The reactions of radioactive chlorine atoms recoiling from two nuclear processes, "Cl(n, 2n)"'"a and "Cl(n, 7)"CI have been studied in different liquid halobenzenes. The ratio "Q-for-Cl vs.^'Cl-for-H yield for C^HjCl is dose-dependent, indicative for radiation-enhanced ^' Cl-for-Cl exchange. The Cl-for-Cl replacement yield is lowered on the addition of Ij, but much more for "CI than for It is suggested that the precursors to the exchange is a charge-transfer 7r-complex, rather than a a-complex. The fraction of the * Cl-for-Cl replacement yield not affected by Ij is ascribed to direct hot one-step processes. The behaviour of m-Cj H4 Clj is different to the extent that -as measured for 34 mQ _ thermal exchange seems to take place. Also, isomeric dichlorobenzenes irradiated in a mixture show differences among each other in exchange behaviour. Part of the unidentified products is attributed to an excited jr-complex subject to ring-opening and/or fragmentation. The ""'Cl vs."Cl isotope effect is not due to external radiation and is discussed in terms of differences in the recoil energy distribution and the life-time of lower levels of nuclear excitation.
Abstract-We determined the cosmic-ray exposure age of 20 diogenites from measured cosmogenic noble gas isotopes and calculated production rates of 3He, 21Ne and 38Ar. The production rates were calculated on the basis of the measured chemical composition and the cosmogenic 22Ne/21Ne ratio of each sample. The shielding conditions of each sample were also checked on the basis of the measured I0Be and 26A1 concentrations. The exposure ages range from 6 to 50 Ma but do not form a continuous distribution: ten ages cluster at 2 1-25 Ma and four at 3 5 4 2 Ma. The two diogenite clusters coincide with the 22 Ma and 38 Ma peaks in the exposure age distribution of eucrites and howardites. After the selection from literature data of 32 eucrites and 11 howardites with reliable ages, we find a total of 23 howardite, eucrite and diogenite (HED) group meteorites at 20-25 Ma and 10 at 3 5 4 2 Ma. The shape of the two peaks is consistent with single impact events, and random number statistics show that they are statistically significant at the 99% level. Altogether, this provides strong evidence for two major impact events 22 Ma and 39 Ma ago. Although these two events can explain more than half of all HED exposure ages, it takes at least five impact events to explain all ages <50 Ma. An impact frequency of one per 10 Ma corresponds to projectiles of at least 2 4 km in diameter for Vesta and of 60-300 m for the lOOx smaller Vesta-derived "vestoids." Based on the HED exposure-age distribution, the size distribution of the main-belt asteroids and the difference in size between Vesta and the kilometer size vestoids, we favor Vesta as the major source of HED meteorites, although some of the meteorites may have been ejected from the vestoids rather than directly from Vesta.
During leg 2 (November 1988–January 1989) of the European Polarstern study (EPOS) in the Weddell‐Scotia Confluence zone, we implemented a new, simple, rapid, and sensitive method based on 32Si (a β‐emitter in equilibrium with 32P) to determine biogenic silica (BSi) production. This method provides simultaneous determination of the production of organic P. Rates of BSi production ranged from 0.09 to 0.95 mmol Si m‒2 h‒1, and the BSi content of the photic layer ranged from 7 to 220 mmol m‒2. The Si: C production molar ratios varied between 0.08 (small cell density of diatoms in communities dominated by cryptophyceans and dinophyceans) and 0.81 (diatom‐dominated populations), while the C: P production ratios varied between 47 and 151 with an average of 105, in agreement with the Redfield ratio. We give here the first direct evidence that in the well‐mixed waters of the Scotia Sea highly silicified diatom blooms may occur and that unlike earlier descriptions, because of the intense grazing pressure of krill in the marginal ice zone (MIZ) of the Weddell Sea, the remaining siliceous phytoplankton can grow with only moderate Si production rates. We conclude that the contribution of the MIZ to the total BSi production of the Southern Ocean must not be overstated.
Abstract-We measured the concentrations of the cosmogenic radionuclides IoBe (half-life = 1.51 x 106 a), *6Al(7.05 x lo5 a) and 36Cl (3.01 x los a) in Lewis Cliff (LEW) 86360, an L-chondrite from the Lewis Cliff stranding area, East Antarctica. In addition, the concentrations and isotopic compositions of He, Ne and Ar were measured. The combined results yield a terrestrial age of 2.35 ? 0.15 Ma. Only one other stony meteorite with a similar terrestrial age (-2 Ma) is known from the Allan Hills stranding area (ALH 88019), whereas all previously dated stony meteorites from Antarctica are younger than 1 Ma. We argue that LEW 86360 spent most of its terrestrial residence time deep inside the ice, near the base of the glacier, where ice flow rates are much lower than at the surface. The terrestrial ages of LEW 86360 and ALH 88019 are consistent with existing hypotheses concerning the stability and persistence of the East Antarctic ice sheet.
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