全文获取类型
收费全文 | 59983篇 |
免费 | 700篇 |
国内免费 | 420篇 |
专业分类
测绘学 | 1213篇 |
大气科学 | 3499篇 |
地球物理 | 11700篇 |
地质学 | 22565篇 |
海洋学 | 5338篇 |
天文学 | 13951篇 |
综合类 | 143篇 |
自然地理 | 2694篇 |
出版年
2022年 | 557篇 |
2021年 | 811篇 |
2020年 | 906篇 |
2019年 | 990篇 |
2018年 | 2064篇 |
2017年 | 1884篇 |
2016年 | 2103篇 |
2015年 | 997篇 |
2014年 | 1918篇 |
2013年 | 3091篇 |
2012年 | 2218篇 |
2011年 | 2754篇 |
2010年 | 2511篇 |
2009年 | 3019篇 |
2008年 | 2645篇 |
2007年 | 2884篇 |
2006年 | 2556篇 |
2005年 | 1625篇 |
2004年 | 1569篇 |
2003年 | 1522篇 |
2002年 | 1397篇 |
2001年 | 1373篇 |
2000年 | 1210篇 |
1999年 | 945篇 |
1998年 | 963篇 |
1997年 | 991篇 |
1996年 | 751篇 |
1995年 | 744篇 |
1994年 | 685篇 |
1993年 | 589篇 |
1992年 | 573篇 |
1991年 | 552篇 |
1990年 | 607篇 |
1989年 | 531篇 |
1988年 | 479篇 |
1987年 | 562篇 |
1986年 | 432篇 |
1985年 | 633篇 |
1984年 | 669篇 |
1983年 | 643篇 |
1982年 | 596篇 |
1981年 | 549篇 |
1980年 | 548篇 |
1979年 | 472篇 |
1978年 | 544篇 |
1977年 | 453篇 |
1976年 | 421篇 |
1975年 | 443篇 |
1974年 | 397篇 |
1973年 | 426篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
In the atmospheric Čerenkov technique γ-rays are detected against the abundant background produced by hadronic showers. In
order to improve the signal to noise ratio of theexperiment, it is necessary to reject a significant fraction of hadronic
showers. Traditional background rejection methods based on image shape parameters have been extensively used for the data
from imaging telescopes. However, non-imaging Čerenkov telescopes have to develop very different means of statistically identifying
and removing cosmic ray events. Some of the parameters, which could be potentially important for non-imaging arrays, are the
temporal and spectral differences, the lateral distributions and density fluctuations of Čerenkov photons generated by γ-ray
and hadron primaries. Here we study the differences in fluctuations of Čerenkov photon density in the light pool at the observation
level from showers initiated by photons and those initiated by protons or heavier nuclei. The database of simulated events
for the PACT array has been used to evaluate the efficiency of the new technique. Various types of density fluctuations like
the short range and medium range fluctuations as well as flatness parameter are studied. The estimated quality factors reflect
the efficiencies with which the hadrons can be rejected from the data. Since some of these parameters are independent, the
cuts may be applied in tandem and we demonstrate that the proton rejection efficiency of ∼90% can be achieved. Use of density
fluctuations is particularly suited for wavefront sampling observations and it seems to be a good technique to improve the
signal to noise ratio.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
14.
Roberto F. Viotti Lucio Angelo Antonelli Sonja Rebecchi Corinne Rossi 《Journal of Astrophysics and Astronomy》2002,23(1-2):19-22
We have investigated with BeppoSAX the long term behaviour of the harder X-ray component of the supposed supermassive binary
system η Car along its 5.52 year cycle. We have found that in March 1998 during egress from the last December 1997 eclipse,
this component was the same as outside eclipse, but for a large (×3.5) increase of NH
h
, that can be attributed to the presence or formation of opaque matter in front of the source near periastron. Unexpectedly,
at that time the iron 6.7 keV emission line was 40% stronger. BeppoSAX has for the first time found ahard X-ray tail extending to at least 50 keV, that cannot be adequately fitted with an additional hotter thermal component. The 2–100 keV
spectrum of η Car is instead well fitted with an absorbed powerlaw spectrum with photon index 2.53, suggesting non-thermal
emission as an alternative model for the core source. 相似文献
15.
16.
17.
M. J. Freyberg H. Bräuninger W. Burkert G. D. Hartner O. Citterio F. Mazzoleni G. Pareschi D. Spiga S. Romaine P. Gorenstein B. D. Ramsey 《Experimental Astronomy》2005,20(1-3):405-412
The Max-Planck-Institut für extraterrestrische Physik (MPE) in Garching, Germany, uses its large X-ray beam line facility PANTER for testing X-ray astronomical instrumentation. A number of telescopes, gratings, filters, and detectors, e.g. for astronomical satellite missions like Exosat, ROSAT, Chandra (LETG), BeppoSAX, SOHO (CDS), XMM-Newton, ABRIXAS, Swift (XRT), have been successfully calibrated in the soft X-ray energy range (< 15keV). Moreover, measurements with mirror test samples for new missions like ROSITA and XEUS have been carried out at PANTER. Here we report on an extension of the energy range, enabling calibrations of hard X-ray optics over the energy range 15–50 keV. Several future X-ray astronomy missions (e.g., Simbol-X, Constellation-X, XEUS) have been proposed, which make use of hard X-ray optics based on multilayer coatings. Such optics are currently being developed by the Osservatorio Astronomico di Brera (OAB), Milano, Italy, and the Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, MA, USA. These optics have been tested at the PANTER facility with a broad energy band beam (up to 50 keV) using the XMM-Newton EPIC-pn flight spare CCD camera with its good intrinsic energy resolution, and also with monochromatic X-rays between C-K (0.277 keV) and Cu-Kα (8.04 keV).
PACS: 95.55.Ka, 95.55.Aq, 41 50.+h, 07.85.Fv 相似文献
18.
19.
20.