The seasonal variation of surface refractivity over Nigeria was studied using two years in-situ meteorological data from eight locations over Nigeria. The result shows that the surface refractivity generally has higher value during rainy season than dry season at all location studied. The results also show that the value of surface refractivity increases from arid region in the north to the coastal area in south. The results also show that local meteorology plays a very important role in refractivity variation. 相似文献
We propose to study the radiation environment on board different flight vehicles: cosmos-type satellites, orbital stations, Space Shuttles and civil (sonic and supersonic) aircraft. These investigations will be carried out with single type of passive detector, namely, nuclear photoemulsions (NPE) with adjustable threshold of particle detection within broad range of linear energy transfer (LET) that is done by means of the technique of selective development of NPE exposed in space.
These investigations will allow one to determine:
• integral spectra of LET of charged particles of cosmic ray (CR) over a wide range from 2.0 to 5×104 MeV/cm in biological tissue;
• differential energy spectra of fast neutrons (1–20 MeV);
• estimation of absorbed and equivalent doses from charged and neutral component CR;
• charge and energy spectra of low energy nuclei (E≤100 MeV) with Z≥2 having in view the extreme hazard radiation to biological objects and microelectronic schemes taken on board inside and outside of these different flight vehicles with exposures from several days to several months.
The investigation of radiation environment on board the airplanes depending on the flight parameters will be conducted using emulsions of different sensitivity without any controlling of threshold sensitivity (Akopova et al., 1996). The proposed detector can be used in the joint experiments on the new International Cosmic Station “Alpha”. 相似文献
We have studied the relationship between cell killing and the induction of residual chromatin breaks on various human cell lines and primary cultured cells obtained by biopsy from patients irradiated with either X-rays or heavy-ion beams to identify potential bio-marker of radiosensitivity for radiation-induced cell killing. The carbon-ion beams were accelerated with the Heavy Ion Medical Accelerator in Chiba (HIMAC). Six primary cultures obtained by biopsy from 6 patients with carcinoma of the cervix were irradiated with two different mono-LET beams (LET= 13 keV/μm, 76 keV/μm) and 200kV X rays. Residual chromatin breaks were measured by counting the number of non-rejoining chromatin fragments detected by the premature chromosome condensation (PCC) technique after a 24 hour postirradiation incubation period. The induction rate of residual chromatin breaks per cell per Gy was the highest for 76 keV/μm beams on all of the cells. Our results indicated that cell which was more sensitive to the cell killing was similarly more susceptible to induction of residual chromatin breaks. Furthermore there is a good correlation between these two end points in various cell lines and primary cultured cells. This suggests that the detection of residual chromatin breaks by the PCC technique may be useful as a predictive assay of tumor response to cancer radiotherapy. 相似文献
The drop tube which will be available in the “Centre d'Etudes Nucléaires de Grenoble” is described. Its main features are the following: - Dimensions : Drop height : 47.1 m Drop time : 3.1 s Tube inside diameter : 0.2 m - Experimental atmosphere : 1 Ultra-vacuum : 10−6 to 10−7 Pa - Residual gravity level : 10−8 to 10−9 g according to the vacuum level and drop diameter.
This facility is unique insofar as it enables experiments to be performed under ultra-vacuum conditions which, by delaying the formation of surface oxides, should contribute to improving maximum undercooling values.
The techniques used for obtaining small metallic drops (0.5 to 3 mm) are described. The availability of this instrument for the scientific community is also foreseen by the french sponsoring organizations (CEA, CNES, CNRS) ; some practicle informations will be given to potential experimenters. 相似文献
Modelisation and solution of heat and mass transfer problems relevant for material processing are generally hard to be handled, as they often involve 3D unsteady flows, viscous mixtures, phase changes, moving liquid-solid fronts, deforming liquid-gas interfaces, etc.… For space applications, material processing benefits of reduced buoyancy convection but can be faced to a strongly increased complexity due to variable g, mainly in manned flight.
Computational techniques used to analyse fluid motions in material processing, accounting for free surface, crystallization front and bulk convection in melt, are reviewed with emphasis to directional crystallization. Hydrodynamics stability and bifurcation analysis are shown to be useful complementary tools for correlating data, and for a better understanding of the physical laws. This last point will be illustrated in the case of the onset of oscillations in metallic melts. 相似文献
The paper is concerned with the numerical simulation and the analysis of some kinds of flow regimes which can develop in Bridgman and Czochralski systems for material processings. The flows in the liquid phase are investigated considering two-dimensional and axisymmetric models. The time-dependent regimes were studied for a zero-Prandtl-number fluid layer confined inside a two-dimensional cavity of aspect ratio (length-to-height) A=4, involving a stress-free upper surface and submitted to a horizontal temperature gradient. The range of Grashof number was varied up to the conditions at which the flow goes from oscillatory to chaotic type behaviours. The combined influence of the temperature gradients and of the rotations of the crucible and of the seed/crystal was investigated for a Czochralski model. The axisymmetric regimes were studied for a Prm=0.015 liquid melt confined inside a cylindrical crucible of aspect ratio (height-to-radius) Am=2, and coupled to a viscous encapsulant liquid layer (10<Pre<1200) of aspect ratio Ae=0.5. A number of steady and (transient) time-dependent flow patterns are identified. 相似文献
This paper reports the results of a general study carried out on the Periodic Multi-SunSynchronous Orbits (PMSSOs), which the classical Periodic SunSynchronous Orbits (PSSOs) represent a specific solution of. Such orbits allow to obtain cycles of observation of the same region in which the solar illumination regularly varies according to the value of the orbit elements and comes back to the initial condition after a time interval which is multiple of the revisit time. Therefore this kind of orbits meets all the remote sensing applications that need observations of the same area at different local times (for example the reconstruction of the day-nighttime trend of the surface temperature of the planet) and it is particularly suitable to the study of several terrestrial and martian phenomena (diurnal cycle of the hazes and clouds, dynamics of the thermal tides, density variations, meteorology phenomena, etc.). The design of PMSSO is based on the variation of the Right Ascension of the Ascending Node due to the Earth oblateness (referred as basic solution). However, with respect to the basic solution, the analysis of the perturbative effects has demonstrated the need, especially in the case of Mars, to take into account all the superior harmonics of the gravitational field. To this end a corrective factor, to add to the basic equations, has been proposed, allowing a significant saving of propellant (of the order of 2 km/s per year). Besides, single and multi-plane satellite constellations have been taken into account in order to improve the repetition of observation and the ground spatial resolution. 相似文献