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- Author or Editor: V. Michel x
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The basic idea behind selective multiscale reconstruction of functions from error- afiected data is outlined on the sphere. The selective reconstruction mechanism is based on the premise that multiscale approximation can be well-represented in terms of only a relatively small number of expansion coeficients at various resolution levels. An attempt is made within a tree algorithm (pyramid scheme) to remove the noise component from each scale coefilient usinga priori statistical information (provided by an error covariance kernel of a Gaussian, stationary stochastic model)
Abstract
The possible application of cyclotron-produced fast neutrons to activation analysis for oxygen based on the16O(n, p)16N reaction has been investigated. Neutrons were produced by bombarding a thick beryllium target with 22 to 45 MeV deuterons. It was found that the sensitivity increases rapidly with the energy of the deuterons. Using 45 MeV deuterons and a 10 μA beam current a sensitivity of about 20 counts per 1 μg oxigen could be achieved, enabling the determination of less than 1 μg oxigen. In a direct comparison it was experimentally established that the sensitivity for cyclotron-produced neutrons assuming a deuteron beam of about 10 μA, is up to two orders of magnitude higher than that achievable for 14 MeV neutrons with a flux of about 1010 n/s. The interference of fluorine is at about the same level for both the cyclotron-produced and 14 MeV neutrons. Using cyclotron-produced fast neutrons in the investigated energy range, sodium and magnesium can also interfere, but only to a very much lower extent.
Abstract
Nickel-63 is a low energy beta-emitter needing a radiochemical separation before β-counting. Several papers described the separation and the detection of 63Ni in environmental samples. The method used in our work was first developed by Skwarzec and Holm. The main steps of this separation are the selective precipitation of nickel with dimethylglyoxime (DMG) and the use of the non-sorption of nickel on anion resin in hydrochloric acid solutions. In this paper, a new step is added to eliminate high quantities of calcium and magnesium phosphate disturbing the iron hydroxide precipitation step. The procedure has been applied to soft water samples: sediments, plants and fishes. The chemical yields are 60, 70 and 20%, respectively. The detection limit of 63Ni by liquid scintillation spectrometer is 10 mBq.
Abstract
Four cores were collected in weathered rocks and soils in the Boréon forest area (1765 m, Mercantour Massif, France). The samples were analyzed for the isotopes 230Th, 232Th, 234U and 238U. The activity and isotopic ratio profiles suggest that uranium was mobilized (leaching and precipitation) during the weathering process, as well as thorium but in a much less proportion. A model was drawn up to evaluate the U leaching rate and the time that some levels of the weathered rocks have been subjected to weathering. It utilizes LATHAM and SCHWARCZ’s two equations,15 expressed as 234U/238U and 230Th/238U activity ratios, which assume that the alpha recoil effect allows easier leaching for 234U than 238U and no Th mobility. But this last assumption does not correspond to the observations made in the Boréon area, since it appears that in some soil deeper layers 230Th and 228Th are in radioactive deficit relatively to their parents. As there are four unknown quantities (the time, the leaching rates of 238U, 234U, 230Th), the problem to be solved requires two more equations; these can be obtained utilizing the U activity ratio in water, and taking into account the 232Th behavior. In some sites the 238U leaching rate is high in deeper soil levels (near the fresh rocks); this would correspond to a loss of half the U amount in less than 24 000 years.
