1,363 research outputs found
Thermal detectors as X-ray spectrometers
Sensitive thermal detectors should be useful for measuring very small energy pulses, such as those produced by the absorption of X-ray photons. The measurement uncertainty can be very small, making the technique promising for high resolution nondispersive X-ray spectroscopy. The limits to the energy resolution of such thermal detectors are derived and used to find the resolution to be expected for a detector suitable for X-ray spectroscopy in the 100 eV to 10,000 eV range. If there is no noise in the thermalization of the X-ray, resolution better than 1 eV full width at half maximum is possible for detectors operating at 0.1 K. Energy loss in the conversion of the photon energy to heat is a potential problem. The loss mechanisms may include emission of photons or electrons, or the trapping of energy in long lived metastable states. Fluctuations in the phonon spectrum could also limit the resolution if phonon relaxation times are very long. Conceptual solutions are given for each of these possible problems
Simple estimation of absolute free energies for biomolecules
One reason that free energy difference calculations are notoriously difficult
in molecular systems is due to insufficient conformational overlap, or
similarity, between the two states or systems of interest. The degree of
overlap is irrelevant, however, if the absolute free energy of each state can
be computed. We present a method for calculating the absolute free energy that
employs a simple construction of an exactly computable reference system which
possesses high overlap with the state of interest. The approach requires only a
physical ensemble of conformations generated via simulation, and an auxiliary
calculation of approximately equal central-processing-unit (CPU) cost.
Moreover, the calculations can converge to the correct free energy value even
when the physical ensemble is incomplete or improperly distributed. As a "proof
of principle," we use the approach to correctly predict free energies for test
systems where the absolute values can be calculated exactly, and also to
predict the conformational equilibrium for leucine dipeptide in implicit
solvent.Comment: To appear in J. Chem. Phys., 10 pages, 6 figure
Report of the x ray and gamma ray sensors panel
Overall five major areas of technology are recommended for development in order to meet the science requirements of the Astrotech 21 mission set. These are: detectors for high resolution gamma ray spectroscopy, cryogenic detectors for improved x ray spectral and spatial resolution, advanced x ray charge coupled devices (CCDs) for higher energy resolution and larger format, extension to higher energies, liquid and solid position sensitive detectors for improving stopping power in the energy range 5 to 500 keV and 0.2 to 2 MeV. Development plans designed to achieve the desired capabilities on the time scales required by the technology freeze dates have been recommended in each of these areas
Coupling hydrophobic, dispersion, and electrostatic contributions in continuum solvent models
Recent studies of the hydration of micro- and nanoscale solutes have
demonstrated a strong {\it coupling} between hydrophobic, dispersion and
electrostatic contributions, a fact not accounted for in current implicit
solvent models. We present a theoretical formalism which accounts for coupling
by minimizing the Gibbs free energy with respect to a solvent volume exclusion
function. The solvent accessible surface is output of our theory. Our method is
illustrated with the hydration of alkane-assembled solutes on different length
scales, and captures the strong sensitivity to the particular form of the
solute-solvent interactions in agreement with recent computer simulations.Comment: 11 pages, 2 figure
Vibration Isolation Design for the Micro-X Rocket Payload
Micro-X is a NASA-funded, sounding rocket-borne X-ray imaging spectrometer
that will allow high precision measurements of velocity structure, ionization
state and elemental composition of extended astrophysical systems. One of the
biggest challenges in payload design is to maintain the temperature of the
detectors during launch. There are several vibration damping stages to prevent
energy transmission from the rocket skin to the detector stage, which causes
heating during launch. Each stage should be more rigid than the outer stages to
achieve vibrational isolation. We describe a major design effort to tune the
resonance frequencies of these vibration isolation stages to reduce heating
problems prior to the projected launch in the summer of 2014.Comment: 6 pages, 7 figures, LTD15 Conference Proceeding
Application of the level-set method to the implicit solvation of nonpolar molecules
A level-set method is developed for numerically capturing the equilibrium
solute-solvent interface that is defined by the recently proposed variational
implicit solvent model (Dzubiella, Swanson, and McCammon, Phys. Rev. Lett. {\bf
104}, 527 (2006) and J. Chem.\Phys. {\bf 124}, 084905 (2006)). In the level-set
method, a possible solute-solvent interface is represented by the zero
level-set (i.e., the zero level surface) of a level-set function and is
eventually evolved into the equilibrium solute-solvent interface. The evolution
law is determined by minimization of a solvation free energy {\it functional}
that couples both the interfacial energy and the van der Waals type
solute-solvent interaction energy. The surface evolution is thus an energy
minimizing process, and the equilibrium solute-solvent interface is an output
of this process. The method is implemented and applied to the solvation of
nonpolar molecules such as two xenon atoms, two parallel paraffin plates,
helical alkane chains, and a single fullerene . The level-set solutions
show good agreement for the solvation energies when compared to available
molecular dynamics simulations. In particular, the method captures solvent
dewetting (nanobubble formation) and quantitatively describes the interaction
in the strongly hydrophobic plate system
Making Good Lawyers
Today, the criticism of law schools has become an industry. Detractors argue that legal education fails to effectively prepare students for the practice of law, that it is too theoretical and detached from the profession, that it dehumanizes and alienates students, too expensive and inapt in helping students develop a sense of professional identity, professional values, and professionalism. In this sea of criticisms it is hard to see the forest from the trees. “There is so much wrong with legal education today,” writes one commentator, “that it is hard to know where to begin.” This article argues that any reform agenda will fall short if it does not start by recognizing the dominant influence of the culture of autonomous self-interest in legal education. Law schools engage in a project of professional formation and instill a very particular brand of professional identity. They educate students to become autonomously self-interested lawyers who see their clients and themselves as pursuing self-interest as atomistic actors. As a result, they understand that their primary role is to serve as neutral partisans who promote the narrow self-interest of clients without regard to the interests of their families, neighbors, colleagues, or communities and to the exclusion of counseling clients on the implications of those interests. They view as marginal their roles as an officer of the legal system and as a public citizen and accordingly place a low priority on traditional professional values, such as the commitment to the public good, that conflict with their primary allegiance to autonomous self-interest. In this work of professional formation, law schools are reflecting the values and commitments of the autonomously self-interested culture that is dominant in the legal profession. Therefore, even if law schools sought to form a professional identity outside of the mold of autonomous self-interest, such a commitment would require much more than curricular reform. It would, at minimum, require the construction of a persuasive alternative understanding of the lawyer’s role. The article seeks to offer such an understanding grounded in a relational perspective on lawyers and clients. Part I offers workable definitions of professionalism and professional identity that enable an informed discussion of the formation of professional identity in and by law schools. Part II explores what and how legal education teaches students showing that both institutionally (at the law school level) and individually (at the law professor level) legal education is proactively engaged in the formation of a professional identity of autonomous self-interest. Part II further explains that its dominance in legal education notwithstanding, autonomous self-interest is but one, often unpersuasive, account of professionalism and professional identity. Part III turns to the competing vision of relationally self-interested professionalism and professional identity and develops an outline for legal education grounded in these conceptions. Because legal education reflects a deep commitment to the dominant culture of autonomous self-interest, it is unlikely that reform proposals that are inconsistent with that culture are likely to succeed in the near future. Yet proposing an alternative account of professional identity that exposes the assumptions of the dominant culture, explains their limitations, and develops a more persuasive understanding is a necessary step toward providing a workable framework for reformers committed to promoting professional values in the long term
Substrate concentration dependence of the diffusion-controlled steady-state rate constant
The Smoluchowski approach to diffusion-controlled reactions is generalized to
interacting substrate particles by including the osmotic pressure and
hydrodynamic interactions of the nonideal particles in the Smoluchoswki
equation within a local-density approximation. By solving the strictly
linearized equation for the time-independent case with absorbing boundary
conditions, we present an analytic expression for the diffusion-limited
steady-state rate constant for small substrate concentrations in terms of an
effective second virial coefficient B_2*. Comparisons to Brownian dynamics
simulations excluding HI show excellent agreement up to bulk number densities
of B_2* rho_0 < 0.4 for hard sphere and repulsive Yukawa-like interactions
between the substrates. Our study provides an alternative way to determine the
second virial coefficient of interacting macromolecules experimentally by
measuring their steady-state rate constant in diffusion-controlled reactions at
low densities.Comment: 7 pages, 3 figure
Dewetting-controlled binding of ligands to hydrophobic pockets
We report on a combined atomistic molecular dynamics simulation and implicit
solvent analysis of a generic hydrophobic pocket-ligand (host-guest) system.
The approaching ligand induces complex wetting/dewetting transitions in the
weakly solvated pocket. The transitions lead to bimodal solvent fluctuations
which govern magnitude and range of the pocket-ligand attraction. A recently
developed implicit water model, based on the minimization of a geometric
functional, captures the sensitive aqueous interface response to the
concave-convex pocket-ligand configuration semi-quantitatively
- …