X-ray emission from betatron motion in a plasma wiggler

The successful utilization of an ion channel in a plasma to wiggle a 28.5-GeV electron beam to obtain broadband x-ray radiation is reported. The ion channel is induced by the electron bunch as it propagates through an underdense 1.4-meter-long lithium plasma. The quadratic density dependence of the spontaneously emitted betatron x-ray radiation and the divergence angle of approximately (1-3)x10(-4) radian of the forward-emitted x-rays as a consequence of betatron motion in the ion channel are in good agreement with theory. The absolute photon yield and the peak spectral brightness at 14.2-keV photon energy are estimated.     

Transverse envelope dynamics of a 28.5-GeV electron beam in a long plasma

The transverse dynamics of a 28.5-GeV electron beam propagating in a 1.4 m long, (0-2)x10(14) cm(-3) plasma are studied experimentally in the underdense or blowout regime. The transverse component of the wake field excited by the short electron bunch focuses the bunch, which experiences multiple betatron oscillations as the plasma density is increased. The spot-size variations are observed using optical transition radiation and Cherenkov radiation. In this regime, the behavior of the spot size as a function of the plasma density is well described by a simple beam-envelope model. Dynamic changes of the beam envelope are observed by time resolving the Cherenkov light.     

The 10,000,000,000-Volt question: What is the best choice of proton energy to drive a pulsed spallation neutron source?

In pulsed spallation neutron sources, higher proton beam current or higher proton energy brings higher proton beam power, and from greater beam power come more neutrons. Consequently, accelerator scientists and target systems designers conceive systems to produce ever-higher intensities of neutrons, and the question arises: What is the best choice of proton energy to drive a pulsed spallation neutron source? Meanwhile target engineering, radiation shielding, and neutronic coupling to moderators must be accomplished effectively. This paper addresses the title question from the target side, assessing the neutron yield, the distribution of leakage neutrons, and the power density distribution as functions of proton energy.     

Amyloid-like behavior in abiotic, amphiphilic foldamers

Previously, we reported an abiotic amphiphilic foldamer that, upon heating, undergoes an irreversible conformational change to a highly aggregated state (Nguyen, J.Q.; Iverson, B.L. J. Am. Chem. Soc. 1999, 121, 2639-2640.). Herein, we extend this work through the study of a series of structurally related amphiphilic foldamers and present a more refined model of their conformational switching behavior. Prior to heating, all foldamers of the series exhibited spectral characteristics consistent with folding in the pleated, stacked geometry characteristic of this class of foldamer. Following heating at 80 degrees C, three of the four molecules exhibited irreversible aggregation to produce hydrogels. The hydrogels were characterized by rheology measurements, and circular dichroism spectra revealed that hydrogel formation was dependent on highly ordered intermolecular assembly, conceptually analogous to protein amyloid formation. Hydrogel formation had the effect of amplifying the subtle structural differences between molecules, as the three amphiphilic foldamer constitutional isomers that formed hydrogels upon heating displayed significant differences in hydrogel properties. Taking a global view, our results indicate that amyloid-like behavior is not unique to proteins but may be a relatively general property of amphiphilic folding molecules in aqueous solution.     

Vowel recognition via cochlear implants and noise vocoders: effects of formant movement and duration

Previous work has demonstrated that normal-hearing individuals use fine-grained phonetic variation, such as formant movement and duration, when recognizing English vowels. The present study investigated whether these cues are used by adult postlingually deafened cochlear implant users, and normal-hearing individuals listening to noise-vocoder simulations of cochlear implant processing. In Experiment 1, subjects gave forced-choice identification judgments for recordings of vowels that were signal processed to remove formant movement and/or equate vowel duration. In Experiment 2, a goodness-optimization procedure was used to create perceptual vowel space maps (i.e., best exemplars within a vowel quadrilateral) that included F1, F2, formant movement, and duration. The results demonstrated that both cochlear implant users and normal-hearing individuals use formant movement and duration cues when recognizing English vowels. Moreover, both listener groups used these cues to the same extent, suggesting that postlingually deafened cochlear implant users have category representations for vowels that are similar to those of normal-hearing individuals.     

Peptide bis-intercalator binds DNA via threading mode with sequence specific contacts in the major groove

BACKGROUND: We previously described a general class of DNA polyintercalators in which 1,4,5,8-naphthalenetetracarboxylic diimide (NDI) intercalating units are connected via peptide linkers, resulting in the first known tetrakis- and octakis-intercalators. We showed further that changes in the composition of the peptide tether result in novel DNA binding site specificities. We now examine in detail the DNA binding mode and sequence specific recognition of Compound 1, an NDI bis-intercalator containing the peptide linker gly-gly-gly-lys. RESULTS: 1H-NMR structural studies of Compound 1 bound to d(CGGTACCG)(2) confirmed a threading mode of intercalation, with four base pairs between the diimide units. The NMR data, combined with DNAse I footprinting of several analogs, suggest that specificity depends on a combination of steric and electrostatic contacts by the peptide linker in the floor of the major groove. CONCLUSIONS: In view of the modular nature and facile synthesis of our NDI-based polyintercalators, such structural knowledge can be used to improve or alter the specificity of the compounds and design longer polyintercalators that recognize correspondingly longer DNA sequences with alternating access to both DNA grooves.