7,359 research outputs found
Visibility Fringe Reduction Due to Noise-Induced Effects: Microscopic Approach to Interference Experiments
Decoherence is the main process behind the quantum to classical transition.
It is a purely quantum mechanical effect by which the system looses its ability
to exhibit coherent behavior. The recent experimental observation of
diffraction and interference patterns for large molecules raises some
interesting questions. In this context, we identify possible agents of
decoherence to take into account when modeling these experiments and study
theirs visible (or not) effects on the interference pattern. Thereby, we
present an analysis of matter wave interferometry in the presence of a dynamic
quantum environment and study how much the visibility fringe is reduced and in
which timescale the decoherence effects destroy the interference of massive
objects. Finally, we apply our results to the experimental data reported on
fullerenes and cold neutrons.Comment: 14 pages, 5 figures. Version to appear in Mod. Phys.
Unconventional antiferromagnetic correlations of the doped Haldane gap system YBaNiZnO
We make a new proposal to describe the very low temperature susceptibility of
the doped Haldane gap compound YBaNiZnO. We propose a new
mean field model relevant for this compound. The ground state of this mean
field model is unconventional because antiferromagnetism coexists with random
dimers. We present new susceptibility experiments at very low temperature. We
obtain a Curie-Weiss susceptibility as expected
for antiferromagnetic correlations but we do not obtain a direct signature of
antiferromagnetic long range order. We explain how to obtain the ``impurity''
susceptibility by subtracting the Haldane gap contribution to
the total susceptibility. In the temperature range [1 K, 300 K] the
experimental data are well fitted by . In the temperature range [100 mK, 1 K] the experimental data are
well fitted by , where increases with
. This fit suggests the existence of a finite N\'eel temperature which is
however too small to be probed directly in our experiments. We also obtain a
maximum in the temperature dependence of the ac-susceptibility which
suggests the existence of antiferromagnetic correlations at very low
temperature.Comment: 19 pages, 17 figures, revised version (minor modifications
Production and relevance of cosmogenic radionuclides in NaI(Tl) crystals
The cosmogenic production of long-lived radioactive isotopes in materials is
an hazard for experiments demanding ultra-low background conditions. Although
NaI(Tl) scintillators have been used in this context for a long time, very few
activation data were available. We present results from two 12.5 kg NaI(Tl)
detectors, developed within the ANAIS project and installed at the Canfranc
Underground Laboratory. The prompt data taking starting made possible a
reliable quantification of production of some I, Te and Na isotopes with
half-lives larger than ten days. Initial activities underground were measured
and then production rates at sea level were estimated following the history of
detectors; a comparison of these rates with calculations using typical cosmic
neutron flux at sea level and a selected description of excitation functions
was also carried out. After including the contribution from the identified
cosmogenic products in the detector background model, we found that the
presence of 3H in the crystal bulk would help to fit much better our background
model and experimental data. We have analyzed the cosmogenic production of 3H
in NaI, and although precise quantification has not been attempted, we can
conclude that it could imply a very relevant contribution to the total
background below 15 keV in NaI detectors.Comment: Proceedings of the Low Radioactivity Techniques 2015 workshop, March
2015, Seattle (US
Background analysis and status of the ANAIS dark matter project
ANAIS (Annual modulation with NaI Scintillators) is a project aiming to set
up at the new facilities of the Canfranc Underground Laboratory (LSC), a large
scale NaI(Tl) experiment in order to explore the DAMA/LIBRA annual modulation
positive result using the same target and technique. Two 12.5 kg each NaI(Tl)
crystals provided by Alpha Spectra took data at the LSC in the ANAIS-25 set-up.
The comparison of the background model for the ANAIS-25 prototypes with the
experimental results is presented. ANAIS crystal radiopurity goals have been
achieved for Th-232 and U-238 chains, but a Pb-210 contamination
out-of-equilibrium was identified, whose origin has been studied. The high
light collection efficiency obtained with these prototypes allows to anticipate
an energy threshold of the order of 1 keVee. A new detector, with improved
performances, was received in March 2015 and very preliminary results are
shown.Comment: 6 pages, 7 figure
Geometric phases under the presence of a composite environment
We compute the geometric phase for a spin-1/2 particle under the presence of
a composite environment, composed of an external bath (modeled by an infinite
set of harmonic oscillators) and another spin-1/2 particle. We consider both
cases: an initial entanglement between the spin-1/2 particles and an initial
product state in order to see if the initial entanglement has an enhancement
effect on the geometric phase of one of the spins. We follow the nonunitary
evolution of the reduced density matrix and evaluate the geometric phase for a
single two-level system. We also show that the initial entanglement enhances
the sturdiness of the geometric phase under the presence of an external
composite environment.Comment: 10 pages, 12 figures. Version to appear in Phys. Rev.
Preliminary results of ANAIS-25
The ANAIS (Annual Modulation with NaI(Tl) Scintillators) experiment aims at
the confirmation of the DAMA/LIBRA signal using the same target and technique
at the Canfranc Underground Laboratory. 250 kg of ultrapure NaI(Tl) crystals
will be used as a target, divided into 20 modules, each coupled to two
photomultipliers. Two NaI(Tl) crystals of 12.5 kg each, grown by Alpha Spectra
from a powder having a potassium level under the limit of our analytical
techniques, form the ANAIS-25 set-up. The background contributions are being
carefully studied and preliminary results are presented: their natural
potassium content in the bulk has been quantified, as well as the uranium and
thorium radioactive chains presence in the bulk through the discrimination of
the corresponding alpha events by PSA, and due to the fast commissioning, the
contribution from cosmogenic activated isotopes is clearly identified and their
decay observed along the first months of data taking. Following the procedures
established with ANAIS-0 and previous prototypes, bulk NaI(Tl) scintillation
events selection and light collection efficiency have been also studied in
ANAIS-25.Comment: 4 pages, 6 Figure
Status of the ANAIS Dark Matter Project at the Canfranc Underground Laboratory
The ANAIS experiment aims at the confirmation of the DAMA/LIBRA signal. A
detailed analysis of two NaI(Tl) crystals of 12.5 kg each grown by Alpha
Spectra will be shown: effective threshold at 1 keVee is at reach thanks to
outstanding light collection and robust PMT noise filtering protocols and the
measured background is well understood down to 3 keVee, having quantified K, U
and Th content and cosmogenic activation in the crystals. A new detector was
installed in Canfranc in March 2015 together with the two previous modules and
preliminary characterization results will be presented. Finally, the status and
expected sensitivity of the full experiment with 112 kg will be reviewed.Comment: Contributed to the 11th Patras Workshop on Axions, WIMPs and WISPs,
Zaragoza, June 22 to 26, 201
Bulk NaI(Tl) scintillation low energy events selection with the ANAIS-0 module
Dark matter particles scattering off some target nuclei are expected to
deposit very small energies in form of nuclear recoils (below 100 keV). Because
of the low scintillation efficiency for nuclear recoils vs. electron recoils,
in most of the scintillating targets considered in the search for dark matter,
the region below 10 keVee concentrates most of the expected dark matter signal.
For this reason, very low energy threshold (at or below 2 keVee) and very low
background are required. This is the case of the ANAIS (Annual modulation with
NaI Scintillators) experiment. A good knowledge of the detector response
function for real scintillation events, a good characterization of other
anomalous or noise event populations contributing in that energy range, and the
development of convenient filtering procedures for the latter are mandatory to
achieve the required low background at such a low energy. In this work we will
present the specific protocols developed to select bulk scintillation events in
NaI(Tl), and its application to data obtained with the ANAIS-0 prototype.
Slight differences in time constants are expected in scintillation pulses
produced by nuclear or electron recoils in NaI(Tl), so in order to analyze the
effect of these filtering procedures in the case of a recoil population
attributable to dark matter, data from a neutron calibration have been used.Comment: 13 pages, 22 figure
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