24 research outputs found
MHSP in reversed-biased operation mode for ion blocking in gas-avalanche multipliers
We present recent results on the operation of gas-avalanche detectors
comprising a cascade of gas electron multipliers (GEMs) and Micro-Hole and
Strip Plates (MHSPs) multiplier operated in reversed-bias (R-MHSP) mode. The
operation mechanism of the R-MHSP is explained and its potential contribution
to ion-backflow (IBF) reduction is demonstrated. IBF values of 4E-3 were
obtained in cascaded R-MHSP and GEM multipliers at gains of about 1E+4, though
at the expense of reduced effective gain in the first R- MHSP multiplier in the
cascade.Comment: 23 pages, 8 figure
Ion-induced effects in GEM & GEM/MHSP gaseous photomultipliers for the UV and the visible spectral range
We report on the progress in the study of cascaded GEM and GEM/MHSP gas
avalanche photomultipliers operating at atmospheric pressure, with CsI and
bialkali photocathodes. They have single-photon sensitivity, ns time resolution
and good localization properties. We summarize operational aspects and results,
with the highlight of a high-gain stable gated operation of a visible-light
device. Of particular importance are the results of a recent ion-backflow
reduction study in different cascaded multipliers, affecting the detector's
stability and the photocathode's liftime. We report on the significant progress
in ion-blocking and provide first results on bialkali-photocathode aging under
gas multiplication.Comment: 6 pages, 8 figure
A concise review on THGEM detectors
We briefly review the concept and properties of the Thick GEM (THGEM); it is
a robust, high-gain gaseous electron multiplier, manufactured economically by
standard printed-circuit drilling and etching technology. Its operation and
structure resemble that of GEMs but with 5 to 20-fold expanded dimensions. The
millimeter-scale hole-size results in good electron transport and in large
avalanche-multiplication factors, e.g. reaching 10^7 in double-THGEM cascaded
single-photoelectron detectors. The multiplier's material, parameters and shape
can be application-tailored; it can operate practically in any counting gas,
including noble gases, over a pressure range spanning from 1 mbar to several
bars; its operation at cryogenic (LAr) conditions was recently demonstrated.
The high gain, sub-millimeter spatial resolution, high counting-rate
capability, good timing properties and the possibility of industrial production
capability of large-area robust detectors, pave ways towards a broad spectrum
of potential applications; some are discussed here in brief.Comment: 8 pages, 11 figures; Invited Review at INSTR08, Novosibirsk, Feb
28-March 5 200
The accelerating universe in brane-world cosmology
The standard Friedmann universe embedded in a five dimensional and constant
curvature bulk is examined without any a priori junction condition between the
brane and the bulk. A geometrical explanation for the accelerated expansion of
the universe is derived by using a minimum set of assumptions consistent with
the brane-world program. It is shown that the extrinsic curvature of the brane
can be associated to the dark energy which presumably drives the universe
expansion.Comment: Revtex, 4 page
High-gain DC-mode operated Gaseous Photomultipliers for the visible spectral range
We shortly describe recent progress in photon detectors combining bi-alkali
photocathodes and cascaded patterned gas-avalanche electron multipliers. It
permitted the development and the first feasibility demonstration of high-gain
gaseous photomultipliers sensitive in the visible spectral range, operated in
DC mode with single-photon sensitivity.Comment: Proceedings to the 5th International Conference on New Developments
In Photodetection 2008, Aix-les-Bains, France, June 15-20, 2008, submitted to
NIM
(An)Isotropic models in scalar and scalar-tensor cosmologies
We study how the constants and may vary in different
theoretical models (general relativity with a perfect fluid, scalar
cosmological models (\textquotedblleft quintessence\textquotedblright) with and
without interacting scalar and matter fields and a scalar-tensor model with a
dynamical ) in order to explain some observational results. We apply
the program outlined in section II to study three different geometries which
generalize the FRW ones, which are Bianchi \textrm{V}, \textrm{VII} and
\textrm{IX}, under the self-similarity hypothesis. We put special emphasis on
calculating exact power-law solutions which allow us to compare the different
models. In all the studied cases we arrive to the conclusion that the solutions
are isotropic and noninflationary while the cosmological constant behaves as a
positive decreasing time function (in agreement with the current observations)
and the gravitational constant behaves as a growing time function
Scalar perturbation spectra from warm inflation
We present a numerical integration of the cosmological scalar perturbation
equations in warm inflation. The initial conditions are provided by a
discussion of the thermal fluctuations of an inflaton field and thermal
radiation using a combination of thermal field theory and thermodynamics. The
perturbation equations include the effects of a damping coefficient
and a thermodynamic potential . We give an analytic expression for the
spectral index of scalar fluctuations in terms of a new slow-roll parameter
constructed from . A series of toy models, inspired by spontaneous
symmetry breaking and a known form of the damping coefficient, lead to a
spectrum with on large scales and on small scales.Comment: 12 pages, 5 figures, RevTeX 4, revised with extra figure
Captura de enxames de abelhas sem ferrão (Hymenoptera, Apidae, Meliponinae) sem destruição de árvores
Some Observational Consequences of Brane World Cosmologies
The presence of dark energy in the Universe is inferred directly and
indirectly from a large body of observational evidence. The simplest and most
theoretically appealing possibility is the vacuum energy density (cosmological
constant). However, although in agreement with current observations, such a
possibility exacerbates the well known cosmological constant problem, requiring
a natural explanation for its small, but nonzero, value. In this paper we focus
our attention on another dark energy candidate, one arising from gravitational
\emph{leakage} into extra dimensions. We investigate observational constraints
from current measurements of angular size of high- compact radio-sources on
accelerated models based on this large scale modification of gravity. The
predicted age of the Universe in the context of these models is briefly
discussed. We argue that future observations will enable a more accurate test
of these cosmologies and, possibly, show that such models constitute a viable
possibility for the dark energy problem.Comment: 6 pages, 4 figures, to appear in Phys. Rev. D (minor revisions
Bianchi {VI} in Scalar and Scalar-Tensor Cosmologies
We study several cosmological models with Bianchi \textrm{VI}
symmetries under the self-similar approach. In order to study how the
\textquotedblleft constants\textquotedblright\ and may vary, we
propose three scenarios where such constants are considered as time functions.
The first model is a perfect fluid. We find that the behavior of and
are related. If behaves as a growing time function then
is a positive decreasing time function but if is decreasing then
is negative. For this model we have found a new solution. The second model is a
scalar field, where in a phenomenological way, we consider a modification of
the Klein-Gordon equation in order to take into account the variation of .
Our third scenario is a scalar-tensor model. We find three solutions for this
models where is growing, constant or decreasing and is a positive
decreasing function or vanishes. We put special emphasis on calculating the
curvature invariants in order to see if the solutions isotropize.Comment: Typos corrected. References added, minor corrections. arXiv admin
note: text overlap with arXiv:0905.247