162 research outputs found
Sensitivity of the NEXT experiment to Xe-124 double electron capture
Artículo escrito por un elevado número de autores, solo se referencian el que aparece en primer lugar, el nombre del grupo de colaboración, si le hubiere, y los autores pertenecientes a la UAMDouble electron capture by proton-rich nuclei is a second-order nuclear process analogous to double beta decay. Despite their similarities, the decay signature is quite different, potentially providing a new channel to measure the hypothesized neutrinoless mode of these decays. The Standard-Model-allowed two-neutrino double electron capture (2νEC EC) has been predicted for a number of isotopes, but only observed in 78Kr, 130Ba and, recently, 124Xe. The sensitivity to this decay establishes a benchmark for the ultimate experimental goal, namely the potential to discover also the lepton-number-violating neutrinoless version of this process, 0νEC EC. Here we report on the current sensitivity of the NEXT-White detector to 124Xe 2νEC EC and on the extrapolation to NEXT-100. Using simulated data for the 2νEC EC signal and real data from NEXT-White operated with 124Xe-depleted gas as background, we define an optimal event selection that maximizes the NEXT-White sensitivity. We estimate that, for NEXT-100 operated with xenon gas isotopically enriched with 1 kg of 124Xe and for a 5-year run, a sensitivity to the 2νEC EC half-life of 6 × 1022 y (at 90% confidence level) or better can be reache
Electron transfer efficiency in liquid xenon across THGEM holes
Dual-phase liquid-xenon time projection chambers (LXe TPCs) deploying a few
tonnes of liquid are presently leading the search for WIMP dark matter. Scaling
these detectors to 10-fold larger fiducial masses, while improving their
sensitivity to low-mass WIMPs presents difficult challenges in detector design.
Several groups are considering a departure from current schemes, towards either
single-phase liquid-only TPCs, or dual-phase detectors where the
electroluminescence region consists of patterned electrodes. Here, we discuss
the possible use of Thick Gaseous Electron Multipliers (THGEMs) coated with a
VUV photocathode and immersed in LXe as a building block in such designs. We
focus on the transfer efficiencies of ionization electrons and photoelectrons
emitted from the photocathode through the electrode holes, and show
experimentally that efficiencies approaching 100 % can be achieved with
realistic voltage settings. The observed voltage dependence of the transfer
efficiencies is consistent with electron transport simulations once diffusion
and charging-up effects are included
Quasiparticle photoemission intensity in doped two-dimensional quantum antiferromagnets
Using the self-consistent Born approximation, and the corresponding wave
function of the magnetic polaron, we calculate the quasiparticle weight
corresponding to destruction of a real electron (in contrast to creation of a
spinless holon), as a funtion of wave vector for one hole in a generalized
model and the strong coupling limit of a generalized Hubbard model. The
results are in excellent agreement with those obtained by exact diagonalization
of a sufficiently large cluster. Only the Hubbard weigth compares very well
with photoemission measurements in Sr_2CuO_2Cl_2.Comment: 11 pages, latex, 3 figure
Electron momentum distribution in underdoped cuprates
We investigate the electron momentum distribution function (EMD) in a weakly
doped two-dimensional quantum antiferromagnet (AFM) as described by the t-J
model. Our analytical results for a single hole in an AFM based on the
self-consistent Born approximation (SCBA) indicate an anomalous momentum
dependence of EMD showing 'hole pockets' coexisting with a signature of an
emerging large Fermi surface. The position of the incipient Fermi surface and
the structure of the EMD is determined by the momentum of the ground state. Our
analysis shows that this result remains robust in the presence of next-nearest
neighbor hopping terms in the model. Exact diagonalization results for small
clusters are with the SCBA reproduced quantitatively.Comment: 5 pages, submitted to PR
Charge and spin excitations of insulating lamellar copper oxides
A consistent description of low-energy charge and spin responses of the
insulating Sr_2CuO_2Cl_2 lamellar system is found in the framework of a
one-band Hubbard model which besides includes hoppings up to 3^{rd}
nearest-neighbors. By combining mean-field calculations, exact diagonalization
(ED) results, and Quantum Monte Carlo simulations (QMC), we analyze both charge
and spin degrees of freedom responses as observed by optical conductivity,
ARPES, Raman and inelastic neutron scattering experiments. Within this
effective model, long-range hopping processes flatten the quasiparticle band
around . We calculate also the non-resonant A_{1g} and B_{1g} Raman
profiles and show that the latter is composed by two main features, which are
attributed to 2- and 4-magnon scattering.Comment: 6 pages, 3 figures, To be published in PRB (july
Evaluation of turbulent dissipation rate retrievals from Doppler Cloud Radar
Turbulent dissipation rate retrievals from cloud radar Doppler velocity measurements are evaluated using independent, in situ observations in Arctic stratocumulus clouds. In situ validation data sets of dissipation rate are derived using sonic anemometer measurements from a tethered balloon and high frequency pressure variation observations from a research aircraft, both flown in proximity to stationary, ground-based radars. Modest biases are found among the data sets in particularly low- or high-turbulence regimes, but in general the radar-retrieved values correspond well with the in situ measurements. Root mean square differences are typically a factor of 4-6 relative to any given magnitude of dissipation rate. These differences are no larger than those found when comparing dissipation rates computed from tetheredballoon and meteorological tower-mounted sonic anemometer measurements made at spatial distances of a few hundred meters. Temporal lag analyses suggest that approximately half of the observed differences are due to spatial sampling considerations, such that the anticipated radar-based retrieval uncertainty is on the order of a factor of 2-3. Moreover, radar retrievals are clearly able to capture the vertical dissipation rate structure observed by the in situ sensors, while offering substantially more information on the time variability of turbulence profiles. Together these evaluations indicate that radar-based retrievals can, at a minimum, be used to determine the vertical structure of turbulence in Arctic stratocumulus clouds
Radon and material radiopurity assessment for the NEXT double beta decay experiment
The Neutrino Experiment with a Xenon TPC (NEXT), intended to investigate the
neutrinoless double beta decay using a high-pressure xenon gas TPC filled with
Xe enriched in 136Xe at the Canfranc Underground Laboratory in Spain, requires
ultra-low background conditions demanding an exhaustive control of material
radiopurity and environmental radon levels. An extensive material screening
process is underway for several years based mainly on gamma-ray spectroscopy
using ultra-low background germanium detectors in Canfranc but also on mass
spectrometry techniques like GDMS and ICPMS. Components from shielding,
pressure vessel, electroluminescence and high voltage elements and energy and
tracking readout planes have been analyzed, helping in the final design of the
experiment and in the construction of the background model. The latest
measurements carried out will be presented and the implication on NEXT of their
results will be discussed. The commissioning of the NEW detector, as a first
step towards NEXT, has started in Canfranc; in-situ measurements of airborne
radon levels were taken there to optimize the system for radon mitigation and
will be shown too.Comment: Proceedings of the Low Radioactivity Techniques 2015 workshop
(LRT2015), Seattle, March 201
Spatial Structure of Spin Polarons in the t-J Model
The deformation of the quantum Neel state induced by a spin polaron is
analyzed in a slave fermion approach. Our method is based on the selfconsistent
Born approximation for Green's and the wave function for the quasiparticle. The
results of various spin-correlation functions relative to the position of the
moving hole are discussed and shown to agree with those available from small
cluster calculations. Antiferromagnetic correlations in the direct neighborhood
of the hole are reduced, but they remain antiferromagnetic even for J as small
as 0.1 t. These correlation functions exhibit dipolar distortions in the spin
structure, which sensitively depend on the momentum of the quasiparticle. Their
asymptotic decay with the distance from the hole is governed by power laws, yet
the spectral weight of the quasiparticles does not vanish.Comment: 12 pages, 2 postscipt files with figures; uses REVTeX, to be
published in Phys. Rev. B, Feb. 199
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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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