The photoelectric meteor detector consists of 19 type 931-A photomultiplier tubes arranged to cover the entire sky. Signals from the photocells are repeatedly sampled in time sequence and displayed on a cathode ray tube. Photoelectric meteor signals, radar meteor echoes, and a marker light operated by visual observers are photographed by a continuous-film camera.Preliminary tests indicated that the average sensitivity of the photoelectric detector to meteors was about one visual magnitude inferior to that of the human eye. Approximate meteor positions and qualitative light intensities could be obtained from the photoelectric record. Timing comparisons between the photoelectric and the radar records could be made to 0.05 sec. Methods of improving the sensitivity and other operational characteristics are discussed.

The Differential Scanning Calorimeter (DSC) was used to discriminate among 25 commercial high density polyethylenes (HDPE) on the basis of their degree of crystallinity and melting temperature. The area under the melting endotherm correlated directly with density and inversely with creep and thermal expansion measurements. Since high crystallinity was related to the design required properties of density, creep, and thermal expansion, DSC studies readily identified eight of the more promising polymers from the group of 25. The overall crystallization kinetics of polyethylenes with 75 percent crystallinity were analyzed by the Avrami and Fischer-Turnbull equations. Results indicate small disk-like spherulites (Avrami n = 2 ) following nucleation-controlled growth kinetics. These conclusions are in reasonable agreement with polarizing microscope observations. An equilibrium melting temperature between 141 and 142°C was estimated from Hoffman-Weeks plots. Processing thick parts from highly crystalline polyethylene is difficult because of the 14 percent volume change on crystallization. Higher degrees of crystallinity are associated with moderate molecular weight, so the viscosity range of these polyethylenes is not especially suited for processing by extrusion. These caveates necessitate tradeoffs between optimal design properties and processing requirements for HDPE parts. s a result of a design change by engineers at A Lawrence Livermore National Laboratory (LLNL), the polymer group was asked to select and characterize high density polyethylene (HDPE) on the basis of four design requirements. The objective was to identify a +CH& polymer which is of (1) high purity, (2) high density, (3) low thermal expansion, and (4) maximized mechanical properties. The polymer must also be readily available and easily processed into large (10 by 10 b y 6 inch) parts. Since all polyethylenes are semicrystalline, the degree of crystallinity significantly affects the density, thermal expansion, and mechanical properties. Assuming a two phase model [l] for semicrystalline polymers, the density of the polymer, p(HDPE) is given by: p(HDPE) = p u p c / [ p < ( 1 -XJ + PJLI (1) where p is the density, X , is the degree of crystallinity, and the subscripts a and c indicate the amorphous or crystalline phase, respectively. As the fraction of crystallinity approaches 1 .O, the polymer density approaches the density of the perfect HDPE crystal, p c . ' Work performed under the auspices of the U S Department of Energy by Lawrence Livermore National Laboratorv under Contr No W 7405 Eng 48Based on analogous reasoning, [2] the thermal expansion coefficient of the amorphous, or liquid, phase will be greater than that of the crystalline phase. Again, a higher degree of crystallinity should result in a lower thermal expansion coefficient.Similarly, 131 the modulus of t h e amorphous phase will be lower than that of the crystalline phase. Clearly, as the degree of crystallinity increases, the thermal expansion should decrease, the mechanical properti...

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