10,119 research outputs found
Substrate Specificity of Peptide Adsorption: A Model Study
Applying the contact density chain-growth algorithm to lattice
heteropolymers, we identify the conformational transitions of a nongrafted
hydrophobic-polar heteropolymer with 103 residues in the vicinity of a polar, a
hydrophobic, and a uniformly attractive substrate. Introducing only two system
parameters, the numbers of surface contacts and intrinsic hydrophobic contacts,
respectively, we obtain surprisingly complex temperature and solvent dependent,
substrate-specific pseudo-phase diagrams.Comment: 5 pages, 2 figure
Study of ion feedback in multi-GEM structures
We study the feedback of positive ions in triple and quadruple Gas Electron
Multiplier (GEM) detectors. The effects of GEM hole diameter, detector gain,
applied voltages, number of GEMs and other parameters on ion feedback are
investigated in detail. In particular, it was found that the ion feedback is
independent of the gas mixture and the pressure. In the optimized multi-GEM
structure, the ion feedback current can be suppressed down to 0.5% of the anode
current, at a drift field of 0.1 kV/cm and gain of 10^4. A simple model of ion
feedback in multi-GEM structures is suggested. The results obtained are
relevant to the performance of time projection chambers and gas
photomultipliers.Comment: 9 pages, 11 figures. Submitted to Nucl. Instr. and Meth.
Development of a triple GEM UV-photon detector operated in pure CF4 for the PHENIX experiment
Results obtained with a triple GEM detector operated in pure CF4 with and
without a reflective CsI photocathode are presented. The detector operates in a
stable mode at gains up to 10^4. A deviation from exponential growth starts to
develop when the total charge exceeds ~ 4 10^6 e leading to gain saturation
when the total charge is ~ 2 10^7 e and making the structure relatively robust
against discharges. No aging effects are observed in the GEM foils after a
total accumulated charge of ~ 10 mC/cm^2 at the anode. The ion back-flow
current to the reflective photocathode is comparable to the electron current to
the anode. However, no significant degradation of the CsI photocathode is
observed for a total ion back-flow charge of ~ 7 mC/cm^2.Comment: 14 pages, 11 figures, Submitted to NIM
Mid-infrared sub-wavelength grating mirror design: tolerance and influence of technological constraints
High polarization selective Si/SiO2 mid-infrared sub-wavelength grating
mirrors with large bandwidth adapted to VCSEL integration are compared. These
mirrors have been automatically designed for operation at \lambda = 2.3 m
by an optimization algorithm which maximizes a specially defined quality
factor. Several technological constraints in relation with the grating
manufacturing process have been imposed within the optimization algorithm and
their impact on the optical properties of the mirror have been evaluated.
Furthermore, through the tolerance computation of the different dimensions of
the structure, the robustness with respect to fabrication errors has been
tested. Finally, it appears that the increase of the optical performances of
the mirror imposes a less tolerant design with severer technological
constraints resulting in a more stringent control of the manufacturing process.Comment: The final publication is available at
http://iopscience.iop.org/2040-8986/13/12/125502
Light multi-GEM detector for high-resolution tracking systems
Controlled etching of copper electrodes in Gas Electron Multiplier foils
allows a reduction of the material budget by more than a factor of two for a
triple-GEM detector. Detectors making use of thinned foils provide performances
similar to those obtained with standard devices: a gain above 10^4 for a
double-GEM, with energy resolution of 27 % fwhm for 5.9 keV X-rays.Comment: Submitted to Nucl.Instr.& Meth.
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