Autoradiographic study on the distribution of 241Am in the shell of the freshwater zebra mussel Dreissena polymorpha

Autoradiography was used to identify α-track distributions in a series of shell sections from live mussels Dreissena polymorpha Pallas and dissected shells of dead mussels obtained from laboratory experiments using relatively high concentrations of ²⁴¹Am in the exposure media, a required condition for successful use of this autoradiographic technique. A comparable distribution of α-tracks was recorded on autoradiographs from both live and dead shells suggesting that metabolism does not lead to any sizable changes in the process of ²⁴¹Am adsorption (present in the extrapallial fluid) onto the inner surface of shell. Autoradiographs showed a preferential accumulation of ²⁴¹Am in the organic periostracum, whereas the outer and inner shell layers were characterized by a relatively low α-tracks density. No α-tracks were observed in the central part of the shell in any of the samples. These observations will be useful for the development of a general model to explain bioaccumulation and biosorption processes of radionuclides into mollusk shells.     

Spin-diffusion lengths in dilute Cu(Ge) and Ag(Sn) alloys

We used current-perpendicular-to-plane (CPP) exchange-biased spin valves to directly measure spin-diffusion lengths ?_sf^N for N=Cu(2.1 at% Ge) and Ag(3.6 at% Sn) alloys. We found ?_sf^{Cu(2% Ge)}=117₋₆⁺¹⁰ nm and ?_sf^{Ag(4% Sn)}=39±3 nm. The good agreement of this ?_sf^{Cu(2% Ge)} with the value ?_sf^{Cu(2% Ge)}=121±10 nm derived from an independent spin-orbit cross-sectional measurement for Ge in Cu quantitatively validates the use of Valet-Fert theory for CPP-MR data analysis to layer thicknesses several times larger than had been done before. From the value of ?_sf^{Ag(4% Sn)}, we predicted the ESR spin-orbit cross-section for Sn impurities in Ag.     

Bin covering algorithms in the second stage of the lot to order matching problem

We formulate the problem of effectively assigning semiconductor fabrication wafer lots to customer orders of various sizes, or the lot-to-order matching problem, as an integer programming problem. Our goal in this paper is to develop an efficient, practical method for solving this problem for various performance measures. Because of its complexity we decompose the problem into a knapsack problem coupled with a generalized bin-covering problem, and solve these subproblems sequentially using heuristic methods. We restrict our attention to solution methods for the less-common second subproblem, and analyze the performance of several heuristics using a data set representative of real situations in a semiconductor back-end. Based on this analysis, we show that these heuristics perform significantly better than current industrial practice in the context of the overall problem.     

The electronic structure of inorganic benzenes: valence bond and ring-current descriptions

Valence bond (VB) theory and ring-current maps have been used to study the electronic structure of inorganic benzene analogues X(6)H(6) (X = C (1), Si (2)), X(6) (X = N (3), P (4)), X(3)Y(3)H(6) (X,Y = B,N (5), B,P (6), Al,N (7), Al,P (8)), and B(3)Y(3)H(3) (Y = O (9), S (10)). It is shown that the homonuclear compounds possess benzene-like character, with resonance between two Kekulé-like structures and induced diatropic ring currents. Heteronuclear compounds typically show localization of the lone pairs on the electronegative atoms; Kekulé-like structures do not contribute. Of the heteronuclear compounds, only B(3)P(3)H(6) (6) has some benzene-like features with a significant contribution of two Kekulé-like structures to its VB wave function, an appreciable resonance energy, and a discernible diatropic ring current in planar geometry. However, relaxation of 6 to the optimal nonplanar chair conformation is accompanied by the onset of localization of the ring current.     

Rotation of molecular hydrogen in Si: unambiguous identification of ortho-H(2) and para-D(2)

The 3618.4 and 2642.6 cm(-1) infrared absorption lines of interstitial H(2) and D(2) in silicon have been studied under applied uniaxial stresses. The resulting splittings and their small dependence on isotope establish that H(2) in Si is a nearly free rotor and that these lines arise from vibrational transitions between rovibrational states with rotational quantum number J = 1 (T(2) in T(d) symmetry) for ortho-H(2) and para-D(2).     

Results of vacuum cleaning techniques on the performance of LiFfield-threshold ion sources on extraction applied-B ion diodes at 1-10TW

Uncontrolled plasma formation on electrode surfaces limits performance in a wide variety of pulsed power devices such as electron and ion diodes, transmission lines, radio frequency (RF) cavities, and microwave devices. Surface and bulk contaminants on the electrodes in vacuum dominate the composition of these plasmas, formed through processes such as stimulated and thermal desorption followed by ionization. We are applying RF discharge cleaning, anode heating, cathode cooling, and substrate surface coatings to the control of the effects of these plasmas in the particular case of applied-B ion diodes on the SABRE (1 TW) and PBFA-X (30 TW) accelerators. Evidence shows that our LiF ion source provides a 200-700 A/cm² lithium beam for 10-20 ns which is then replaced by a contaminant beam of protons and carbon. Other ion sources show similar behavior. Our electrode surface and substrate cleaning techniques reduce beam contamination, anode and cathode plasma formation, delay impedance collapse, and increase lithium energy, power, and production efficiency. Theoretical and simulation models of electron-stimulated and thermal-contaminant desorption leading to anode plasma formation show agreement with many features from experiment. Decrease of the diode electron loss by changing the shape and magnitude of the insulating magnetic field profiles increases the lithium output and changes the diode response to cleaning. We also show that the LiF films are permeable, allowing substrate contaminants to affect diode behavior. Substrate coatings of Ta and Au underneath the LiF film allow some measure of control of substrate contaminants, and provide direct evidence for thermal desorption. We have increased lithium current density by a factor of four and lithium energy by a factor of five through a combination of in situ surface and substrate cleaning, substrate coatings, and field profile modifications     

Branching Constraint Satisfaction Problems and Markov Decision Problems Compared

Branching Constraint Satisfaction Problems (BCSPs) model a class of uncertain dynamic resource allocation problems. We describe the features of BCSPs, and show that the associated decision problem is NP-complete. Markov Decision Problems could be used in place of BCSPs, but we show analytically and empirically that, for the class of problems in question, the BCSP algorithms are more efficient than the related MDP algorithms.     

Sound-producing sources as objects of perception: rate normalization and nonspeech perception

In a variety of experiments and paradigms, researchers have attempted to determine whether or not speech perception is specialized by comparing perception of speech syllables to perception of nonspeech analogs. While nonspeech analogs appear optimal as comparisons to speech because they are acoustically similar without being recognized as speechlike, it is argued that the comparison they offer is confounded and uninterpretable. Two experiments are designed to show that, in auditory perception generally where acoustic signals are causal consequences of mechanical events, perceptual experiences are of the mechanical events themselves, not of the acoustic signal. This has two consequences. One is that there is a confounding in comparisons of speech with sine wave analogs that, whereas the one perceived as speech also has a definite causal source, the other, perceived as nonspeech, has an indeterminate or ambiguous source. A second is that response patterns in classification tasks such as those used in the literature comparing speech to nonspeech will reflect properties of the perceived sound-producing event; they will not provide a clear window on auditory system processes used to recover event properties. Experiment 3 is designed to show that perception of many acoustic-signal-producing events can appear to be special by the logic of speech-sine wave comparisons--even events that cannot plausibly be supposed to involve a specialization.     

Homoclinic Bifurcations in n Dimensions

Bifurcations near homoclinic orbits in n dimensions are described. Depending on the eigenvalues of the Jacobian at the fixed point whose real parts are closest to zero, a strange invariant set of periodic and aperiodic orbits can be produced, which can be described by a Bernoulli shift on a finite set of symbols. These results generalize earlier ones of Shil'nikov, Gaspard, Tresser, Glendinning, and Sparrow, amongst others.