5,543 research outputs found
Self-vapor cooled targets for production of I-123 at high current accelerators
The basic elements of the vapor cooled target system are shown. This system can be operated as a heat pipe or as a conventional condenser. The choice of target fluid is based on the specific nuclear reaction chosen to produce Xe-123. The reaction using I-127 was studied and shown to have a significant yield for bombarding energies from 47 to 63 MeV. The Cs-133 reaction is also included. Xenon-123 is applied to I-123 production in a purer form for thyroid studies
Frictional sliding without geometrical reflection symmetry
The dynamics of frictional interfaces play an important role in many physical
systems spanning a broad range of scales. It is well-known that frictional
interfaces separating two dissimilar materials couple interfacial slip and
normal stress variations, a coupling that has major implications on their
stability, failure mechanism and rupture directionality. In contrast,
interfaces separating identical materials are traditionally assumed not to
feature such a coupling due to symmetry considerations. We show, combining
theory and experiments, that interfaces which separate bodies made of
macroscopically identical materials, but lack geometrical reflection symmetry,
generically feature such a coupling. We discuss two applications of this novel
feature. First, we show that it accounts for a distinct, and previously
unexplained, experimentally observed weakening effect in frictional cracks.
Second, we demonstrate that it can destabilize frictional sliding which is
otherwise stable. The emerging framework is expected to find applications in a
broad range of systems.Comment: 14 pages, 5 figures + Supplementary Material. Minor change in the
title, extended analysis in the second par
Twisted mass fermions: neutral pion masses from disconnected contributions
Twisted mass fermions allow light quarks to be explored but with the
consequence that there are mass splittings, such as between the neutral and
charged pion. Using a direct calculation of the connected neutral pion
correlator and stochastic methods to evaluate the disconnected correlations, we
determine the neutral pion mass. We explore the dependence on lattice spacing
and quark mass in quenched QCD. For dynamical QCD, we determine the sign of the
splitting which is linked, via chiral PT, to the nature of the phase transition
at small quark mass.Comment: 6 pages, poster (hadron spectrum and quark masses) at Lattice
2005,Dublin, July 25-3
The large-scale disk fraction of brown dwarfs in the Taurus cloud as measured with Spitzer
Aims. The brown dwarf (BD) formation process has not yet been completely
understood. To shed more light on the differences and similarities between star
and BD formation processes, we study and compare the disk fraction among both
kinds of objects over a large angular region in the Taurus cloud. In addition,
we examine the spatial distribution of stars and BD relative to the underlying
molecular gas Methods. In this paper, we present new and updated photometry
data from the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope
on 43 BDs in the Taurus cloud, and recalculate of the BD disk fraction in this
region. We also useed recently available CO mm data to study the spatial
distribution of stars and BDs relative to the cloud's molecular gas. Results.
We find that the disk fraction among BDs in the Taurus cloud is 41 \pm 12%, a
value statistically consistent with the one among TTS (58 \pm 9%). We find that
BDs in transition from a state where they have a disk to a diskless state are
rare, and we study one isolated example of a transitional disk with an inner
radius of \approx 0.1 AU (CFHT BD Tau 12, found via its relatively small mid-IR
excess compared to most members of Taurus that have disks. We find that BDs are
statistically found in regions of similar molecular gas surface density to
those associated with stars. Furthermore, we find that the gas column density
distribution is almost identical for stellar and substellar objects with and
without disks.Comment: 8 page, 6 figures, Accepted in Astronomy & Astrophysics
Power Laws, Precursors and Predictability During Failure
We investigate the dynamics of a modified Burridge-Knopoff model by
introducing a dissipative term to mimic the bursts of acoustic emission (AE)
from rock samples. The model explains many features of the statistics of AE
signals observed in experiments such as the crossover in the exponent value
from relatively small amplitude AE signals to larger regime, and their
dependence on the pulling speed. Significantly, we find that the cumulative
energy dissipated identified with acoustic emission can be used to predict a
major slip event. We also find a data collapse of the acoustic activity for
several major slip events describable by a universal stretched exponential with
corrections in terms of time-to-failure.Comment: 7 pages, 6 figures, Final version with minor change
Non-locality of non-Abelian anyons
Topological systems, such as fractional quantum Hall liquids, promise to
successfully combat environmental decoherence while performing quantum
computation. These highly correlated systems can support non-Abelian anyonic
quasiparticles that can encode exotic entangled states. To reveal the non-local
character of these encoded states we demonstrate the violation of suitable Bell
inequalities. We provide an explicit recipe for the preparation, manipulation
and measurement of the desired correlations for a large class of topological
models. This proposal gives an operational measure of non-locality for anyonic
states and it opens up the possibility to violate the Bell inequalities in
quantum Hall liquids or spin lattices.Comment: 7 pages, 3 figure
GI2T/REGAIN spectro-interferometry with a new infrared beam combiner
We have built an infrared beam combiner for the GI2T/REGAIN interferometer of
the Observatoire de la Cote d'Azur. The beam combiner allows us to record
spectrally dispersed Michelson interference fringes in the near-infrared J-, H-
or K-bands. The beam combiner has the advantage that Michelson interferograms
can simultaneously be recorded in about 128 different spectral channels. The
tilt of the spectrally dispersed fringes is a measure of the instantaneous
optical path difference. We present the optical design of the beam combiner and
GI2T/REGAIN observations of the Mira star R Cas with this beam combiner in the
spectral range of 2.00 micron - 2.18 micron (observations on 22 and 25 August
1999; variability phase 0.08; V-magnitude approx. 6; seven baselines between
12m and 24m; reference stars Vega and Beta Peg). The spectrograph of the beam
combiner consists of an anamorphotic cylindrical lens system, an image plane
slit, and a grism. A system of digital signal processors calculates the
ensemble average power spectrum of the spectrally dispersed Michelson
interferograms and the instantaneous optical path difference error in real
time. From the observed R Cas visibilities at baselines 12.0m, 13.8m and 13.9m,
a 2.1 micron uniform-disk diameter of 25.3mas +/-3.3mas was derived. The
unusually high visibility values at baselines >16m show that the stellar
surface of R Cas is more complex than previously assumed. The visibility values
at baselines >16m can be explained by high-contrast surface structure on the
stellar surface of R Cas or other types of unexpected center-to-limb
variations. The R Cas observations were compared with theoretical Mira star
models yielding a linear Rosseland radius of 276Rsun +/-66Rsun and an effective
temperature of 2685K+/-238K for R Cas at phase 0.08.Comment: 10 pages, 6 figures, see also
http://www.mpifr-bonn.mpg.de/div/speckle, SPIE conf 4006 "Interferometry in
Optical Astronomy", in pres
Cross-section measurement of the Ba 130 (p,γ) La 131 reaction for γ -process nucleosynthesis
Background: Deviations between experimental data of charged-particle-induced reactions and calculations within the statistical model are frequently found. An extended data base is needed to address the uncertainties regarding the nuclear-physics input parameters in order to understand the nucleosynthesis of the neutron-deficient p nuclei. Purpose: A measurement of total cross-section values of the Ba130(p,γ)La131 reaction at low proton energies allows a stringent test of statistical model predictions with different proton+nucleus optical model potentials. Since no experimental data are available for proton-capture reactions in this mass region around A ≈130, this measurement can be an important input to test the global applicability of proton+nucleus optical model potentials. Method: The total reaction cross-section values were measured by means of the activation method. After the irradiation with protons, the reaction yield was determined by use of γ-ray spectroscopy using two clover-type high-purity germanium detectors. In total, cross-section values for eight different proton energies could be determined in the energy range between 3.6 MeV ≤Ep≤ 5.0 MeV, thus, inside the astrophysically relevant energy region. Results: The measured cross-section values were compared to Hauser-Feshbach calculations using the statistical model codes TALYS and SMARAGD with different proton+nucleus optical model potentials. With the semimicroscopic JLM proton+nucleus optical model potential used in the SMARAGD code, the absolute cross-section values are reproduced well, but the energy dependence is too steep at the lowest energies. The best description is given by a TALYS calculation using the semimicroscopic Bauge proton+nucleus optical model potential using a constant renormalization factor. Conclusions: The statistical model calculation using the Bauge semimicroscopic proton+nucleus optical model potential deviates by a constant factor of 2.1 from the experimental data. Using this model, an experimentally supported stellar reaction rate for proton capture on the p nucleus Ba130 was calculated. At astrophysical temperatures, an increase in the stellar reaction rate of 68% compared to rates obtained from the widely used NON-SMOKER code is found. This measurement extends the scarce experimental data base for charged-particle-induced reactions, which can be helpful to derive a more globally applicable proton+nucleus optical model potential.Peer reviewedFinal Accepted Versio
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