2,622 research outputs found
Novel Approaches towards Highly Selective Self-Powered Gas Sensors
The prevailing design approaches of semiconductor gas sensors struggle to overcome most of their current limitations such as poor selectivity, and high power consumption. Herein, a new sensing concept based on devices that are capable of detecting gases without the need of any external power sources required to activate interaction of gases with sensor or to generate the sensor read out signal. Based on the integration of complementary functionalities (namely; powering and sensing) in a singular nanostructure, self-sustained gas sensors will be demonstrated. Moreover, a rational methodology to design organic surface functionalization that provide high selectivity towards single gas species will also be discussed. Specifically, theoretical results, confirmed experimentally, indicate that precisely tuning of the sterical and electronic structure of sensor material/organic interfaces can lead to unprecedented selectivity values, comparable to those typical of bioselective processes. Finally, an integrated gas sensor that combine both the self-powering and selective detection strategies in one single device will also be presented. © 2015 Published by Elsevier Ltd.Peer ReviewedPostprint (published version
Characteristics of the Multi-Telescope Coincidence Trigger of the HEGRA IACT System
The HEGRA--collaboration is operating a system of imaging atmospheric
Cherenkov telescopes to search for sources of TeV-gamma-rays. Air showers are
observed in stereoscopic mode with several telescopes simultaneously. To
trigger the telescope system a versatile two-level trigger scheme has been
implemented, which allows a significant reduction of the energy threshold with
respect to single telescopes. The technical implementation of this trigger
scheme and the performance of the trigger system are described. Results include
the dependence of single- and multi-telescope trigger rates on the trigger
thresholds, on the orientation of the telescopes, and on the type of the
primary particle.Comment: 17 Pages, 10 figures, Late
Comparison of ultracold neutron sources for fundamental physics measurements
Ultracold neutrons (UCNs) are key for precision studies of fundamental
parameters of the neutron and in searches for new CP violating processes or
exotic interactions beyond the Standard Model of particle physics. The most
prominent example is the search for a permanent electric dipole moment of the
neutron (nEDM). We have performed an experimental comparison of the leading UCN
sources currently operating. We have used a 'standard' UCN storage bottle with
a volume of 32 liters, comparable in size to nEDM experiments, which allows us
to compare the UCN density available at a given beam port.Comment: 20 pages, 30 Figure
Solid deuterium surface degradation at ultracold neutron sources
Solid deuterium (sD_2) is used as an efficient converter to produce ultracold
neutrons (UCN). It is known that the sD_2 must be sufficiently cold, of high
purity and mostly in its ortho-state in order to guarantee long lifetimes of
UCN in the solid from which they are extracted into vacuum. Also the UCN
transparency of the bulk sD_2 material must be high because crystal
inhomogeneities limit the mean free path for elastic scattering and reduce the
extraction efficiency. Observations at the UCN sources at Paul Scherrer
Institute and at Los Alamos National Laboratory consistently show a decrease of
the UCN yield with time of operation after initial preparation or later
treatment (`conditioning') of the sD_2. We show that, in addition to the
quality of the bulk sD_2, the quality of its surface is essential. Our
observations and simulations support the view that the surface is deteriorating
due to a build-up of D_2 frost-layers under pulsed operation which leads to
strong albedo reflections of UCN and subsequent loss. We report results of UCN
yield measurements, temperature and pressure behavior of deuterium during
source operation and conditioning, and UCN transport simulations. This,
together with optical observations of sD_2 frost formation on initially
transparent sD_2 in offline studies with pulsed heat input at the North
Carolina State University UCN source results in a consistent description of the
UCN yield decrease.Comment: 15 pages, 22 figures, accepted by EPJ-
The KARMEN Time Anomaly: Search for a Neutral Particle of Mass 33.9 MeV in Pion Decay
We have searched for the pion decay pi^+ --> mu^+ X, where X is a neutral
particle of mass 33.905 MeV. This process was suggested by the KARMEN
Collaboration to explain an anomaly in their observed time distribution of
neutrino induced reactions. Having measured the muon momentum spectrum of
charged pions decaying in flight, we find no evidence for this process and
place an upper limit on the branching fraction eta leq 6.0 * 10^{-10} of such a
decay at a 95% confidence level.Comment: 17 pages including 4 for figure
Three-flavor atmospheric neutrino anomaly
We investigate the indications of flavor oscillations that come from the
anomalous flavor composition of the atmospheric neutrino flux observed in some
underground experiments. We study the information coming from the
neutrino-induced -like and -like events both in the sub-GeV energy
range (Kamiokande, IMB, Fr{\'e}jus, and NUSEX experiments) and in the multi-GeV
energy range (Kamiokande experiment). First we analyze all the data in the
limits of pure and
oscillations. We obtain that
oscillations provide a better fit, in particular
to the multi-GeV data. Then we perform a three-flavor analysis in the
hypothesis of dominance of one neutrino square mass difference, , implying
that the neutrino mixing is parametrized by two angles,
. We explore the space
exhaustively, and find the regions favored by the oscillation hypothesis. The
results are displayed in a form suited to the comparison with other flavor
oscillation searches at accelerator, reactor, and solar experiments. In
the analysis, we pay particular attention to the earth matter effects, to the
correlation of the uncertainties, and to the symmetry properties of the
oscillation probability.Comment: 25 pages (RevTeX) + 12 figures, requires epsfig.sty. All the figures
are bitmapped. Postscript figures with full resolution are available at
ftp://ftp.sns.ias.edu/pub/lisi/atmpaper
Beyond Einstein-Cartan gravity: Quadratic torsion and curvature invariants with even and odd parity including all boundary terms
Recently, gravitational gauge theories with torsion have been discussed by an
increasing number of authors from a classical as well as from a quantum field
theoretical point of view. The Einstein-Cartan(-Sciama-Kibble) Lagrangian has
been enriched by the parity odd pseudoscalar curvature (Hojman, Mukku, and
Sayed) and by torsion square and curvature square pieces, likewise of even and
odd parity. (i) We show that the inverse of the so-called Barbero-Immirzi
parameter multiplying the pseudoscalar curvature, because of the topological
Nieh-Yan form, can only be appropriately discussed if torsion square pieces are
included. (ii) The quadratic gauge Lagrangian with both parities, proposed by
Obukhov et al. and Baekler et al., emerges also in the framework of Diakonov et
al.(2011). We establish the exact relations between both approaches by applying
the topological Euler and Pontryagin forms in a Riemann-Cartan space expressed
for the first time in terms of irreducible pieces of the curvature tensor.
(iii) Only in a Riemann-Cartan spacetime, that is, in a spacetime with torsion,
parity violating terms can be brought into the gravitational Lagrangian in a
straightforward and natural way. Accordingly, Riemann-Cartan spacetime is a
natural habitat for chiral fermionic matter fields.Comment: 12 page latex, as version 2 an old file was submitted by mistake,
this is now the real corrected fil
Renormalization Group Flow in Scalar-Tensor Theories. II
We study the UV behaviour of actions including integer powers of scalar
curvature and even powers of scalar fields with Functional Renormalization
Group techniques. We find UV fixed points where the gravitational couplings
have non-trivial values while the matter ones are Gaussian. We prove several
properties of the linearized flow at such a fixed point in arbitrary dimensions
in the one-loop approximation and find recursive relations among the critical
exponents. We illustrate these results in explicit calculations in for
actions including up to four powers of scalar curvature and two powers of the
scalar field. In this setting we notice that the same recursive properties
among the critical exponents, which were proven at one-loop order, still hold,
in such a way that the UV critical surface is found to be five dimensional. We
then search for the same type of fixed point in a scalar theory with minimal
coupling to gravity in including up to eight powers of scalar curvature.
Assuming that the recursive properties of the critical exponents still hold,
one would conclude that the UV critical surface of these theories is five
dimensional.Comment: 14 pages. v.2: Minor changes, some references adde
S-layers at second glance? Altiarchaeal grappling hooks (hami) resemble archaeal S-layer proteins in structure and sequence
This is the final version of the article. Available from Frontiers Media via the DOI in this recordResearch on SM1-MSI was supported by the German Research Foundation (Deutsche Forschungsgemeinschaft), grant no. MO 1977/3-1 given to CM-E. AJP was supported by the German National Academic Foundation (Studienstiftung des deutschen Volkes)
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