measurement of noise power spectrum in photon counting PMT's and the effect of the observed (1/f) noise on astronomical photometry     

S. S. Raouf  A. R. Sadik  N. K. Wafi  K. M. Aboud 《Astrophysics and Space Science》1986,127(2):207-218

In astronomical photometry, the sensitivity of observations is limited by the dark counts of the photomultiplier tube. In the present work, the effect of dark count noise in photon counting systems is investigated by theory and experimental measurements. Dark counts are considered to be originating from two sources, namely: dc fluctuations and random pulses.Experimental measurements were carried out to determine noise effects in different operating regions of noise dominance. The results provide strong evidence that: in normal operating mode, where the effect of random pulses is dominant, dark counts do not follow Poisson statistics. The observed noise shows strong (1/f) power spectrum, where the observed noise power is found to increase with time of observation.The results are important in photon counting systems operating under dark count limited mode. The conclusions drawn can be useful in obtaining more accurate error estimates and in assessing astronomical photometric observations and data reduction techniques.  相似文献   

The discovery of new galaxy members in the NGC 5044 and 1052 groups     

N. P. F. MKay  C. G. Mundell †  S. Brough  Duncan A. Forbes  D. G. Barnes  P. A. James  P. Goudfrooij  V. Kozhurina-Platais  R. Whitaker 《Monthly notices of the Royal Astronomical Society》2004,352(4):1121-1134

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Ca ii K interstellar observations towards early-type disc and halo stars – distances to intermediate- and high-velocity clouds     

J. V. Smoker  B. B. Lynn  W. R. J. Rolleston  H. R. M. Kay  E. Bajaja  W. G. L. Poppel  F. P. Keenan  P. M. W. Kalberla  C. J. Mooney  P. L. Dufton  R. S. I. Ryans 《Monthly notices of the Royal Astronomical Society》2004,352(4):1279-1290

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Is the transition impact to post‐impact rock complete? Some remarks based on XRF scanning,electron microprobe,and thin section analyses of the Yaxcopoil‐1 core in the Chicxulub crater     

J. SMIT  S. VAN DER GAAST  W. LUSTENHOUWER 《Meteoritics & planetary science》2004,39(7):1113-1126

Abstract The transition from impact to post‐impact rocks in the Yaxcopoil‐1 (Yax‐1) core is marked by a 2 cm‐thick clay layer characterized by dissolution features. The clay overlies a 9 cm‐thick hardground, overlying a 66 cm‐thick crossbedded unit, consisting of dolomite sandstone alternating with thin micro‐conglomerate layers with litho‐ and bioclasts and the altered remains of impact glass, now smectite. The micro‐conglomerates mark erosion surfaces. Microprobe and backscatter SEM analysis of the dolomite rhombs show an early diagenetic, complex‐zoned, idiomorphic overgrowth, with Mn‐rich zones, possibly formed by hot fluids related to cooling melt sheet in the crater. The pore spaces are filled with several generations of coelestite, barite, K‐feldpar, and sparry calcite. XRF core scanning analysis detected high Mn values in the crossbedded sediments but no anomalous enrichment of the siderophile elements Cr, Co, Fe, and Ni in the clay layer. Shocked quartz occurs in the crossbedded unit but is absent in the clay layer. The basal Paleocene marls are strongly dissolved and do not contain a basal Paleocene fauna. The presence of a hardground, the lack of siderophile elements, shocked quartz, or Ni‐rich spinels in the clay layer, and the absence of basal Paleocene biozones P0 and Pa all suggest that the top of the ejecta sequence and a significant part of the lower Paleocene is missing. Due to the high energy sedimentation infill, a hiatus at the top of the impactite is not unexpected, but there is nothing in the biostratigraphy, geochemistry, and petrology of the Yax‐1 core that can be used to argue against the synchroneity of the end‐Cretaceous mass‐extinctions and the Chicxulub crater.  相似文献   

Self-similar solutions for the blast wave problem in radiative gasdynamics,I     

J. B. Singh  K. S. Singh 《Astrophysics and Space Science》1985,111(1):79-85

Similarity solutions, describing the flow of a perfect gas behind spherical shock waves, are investigated including the radiation heat flux. The shock is assumed to be propagating in a medium at rest. Shock radius varies exponentially with time and density is inversely proportional to fifth power of the shock radius immediately ahead of the shock front.  相似文献   

Aircraft Observation of CO₂, CO, O₃ and H₂ over the North Pacific during the PACE-7 Campaign     

Y. SAWA  H. MATSUEDA  Y. MAKINO  H. Y. INOUE  S. MURAYAMA  M. HIROTA  Y. TSUTSUMI  Y. ZAIZEN  M. IKEGAMI  K. OKADA 《Tellus. Series B, Chemical and physical meteorology》2004,56(1):2-20

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Equilibrium response of thermohaline circulation to large changes in atmospheric CO2 concentration     

R. J. Stouffer  S. Manabe 《Climate Dynamics》2004,22(2-3):325-326

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Can the big-bang singularity be avoided in the scale-covariant theory?     

Hafizur Rahman  S. Banerji 《Astrophysics and Space Science》1985,113(2):405-412

The Raychaudhuri equation is written in terms of atomic units in the scale-covariant theory of Canutoet al. (1977) and it is pointed out that even dust-filled nonrotating cosmological models may not have an observable singularity of infinite density in the beginning. Simple explicit solutions of Einstein's equations are also constructed in terms of atomic units and its is shown that the Einstein-de Sitter solution in gravitational units may appear to be a static or an oscillating universe in atomic units.  相似文献   

The dust coma of periodic comet Churyumov-Gerasimenko (1982 VIII)     

M.S. Hanner  E. Tedesco  A.T. Tokunaga  G.J. Veeder  D.F. Lester  F.C. Witteborn  J.D. Bregman  J. Gradie  L. Lebofsky 《Icarus》1985,64(1):11-19

The dust coma of Comet P/Churyumov-Gerasimenko was monitored in the infrared (1–20 μm) from September 1982 to March 1983. Maximum dust production rate of ～2 × 10⁵ g/sec occured in December, 1 month postperihelion. The ratio of dust/gas production was higher than that in other short-period comets. No silicate feature was visible in the 8- to 13-μm spectrum on 23 October. The mean geometric albedo of the grains was ～0.04 at 1.25 μm and ～0.05 at 2.2 μm.  相似文献   

Zircon U–Pb age, trace element and Hf isotope composition of Kongling terrane in the Yangtze Craton: refining the timing of Palaeoproterozoic high-grade metamorphism     

Y.-B. WU  S. GAO  H.-J. GONG  H. XIANG  W.-F. JIAO  S.-H. YANG  Y.-S. LIU  H.-L. YUAN 《Journal of Metamorphic Geology》2009,27(6):461-477

U–Pb age, trace element and Hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the Kongling terrane in the northern part of the Yangtze Craton. The zircon shows distinct morphological and chemical characteristics. Most zircon in an amphibolite shows oscillatory zoning, high Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, high formation temperature, high trace element contents, clear negative Eu anomaly, as well as HREE-enriched patterns, suggesting that it is igneous. The zircon yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2857 ± 8 Ma, representing the age of the magmatic protolith. The zircon in the other two samples is metamorphic. It has low Th/U ratios, low trace element concentrations, variable HREE contents (33.8 ≥ Lu_N≥2213; 14.7 ≤ Lu_N/Sm_N ≤ 354) and ¹⁷⁶Lu/¹⁷⁷Hf ratios (0.000030–0.001168). The data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions. The metamorphic zircon yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2010 ± 13 Ma. These results combined with previously obtained Palaeoproterozoic metamorphic ages suggest a c. 2.0 Ga Palaeoproterozoic collisional event in the Yangtze Craton, which may result from the assembly of the supercontinent Columbia. The zircon in two samples yields weighted mean two-stage Hf model ( T _DM2) ages of 3217 ± 110 and 2943 ± 50 Ma, respectively, indicating that their protoliths were mainly derived from Archean crust.  相似文献