are presented.
This paper describes the instrumentation and technique for the routine characterization of thin Films, multi-layers and substrates by grazing incidence X-ray reflectometry with a rotating anode and CuKα1, line source. Ge 220 channel monochromators with two reflections per channel were used with one CM placed before and the other after the specimen. In many applications the second CM could be replaced with a narrow receiving slit. The specimen holder was designed to make fine translations of the specimen for alignment, a base was designed to permit precise positioning of the diffractometer, and a high-speed Rigaku scintillation counter was used. The alignment and calibration are described and some typical data given to illustrate the capability of the method.
Character of the metal-insulator transition which occurs at about 23 GPa in bulk GaN crystals has been studied by means of high pressure Raman spectroscopy. The related freeze-out of electrons is caused by the localized donor state formed by most likely oxygen and emerging at high pressures to the band gap of GaN. As a result, the electron concentration drops from its initial value of 5.1019 cm-3 to about 3. 1018 cm-3. These remaining electrons originate likely from another donor center with effective mass character, probably carbon. The obtained results raise a question whether the nitrogen vacancy is abundant enough to be observed in bulk GaN crystals.
SIMS analysis was applied to the characterization of GaN, AlGaN/GaN and InGaN/GaN grown by MOCVD. Such characterization enables the control of purity and doping, and the determination of growth rate and alloy composition. The analysis can be performed on finished optoelectronic and electronic devices and this makes SIMS technique a powerful tool for failure analysis, reverse engineering, and concurrent engineering.