Shifting of the electron-capture-to-the-continuum peak in proton-helium collisions at 10 and 20 keV

A refined theoretical approach has been developed to study the double-differential cross sections (DDCS's) in proton-helium collisions as a function of the ratio of ionized electron velocity to the incident proton velocity. The refinement is done in the present coupled-channel calculation by introducing a continuum distorted wave in the final state coupled with discrete states including direct as well as charge transfer channels. It is confirmed that the electron-capture-to-the-continuum (ECC) peak is slightly shifted to a lower electron velocity than the equivelocity position. Comparing measurements and classical trajectory Monte Carlo (CTMC) calculations at 10 and 20 keV proton energies, excellent agreement of the ECC peak heights is achieved at both energies. However, a minor disagreement in the peak positions between the present calculation and the CTMC results is noted. A smooth behavior of the DDCS is found in the present calculation on both sides of the peak whereas the CTMC results show some oscillatory behavior particularly to the left of the peak, associated with the statistical nature of CTMC calculations.

A dielectric continuum molecular dynamics method

We introduce a novel method to simulate hydrated macromolecules with a dielectric continuum representation of the surrounding solvent. In our approach, the interaction between the solvent and the molecular degrees of freedom is described by means of a polarization density free energy functional which is minimum at electrostatic equilibrium. After a pseudospectral expansion of the polarization and a discretization of the functional, we construct the equations of motion for the system based on a Car-Parrinello technique. In the limit of the adiabatic evolution of the polarization field variables, our method provides the solution of the dielectric continuum problem "on the fly," while the molecular coordinates are propagated. In this first study, we show how our dielectric continuum molecular dynamics method can be successfully applied to hydrated biomolecules, with low cost compared to free energy simulations with explicit solvent. To our knowledge, this is the first time that stable and conservative molecular dynamic simulations of solutes can be performed for a dielectric continuum model of the solvent. (C) 2001 American Institute of Physics.

Damage to plasmid DNA induced by low energy carbon ions

The damage induced in supercoiled plasmid DNA molecules by 1-6 keV carbon ions has been investigated as a function of ion exposure, energy and charge state. The production of short linear fragments through multiple double strand breaks has been demonstrated and exponential exposure responses for each of the topoisomers have been found. The cross section for the loss of supercoiling was calculated to be (2.2 +/- 0.5) x 10(-14) cm(2) for 2 keVC(+) ions. For singly charged carbon ions, increased damage was observed with increasing ion energy. In the case of 2 keV doubly charged ions, the damage was greater than for singly charged ions of the same energy. These observations demonstrate that ion induced damage is a function of both the kinetic and potential energies of the ion.

Creation of damage-free ferroelectric nanocapacitors via focused ion beam milling

We present a novel method for creating damage-free ferroelectric nanostructures with a focused ion beam milling machine. Using a standard e-beam photoresist followed by a dilute acid wash, nanostructures ranging in size from 1 mu m down to 250 nm were created in a 90 nm thick lead zirconate titanate ( PZT) wafer. Transmission electron microscopy and piezoresponse force microscopy ( PFM) confirmed that the surfaces of the nanostructures remained damage free during fabrication, and showed no gallium implantation, and that there was no degradation of ferroelectric properties. In fact DC strain loops, obtained using PFM, demonstrated that the nanostructures have a higher piezoresponse than unmilled films. As the samples did not have any top hard mask, the method presented is unique as it allows for imaging of the top surface to understand edge effects in well-defined nanostructures. In addition, as no post-mill annealing was necessary, it facilitates investigation of nanoscale domain mechanisms without process-induced artefacts.

Sandstone response to salt weathering following simulated fire damage: a comparison of furnace heating and fire.

Fire has long been recognized as an agent of rock weathering. Our understanding of the impact of fire on stone comes either from early anecdotal evidence, or from more recent laboratory simulation studies, using furnaces to simulate the effects of fire. This paper suggests that knowledge derived from simulated heating experiments is based on the preconceptions of the experiment designer – when using a furnace to simulate fire, the operator decides on the maximum temperature and the duration of the experiment. These are key factors in determining the response of the stone to fire, and if these are removed from realworld observations then knowledge based on these simulations must be questioned. To explore the differences between heating sandstone in a furnace and a real fire, sample blocks of Peakmoor Sandstone were subjected to different stress histories in combination (lime rendering and removal, furnace heating or fire, frost and salt weathering). Block response to furnace heating and fire is discussed, with emphasis placed on the non-uniformity of the fire and of block response to fire in contrast to the uniform response to surface heating in a furnace. Subsequent response to salt weathering (by a 10% solution of sodium chloride and magnesium sulphate) was then monitored by weight loss. Blocks that had experienced fire showed a more unpredictable response to salt weathering than those that had undergone furnace heating – spalling of corners and rapid catastrophic weight loss were evidenced in blocks that had been subjected to fire, after periods of relative quiescence. An important physical side-effect of the fire was soot accumulation, which created a waxy, relatively impermeable layer on some blocks. This layer repelled water and hindered salt ingress, but eventually detached when salt, able to enter the substrate through more permeable areas, concentrated and crystallized behind it, resulting in rapid weight loss and accelerated decay. Copyright ©2007 John Wiley & Sons, Ltd.

Chk1 Suppresses a Caspase-2 Apoptotic Response to DNA Damage that Bypasses p53, Bcl-2, and Caspase-3

Evasion of DNA damage-induced cell death, via mutation of the p53 tumor suppressor or overexpression of prosurvival Bcl-2 family proteins, is a key step toward malignant transformation and therapeutic resistance. We report that depletion or acute inhibition of checkpoint kinase 1 (Chk1) is sufficient to restore ?-radiation-induced apoptosis in p53 mutant zebrafish embryos. Surprisingly, caspase-3 is not activated prior to DNA fragmentation, in contrast to classical intrinsic or extrinsic apoptosis. Rather, an alternative apoptotic program is engaged that cell autonomously requires atm (ataxia telangiectasia mutated), atr (ATM and Rad3-related) and caspase-2, and is not affected by p53 loss or overexpression of bcl-2/xl. Similarly, Chk1 inhibitor-treated human tumor cells hyperactivate ATM, ATR, and caspase-2 after ?-radiation and trigger a caspase-2-dependent apoptotic program that bypasses p53 deficiency and excess Bcl-2. The evolutionarily conserved "Chk1-suppressed" pathway defines a novel apoptotic process, whose responsiveness to Chk1 inhibitors and insensitivity to p53 and BCL2 alterations have important implications for cancer therapy. © 2008 Elsevier Inc. All rights reserved.