High Resolution Imaging Systems For Spin-Stabilized Probe Spacecraft

A novel design for a high-resolution imaging system which includes on-board data editing and optical navigation, suggests high quality images can be acquired from spin-stabilized spacecraft oriented towards high velocity, short duration planetary missions ("Probes"). The approach to designing imaging systems requires that mission objectives be met within the physical and fiscal constraints imposed by the spacecraft and mission design. Severe constraints imposed on a Comet Halley probe (for example, 57km/sec encounter velocity with a small, 10km diameter, object coupled with a great uncertainty in encounter time and distance, were overcome by innovative use of existing technology. Such designs suggest that 3-axis stabilization or non-spinning platforms are not necessary to acquire high resolution, high quality planetary images

Measurement of minority carrier lifetimes in semiconductors

The bulk lifetimes of minority ,carriers in n-type l :germanium, in both n and p-type silicon and in n-type magnesium germanide have been investigated at room temperature. The sample was illuminated with periodic light ·flashes produced by a spark gap. The transient change in conductance of the sample after illumination gave the lifetime. In germanium and silicon the measured bulk lifetimes had values from 15 to 140 microseconds with estimated errors of 4 to 15 per cent, respectively. It was not possible to measure the lifetime in magnesium germanide because the particular sample used in this experiment was not sufficiently pure. The general solution of the equation of continuity for the minority carriers in a .rectangular sample has been obtained. The particular solution for the boundary and initial conditions of this experiment is discussed

Microwave mobilities of holes in p-type geranium

Hall mobilities of an n-type (GN l) and a p-type (GP 2) germanium single crystal were measured at a microwave frequency of 9 Gc/ sec from 80 °K to 300 °K. A bimodal rectangular cavity designed by Nishina was used in the present investigation. The microwave circuit was nearly the same as that described by Nishina except that the microwave signal was modulated by 1000 cycle per second square-wave signal. The microwave mobilities measured (with sample size correction factor of 0. 423 for n-type and 0. 687 for p-type germanium) were compared with the corresponding d. c. Hall mobilities. For n-type germanium, the discrepancy between the d. c. and microwave mobilities was believed to be predominatly due to the E: -1/2 dependence of the relaxation time (acoustical mode scattering). For p-type germanium, a large deviation occurred at low temperatures and was in agreement with the results obtained by Hambleton et al. and by Watanabe. This result might be explained qualitatively as a combined effect of lattice and impurity scattering, particularly the effect of impurity scattering on the light mass holes

Thermal conductivity of metals at high temperatures

A new method of measuring thermal diffusivity and hence thermal conductivity of metals is suggested. Like previously reported dynamic methods, this method uses a heat source, whose temperature varies sinusoidally located at one end of an effectively infinite rod. Unlike these methods only one period of the heat wave is required to eliminate the unknown coefficient determining the heat lost by radiation since both velocity and amplitude decrement of the heat wave are measured. The new method is faster in taking data and simpler in computation. The thermoelectric potentials from two thermojunctions are amplified and plotted on a Brown Electronic recorder in order to obtain a permanent record of all necessary data for computing the thermal diffusivity. Results for copper over the temperature range 0-560°C and for thorium over the temperature range 0-430°C are given

Electrical properties of magnesium silicide and magnesium germanide

Single crystals of Mg2Si and Mg2Ge, of high purity, on the order of 1 mm x 1 mm x 1 em in size were obtained, and measurements were made of their electrical resistivities and Hall coefficients in the temperature range 60°K- 1000°K. Both compounds behaved typically as excess impurity semiconductors and exhibited apparent intrinsic conduction above 450°K

Variations of electrophotoluminescence phenomena with frequency of applied field

Luminescence is a general term which includes the many phenomena involving the absorption of energy by a substance and its re=emission as visible or near visible radiation. Photoluminescence implies excitation 7 by irradiation with electromagnetic radiation; cathodoluminescence implies excitation by electron bombardment; thermoluminescence or glow refers to additional excitation by heat of a phosphor previously irradiated at a low temperature; and electroluminescence implies excitation by an electric field. The most recently observed phenomena, and the subjects of this investigation are those included in the general term electrophotoluminescence which refers to the effect of an electric field on photoluminescence. Luminescence includes both fluorescence and phosphorescence. Fluoresgence refers-to the emission observed either during excitation or within 10^-8 sec after excitation has ceased. Phosphorescence refers to any omission observed later than 10^-8 sec after excitation has ceased. Most substances are not luminescent but dissipate absorbed energy in heat. A few substances dissipate absorbed energy in photoelectrons, or in other forms. This discussion will be concerned exclusively with that class of crystalline inorganic solids which exhibit phosphorescence due to controlled traces of impurity. These solids are known as phosphors

Measurements of Specific Heats by a Pulse Method

Calorimetric methods of measuring specific heats at high temperatures are slow and often involve laborious precautions to eliminate heat losses. Pulse heating methods are rapid and heat losses can be made negligible. The accuracy of the calorimetric methods, however, has been greater than that of the pulse heating methods, which need further development. The purpose of the present investigation was to develop further and to evaluate again one of the more promising pulse heating methods. Kurrelmeyer, Mais and Green (1) suggested in 1941 the use of pulse heating. Baxter (2) described in 1944 a pulse method in which the current and voltage across a thin wire sample were recorded simultaneously. This information, combined with knowledge of the resistance as a function of temperature, gave the specific heat. The principle of Baxter\u27s method has been used in preliminary measurements by Khotkovitch and Bagrov (3) and by Nathahn (4), and is also used in this investigation

Applications systems verification and transfer project. Volume 4: Operational applications of satellite snow cover observations. Colorado Field Test Center

The study was conducted on six watersheds ranging in size from 277 km to 3460 km in the Rio Grande and Arkansas River basins of southwestern Colorado. Six years of satellite data in the period 1973-78 were analyzed and snowcover maps prepared for all available image dates. Seven snowmapping techniques were explored; the photointerpretative method was selected as the most accurate. Three schemes to forecast snowmelt runoff employing satellite snowcover observations were investigated. They included a conceptual hydrologic model, a statistical model, and a graphical method. A reduction of 10% in the current average forecast error is estimated when snowcover data in snowmelt runoff forecasting is shown to be extremely promising. Inability to obtain repetitive coverage due to the 18 day cycle of LANDSAT, the occurrence of cloud cover and slow image delivery are obstacles to the immediate implementation of satellite derived snowcover in operational streamflow forecasting programs

Vibrational Feshbach Resonances Mediated by Nondipole Positron-Molecule Interactions

Measurements of energy-resolved positron-molecule annihilation show the existence of positron binding and vibrational Feshbach resonances. The existing theory describes this phenomenon successfully for the case of infrared-active vibrational modes which allow dipole coupling between the incident positron and the vibrational motion. Presented here are measurements of positron-molecule annihilation made using a recently developed cryogenic positron beam capable of significantly improved energy resolution. The results provide evidence of resonances associated with infrared-inactive vibrational modes, indicating that positron-molecule bound states may be populated by nondipole interactions. The anticipated ingredients for a theoretical description of such interactions are discussed.Comment: 5 pages, 2 figures, Phys. Rev. Lett. (in press

Mode coupling and multiquantum vibrational excitations in Feshbach-resonant positron annihilation in molecules

The dominant mechanism of low-energy positron annihilation in polyatomic molecules is through positron capture in vibrational Feshbach resonances (VFR). In this paper we investigate theoretically the effect of anharmonic terms in the vibrational Hamiltonian on the positron annihilation rates. Such interactions enable positron capture in VFRs associated with multiquantum vibrational excitations, leading to enhanced annihilation. Mode coupling can also lead to faster depopulation of VFRs, thereby reducing their contribution to the annihlation rates. To analyze this complex picture, we use coupled-cluster methods to calculate the anharmonic vibrational spectra and dipole transition amplitudes for chloroform, chloroform-$d_1$, 1,1-dichloroethylene, and methanol, and use these data to compute positron resonant annihilation rates for these molecules. Theoretical predictions are compared with the annihilation rates measured as a function of incident positron energy. The results demonstrate the importance of mode coupling in both enhancement and suppression of the VFR. There is also experimental evidence for the direct excitation of multimode VFR. Their contribution is analyzed using a statistical approach, with an outlook towards more accurate treatment of this phenomenon.Comment: 16 pages, 10 figures, submitted to Phys. Rev.