Radioisotopes and the history of nucleosynthesis in the galaxy

Nearly all of the heavier elements seem to have been assembled by successive neutron captures occurring in two distinct processes: the s (slow) process refers to neutron capture at a rate which is slow compared to the intervening beta-decay; the r (rapid) process refers to neutron capture at a rate which is rapid compared to the beta process. It is becoming increasingly apparent that simple models for galactic r-process nucleosynthesis are inadequate. Modern astronomical observations, which indicate that the bulk of r-process synthesis may have occurred early in the life of the galaxy, cannot be ignored. Recent data on the fissiogenic xenon in whitlockite from the St. Severin meteorite also place stringent conditions on permissible models for element synthesis. It appears that neither sudden nor continuous models for element formation are consistent with isotopic data now available. I propose a more complex model for the synthesis of solar system material in which the r-process is allowed to occur in three distinct modes: a "prompt" synthesis early in the history of the galaxy, a "continuous" synthesis whereby r-process products are continuously added to the galactic mix, and a "last-minute" synthesis which enriches the solar nebula with r-process material shortly before the formation of the solar system. Calculations based on the present abundances of uranium-235, uranium-238, and thorium-232 and the measured abundances of iodine-129 and plutonium-244 present when meteorites began to retain xenon indicate that the galactic age is between 8.0 and 8.8 billion years, with the initial "prompt" synthesis accounting for 81 to 89 percent of the total r-process material ever produced, the "last-minute" synthesis contributing between 11 and 13 percent, and 0 to 8 percent occurring in the continuous mode. The time interval between the isolation of the solar nebula from galactic r-process and the onset of xenon retention in meteorites lies between 176 and 179 million years.     

Molecular imaging using a targeted magnetic resonance hyperpolarized biosensor

A magnetic resonance approach is presented that enables high-sensitivity, high-contrast molecular imaging by exploiting xenon biosensors. These sensors link xenon atoms to specific biomolecular targets, coupling the high sensitivity of hyperpolarized nuclei with the specificity of biochemical interactions. We demonstrated spatial resolution of a specific target protein in vitro at micromolar concentration, with a readout scheme that reduces the required acquisition time by >3300-fold relative to direct detection. This technique uses the signal of free hyperpolarized xenon to dramatically amplify the sensor signal via chemical exchange saturation transfer (CEST). Because it is approximately 10,000 times more sensitive than previous CEST methods and other molecular magnetic resonance imaging techniques, it marks a critical step toward the application of xenon biosensors as selective contrast agents in biomedical applications.     

Interstellar carbon in meteorites

The Murchison and Allende chondrites contain up to 5 parts per million carbon that is enriched in carbon-13 by up to + 1100 per mil (the ratio of carbon-12 to carbon-13 is approximately 42, compared to 88 to 93 for terrestrial carbon). This "heavy" carbon is associated with neon-22 and with anomalous krypton and xenon showing the signature of the s-process (neutron capture on a slow time scale). It apparently represents interstellar grains ejected from late-type stars. A second anomalous xenon component ("CCFXe") is associated with a distinctive, light carbon (depleted in carbon-13 by 38 per mil), which, however, falls within the terrestrial range and hence may be of either local or exotic origin.     

Off-Resonance Conduction Through Atomic Wires

The electrical resistance of wires consisting of either a single xenon atom or two xenon atoms in series was measured and calculated on the basis of an atom-jellium model. Both the measurement and the calculation yielded a resistance of 10(5) ohms for the single-xenon atom system and 10(7) ohms for the two-xenon atom system. These resistances greatly exceeded the 12,900-ohm resistance of an ideal one-dimensional conduction channel because conduction through the xenon atoms occurs through the tail of the xenon 6s resonance, which lies far above the Fermi level. This conduction process in an atom-sized system can now be understood in terms of the electronic states of individual atoms.     

Barium isotopes in allende meteorite: evidence against an extinct superheavy element

Carbon and chromite fractions from the Allende meteorite that contain isotopically anomalous xenon-131 to xenon-136 (carbonaceous chondrite fission or CCF xenon) at up to 5 x 10(11) atoms per gram show no detectable isotopic anomalies in barium-130 to barium-138. This rules out the possibility that the CCF xenon was formed by in situ fission of an extinct superheavy element. Apparently the CCF xenon and its carbonaceous carrier are relics from stellar nucleosynthesis.     

Thermochemical Properties of Xenon Difluoride and Xenon Tetrafluoride from Mass Spectra

The standard heats of formation for gaseous xenon tetrafluoride and xenon difluoride and the average strength of the bonds in these molecules have been determined from appearance-potential data obtained with a mass spectrometer. The experimental values are compatible with theoretical estimates of these quantities.     

Ordering in a fluid inert gas confined by flat surfaces

High-resolution transmission electron microscopy images of room-temperature fluid xenon in small faceted cavities in aluminum reveal the presence of three well-defined layers within the fluid at each facet. Such interfacial layering of simple liquids has been theoretically predicted, but observational evidence has been ambiguous. Molecular dynamics simulations indicate that the density variation induced by the layering will cause xenon, confined to an approximately cubic cavity of volume approximately 8 cubic nanometers, to condense into the body-centered cubic phase, differing from the face-centered cubic phase of both bulk solid xenon and solid xenon confined in somewhat larger (>/=20 cubic nanometer) tetradecahedral cavities in face-centered cubic metals. Layering at the liquid-solid interface plays an important role in determining physical properties as diverse as the rheological behavior of two-dimensionally confined liquids and the dynamics of crystal growth.     

Retention of xenon in quartz and Earth's missing xenon

The reactivity of xenon with terrestrial oxides was investigated by in situ synchrotron x-ray diffraction. At high temperature (T > 500 kelvin), some silicon was reduced, and the pressure stability of quartz was expanded, attesting to the substitution of some xenon for silicon. When the quartz was quenched, xenon diffused out and only a few weight percent remained trapped in samples. These results show that xenon can be covalently bonded to oxygen in quartz in the lower continental crust, providing an answer to the missing xenon problem; synthesis paths of rare gas compounds are also opened.     

Xenon Tetrafluoride: Reaction with Aqueous Solutions

Xenon tetrafluoride reacts with water to yield xenon, oxygen, hydrofluoric acid, and a very soluble species containing xenon. Evaporation of the solution yields a white, crystal-line substance which has been identified as xenon (VI) oxide, XeO(3).     

Martian gases in an antarctic meteorite?

Significant abundances of trapped argon, krypton, and xenon have been measured in shock-altered phases of the achondritic meteorite Elephant Moraine 79001 from Antarctica. The relative elemental abundances, the high ratios of argon-40 to argon-36 (>/= 2000), and the high ratios of xenon-129 to xenon-132 (>/= 2.0) of the trapped gas more closely resemble Viking data for the martian atmosphere than data for noble gas components typically found in meteorites. These findings support earlier suggestions, made on the basis of geochemical evidence, that shergottites and related rare meteorites may have originated from the planet Mars.     

Xenon as a complex ligand: the tetra xenono Gold(II) cation in AuXe(4)2+(Sb(2)F(11)-)(2)

The first metal-xenon compound with direct gold-xenon bonds is achieved by reduction of AuF(3) with elemental xenon. The square planar AuXe(4)2+ cation is established by a single-crystal structure determination, with a gold-xenon bond length of approximately 274 picometers. The bonding between gold and xenon is of the final sigma donor type, resulting in a charge of approximately 0.4 per xenon atom.     

呋虫胺在有机溶剂和水中的手性稳定性

【目的】在人工氙灯光照、室内自然光照和避光培养条件下,探究呋虫胺外消旋体（Rac-呋虫胺）及其对映体（S-呋虫胺和R-呋虫胺）在甲醇、乙醇、异丙醇、乙酸乙酯、乙腈、二氯甲烷、超纯水以及β-环糊精水溶液中的手性稳定性,为大宗用量的手性新烟碱类杀虫剂呋虫胺的准确检测分析、药效和环境安全的正确评价提供依据。【方法】培养周期内分段取样,以直链淀粉-三（3, 5-二甲基苯基氨基甲酸酯）为手性固定相,正己烷/甲醇/乙醇（85/10/5, v/v/v）为流动相,柱温为30℃,流速为1.0 mL?min^-1,紫外检测波长为270 nm,进样量为20 µL,高效液相色谱/二极管阵列检测器（HPLC/DAD）进行分离分析,S-呋虫胺和R-呋虫胺的保留时间分别为8.3和9.7 min。外标法定量。【结果】人工氙灯光照、室内自然光照和避光培养条件下,S-呋虫胺和R-呋虫胺在甲醇、乙醇、异丙醇、乙酸乙酯、乙腈、二氯甲烷、超纯水以及β-环糊精水溶液中均不存在相互转化现象;配对样品t-test分析Rac-呋虫胺在每种溶剂中两个对映体的降解残留量,证实无对映体选择性降解。对映体浓度比值（EF值）为0.4746-0.5116。但在人工氙灯光照下,Rac-呋虫胺、S-呋虫胺和R-呋虫胺迅速降解,顺序为二氯甲烷＞乙腈＞乙酸乙酯≈异丙醇≈乙醇＞甲醇＞超纯水＞β-环糊精水溶液。相比水溶液,呋虫胺在有机溶剂中更容易降解,半衰期分别为3.3-3.6 h和1.2-2.3 h。光解动态符合一级动力学模型,相关系数为0.9550-0.9959。室内自然光照与避光培养条件下,Rac-呋虫胺、S-呋虫胺和R-呋虫胺在二氯甲烷、乙腈、乙酸乙酯、异丙醇、乙醇、甲醇、超纯水和β-环糊精水溶液中无明显降解,实测浓度和相对标准偏差分别为9.5-10.4 mg?L^-1,1.0%-3.2%和9.5-10.5 mg?L^-1,1.4%-2.8%。【结论】在（25±2）℃人工氙灯光照、室内自然光照和避光条件下,呋虫胺在甲醇、乙醇、异丙醇、乙酸乙酯、乙腈、二氯甲烷、超纯水以及β-环糊精水溶液中手性构型稳定,在此条件下进行定性定量分析、药效和环境安全评价是准确的。     

Noble Gases in the Murchison Meteorite: Possible Relics of s-Process Nucleosynthesis

The Murchison carbonaceous chondrite contains a new type of xenon component, enriched by up to 50 percent in five of the nine stable xenon isotopes, mass numbers 128 to 132. This component, comprising 5 x 10(-5) of the total xenon in the meteorite, is released at 1200 degrees to 1600 degrees C from a severely etched mineral fraction, and probably resides in some refractory mineral. Krypton shows a similar but smaller enrichment in the isotopes 80 and 82. Neon and helium released in the same interval also are quite anomalous, being highly enriched in the isotopes 22 and 3. These patterns are strongly suggestive of three nuclear processes believed to take place in red giants: the s process (neutron capture on a slow time scale), helium burning, and hydrogen shell burning. If this interpretation is correct, then primitive meteorites contain yet another kind of alien, presolar material: dust grains ejected from red giants.     

Solar-type xenon: a new isotopic composition of xenon in the pesyanoe meteorite

Xenon in the Pesyanoe meteorite is a mixture of several componenits. Solar-type xenon is a new component deficient in the neutron-rich isotopes as compared to both trapped chondritic and terrestrial atmospheric xenon.     

Xenon: effect on radiation sensitivity of HeLa cells

HeLa cells, plated onto plastic petri dishes, were exposed to various atmospheres composed of air and carbon dioxide; helium, oxygen, and carbon dioxide; and xenon, oxygen, and carbon dioxide in a pressure vessel. Survival curves with x-rays, 280 kilovolts (peak), show that air and helium have the same effect, but that xenon potentiates x-irradiation to the extent that the dose to produce a given level of survival with xenon is 0.58 of the dose required with air.     

The xenon record of extinct radioactivities in the Earth

Analyses of xenon from well gas rich in carbon dioxide reveal a large excess of radiogenic xenon-129 from the decay of extinct iodine-129. Smaller excesses observed in the heavy xenon isotopes are from fission. These results place narrow limits on any age difference between the earth and the oldest meteorites. The occurrence of excess radiogenic xenon-129 in well gas also suggests that any quantitative degassing of existing solid materials to form the atmosphere must have been limited to a very early period of the earth's history, approximately the first 10(8) years. Alternatively, this observation is consistent with a model of the earth's continuous, but still incomplete, degassing since its time of formation.     

Plutonium-fission xenon found in Earth's mantle

Data from mid-ocean ridge basalt glasses indicate that the short-lived radionuclide plutonium-244 that was present during an early stage of the development of the solar system is responsible for roughly 30 percent of the fissiogenic xenon excesses in the interior of Earth today. The rest of the fissiogenic xenon can be ascribed to the spontaneous fission of still live uranium-238. This result, in combination with the refined determination of xenon-129 excesses from extinct iodine-129, implies that the accretion of Earth was finished roughly 50 million to 70 million years after solar system formation and that the atmosphere was formed by mantle degassing.     

Plutonium-244: confirmation as an extinct radioactivity

The mass spectrum of xenon from spontaneous fission in a laboratory sample of plutonium-244 is precisely what meteoriticists predicted it would be; this discovery completes a web of proof that this nuclide is a bona fide extinct radioactivity of galactic origin, that r-process nucleosynthesis was ongoing in the galaxy at the time of the birth of the sun, and that the early meteoritic abundances of plutonium-244, heretofore tentative, can be utilized with confidence in models for the chronology of galactic nucleosynthesis. The search for an explanation for anomalous fission-like xenon in carbonaceous chondrites can now be narrowed.     

Xenon Tetrafluoride: Crystal Structure

On the basis of a three-dimensional x-ray analysis, the xenon tetrafluoride molecule in the solid is planar; the approximate symmetry is D(4h). The average distance between the xenon and the fluorine is 1.92 +/- 0.03 A.     

Nucleon stability: a geochemical test independent of decay mode

By analyzing published geochemical data on xenon isotopes measured in a 2.46 x 10(9)-year-old telluride ore, a lower limit of 1.6 x 10(25) years has been obtained for the mean lifetime of the nucleons in the tellurium-130 nucleus. This result is insensitive to the particular mode by which the nucleons decay and therefore provides a rigorous limit on possible baryon number nonconservation. The new limit is about two orders of magnitude better than the previous rigorous limit on nucleon stability.