96 research outputs found
Finite element optimizations for efficient non-linear electrical tomography reconstruction
Electrical Tomography can produce accurate results only if the underlying 2D or 3D volume discretization is chosen suitably for the applied numerical algorithm. We give general indications where and how to optimize a finite element discretization of a volume under investigation to enable efficient computation of potential distributions and the reconstruction of materials. For this, we present an error estimator and material-gradient indicator as a driver for adaptive mesh refinement and show how finite element mesh properties affect the efficiency and accuracy of the solutions
Relative entropy in 2d Quantum Field Theory, finite-size corrections and irreversibility of the Renormalization Group
The relative entropy in two-dimensional Field Theory is studied for its
application as an irreversible quantity under the Renormalization Group,
relying on a general monotonicity theorem for that quantity previously
established. In the cylinder geometry, interpreted as finite-temperature field
theory, one can define from the relative entropy a monotonic quantity similar
to Zamolodchikov's c function. On the other hand, the one-dimensional quantum
thermodynamic entropy also leads to a monotonic quantity, with different
properties. The relation of thermodynamic quantities with the complex
components of the stress tensor is also established and hence the entropic c
theorems are proposed as analogues of Zamolodchikov's c theorem for the
cylinder geometry.Comment: 5 pages, Latex file, revtex, reorganized to best show the generality
of the results, version to appear in Phys. Rev. Let
Epsilon Expansion for Multicritical Fixed Points and Exact Renormalisation Group Equations
The Polchinski version of the exact renormalisation group equations is
applied to multicritical fixed points, which are present for dimensions between
two and four, for scalar theories using both the local potential approximation
and its extension, the derivative expansion. The results are compared with the
epsilon expansion by showing that the non linear differential equations may be
linearised at each multicritical point and the epsilon expansion treated as a
perturbative expansion. The results for critical exponents are compared with
corresponding epsilon expansion results from standard perturbation theory. The
results provide a test for the validity of the local potential approximation
and also the derivative expansion. An alternative truncation of the exact RG
equation leads to equations which are similar to those found in the derivative
expansion but which gives correct results for critical exponents to order
and also for the field anomalous dimension to order . An
exact marginal operator for the full RG equations is also constructed.Comment: 40 pages, 12 figures version2: small corrections, extra references,
final appendix rewritten, version3: some corrections to perturbative
calculation
Four-Dimensional Computational Ultrasound Imaging of Brain Haemodynamics
Four-dimensional ultrasound imaging of complex biological systems such as the
brain is technically challenging because of the spatiotemporal sampling
requirements. We present computational ultrasound imaging (cUSi), a new imaging
method that uses complex ultrasound fields that can be generated with simple
hardware and a physical wave prediction model to alleviate the sampling
constraints. cUSi allows for high-resolution four-dimensional imaging of brain
haemodynamics in awake and anesthetized mice
Measurement of the scintillation resolution in liquid xenon and its impact for future segmented calorimeters
We report on a new measurement of the energy resolution that can be attained
in liquid xenon when recording only the scintillation light. Our setup is
optimised to maximise light collection, and uses state-of-the-art, high-PDE,
VUV-sensitive silicon photomultipliers. We find a value of 2.7% +- 0.3% FWHM at
511 keV, a result much better than previous measurements and very close to the
Poissonian resolution that we expect in our setup (3.0% +- 0.7% FWHM at 511
keV). Our results are compatible with a null value of the intrinsic energy
resolution in xenon, with an upper bound of 1.5% FWHM at 95% CL at 511 keV, to
be compared with 3--4% FWHM in the same region found by theoretical estimations
which have been standing for the last twenty years. Our work opens new
possibilities for apparatus based on liquid xenon and using scintillation only.
In particular it suggests that modular scintillation detectors using liquid
xenon can be very competitive as building blocks in segmented calorimeters,
with applications to nuclear and particle physics as well as Positron Emission
Tomography technology
Measurement of radon-induced backgrounds in the NEXT double beta decay experiment
The measurement of the internal Rn activity in the NEXT-White
detector during the so-called Run-II period with Xe-depleted xenon is
discussed in detail, together with its implications for double beta decay
searches in NEXT. The activity is measured through the alpha production rate
induced in the fiducial volume by Rn and its alpha-emitting progeny.
The specific activity is measured to be ~mBq/m. Radon-induced electrons have also been
characterized from the decay of the Bi daughter ions plating out on the
cathode of the time projection chamber. From our studies, we conclude that
radon-induced backgrounds are sufficiently low to enable a successful NEXT-100
physics program, as the projected rate contribution should not exceed
0.1~counts/yr in the neutrinoless double beta decay sample.Comment: 28 pages, 10 figures, 6 tables. Version accepted for publication in
JHE
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Demonstration of the event identification capabilities of the NEXT-White detector
In experiments searching for neutrinoless double-beta decay, the possibility of identifying the two emitted electrons is a powerful tool in rejecting background events and therefore improving the overall sensitivity of the experiment. In this paper we present the first measurement of the efficiency of a cut based on the different event signatures of double and single electron tracks, using the data of the NEXT-White detector, the first detector of the NEXT experiment operating underground. Using a 228Th calibration source to produce signal-like and background-like events with energies near 1.6 MeV, a signal efficiency of 71.6 ± 1.5 stat± 0.3 sys% for a background acceptance of 20.6 ± 0.4 stat± 0.3 sys% is found, in good agreement with Monte Carlo simulations. An extrapolation to the energy region of the neutrinoless double beta decay by means of Monte Carlo simulations is also carried out, and the results obtained show an improvement in background rejection over those obtained at lower energies. [Figure not available: see fulltext.
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Radiogenic backgrounds in the NEXT double beta decay experiment
Natural radioactivity represents one of the main backgrounds in the search for neutrinoless double beta decay. Within the NEXT physics program, the radioactivity- induced backgrounds are measured with the NEXT-White detector. Data from 37.9 days of low-background operations at the Laboratorio Subterráneo de Canfranc with xenon depleted in 136Xe are analyzed to derive a total background rate of (0.84±0.02) mHz above 1000 keV. The comparison of data samples with and without the use of the radon abatement system demonstrates that the contribution of airborne-Rn is negligible. A radiogenic background model is built upon the extensive radiopurity screening campaign conducted by the NEXT collaboration. A spectral fit to this model yields the specific contributions of 60Co, 40K, 214Bi and 208Tl to the total background rate, as well as their location in the detector volumes. The results are used to evaluate the impact of the radiogenic backgrounds in the double beta decay analyses, after the application of topological cuts that reduce the total rate to (0.25±0.01) mHz. Based on the best-fit background model, the NEXT-White median sensitivity to the two-neutrino double beta decay is found to be 3.5σ after 1 year of data taking. The background measurement in a Qββ±100 keV energy window validates the best-fit background model also for the neutrinoless double beta decay search with NEXT-100. Only one event is found, while the model expectation is (0.75±0.12) events. [Figure not available: see fulltext.]
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