Microcanonical Lattice Gas Model for Nuclear Disassembly

Microcanonical calculations are no more difficult to implement than canonical calculations in the Lattice Gas Model. We report calculations for a few observables where we compare microcanonical model results with canonical model results.Comment: 7 pages, Revtex, 3 postscript figure

Influence of the Coulomb Interaction on the Chemical Equilibrium of Nuclear Systems at Break-Up

The importance of a Coulomb correction to the formalism proposed by Albergo et al. for determining the temperatures of nuclear systems at break-up and the ensities of free nucleon gases is discussed. While the proposed correction has no effect on the temperatures extracted based on double isotope ratios, it becomes non-negligible when such temperatures or densities of free nucleon gases are extracted based on multiplicities of heavier fragments of different atomic numbers

Elaboration and characterization of Fe1–xO thin films sputter deposited from magnetite target

Majority of the authors report elaboration of iron oxide thin films by reactive magnetron sputtering from an iron target with Ar–O2 gas mixture. Instead of using the reactive sputtering of a metallic target we report here the preparation of Fe1–xOthin films, directly sputtered froma magnetite target in a pure argon gas flow with a bias power applied. This oxide is generally obtained at very low partial oxygen pressure and high temperature.We showed that bias sputtering which can be controlled very easily can lead to reducing conditions during deposition of oxide thin film on simple glass substrates. The proportion of wustite was directly adjusted bymodifying the power of the substrate polarization. Atomic force microscopy was used to observe these nanostructured layers. Mössbauer measurements and electrical properties versus bias polarization and annealing temperature are also reported

Mechanistic studies on chemical instabilities of recombinant proteins

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2009.Includes bibliographical references.Protein molecules are being widely used as pharmaceuticals for treating diseases ranging from diabetes and haemophilia to various types of cancers due to their great potency and specificity. However, these macromolecules are intrinsically unstable in aqueous solutions, due to the existence of various physical and chemical degradation pathways. Degraded protein molecules have much reduced biological functions, and may also have adverse effects such as immunogenriicity or pharmacokinetic issues. Thus, understanding the underlying mechanisms of these degradation pathways is essential for rationally devising better ways to stabilize protein pharmaceuticals and extends their applicability. In this thesis, two important types of chemical degradation pathways, the oxidation of methionine residues and the hydrolysis of peptide bonds in monoclonal antibody molecules, are investigated from a mechanistic point of view. In the first half of the thesis, oxidation 'of methionine residues in a model protein G-CSF (Granulocyte-Colony Stimulating Factor) was studied to address the issue of how protein structure affects its reactivity. Comparative oxidation studies were performed where the kinetics of oxidation of methionine residues by hydrogen peroxide (H₂0₂) in G-CSF and corresponding chemically synthesized peptides thereof were measured at different temperatures. To assess structural effects, equilibrium denaturation experiments also were conducted on G-CSF to obtain the free energy of unfolding as a function of temperature.(cont.) A comparison of the relative rates of oxidation of methionine residues in short peptides with those of corresponding methionine residues in rhG-CSF yields an understanding of how protein tertiary structure affects oxidation reactions. For the temperature range studied, 4°C to 45°C, the oxidation rate constants followed an Arrhenius equation quite well, suggesting the lack of temperature-induced local structural perturbations that affect chemical degradation rates. One out of the four methionine residues, Met122, showed an activation energy significantly different than that of the corresponding peptide. Extrapolation of kinetic data predicts non-Arrhenius behavior around the melting temperature. Phenomenological modeling trying to understand the temperature dependence of rate constants was pursued. Finally, we show that the data obtained from accelerated oxidation can be used in conjunction with our models to get predictions about the long-term shelf-life oxidation comparable with experimental results. In the latter half of this thesis, three approaches in a hierarchical order were taken in order to explain the higher rate of un-catalyzed hydrolysis of peptide bonds only in the hinge region of antibody molecules. First, ab initio molecular dynamic simulations were performed to understand the reaction mechanism of the hydrolysis of peptide bonds. The system solvated in explicit water molecules was modeled quantum mechanically and dynamic transition trajectories of the chemical reaction were computed at ambient conditions.(cont.) Since no unique pathway can be used to describe the reaction process due to fluctuations at finite temperature, path sampling technique was applied to obtain an ensemble of trajectories. A statistical tool, likelihood maximization, was used to extract physically important degrees of freedom by screening a large number of reaction coordinate models. The same approach was applied to the hydrolytic reactions under both acidic and neutral pH conditions, which are the most relevant to the formulation of antibody molecules. In both cases, changes in local bonding pattern close to the reaction center, as well as the solvent network, showed importance in determining the reaction dynamics of the hydrolysis of the peptide bond. Then classical molecular dynamic simulations were performed to study the dynamics of a free hinge fragment and -the hinge fragments in the antibody molecule. Important structural and dynamic differences between the two situations were revealed, especially the observation that the free hinge fragment takes on configurations much less frequently accessed by the hinge fragment when situated inside the antibody molecule. In the third approach, a coarse-grained reaction rate model was proposed in order to explain the experimentally observed higher rate of hydrolysis of peptide bonds. A hypothesis involving a mechano-chemical mechanism was motivated by the essential constraining effect of Fab and Fc domains on the hinge region in the antibody molecule revealed in the second approach.(cont.) Combining the information obtained from the previous two approaches, force was calculated along the reaction coordinate direction that was determined and verified previously. This information was integrated into a reaction rate model in order to compute the reaction rate constants. The computational results show that the mechano-chemical mechanism can yield reasonable rate constants comparable with available experimental data.by Bin Pan.Ph.D

A scaler-based data acquisition system for measuring parity violation asymmetry in deep inelastic scattering

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Physics, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 79-81).An experiment that measured the parity violating (PV) asymmetry Ad in e- 2H deep inelastic scattering (DIS) at Q2 ~ 1.10 and 1.90 (GeV/c) 2 and XB ~ 0.3 was completed in experimental Hall A at the Thomas Jefferson National Accelerator Facility. The asymmetry can be used to extract the neutral weak coupling combination (2C2u-C2d), providing a factor of five to six improvement over the current world data. To achieve this precision, asymmetries of the 10- level needed to be measured at event rates up to 500 kHz with high electron detection efficiency and high pion background rejection capability. A specialized scaler-based counting data acquisition system (DAQ) with hardware-based particle identification was successfully implemented. The overall pion contamination in the electron sample was controlled to approximately 2 x 10-4 or lower with an electron efficiency above 91% throughout the experiment. The DAQ deadtime contributed an approximately 0.2% uncertainty to the final asymmetries. The statistical quality of the asymmetry measurement agreed with the Gaussian distribution to over five orders of magnitudes and the experimental goal of 3-4% statistical uncertainty was achieved. The results presented here demonstrate that this type of scaler-based DAQ is able to perform accurate measurements of small asymmetries at the 1ppm level. The design of the DAQ system is presented including the analysis of PID performance, deadtime effect and the capability of measuring small asymmetries.by Kai Pan.S.M

Incorporating Radial Flow in the Lattice Gas Model for Nuclear Disassembly

We consider extensions of the lattice gas model to incorporate radial flow. Experimental data are used to set the magnitude of radial flow. This flow is then included in the Lattice Gas Model in a microcanonical formalism. For magnitudes of flow seen in experiments, the main effect of the flow on observables is a shift along the $E^*/A$ axis.Comment: Version accepted for publication in Phys. Rev. C, Rapid Communicatio

Existence of weak solutions for the generalized Navier-Stokes equations with damping

In this work we consider the generalized Navier-Stokes equations with the presence of a damping term in the momentum equation. The problem studied here derives from the set of equations which govern isothermal flows of incompressible and homogeneous non-Newtonian fluids. For the generalized Navier-Stokes problem with damping, we prove the existence of weak solutions by using regularization techniques, the theory of monotone operators and compactness arguments together with the local decomposition of the pressure and the Lipschitz-truncation method. The existence result proved here holds for any and any sigma > 1, where q is the exponent of the diffusion term and sigma is the exponent which characterizes the damping term.MCTES, Portugal [SFRH/BSAB/1058/2010]; FCT, Portugal [PTDC/MAT/110613/2010]info:eu-repo/semantics/publishedVersio

Model of multifragmentation, Equation of State and phase transition

We consider a soluble model of multifragmentation which is similar in spirit to many models which have been used to fit intermediate energy heavy ion collision data. We draw a p-V diagram for the model and compare with a p-V diagram obtained from a mean-field theory. We investigate the question of chemical instability in the multifragmentation model. Phase transitions in the model are discussed.Comment: Revtex, 9 pages including 6 figures: some change in the text and Fig.

Dynamic large deformation analysis of a cantilever beam

National Natural Science Foundation of Chin