137 research outputs found
Heavy metals transport in typical portuguese loamy sand soils
Illegal discharges are of great concern among industry activities, since they occur under
uncontrolled conditions. In most cases, effluents are acidic and the concentrations of heavy
metals are very high. With this in mind, the main goal of this study was to evaluate the
sorption of two of the most toxic heavy metals, Cr(VI) and Pb(II), in those conditions.
A loamy sand soil was collected in Oporto, Portugal. Batch equilibrium and kinetic sorption
experiments were performed using both metals solutions, with concentrations among 50 mg
L-1 and 200 mg L-1, at pH 2 and 5, between 2 h and 288 h.
To evaluate the sorption equilibrium, eight isotherm models were fitted. Better adjustments
were observed for the Redlich-Peterson and Khan models for the adsorption of chromium (R2
= 0.99), and of lead (R2 = 0.99), respectively. The sorption kinetics was evaluated using three
models – Elovich, Pseudo first order and an empirical power function. The retention of lead
was almost instantaneous and the empirical power function described better the sorption
kinetics of chromium (0.89 < R2 < 0.99).
In addition, flow experiments were performed with effluents of both metals (50 mg L-1) at pH
2 and 5, for about 90 h. Results revealed a high retention of chromium, and a weak retention
of lead, for low pH values. FTIR analyses to the columns samples revealed that clay minerals
have an important role in the retention of both metals
Inverse Low Gain Avalanche Detectors (iLGADs) for precise tracking and timing applications
Low Gain Avalanche Detector (LGAD) is the baseline sensing technology of the
recently proposed Minimum Ionizing Particle (MIP) end-cap timing detectors
(MTD) at the Atlas and CMS experiments. The current MTD sensor is designed as a
multi-pad matrix detector delivering a poor position resolution, due to the
relatively large pad area, around 1 ; and a good timing resolution,
around 20-30 ps. Besides, in his current technological incarnation, the timing
resolution of the MTD LGAD sensors is severely degraded once the MIP particle
hits the inter-pad region since the signal amplification is missing for this
region. This limitation is named as the LGAD fill-factor problem. To overcome
the fill factor problem and the poor position resolution of the MTD LGAD
sensors, a p-in-p LGAD (iLGAD) was introduced. Contrary to the conventional
LGAD, the iLGAD has a non-segmented deep p-well (the multiplication layer).
Therefore, iLGADs should ideally present a constant gain value over all the
sensitive region of the device without gain drops between the signal collecting
electrodes; in other words, iLGADs should have a 100 fill-factor by
design. In this paper, tracking and timing performance of the first iLGAD
prototypes is presented.Comment: Conference Proceedings of VCI2019, 15th Vienna Conference of
Instrumentation, February 18-22, 2019, Vienna, Austri
Finite-key analysis of loss-tolerant quantum key distribution based on random sampling theory
The core of security proofs of quantum key distribution (QKD) is the estimation of a parameter that determines the amount of privacy amplification that the users need to apply in order to distill a secret key. To estimate this parameter using the observed data, one needs to apply concentration inequalities such as random sampling theory or Azuma's inequality. The latter can be straightforwardly employed in a wider class of QKD protocols, including those that do not rely on basis-independent sources such as the loss-tolerant (LT) protocol. However, when applied to real-life finite-length QKD experiments, Azuma's inequality typically results in substantially lower secret-key rates. Here we propose an alternative security analysis of the LT protocol against general attacks, for both its prepare-and-measure and measurement-device-independent versions, that is based on random sampling theory. Consequently, our security proof provides considerably higher secret-key rates than the previous finite-key analysis based on Azuma's inequality. This work opens up the possibility of using random sampling theory to provide alternative security proofs for other QKD protocols
Late Holocene Aleppo pine (Pinus halepensis Miller) woodlands in Mallorca (Balearic Islands, Western Mediterranean): Investigation of their distribution and the role of human management based on anthracological, dendro-anthracological and archaeopalynological data
The pioneering nature of Mediterranean pines and their phytosociological role have been largely discussed in relation to different agents (e.g., edaphic, climatic or anthropogenic). In this context, Aleppo pine is one of the most widespread pine species in the Mediterranean basin, as it is especially adapted to climatic constraints, such as drought and high seasonality, and has a high tolerance for salinity and strong coastal winds. It is also well adapted to regeneration after anthropogenic landscape disturbances, highlighting its important after-fire regeneration rates. In this sense, phytosociological studies conducted in Mediterranean landscapes have found that this species'' wide distribution is mostly due to its rapid regeneration after human landscape transformation, including fire, and the abandonment of agricultural lands. Aleppo pine is considered to broadly develop after human action in sclerophyllous formation, in which it would be scarce or absent without human intervention. Parallel, paleoenvironmental and archaeobotanical studies have attempted to trace these trends back to prehistoric times to investigate this species'' role in Late Pleistocene and Holocene vegetation and evaluate the role of climate and human action in its diachronic dynamics. In this study, we present a compendium of anthracological, dendro-anthracological and archaeopalyonological data with the objective of (i) investigating the nature and distribution of Aleppo pine on the island of Mallorca and (ii) evaluating the possibility that human action could have resulted in the spread of this pine species during the first two millennia of permanent human occupation of the island (c. 2300 cal. BCE–1st-century ACE). Investigating these archaeobotanical datasets, as well as making comparisons with anthracological and paleoenvironmental studies in neighbouring Mediterranean zones (Iberia), allowed us to attest that Aleppo pine is a natural, pre-human component of the Holocene vegetation of the island, and it is especially well-adapted to coastal environments. Moreover, we describe the trends and characteristics of the human management of pine woodlands through anthracology and dendro-anthracology, suggesting that human action did not provoke widespread growth of Aleppo pine in Mallorca at the expense of other vegetation types during prehistory. Such processes, well-documented by current phytosociological studies, probably began at some unknown point after the Romanisation of the island
Nernst Effect and Anomalous Transport in Cuprates: A Preformed-Pair Alternative to the Vortex Scenario
We address those puzzling experiments in underdoped high
superconductors which have been associated with normal state "vortices" and
show these data can be understood as deriving from preformed pairs with onset
temperature . For uncorrelated bosons in small magnetic fields, and
arbitrary , we present the exact contribution to \textit{all}
transport coefficients. In the overdoped regime our results reduce to those of
standard fluctuation theories (). Semi-quantitative agreement
with Nernst, ac conductivity and diamagnetic measurements is quite reasonable.Comment: 9 pages, 4 figures; Title, abstract and contents modified, new
references added, figures changed, one more figure added; to be published on
PR
Normal-superconducting transition induced by high current densities in YBa2Cu3O7-d melt-textured samples and thin films: Similarities and differences
Current-voltage characteristics of top seeded melt-textured YBa2Cu3O7-d are
presented. The samples were cut out of centimetric monoliths. Films
characteristics were also measured on microbridges patterned on thin films
grown by dc sputtering. For both types of samples, a quasi-discontinuity or
quenching was observed for a current density J* several times the critical
current density Jc. Though films and bulks much differ in their magnitude of
both Jc and J*, a proposal is made as to a common intrinsic origin of the
quenching phenomenon. The unique temperature dependence observed for the ratio
J*/Jc, as well as the explanation of the pre-quenching regime in terms of a
single dissipation model lend support to our proposal.Comment: 10 pages, 10 figures, submitted to Physical Review
Tight finite-key security for twin-field quantum key distribution
Quantum key distribution (QKD) offers a reliable solution to communication problems that require long-term data security. For its widespread use, however, the rate and reach of QKD systems must be improved. Twin-field (TF) QKD is a step forward toward this direction, with early demonstrations suggesting it can beat the current rate-versus-distance records. A recently introduced variant of TF-QKD is particularly suited for experimental implementation, and has been shown to offer a higher key rate than other variants in the asymptotic regime, where users exchange an infinite number of signals. Here, we extend the security of this protocol to the finite-key regime, showing that it can overcome the fundamental bounds on point-to-point QKD with ~1010 transmitted signals. In many practical regimes of interest, our analysis offers higher key rates than those of alternative variants. Moreover, some of the techniques we develop are applicable to the finite-key analysis of other QKD protocols
Wanted dead or alive : high diversity of macroinvertebrates associated with living and ’dead’ Posidonia oceanica matte
The Mediterranean endemic seagrass Posidonia
oceanica forms beds characterised by a dense leaf canopy
and a thick root-rhizome ‘matte’. Death of P. oceanica
shoots leads to exposure of the underlying matte, which
can persist for many years, and is termed ‘dead’ matte.
Traditionally, dead matte has been regarded as a degraded
habitat. To test whether this assumption was
true, the motile macroinvertebrates of adjacent living
(with shoots) and dead (without shoots) matte of
P. oceanica were sampled in four different plots located
at the same depth (5–6 m) in Mellieha Bay, Malta
(central Mediterranean). The total number of species
and abundance were significantly higher (ANOVA;
P<0.05 and P<0.01, respectively) in the dead matte
than in living P. oceanica matte, despite the presence of
the foliar canopy in the latter. Multivariate analysis
(MDS) clearly showed two main groups of assemblages,
corresponding to the two matte types. The amphipods
Leptocheirus guttatus and Maera grossimana, and the
polychaete Nereis rava contributed most to the dissimilarity
between the two different matte types. Several
unique properties of the dead matte contributing to the
unexpected higher number of species and abundance of
motile macroinvertebrates associated with this habitat
are discussed. The findings have important implications
for the conservation of bare P. oceanica matte, which
has been generally viewed as a habitat of low ecological
value.peer-reviewe
Hacia la reforma educativa: Análisis de un proyecto de investigación en el distrito de San Juan II
How to cite: Cruz Velázquez, J. L., Currás, M. del C., PadÃn Zamot, W., RodrÃguez, C. A., & Quintero Rivera, I. (1990). Hacia la reforma educativa: Análisis de un proyecto de investigación en el distrito de San Juan II. Cuaderno de Investigación en la Educación, 2, 12- 22. Retrieved from https://revistas.upr.edu/index.php/educacion/article/view/16354Cómo citar: Cruz Velázquez, J. L., Currás, M. del C., PadÃn Zamot, W., RodrÃguez, C. A., & Quintero Rivera, I. (1990). Hacia la reforma educativa: Análisis de un proyecto de investigación en el distrito de San Juan II. Cuaderno de Investigación en la Educación, 2, 12- 22. Recuperado a partir de https://revistas.upr.edu/index.php/educacion/article/view/1635
The in-plane paraconductivity in La_{2-x}Sr_xCuO_4 thin film superconductors at high reduced-temperatures: Independence of the normal-state pseudogap
The in-plane resistivity has been measured in (LSxCO)
superconducting thin films of underdoped (), optimally-doped
() and overdoped () compositions. These films were grown
on (100)SrTiO substrates, and have about 150 nm thickness. The in-plane
conductivity induced by superconducting fluctuations above the superconducting
transition (the so-called in-plane paraconductivity, ) was
extracted from these data in the reduced-temperature range
10^{-2}\lsim\epsilon\equiv\ln(T/\Tc)\lsim1. Such a
was then analyzed in terms of the
mean-field--like Gaussian-Ginzburg-Landau (GGL) approach extended to the
high- region by means of the introduction of a total-energy cutoff,
which takes into account both the kinetic energy and the quantum localization
energy of each fluctuating mode. Our results strongly suggest that at all
temperatures above Tc, including the high reduced-temperature region, the
doping mainly affects in LSxCO thin films the normal-state properties and that
its influence on the superconducting fluctuations is relatively moderate: Even
in the high- region, the in-plane paraconductivity is found to be
independent of the opening of a pseudogap in the normal state of the underdoped
films.Comment: 35 pages including 10 figures and 1 tabl
- …