摘要:

There is convincing evidence that the mass of ordinary galaxies, like our own, is much greater than that measured in the form of stars, gas, and dust. If this dark matter is in the form of compact objects it can be detected via gravitational microlensing of background stars. The MACHO project is in its second year of a time resolved photometric survey of the Magellanic Clouds and galactic bulge to search for the rare microlensing signature of compact objects in the halo and disk of our galaxy. We are using a dedicated 1.3‐m telescope at Mt. Stromlo Observatory and a dual focal plane CCD mosaic camera with a 0.5 square degree field to image up to 10 million stars per night simultaneously in two passbands. As of October 1994, 26,000 images have been taken with this system. A preliminary analysis of more than 8 million stars in the LMC for one year has yielded three stars which undergo time‐symmetric achromatic photometric excursions consistent with gravitational microlensing. In a similar analysis of ∼10 million stars in the galactic bulge we have also found more than 40 likely microlensing candidates, several of which reach peak amplifications of greater than ten. The relatively high rate toward the bulge is consistent with a ‘‘maximal’’ disk that accounts for most of the galactic mass interior to the solar radius or microlensing by a galactic bar.

ISSN:0094-243X    

DOI:10.1063/1.48815

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The main characteristics and capabilities of the major radio interferometers either recently completed, in the process of being upgraded, or being proposed for the future will be presented. My review concentrates in major, national instruments in the american continent and includes the recently completed Very Long Baseline Array (VLBA), the project of upgrading the Very Large Array (VLA), and the proposed Millimeter Array (MMA).

ISSN:0094-243X    

DOI:10.1063/1.48816

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Atmospheric opacity measurements by several investigators show that the Antarctic plateau is likely to be the best submillimeter‐wave observatory site on the Earth’s surface. The Antarctic Submillimeter Telescope and Remote Observatory (AST/RO), a 1.7 m offset telescope for heterodyne spectroscopy currently being installed at the South Pole, will be a proof‐of‐concept for a permanent Antarctic submillimeter observatory. In future, a 15‐m class telescope or multi‐element array at an international base on the Antarctic Plateau at 4200‐m altitude could be used to observe protogalaxies and protostars. Such a telescope might be an international collaboration.

ISSN:0094-243X    

DOI:10.1063/1.48817

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Details of the measurement methods used to determine the distribution of ion charge states resulting from K‐shell photoionization by synchrotron radiation of Ar2+ions confined in a Penning ion trap [S. D. Kravisetal., Phys. Rev. Letters66, 2956 (1991)] are discussed. Modifications leading to improvements in technique based on recent experimental advances are suggested.

ISSN:0094-243X    

DOI:10.1063/1.48818

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Drift tube‐mass spectrometers have made possible the unambiguous and accurate measurement of transport coefficients for many ion species in foreign and parent gases. Herein we discuss the principles of these measurements and present examples on the transport properties of daughter ions in SF6parent gas.

ISSN:0094-243X    

DOI:10.1063/1.48819

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The inherent chemical reactivity of a molecule is discussed in the framework of density functional theory. Using the concepts of chemical potential (electronegativity), hardness, softness, and condensed fukui function it is shown that one may characterize the reactive sites of chemical species through the chemical potential equalization principle, the maximum hardness principle, and the local hard and soft acids and bases principle.

ISSN:0094-243X    

DOI:10.1063/1.48820

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We report accurate nonrelativistic CI results for He, Li, Ne, Ca, and Ca−ground states using a new automatized code for large‐scale calculations. Salient features include efficient optimization of nonlinear one‐electron function parameters and a hierarchical strategy to approximate the full CI energy. We have also developed a variational principle for relativistic atomic states. With this principle we evaluate relativistic SCF solutions working on a precise eigenvalue (usually of very high order) of the CI Hamiltonian and implementing Brillouin’s condition through an iterative process. The same variational principle is also applied to full CI expansions, as well as to limited well‐defined CI expansions.

ISSN:0094-243X    

DOI:10.1063/1.48821

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The physical nature of non‐additive forces in many‐atomic system and the methods of their calculations are discussed. The close recurrence formula which expresses the energy of m‐body interaction through the energies of 2,3,...,(m−1) bodies ones is obtained in the symmetry‐adapted perturbation theory and in the variational approach. The comparative analysis is given of the role of non‐additive forces for the stability of the atomic clusters with different electronic shells.

ISSN:0094-243X    

DOI:10.1063/1.48822

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We discuss our efforts to use coherence properties of laser beams to influence the directionality of current flow in a bulk, photoexcited semiconductor. By optically coupling a pair of conduction and valence band states via single and two photon absorption events it is theoretically possible to generate an electrical current flow whose directionality depends on the relative phase, and polarization of the two optical beams. The current should maintain itself for the carrier dephasing time, which is typically of the order of 100 fs, a time sufficiently long so that ‘‘all‐optical’’ switching applications can be contemplated. Some specific geometries for detecting the effect are offered.

ISSN:0094-243X    

DOI:10.1063/1.48823

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Toxic trace elements such as lead (Pb) and Cadmium (Cd) can be measured non‐invasively in humans by radiation physics techniques, particularly x‐ray fluorescence and neutron activation. An analysis is usually made of the content of a particular organ, representing the principal storage site of the element in question. For example, Pb is measured in bone, whereas Cd is measured in liver and kidney. Measuring stored quantities of these elements has contributed to assessment of health effects of chronic occupational and environmental exposure. In addition knowledge of the elemental metabolism has been significantly extended. Results ofinvivostudies have also contributed to assessment and regulation of workplace exposure. Analogous methods are in use or under development forinvivoassay of mercury, aluminum, gold, platinum, and manganese. The principles of these measurements will be outlined and illustrative applications for Pb and Cd will be discussed.

ISSN:0094-243X    

DOI:10.1063/1.48824

出版商:AIP    

年代:1995

数据来源: AIP