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The isotopic abundances of the rare gases in the fines are found to be similar to those previously reported for gas-rich meteorites. Relative to the heavy gases, neon and helium are depleted by factors of 2.5 and 10 respectively. Accurate krypton-krypton ages for rocks 10017, 10047, 10057, and 10071 were obtained covering a range of from 47 to 509 x 10(6) years. Varying relative production rates of the krypton and xenon isotopes in these rocks suggest different irradiation conditions and a complex history for at least some of the rocks. 相似文献
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Sigl G 《Science (New York, N.Y.)》2001,291(5501):73-79
The origin of cosmic rays is one of the major unresolved questions in astrophysics. In particular, the highest energy cosmic rays observed have macroscopic energies up to several 10(20) electron volts and thus provide a probe of physics and astrophysics at energies unattained in laboratory experiments. Theoretical explanations range from astrophysical acceleration of charged particles, to particle physics beyond the established standard model, and processes taking place at the earliest moments of our universe. Distinguishing between these scenarios requires detectors with effective areas in the 1000-square-kilometer range, which are now under construction or in the planning stage. Close connections with gamma-ray and neutrino astrophysics add to the interdisciplinary character of this field. 相似文献
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Beaujean R Schmidt M Enge W Siegmon G Krause J Fischer E 《Science (New York, N.Y.)》1984,225(4658):193-195
A stack of plastic nuclear track detectors was exposed to heavy cosmic rays on the pallet of Spacelab 1. Some layers of the stack were rotated with respect to the main stack to determine the arrival time of the particles. After return of the stack the latent particle tracks are revealed by chemical etching. Under the optical microscope the charge, mass, energy, and impact direction of the particles can be deduced from the track geometry. 相似文献
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Dunlop JS 《Science (New York, N.Y.)》2011,333(6039):178-181
Major advances in observational astronomy over the past 20 years have revolutionized our view of cosmic history, transforming our understanding of how the hot, smooth, early universe evolved into the complex and beautiful universe of stars and galaxies in which we now live. I describe how astronomers have used a range of complementary techniques to map out the rise and fall of star formation over 95% of cosmic time, back to the current observational frontier only ~500 million years after the Big Bang. 相似文献
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Reines F 《Science (New York, N.Y.)》1979,203(4375):11-16
The neutrino hypothesis, put forward by Pauli to account for the apparent loss of energy and momentum in beta decay, was verified by a series of measurements at a nuclear reactor nearly 25 years ago. An account is given of the first observations of the interaction of neutrinos in a target remote from the fission process that produced them. These experiments completed the observations of the particles involved in beta decay and paved the way for use of the free neutrino to probe the nature of the weak interaction. 相似文献
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Macqueen RM 《Science (New York, N.Y.)》1986,233(4770):1333
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The Cambrian conundrum: early divergence and later ecological success in the early history of animals 总被引:1,自引:0,他引:1
Erwin DH Laflamme M Tweedt SM Sperling EA Pisani D Peterson KJ 《Science (New York, N.Y.)》2011,334(6059):1091-1097
Diverse bilaterian clades emerged apparently within a few million years during the early Cambrian, and various environmental, developmental, and ecological causes have been proposed to explain this abrupt appearance. A compilation of the patterns of fossil and molecular diversification, comparative developmental data, and information on ecological feeding strategies indicate that the major animal clades diverged many tens of millions of years before their first appearance in the fossil record, demonstrating a macroevolutionary lag between the establishment of their developmental toolkits during the Cryogenian [(850 to 635 million years ago (Ma)], and the later ecological success of metazoans during the Ediacaran (635 to 541 Ma) and Cambrian (541 to 488 Ma) periods. We argue that this diversification involved new forms of developmental regulation, as well as innovations in networks of ecological interaction within the context of permissive environmental circumstances. 相似文献
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Weldon HA 《Science (New York, N.Y.)》1984,226(4673):431-432
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Herzfeld KF 《Science (New York, N.Y.)》1972,175(4028):1393-1394
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Priest E 《Science (New York, N.Y.)》1989,245(4919):770