Developments in high-resolution positron emission tomography at MGH

The development of high resolution PET systems is important for the wider application of this techniques. The resolution of PET is limited by a number of physical factors such as positron range, small angle deviation, and sampling frequency. The design of the detector array and its sensitivity remain critical factors; designs incorporating analog coding have proven to be useful. PCR-I, a single plane PET system, has demonstrated the concept and has produced useful images in animal studies. PCR-II will extend the concept to a two dimensional detector array resulting in a system with high resolution and high sensitivity.     

Beta-lactamase-producing mutants of Streptococcus cremoris

Penicillin-resistant mutants were isolated for six strains of Streptococcus cremoris used in commercial Cheddar cheese manufacture after treatment with N-methyl-N-nitro-N-nitrosoguanidine. The resistant mutants had an elevated minimal growth inhibitory concentration, 2.5 micrograms (4.13 units)/ml, for penicillin G and other beta-lactam antibiotics as compared with the penicillin-susceptible parent strains, which were each sensitive to .05 micrograms (.08 units)/ml. Penicillin resistance was due to the production of beta-lactamase. Plasmid DNA was not demonstrated in partially purified lysates of four mutants. Mutants had normal cellular morphology but altered phage sensitivity patterns. All except one strain were able to support complete phage adsorption. Resistance was retained after 20 passages in absence of penicillin.     

Multi-H phase-coded modulations with asymmetric modulationindexes

Multi-H phase-coded modulation (MHPM) is a bandwidth-efficient modulation scheme which offers substantial coding gain over conventional digital modulation schemes. MHPM with asymmetric modulation indices corresponding to the bipolar data +1 and -1 is considered, and numerical results for the minimum Euclidean distances are provided. It is shown that performance improvements on the error probability over conventional MHPM are gained with essentially the same bandwidth and a very slight modification in implementation. The upper bounds on the error probabilities as functions of observation intervals and received E_b/N₀ are also investigated in detail. It is concluded that the concept of asymmetric modulation indices for MHPM is attractive for bandwidth and power-efficient modulation     

Construction of transmission lines above valuable standing timber

Problems with the construction of transmission lines above a forest are examined. Construction of lines above standing timber makes it possible to shorten the length of the route, eliminate the clearing of a continuous swath along the line, and reduce the volume of in-service work. To implement this solution, it is necessary to introduce changes and supplements to existing regulatory technical documents. __________ Translated from élektricheskie Stantsii, No. 10, October 2007, pp. 48–51.     

Adaptive control of diamond drilling of nonmetallic materials

The basic principles for selecting the work regimes for diamond drilling of nonmetallic materials are presented. The process of microcutting by a single diamond grain is examined, and relations between the components of the cutting force and the ratio of the rate of the main motion to the feed rate are obtained on the basis of known relationships. It is shown that adaptive control of the drilling nonmetallic materials is possible using two-circuit systems — one for feed-rate control and one for cutting-rate. __________ Translated from Steklo i Keramika, No. 3, pp. 28–30, March, 2007.     

Growth and characterization of GaAs films deposited on Ge/Si composite substrates by metalorganic chemical vapor deposition

We report the results of studies which have been made on heteroepitaxial layers of GaAs and AlGaAs grown by metalorganic chemical vapor deposition on composite substrates that consist of four different types of heteroepitaxial layered structures of Ge and Ge-Si grown by molecular beam epitaxy on (100)-oriented Si substrates. It is found that of the four structures studied, the preferred composite substrate is a single layer of Ge ∼1 μm thick grown directly on a Si buffer layer. The double-crystal X-ray rocking curves of 2 μm thick GaAs films grown on such substrates have FWHM values as small as 168 arc sec. Transmission electron micrographs of these Ge/Si composite substrates has shown that the number of dislocations in the Ge heteroepitaxial layer can be greatly reduced by an anneal at about 750° C for 30 min which is simultaneously carried out during the growth of the GaAs layer. The quality of the GaAs layers grown on these composite substrates can be greatly improved by the use of a five-period GaAs-GaAsP strained-layer superlattice (SLS). Using the results of these studies, low-threshold optically pumped AlGaAs-GaAs DH laser structures have been grown by MOCVD on MBE Ge/Si composite substrates.     

Establishing Effective Information Center User Groups

Short staffing and insufficient technical and business knowledge among staff members can hinder information centers from meeting the needs of end users. Establishing groups of selected users with special skills improves end-user computing by opening communication channels, influencing training objectives, and fostering better user morale. This article explains the types of user groups that an information center can establish and the benefits to be derived from them.     

Excited state absorption of pump radiation as a loss mechanism insolid-state lasers

The characteristics of optical pumping dynamics in laser-pumped, rare-earth-doped, solid-state laser materials are investigated by using a tunable alexandrite laser to pump Y₃Al₅O₁₂ :Nd³⁺ in an optical cavity. It is found that the slope efficiency of the Nd laser operation depends strongly on the wavelength of the pump laser. For pump wavelengths resulting in low slope efficiencies, intense fluorescence emission is observed from the sample in the blue-green spectral region. This is attributed to the excited-state absorption of pump photons which occurs during radiationless relaxation from the pump band to the metastable state. This type of process is an important loss mechanism for monochromatic pumping of laser systems at specific pump wavelengths     

Gene Transfer Systems and Molecular Cloning in Group N Streptococci: A Review

Plasmid deoxyribonucleic acid of group N streptococci codes for a wide variety of important metabolic properties associated with dairy fermentations. The increasing knowledge of the genetics of these bacteria has created exciting possibilities for genetically constructing improved starter strains. A review of current literature is presented on the genetics of group N streptococci, especially the development and implications of gene transfer and molecular cloning systems for further genetic studies and for strain improvement. Functional properties of plasmid deoxyribonucleic acid and gene transfer systems, which include transduction, conjugation, transformation, and protoplast fusion, are reviewed and followed by a discussion of recent advances in gene cloning.