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1.
The cosmic-ray exposure age of the Farmington meteorite was found to be greater than 7000 years from the ratio of the carbon-14 and argon-39 present and less than 25,000 years from the ratio of the aluminum-26 and argon-39 present. This exposure age is about an order of magnitude smaller than was estimated previously. 相似文献
2.
Activities of aluminum-26 and beryllium-10 in marine sediment were measured at 0.01 +/- 0.13 and 4.4 +/- 0.9 disintegrations per minute, per kilogram dry weight, respectively. Only an upper limit of 0.03 could be determined for the ratio of aluminum-26 to beryllium-10 in the sediment. The ratio is probably explained by production by cosmic rays in the atmosphere. 相似文献
3.
The residence times of the soluble fractions of beryllium and aluminum in seawater are estimated to be 1500 years or more. These residence times are estimated from a comparison of the annual deposition rates of cosmogenic beryllium-10 and aluminum-26 with the concentrations of beryllium-10 and aluminum-26 in seawater estimated from the specific activities of these radionuclides in an authigenic mineral assembly such as a manganese nodule. These residence times are greater by an order of magnitude than the mean residence times of beryllium and aluminum estimated from the geochemical balance. 相似文献
4.
The nuclides zinc-65 and cobalt-60 associated with river-borne particulate matter are incorporated in sediment on the Continental Shelf near the Colum- ia River. Changes in the relative concentrations of zinc-65 and cobalt-60 and in the ratio of the activity of zinc-65 and cobalt-60 suggest that radioactive sediment moves northward 12 to 30 kilometers per year along the shelf and 2.5 to 10 kilometers per year westward away from the coast. 相似文献
5.
The radioactivity of aluminum-26 in two cores of Pacific sediments is an order of magnitude higher than was expected, as a result of its production by cosmic-ray interactions in the terrestrial environment. The higher activity can be explained only by postulating influx with extraterrestrial cosmic dust that had been exposed to significant flux of energetic particles capable of producing nuclear interactions. These particles may well be the "solar" cosmic rays that are sporadically accelerated by Sun during certain solar flares, since the steady galactic cosmic-ray flux is inadequate. The long-term average flux of low-energy protons in interplanetary space, required to yield the observed rate of influx of aluminum-26, is deduced on the basis of certain assumptions. 相似文献
6.
Activities of beryllium-10 and aluminum-26 dissolved in 200-year-old Greenland ice were found to be 18.4 (+ 8.4, - 4.8) x 10(-6) and 3.2 +/- 0.9 x 10(-7) disintegration per minute per liter, respectively. From these values and the precipitation rate (30 milliliters of water per square centimeter per year), the production rates of these isotopes are calculated to be 3.6 (+ 1.6, - 0.9) x 10(-2) and 1.7 +/- 0.5 x 10(-4) atom per second * square centimeter. These rates agree with the rates calculated for the production of these isotopes by cosmic rays in the atmosphere. Probably all the Al(26) in the ice is accounted for by such atmospheric production; however, an upper limit for the influx of cosmic dust bearing aluminum-26 is calculated at 3.2 x 10(5) tons per year for Earth. Only upper limits could be found for Al(26) and Be(10) in the undissolved particulate matter in the ice; their addition to the activities in the dissolved material leaves our conclusions unchanged. 相似文献
7.
Despite marine evidence for at least 50 Pliocene-Pleistocene ice sheet advances, only the most recent one has been accurately reconstructed from terrestrial evidence, because there are few techniques for dating older glacial deposits. Here we show that the cosmic ray-produced nuclides beryllium-10 and aluminum-26 can be used to date tills that overlie paleosols. 相似文献
8.
Lower Pliocene hominid remains from Sterkfontein 总被引：2，自引：0，他引：2
Cosmogenic aluminum-26 and beryllium-10 burial dates of low-lying fossiliferous breccia in the caves at Sterkfontein, South Africa, show that associated hominid fossils accumulated in the Lower Pliocene. These dates indicate that the skeleton StW 573 and newly discovered specimens from Jacovec Cavern have much the same age: approximately 4 million years. These specimens are thus of an age similar to Australopithecus anamensis from East Africa. 相似文献
9.
Silicate grains from Tranquillity Base have shock-induced features ranging from internal fragmentation through complete disruption of the lattice to thermal melting. Half the crystalline grains with diameters less than 125 micrometers have features of shock equivalent to those produced in the laboratory at pressures greater than about 40 kilobars. One quarter have features indicative of pressures greater than 90 kilobars. These properties together with great quantities of melt glass and aluminum-26 in the fine-grained material are indicative of repeated shock-loading by meteoritic bombardment over long periods of time. 相似文献
10.
Enrichments in boron-10/boron-11 in a calcium-aluminum-rich inclusion from the Allende carbonaceous chondrite are correlated with beryllium/boron in a manner indicative of the in situ decay of short-lived beryllium-10. Because this radionuclide is produced only by nuclear spallation reactions, its existence in early solar system materials attests to intense irradiation processes in the solar nebula. The particle fluence inferred from the initial beryllium-10/beryllium-9 is sufficient to produce other short-lived nuclides, calcium-41 and manganese-53, found in meteorites, but the high canonical abundance of aluminum-26 may still require seeding of the solar system by radioactive stellar debris. 相似文献
11.
A model for the process of meteorite concentration in blue ice regions of the Antarctic ice sheet is proposed based on data from near the Allan Hills and the assumptions that both meteorite influx and glacial flow have been constant. The meteorite influx is calculated to be 60 x 10(-6) kilogram per square kilometer per year, and the age of the exposed ice to be 0 to 600,000 years, varying with distance from the Allan Hills. These results are in line with other estimates of influx rate and with measurements of the terrestrial ages of the meteorites, providing support for the assumption of steady flow and meteorite influx. This may be the oldest sequence of ice in stratigraphic order yet discovered, and the results imply that this part of the east Antarctic ice sheet has been approximately steady during this time interval. 相似文献
12.
We performed high-resolution computer simulations of impacts into homogeneous and layered martian terrain analogs to try to account for the ages and characteristics of the martian meteorite collection found on Earth. We found that craters as small as approximately 3 kilometers can eject approximately 10(7) decimeter-sized fragments from Mars, which is enough to expect those fragments to appear in the terrestrial collection. This minimum crater diameter is at least four times smaller than previous estimates and depends on the physical composition of the target material. Terrain covered by a weak layer such as an impact-generated regolith requires larger, therefore rarer, impacts to eject meteorites. Because older terrain is more likely to be mantled with such material, we estimate that the martian meteorites will be biased toward younger ages, which is consistent with the meteorite collection. 相似文献
13.
An increase in the rate of mutation has been found in Drosophila melanogaster reared on a basic medium that was irradiated with a sterilizing dose (150,000 rads) of cobalt-60 gamma rays. In Muller-5 tests, sex-linked recessive lethals occurred only in the F(2) progenies of the male test flies obtained from breeding the parent flies on irradiated medium, while visible changes occurred in experimental cultures from both the control and irradiated media. The frequency of sex-linked recessive lethals was 0.35, 0.55, and 0.8 percent in three independent experiments. Visible changes were two to six times more frequent in the irradiated series than in the controls. 相似文献
14.
The occurrence of positive and negative (26)Mg anomalies in inclusions of the Allende meteorite is explained in terms of proton bombardment of a gas of solar composition. A significant fraction of (26)Mg in the irradiated gas is stored temporarily in the form of radioactive (26)Al by the reaction (26)Mg(p,n) (26)Al. Proton fluxes of 10(17) to 10(19) protons per square centimeter per year at l million electron volts are inferred. Aluminum-rich materials condensing from the gas phase have positive (26)Mg anomalies, whereas magnesium-rich materials have negative (26)Mg anomalies. The proton flux required to account for the observed magnesium anomalies is used to investigate possible isotopic anomalies in the elements from oxygen to argon. Detectable isotopic anomalies are predicted only for neon. The anomalous neon is virtually pure (22)Ne from (22)Na decay. The predicted amount of anomalous (22)Ne is about 10(-8) cubic centimeter (at standard temperature and pressure) per milligram of sodium. 相似文献
15.
The amount of ozone formed in air exposed to a 4000-curie cobalt-60 radiation source was determined. In closed glass containers exposed to about I Mrad of gamma radiation, ozone concentrations up to 18 parts per million (by volume) were recorded. In air flowing through a glass structure placed near the source, ozone contents up to 0.1 parts per million were observed. It is suggested that ozone formation might be a factor in the reported germicidal effects of ionizing radiations and that damage to plant tissues, ascribed to radiation, might have resulted from exposure to ozone. The possible health hazards from the ozone produced during irradiation should not be overlooked. 相似文献
16.
Ion exchange, gel filtration chromatography, and gas chromatographymass spectrometry analyses have demonstrated that ethylenediaminetetraacetic acid (EDTA), an extremely strong complexing agent commonly used in decontamination operations at nuclear facilities, is causing the low-level migration of cobalt-60 from intermediate-level liquid waste disposal pits and trenches in the Oak Ridge National Laboratory burial grounds. Because it forms extremely strong complexes with rare earths and actinides, EDTA or similar chelates may also be contributing to the mobilization of these radionuclides from various terrestrial radioactive waste burial sites around the country. 相似文献
17.
Grossman JN 《Science (New York, N.Y.)》2000,290(5490):283-285
Primitive meteorites provide a glimpse into the early history of our solar system, but some of the most primitive meteorites are also rarely found on Earth. In his Perspective, Grossman explains why the fall of the Tagish Lake meteorite on 18 January 2000 in British Columbia, Canada, was a lucky event for meteorite researchers. The first analysis of the meteorite is reported by Brown et al. Well-preserved organic matter in the meteorite provides a unique opportunity to study the nature and origin of organic matter that may have accreted on early Earth and played a role in the origin of life. 相似文献
18.
An iron-sulfur compound (Fe3S2) was synthesized at pressures greater than 14 gigapascals in the system Fe-FeS. The formation of Fe3S2 changed the melting relations from a simple binary eutectic system to a binary system with an intermediate compound that melted incongruently. The eutectic temperature in the system at 14 gigapascals was about 400°C lower than that extrapolated from Usselman's data, implying that previous thermal models of Fe-rich planetary cores could overestimate core temperature. If it is found in a meteorite, the Fe3S2 phase could also be used to infer the minimum size of a parent body. 相似文献
19.
Records of now-extinct short-lived nuclides in meteorites provide information about the formation and evolution of the solar system. We have found excess 10B that we attribute to the decay of short-lived 10Be (half-life 1.5 million years) in hibonite grains from the Murchison meteorite. The grains show no evidence of decay of two other short-lived nuclides-26Al (half-life 700,000 years) and 41Ca (half-life 100,000 years)-that may be present in early solar system solids. One plausible source of the observed 10Be is energetic particle irradiation of material in the solar nebula. An effective irradiation dose of approximately 2 x 10(18) protons per square centimeter with a kinetic energy of >/=10 megaelectronvolts per atomic mass unit can explain our measurements. The presence of 10Be, coupled with the absence of 41Ca and 26Al, may rule out energetic particle irradiation as the primary source of 41Ca and 26Al present in some early solar system solids and strengthens the case of a stellar source for 41Ca and 26Al. 相似文献
20.
Present-day (1967 to 1969) fluxes of alpha particles from solar cosmic rays, determined from satellite measurements, were used to calculate the production rates of cobalt-57, cobalt-58, and nickel-59 in lunar surface samples. Comparisons with the activities of nickel-59 (half-life, 8 x 10(4) years) measured in lunar samples indicate that the long-term and present-day fluxes of solar alpha particles are comparable within a factor of approximately 4. 相似文献