An Electron Spin Resonance Study of Gamma-Irradiated 5,6-Dihydropyrimidines

Electron spin resonance (ESR) spectra are observed from gamma-irradiated, poly-crystalline samples of 5,6-dihydrouracil, 5,6-dihydro-5-methyluracil, and 5,6-dihydro-6-methyluracll. The spectra indicate that the radiation-induced free radicals are formed by the loss of hydrogen atoms from the parent compounds. The crystal structure of the dihydropyrimidines is apparently such that there is a proton near the unpaired electron which can simultaneously flip with the flip of the unpaired electron resulting in the formation of satellite lines relative to the main spectral lines. Annealing the irradiated samples at 100°C produces a decrease in the radical content of all compounds except in dihydro-5-methyluracll where the radical content increases. These differences are discussed in terms of the close proximity of the radicals formed during irradiation of dihydro-5-methyluracil. The     

Efficient solution of two-stage stochastic linear programs using interior point methods

Solving deterministic equivalent formulations of two-stage stochastic linear programs using interior point methods may be computationally difficult due to the need to factorize quite dense search direction matrices (e.g., AA ^T ). Several methods for improving the algorithmic efficiency of interior point algorithms by reducing the density of these matrices have been proposed in the literature. Reformulating the program decreases the effort required to find a search direction, but at the expense of increased problem size. Using transpose product formulations (e.g., A ^T A) works well but is highly problem dependent. Schur complements may require solutions with potentially near singular matrices. Explicit factorizations of the search direction matrices eliminate these problems while only requiring the solution to several small, independent linear systems. These systems may be distributed across multiple processors. Computational experience with these methods suggests that substantial performance improvements are possible with each method and that, generally, explicit factorizations require the least computational effort.     

Symbol dynamic maps of spatial-temporal chaotic vibrations in a string of impact oscillators

Spatially complex, temporally chaotic dynamics of N-coupled impact oscillators connected by a string are studied experimentally using a discrete measure of the motion for each of the masses. For N=8, a binary assignment of symbols, corresponding to whether or not the masses impact an amplitude constraint, is used to code the spatial pattern as a binary number and to store its change in time in a computer. A spatial pattern return map is then used to observe the change in spatial patterns with time. Bifurcations in spatial impact patterns are observed in this experiment. An entropy measure is also used to characterize the dynamics. Numerical simulation shows behavior similar to the experimental system.     

New precision limit on the strange vector form factors of the proton

The parity-violating cross-section asymmetry in the elastic scattering of polarized electrons from unpolarized protons has been measured at a four-momentum transfer squared Q2 = 0.624 GeV2 and beam energy E(b) = 3.48 GeV to be A(PV) = -23.80 ± 0.78(stat) ± 0.36(syst) parts per million. This result is consistent with zero contribution of strange quarks to the combination of electric and magnetic form factors G(E)(s) + 0.517G(M)(s) = 0.003 ± 0.010(stat) ± 0.004(syst) ± 0.009(ff), where the third error is due to the limits of precision on the electromagnetic form factors and radiative corrections. With this measurement, the world data on strange contributions to nucleon form factors are seen to be consistent with zero and not more than a few percent of the proton form factors.     

Quantum mechanics: cleaning up metaphysical baggage

The dynamic electronic structure of atoms and molecules can be directly observed by means of the (e, 2e) reaction, which measures the distribution of energies and momenta of two electrons in coincidence after a knockout reaction initiated by an electron beam of known momentum incident on a molecular gas target. The molecular state for each event is identified by the electron separation energy. The recoil momentum for each event is known from the difference of measured initial and final momenta. It has been verified that values of this momentum are equal under suitable conditions to the momentum of the electron in the target immediately before knockout. Thus the spherically-averaged electron momentum distribution for each molecular orbital is measured. This is directly related to molecular orbitals calculated by the methods of quantum chemistry. Properties obtained by this method for different types of molecules are discussed.     

New equivalent step index methods for designing nonlinear fibres

Abstact The equivalent step index (ESI) framework is shown to provide a convenient approximation for evaluating the performance of different optical fibre designs for potential use in nonlinear signal processing elements. New ESI methods are presented which combine reasonable accuracy with simple expressions for the ESI parameters, allowing fast and reliable solution of fibre design problems, and straightforward comparison between theory and measurements on experimental nonlinear fibres.