ALBERT

Notes: Monte Carlo results on the three-dimensional randomly-diluted Ising antiferromagnet FexZn1−xF2 indicate that its spin-glass-like phase at x=0.25 and zero magnetic field is characterized by the presence of finite antiferromagnetic domains, separated by random vacancies, but strongly correlated in time. The glassy behavior is a consequence of the combined action of short-range interactions and the presence of strong density fluctuations in the highly diluted regime, close to the percolation concentration. By studying the aging of the system, we also find that its dynamics is much alike that of theoretical spin-glass models. © 2000 American Institute of Physics.

Notes: This work combines two state-of-the-art techniques in the area of magnetic nondestructive evaluation: the application of the superconducting quantum interference device (SQUID) as the magnetic field sensor; and the use of artificial neural networks as analysis tools applied to the detected magnetic signals. Pioneering measurements using the SQUID sensor have been made in steel samples containing various types of flaws, and a neural network system, based on the time-delay neural network and radial basis function algorithms, has been implemented to characterize the flaws. The neural network system aims to, based on the measured magnetic field, provide information about defect geometry, thus allowing the assessment of defect severity, as a basis for maintenance and repair procedures. © 2000 American Institute of Physics.

Notes: The back-energy-transfer process from acceptors to donors is included to predict and explain the nonradiative energy-transfer processes among Yb–Er ions in a YAG matrix. This process is in addition to the direct Yb-to-Er energy transfer and the migration of energy among Yb ions. The two measured Yb transients are well fitted by the corresponding solution of the master equations that give the dynamics of the energy-transfer processes. © 2000 American Institute of Physics.

Notes: : Set yogurts were prepared from fortified milk subjected to the following processes: (a) Thermal process (85C,°C,35 min), (b) High hydrostatic pressure process (193 or 676 MPa for 5 or 30 min), and (c) Nonprocessed milk (control). Yogurts from milk treated with 676 MPa for 30 min exhibited similar yield stress and water-holding capacity (WHC) to yogurts from heated milk. Transmission Electron Microscopy (TEM) micrographs exhibited small round casein micelle aggregates without appendages in yogurts from heated milk. Yogurts from milk treated with 193 MPa and untreated milk exhibited low yield stress, low WHC, and large clusters of coalesced micelles. Mechanisms for gel formation are discussed.

Notes: The control of the thermal gradient in single crystal grown by the laser-heated pedestal growth method is demonstrated through focalization of the laser beam into two different regions of the starting feed stock. To create these conditions, an optical component consisting of two incomplete spherical mirrors with the same focal distances was designed and built in copper and coated with aluminum. The performance of this component was tested in the crystal growth of lithium niobate rods with a radius larger than the critical radius calculated from the theoretical model for single focalization of the laser beam. Thermal gradient and temperature profiles along the crystal axis were measured with single and double focalization of the laser beam during the crystal growth process. The measured data indicate that the main differences in the thermal gradients of both configurations are found in the first two millimeters away from the melt–crystal interface. Single crystal rods of lithium niobate grown along the c axis with radius larger than the critical one are demonstrated with the use of this optical component. © 2001 American Institute of Physics.

Notes: Tropical maize inbred lines, eight derived from a Thai synthetic population (BR-105) and 10 from a Brazilian composite population (BR-106), were assayed for restriction fragment length polymorphisms with 185 clone-enzyme combinations. The aim of this study was to investigate genetic distances among tropical maize material and their relationship to heterotic group allocation and hybrid performance. Genetic distances (GDs) were on average greater for BR-105×BR-106 lines (0.77) than for BR-106×BR-106 (0.71) and for BR-105×BR-105 (0.69) lines. Cluster analysis resulted in a clear separation of BR-105 and BR-106 populations and was according to pedigree information. Correlations of parental GDs with single crosses and their heterosis for grain yield were high for line crosses from the same heterotic group and low for line combinations from different heterotic groups. Our results suggest that RFLP-based GDs are efficient and reliable to assess and allocate genotypes from tropical maize populations into heterotic groups. However, RFLP-based GDs are not suitable for predicting the performance of line crosses from genetically different heterotic groups.

Notes: Previous studies on shock–vortex interactions have either been numerical in nature with idealized boundary conditions and prescribed vortex flows, or in complex experimental flow fields, such as where the vortex is shed at the trailing edge of an aerofoil. In this study a bifurcated shock tube facility has been constructed where two plane waves arrive sequentially at the trailing edge of a wedge. The first shock wave results in a spiral vortex being shed, which is then impacted by the second wave. Accurate control of the delay between the two shock waves was achieved using a highly repeatable piston actuated shock tube driver. A number of interesting new features of this interaction have been identified. The work specifically examines the development of a transient pressure spike, physically occupying an area less than 0.5 mm in diameter and having a duration of 15 μs, with a pressure nearly two-and-a-half times that of the surrounding fluid. This has been done both numerically using an adapting mesh Euler code, and experimentally, the latter with the careful use of fast response miniature pressure transducers. Numerically generated holographic interferograms and shadowgraph images have been generated for direct visual comparisons with the equivalent experimental results of the whole flow field, from which the reason for the production of the pressure spike is established as being due to local shock wave focusing resulting from part of the shock being pulled around the vortex to impact on itself. The generation of a second pressure peak is also examined, as is the wave field emanating from the interaction and the influence on the vortex. The use of both experimental images and numerical flow visualization algorithms were found to provide complimentary information, which allowed for detailed investigation and understanding of the shock wave–vortex interaction. © 2001 American Institute of Physics.

Notes: The observation of acclimation in leaf photosynthetic capacity to differences in growth irradiance has been widely used as support for a hypothesis that enables a simplification of some soil-vegetation-atmosphere transfer (SVAT) photosynthesis models. The acclimation hypothesis requires that relative leaf nitrogen concentration declines with relative irradiance from the top of a canopy to the bottom, in 1 : 1 proportion. In combination with a light transmission model it enables a simple estimate of the vertical profile in leaf nitrogen concentration (which is assumed to determine maximum carboxylation capacity), and in combination with estimates of the fraction of absorbed radiation it also leads to simple ‘big-leaf’ analytical solutions for canopy photosynthesis. We tested how forests deviate from this condition in five tree canopies, including four broadleaf stands, and one needle-leaf stand: a mixed-species tropical rain forest, oak (Quercus petraea (Matt.) Liebl), birch (Betula pendula Roth), beech (Fagus sylvatica L.) and Sitka spruce (Picea sitchensis (Bong.) Carr). Each canopy was studied when fully developed (mid-to-late summer for temperate stands). Irradiance (Q, µmol m−2 s−1) was measured for 20 d using quantum sensors placed throughout the vertical canopy profile. Measurements were made to obtain parameters from leaves adjacent to the radiation sensors: maximum carboxylation and electron transfer capacity (Va, Ja, µmol m−2 s−1), day respiration (Rda, µmol m−2 s−1), leaf nitrogen concentration (Nm, mg g−1) and leaf mass per unit area (La, g m−2).Relative to upper-canopy values, Va declined linearly in 1 : 1 proportion with Na. Relative Va also declined linearly with relative Q, but with a significant intercept at zero irradiance (P 〈 0·01). This intercept was strongly related to La of the lowest leaves in each canopy (P 〈 0·01, r2 = 0·98, n= 5). For each canopy, daily lnQ was also linearly related with lnVa(P 〈 0·05), and the intercept was correlated with the value for photosynthetic capacity per unit nitrogen (PUN: Va/Na, µmol g−1 s−1) of the lowest leaves in each canopy (P 〈 0·05). Va was linearly related with La and Na(P 〈 0·01), but the slope of the Va : Na relationship varied widely among sites. Hence, whilst there was a unique Va : Na ratio in each stand, acclimation in Va to Q varied predictably with La of the lowest leaves in each canopy. The specific leaf area, Lm(cm2 g−1), of the canopy-bottom foliage was also found to predict carboxylation capacity (expressed on a mass basis; Vm, µmol g−1 s−1) at all sites (P 〈 0·01). These results invalidate the hypothesis of full acclimation to irradiance, but suggest that La and Lm of the most light-limited leaves in a canopy are widely applicable indicators of the distribution of photosynthetic capacity with height in forests.