Weathering process at the natural fission reactor of Bangombé

 The uranium deposits in the basin of Franceville (Gabon) host the only natural fission reactors known in the world. Unique geological conditions favoured a natural fission reaction 2 Ga ago. This was detected by anomalous isotopic compositions of uranium and rare earth elements (REE), which are produced by the fission reaction. In total, 16 reactor zones were found. Most of them are mined out. The reactor zone of Bangombé, is only 10–11 m below the surface. This site has been influenced by surface weathering processes. Six drill cores have been sampled at the site of the reactor zone of Bangombé during the course of the study and only one drill core (BAX 08) hit the core of the reactor. From these data and previous drilling campaigns, the reactor size is estimated to be 10 cm thick, 2–3 m wide and 4–6 m long. The migration of fission products can be traced by the anomalous isotope ratios of REE because of the fission process. The ¹⁴⁹Sm/¹⁴⁷Sm ratio close to the reactor zone is only 0.28 (normal: 0.92) because of the intense neutron capture of ¹⁴⁹Sm and subsequent transmutation, whereas ¹⁴⁷Sm is enriched by the fission reaction. Similar changes in isotopic patterns are detectable on other REE. The isotope ratios of Sm and Nd of whole rock and fracture samples surrounding the reactor indicate that fission-genic REE migrated only a few decimetres above and mainly below the reactor zone. Organic matter (bitumen) seems to act as a trap for fission-genic REE. Additional REE-patterns show less intense weathering with increasing depth in the log profile and support a simple weathering model. Received: 26 November 1999 · Accepted: 2 May 2000     

Characterization of soil's structural crust by spectral reflectance in the SWIR region (1.2–2.5 μm)

Structural crust is a thin layer formed on the soil surface after a rainstorm. The crust is the result of a physical segregation and rearrangement of soil particles in a way that affects some of the soil properties, such as infiltration, runoff and soil erosion. In practice, there is no rapid, in situ method for monitoring, assessing and mapping crust intensity and quality. In this study, a controlled spectral investigation of the structural crust across the NIR–SWIR spectral region was conducted on three selected Israeli soils, to study the potential of reflectance radiation to detect structural crust in soils. Two major factors served as the driving forces for this study: (1) there is no valid method for in situ assessment of the crust's characteristics in the agriculture field, and (2) the crust might bias thematic remote sensing of soils, because the thin layer of crust blocks photon–matter interaction, which represents the relevant soil body. Through the use of a laboratory rainfall simulator and a sensitive spectrometer, it was revealed that for three selected soils, significant spectral differences occurred between the crust and its bulk soil. The spectral information was found to be related to changes in particle size distribution and texture at the surface of the soil. This conclusion was based on indications of absorption of OH in clay lattice, OH in adsorbed water and CO₃ in carbonates. It was concluded that the structural crust is a phenomenon that should not be ignored by remote-sensing users. In fact, in the field of agriculture, the spectral properties of crust can be used as tools for estimating the crust's intensity.     

Evolution of Polytropic Reimann Disks in Halos

The equilibrium of elliptical Riemann disks with a polytropic equation of state and their evolution under the influence of viscosity and gravitational radiation inside spheroidal halos with a relative surface mass density k. The evolutionary trajectory of a disk inside a halo with k<0.5, which is analogous to the evolution of an isolated disk, differs from that of a disk inside a denser halo.     

Deep-ocean vertical noise directionality

The structure of beam noise measured at the output of a vertical array in a range dependent ocean basin was investigated using the modified wide-angle parabolic equation (PE). Noise sources were distributed throughout the basin, and the field due to each noise source at an array located in the midbasin was calculated. The response of the array to the superposition of the noise sources was found by beamforming. An efficient and direct approach that superimposes the noise sources on the PE field as the field is marched toward the array was developed. Downslope calculations of the midbasin vertical directionality were made between 50 and 400 Hz with this technique. Use of a geoacoustic model shows that the bottom behaves as a low-pass filter     

Type I Radio Bursts Deflected by Lower Hybrid Waves

Solar Physics - What are the observational effects if type I bursts are deflected by lower-hybrid waves? The deflection creates an elliptical radio mirage, with the direct source at one end of the...     

Circumstellar Silicate Mineralogy

This paper reviews spectra obtained with the SWS on board of ISO of dust shells around O-rich objects. These spectra reveal the presence of many new emission features between 10 and 45 μm. These bands are generally much narrower than the well-known 10 and 20 μm silicates features. The strength of these features relative to the underlying broad continuum varies from source to source (≅ 5-50%). The 10 μm region shows evidence for the presence of Al2O3 grains. At longer wavelength, the spectra are dominated by features due to crystalline olivine and pyroxene. The exact peak position of these features shows that the emitting grains consist of the Mg-rich end-members of these minerals with an Fe-content of < 10%. The underlying continuum is attributed to amorphous silicate grains. These observations of aluminum-rich and magnesium-rich compounds compare well with the thermodynamic condensation sequence of minerals expected for O-rich outflows. The observations also imply that freeze out (ie., kinetics) of this condensation sequence at different temperatures is an important characteristic of dust formation in these objects. It is suggested that the absence of Fe-rich silicates is a natural consequence of the low temperature at which gaseous Fe reacts with Mg-rich silicates in these outflows, resulting in amorphous grains with little characterizing spectral detail. This revised version was published online in August 2006 with corrections to the Cover Date.